Using lectures (on the web-page of histology department), lecture presentations textbooks, additional literature and other resourses, students should prepare such theoretical questions:
1. Structural and functional characteristics of connective tissue that distinguish it from the other basic tissue type.
2. Functions carried out by connective tissue.
3. Fundamental components of loose connective tissue.
4. The biochemical composition and the sites of synthesis of extracellular matrix components.
5. Comparison of the collagen, elastic and reticular fibers in terms of their structure and function.
6. Structure and function of the various cell types found in the loose connective tissue.
7. Structural and functional characteristics of connective tissue that distinguish it from the other basic tissue type.
8. Comparison of the different types of dense connective tissues in terms of the types, relative amounts, and arrangement of cells, fibers, and ground substance.
9. Relations of the composition of the various connective tissues with special properties to their specific functions.
10. Name body organs where each connective tissue type may be found and relate these locations to the tissue’s function.
Connective tissue is a term applied to a basic type of tissue of mesodermal origin, which provides structural and metabolic support for other tissues and organs throughout the body. Connective tissue usually contains blood vessels and mediates the exchange of nutrients, metabolites and waste products between tissues and the circulatory system. The traditional term “connective tissue” thus badly does justice to the wide rang of functions of this type of tissue and it is now probably more appropriate to use the term supporting tissue. Connective tissue occurs in many different forms with diverse physical properties.
1. Visceral organs (e.g., kidneys, lungs) contain an abundance of connective tissue that holds the parenchymal epithelial cells together to form the organ.
2. The cardiovascular system is rich in connective tissues. Here, they tie muscle cells and endothelial cells together into a functionally integrated system.
3. Skeletal muscles are bound together by connective tissues and attached to bones by ligaments and tendons, which are types of specialized connective tissue.
4. The central nervous system (CNS) contains less connective tissue than other systems. Little connective tissue is associated with the nervous system parenchyma of neurons and glair cells; however, collagenous fibers and connective tissue cells form a sheath around the brain and spinal cord. Also, some connective tissue exists in and around peripheral nerves and ganglia.
Organs can be divided into parenchima, which is composed of the cells responsible for the main functions typical of the organ, and stroma, which is supporting tissue. Except in the brain and spinal cord, the stroma is made of connective tissue.
The functions of connective tissues, determined chiefly by their mechanical properties, include the binding together, compartmentalization, support, and physical and immunologic protection of other tissues and organs, as well as storage.
Support. Structural support is the major function of connective tissue, which forms the framework upon which all other body tissues are assembled. Its physical properties allow it to bind, to fill spaces, and to separate functional units of other tissues and organs. It thus maintains functional units in their proper 3-dimensional relationships, allowing maintenance and coordination of all body functions.
Defense physical. The viscosity of the extracellular matrix, due largely to hyaluronic acid, slows the progress of many bacteria and foreign particles. Sheets of tightly packed and often interwoven collagen fibers, as in organ capsules, help to confine local infections. However, some bacteria secrete enzymes that break down matrix components; eg, staphylococci, clostridia, streptococci, and pneumococci secrete hyaluronidase.
Defense immunologic. Foreign bodies that successfully penetrate epithelia are intercepted by immunoresponsive cells that inhabit the underlying connective tissue. These cells not only activate a local immune response (inflammation) but mobilize the immune system to supply additional cells via the bloodstream. Recruited cells migrate through capillary and venule walls into the connective tissue, a process called diapedesis.
Repair. Rapidly closing any breaches in the body's protective barriers is an important function of connective tissue. Injury stimulates invasion of the site by immunocom-petent cells and the proliferation of fibroblasts. Macrophages remove clotted blood, damaged tissue, and foreign material, while fibroblasts secrete extracellular matrix materials to fill the breach. Rapidly formed collagenous matrices that close wounds are often less well organized than the original tissues and form scars. Small scars may eventually be completely remodeled; larger scars are only partially remodeled.
Storage. Reserves of water and electrolytes, especially sodium, are stored in the extracellular matrix, owing to the high polyamonic charge density of glycosammoglycans. Energy reserves in the form of lipids are stored in adipocytes.
Transport. Except in the central nervous system, most blood and lymphatic vessels are surrounded by loose connective tissue, which is thus a crossroads for transporting substances to and from other tissues.
All connective tissue cell types derive from cells of the embryonic mesenchyme. Mesenchyme derives from embryonic mesoderm, except head mesenchyme, which derives from the neural crest (mesectoderm).
Simplified representation of the connective tissue cell lineage derived from the multipotential embryonic mesenchyme cell. Dotted arrows indicate that intermediate cell types exist between the examples illustrated. Note that the cells are not drawn in proportion to actual sizes, eg, adipocyte, megakaryocyte, and osteoclast cells are significantly larger than the other cells illustrated.
All connective tissues have two major constituents, cells and extracellular material. Extra cellular material is the constituent, which determines the physical properties of each type of connective tissue. Extra cellular material consists of a matrix of organic material called ground substance within which are embedded a variety of fibers.
Connective tissue types and subtypes are classified according to the amounts, types, and proportions of these components.
There are three main types of connective tissue:
1. Connective tissue proper.
2. Connective tissue with special properties.
3. Skeletal tissues(supporting connective).
PROPER CONNECTIVE TISSUE
Connective tissue proper, found in most organs, is characterized by a predominance of fibers (mainly type I collagen) in the extracellular matrix. Its varied functions are chiefly related to binding cells and tissues into organs and organ systems. Its subclasses are based on the type, density, and orientation of its fibers.
1. Loose connective tissue Loose connective tissue or areolar tissue generally appears very disorganized. It consists of a loosely arranged network of different types of fibers, upon which many kinds of fixed and wandering cells are suspended. The ground substance is abundant and only moderately viscous. This flexible yet delicate tissue surrounds and suspends vessels and nerves as they traverse most organs, underlies and supports most epithelia, and fills spaces between other tissues (e.g., between muscle fibers and their dense connective tissue sheaths). It also supports the serous membranes (mesothelia) of the pleura, pericardium, and peritoneum. Always well vascularized, areolar tissue conveys oxygen and nutrients to avascular epithelia. Its cells function in immune surveillance for foreign substances entering the body through the blood or epithelia.
Simplified representation of the loose connective tissue cells
Loose connective tissue (surface view)
Stained with iron haematoxylin. Medium magnification
2. Collagen fibers.
3. Elastic fibers.
2. Dense connective tissue Fibers are the predominant component of dense connective tissue. Nearly all the fibers are of type I collagen. The cells are predominantly mature fibroblasts (fibrocytes). The ground substance is essentially identical to that of areolar tissue but is present in lesser quantities. There are 2 types of dense connective tissue: regular, with a ropelike arrangement of fiber bundles, and irregular, with a fabriclike arrangement.
a. Dense regular connective tissue The fibers of this tissue are tightly packed into parallel bundles, between which are a few highly attenuated, spindle-shaped fibroblasts. The small, cigar-shaped nuclei of the fibroblasts are oriented parallel to the fibers; the cytoplasm is difficult to distinguish with the light microscope. There is little room for the ground substance, which nevertheless permeates the tissue. The tensile strength of the packed collagen fibers makes them ideal for transmitting mechanical force over long distances using a minimum of material and space, while resisting mechanical forces from other directions. This tissue therefore serves to transmit the force of muscle contraction, to attach bones to one another, and to protect other tissues and organs. It is found in tendons, ligaments, periosteum, perichondrium, deep fascia, and some organ capsules.
Longitudinal section of dense regular connective tissue (tendon). Bundles of collagen fibers fill the spaces between the elongated fibroblasts. H&E stain. Medium magnification.
Longitudinal section of dense regular connective tissue from a tendon. A: Thick bundles of parallel collagen fibers fill the intercellular spaces between fibroblasts. Low magnification. B: Higher magnification view of a tendon of a young animal. Note active fibroblasts with prominent Golgi regions and dark cytoplasm rich in RNA. PT stain.
Electron micrograph of a fibrocyte in dense regular connective tissue. The sparse cytoplasm of the fibrocytes is divided into numerous thin cytoplasmic processes that interdigitate among the collagen fibers. x25,000.
b. Dense irregular connective tissue The components of this tissue are identical to those found in dense regular connective tissue. At first glance, dense irregular connective tissue seems poorly organized, but its collagen bundles have a complex woven pattern that provides resistance to tensile stress from any direction. Dense irregular connective tissue has numerous extra cellular fibers in dense random arrays. Its functions include covering the more fragile tissues and organs and protecting them from multidirectional mechanical stresses. It is found in the reticular layer of the dermis and in most organ capsules.
Dense irregular connective tissue from human dermis contains thick bundles of collagen fibers, fibroblast nuclei (arrowheads), and a few small blood vessels (bv). H&E stain. Medium magnification.
Section of immature dense irregular collagen tissue. This figure shows numerous fibroblasts (arrow) with many thin cytoplasmic extensions (arrowheads). As these cells are pressed by collagen fibers, the appearance of their cytoplasm depends on the section orientation; when the section is parallel to the cell surface, parts of the cytoplasm are visible. PT stain. Medium magnification.
Dense irregular connective tissue contains many randomly oriented bundles of collagen fibers. H&E stain. Medium magnification.
Loose and dense irregular connective tissue of dermis of the skin. H&E stain. Low magnification.
The loose connective tissue contains the following cell types: fibroblasts, undiferentiated (mesenchymal), macrophages, and a varying, but much smaller, number of lymphoid wandering cells, mast cells, eosinophils, plasma cells, pigment cells and fat cells.
Fibroblasts These are the predominant connective tissue cells and are ubiquitous in connective tissue proper. They synthesize, secrete, and maintain all the major components of the extra cellular matrix, bind extra cellular matrix constituents to form tissue, and facilitate wound healing. For example, when the skin is cut, fibroblasts proliferate and migrate toward the wound to fill the gaps in the tissue. As proliferation continues, they secrete large amounts of extra cellular matrix. Fibroblasts form the scar that closes the wound.
Section of rat skin. A connective tissue layer (dermis) shows several fibroblasts (F), which are the elongated cells. H&E stain. Medium magnification.
Structurally, fibroblasts are of 2 types, one of which resembles mesenchymal cells. This type is stellate, with long cytoplasmic processes and a large, ovoid, pale-staining nucleus. The cytoplasm contains abundant rough endoplasmic reticulum and Golgi complex, and this cell type is important in the production of collagen and other matrix components. Also, lysosomes and vacuoles of secretion product are prominent features of fibroblasts. Fibroblast secretions include collagen, fibronectin, glycoproteins, and proteoglycans. Fibroblasts have many actin-containing microfilaments because they are highly motile cells. Fibroblasts also contain numerous microtubules, which probably help maintain the fibroblastic cell morphology.
Quiescent fibroblasts are elongated cells with thin cytoplasmic extensions and condensed chromatin. Pararosaniline-toluidine blue (PT) stain. Medium magnification.
Cells of the second type are less active and are sometimes termed fibrocytes, because they are believed to be more mature. Fibrocytes are smaller and spindle-shaped, with a dark, elongate nucleus and fewer cytoplasmic organelles. They can revert to the fibroblast state and participate in tissue repair.
Active (left) and quiescent (right) fibroblasts. External morphologic characteristics and ultrastructure of each cell are shown. Fibroblasts that are actively engaged in synthesis are richer in mitochondria, lipid droplets, Golgi complex, and rough endoplasmic reticulum than are quiescent fibroblasts (fibrocytes).
Electron micrograph revealing portions of several flattened fibroblasts in dense connective tissue. Abundant mitochondria, rough endoplasmic reticulum, and vesicles distinguish these cells from the less active fibrocytes. Multiple strata of collagen fibrils (C) lie among the fibroblasts. x30,000.
Macrophages These are large, stellate cells derived from cells of the blood monocyte lineage that infiltrate connective tissue and develop into phagocytes. They have a highly variable shape because they move throughout connective tissues. They have a single irregularly shaped nucleus with one or two prominent indentations and a conspicuous mass of euchromatin. Resident macrophages can proliferate and form additional macrophages. Dye particles injected into the body are engulfed by these cells and accumulate in cytoplasmic granules. Macrophages contain many lysosomes, which aid in digesting phagocytosed materials, and a well-developed Golgi complex. Macrophage lysosomes contain specific bacteriocidal agents including lysozyme, a hydrolytic enzyme for degrading bacteria cell walls containing oligosaccharide, and superoxide dismutase, an enzyme that helps generate bacteriocidal oxygen-free radicals and hydrogen peroxide. They help maintain the integrity of connective tissues by removing foreign substances and cellular debris, and they participate in the immune response by presenting phagocytosed antigens to lymphocytes. Macrophages also have a prominent cortical array of actin-contaimng microfilaments and many microtubules and intermediate filaments for locomotion and phagocytosis.
Section of pancreas from a rat injected with the vital dye trypan blue. Note that 3 macrophages (arrows) have engulfed and accumulated the dye in the form of granules. H&E stain. Low magnification.
Accumulation of the dust by macrophages. H&E stain. Medium magnification.
Electron micrograph of a macrophage. Note the secondary lysosomes (L), the nucleus (N), and the nucleolus (Nu). The arrows indicate phagocytic vacuoles.
To remove large foreign objects such as splinters, macrophages may fuse to form multinuclear giant cells. Monocyte-derived phagocytes, which together constitute the mononuclear phagocyte system, include:
(1) Bone marrow stem cells
(2) Monocyte precursors in bone marrow
(3) Monocytes in peripheral blood
(4) Fixed macrophages in connective tissues (histiocytes)
(5) Phagocytic Kupffer cells lining the liver sinusoids
(6) Alveolar macrophages and free macrophages in serous cavities
(7) Free and fixed macrophages in the spleen, lymph nodes, and thymus
(8) Microglia in the CNS. (Other CNS glia such as astrocytes and oligodendroglial cells are derived from embryonic neuroectoderm; however, convincing evidence exists that mi-croglial cells are derived from mesenchyme and are part of the mononuclear phagocyte system.)
(9) Osteoclasts in bone. (Osteoclasts are derived from monocytes and share many of the morphologic and functional characteristics of other mononuclear phagocyte system components.)
Macrophages also exhibit chemotaxis. They can move up a concentration gradient of certain complement system components (materials released during an inflammatory response) and bacterial components. Chemotaxis is important for attracting phagocytic cells and increasing their number in areas of bacterial invasion and inflammation.
Electron micrograph of several macrophages and 2 eosinophils in a region adjacent to a tumor. This figure illustrates the participation of macrophages in tissue reaction to tumor invasion.
Mast cells are a distinct cell type but are morphologically similar to basophils in peripheral blood. Mast cells are widely distributed in loose areolar connective tissues and are especially plentiful in the lamina propria, which is the areolar connective tissue beneath the moist mucosal epithelium of many visceral organs. These derive from bone marrow precursors and are characterized by abundant basophihc cytoplasmic granules. At the EM level, these granules appear as electron-dense granules. Other features of mast cells at this level are many small plasma membrane folds and a well-developed Golgi complex.
Mast cells are involved in inflammatory reactions and immediate hypersensitivity allergic reactions and are relatively common in the lamina propria of the respiratory and digestive systems.
Section of rat tongue. Several mast cells in the connective tissue surround muscle cells and blood vessels. PT stain. Medium magnification.
Mast cells in the connective tissue. Methylene blue. Medium magnification.
(1) Mast cells bind a portion of the immunoglobulin E (IgE) molecules released into the serum during exposure to antigens such as ragweed pollen.
(2) Upon re-exposure to the antigen, the IgE bound to the surface of the mast cells facilitates release of histamine (which promotes capillary leakage and edema), slow reacting substance (which promotes smooth muscle contraction and blood vessel leakage), eosinophil chemotactic factor (which attracts eosinophils), and heparin (which inhibits coagulation).
Electron micrograph of a human mast cell. The granules (G) contain heparin and histamine. Note the characteristic scroll-like structures within the granules. M, mitochondrion; C, collagen fibrils; E, elastic fibril; N, nucleus. x14,700. Inset: Higher magnification view of a mast cell granule. x44,600.
1: IgE molecules are bound to the surface receptors.
2: After a second exposure to an antigen (eg, bee venom), IgE molecules bound to surface receptors are cross-linked by the antigen. This activates adenylate cyclase and results in the phosphorylation of certain proteins.
3: At the same time, Ca2+ enters the cell.
4: These events lead to intracellular fusion of specific granules and exocytosis of their contents.
5: In addition, phospholipases act on membrane phospholipids to produce leukotrienes. The process of extrusion does not damage the cell, which remains viable and synthesizes new granules. ECF-A, eosinophil chemotactic factor of anaphylaxis.
Mesenchymal cells These are the precursors of most cells indigenous to connective tissues, including fibroblasts and adipose cells. Embryonic mesenchyme consists of a loose network of stellate mesenchymal cells and abundant intercellular fluid. Some mesenchymal cells remain undifferentiated in adult connective tissue and constitute a reserve population of stem cells called adventitial cells, which are difficult to distinguish from some fibroblasts and pericytes in the basement membrane of capillary wall.
Plasma cells These differentiate from antigen-stimulated B lymphocytes and are the primary producers of circulating antibodies. They are sparsely distributed throughout the body but abundant in areas susceptible to penetration by bacteria. Plasma cells are large and ovoid, with an eccentric nucleus and abundant rough endoplasmic reticulum. The characteristic "clock face" of the nucleus results from a large, central nucleolus and several large heterochromatin clumps regularly spaced inside the nuclear envelope. These cells usually exhibit a clear juxtanuclear area (cytocenter) containing a well-developed Golgi complex and centrioles.
Portion of a chronically inflamed intestinal villus. The plasma cells are characterized by their size and abundant basophilic cytoplasm (rough endoplasmic reticulum) and are involved in the synthesis of antibodies. A large Golgi complex (arrows) is where the terminal glycosylation of the antibodies (glycoproteins) occurs. Plasma cells produce antibodies of importance in immune reactions. PT stain. Medium magnification.
Plasma cells with a “clockface nucleus” in loose connective tissue. PT stain. High magnification.
Ultrastructure of a plasma cell. The cell contains a well-developed rough endoplasmic reticulum, with dilated cisternae containing immunoglobulins (antibodies). In plasma cells, the secreted proteins do not aggregate into secretory granules. Nu, nucleolus.
Electron micrograph of a plasma cell showing an abundance of rough endoplasmic reticulum (R). Note that many cisternae are dilated. Four profiles of the Golgi complex (G) are observed near the nucleus (N). M, mitochondria.
Section of an inflamed intestinal lamina propria. Inflammation was caused by nematode parasitism. Aggregated eosinophils and plasma cells function mainly in the connective tissue by modulating the inflammatory process. Giemsa stain. Low magnification.
Reticular cells These reticular connective tissue cells make up a functionally diverse yet morphologically similar group. They produce the reticular fibers that form the netlike stroma of hematopoietic and lymphoid tissues. Some apparently can phagocytose antigenic material and cellular debris. Others (antigen-presenting cells) collect antigens on their surfaces and help activate immunocompetent cells to mount an immune response. Reticular cells are typically stellate wth long, thin cytoplasmic processes. Each has a central, pale, irregularly rounded nucleus and a prominent nucleolus. In the cytoplasm, the number of mitochondria and the degree of development of the Golgi complex and rough endoplasmic reticulum is variable. Some reticular cells, particularly those with less developed organelles, may be stem cells of various blood types.
Adipose cells Adipose cells, or adipocytes, are mesenchymal derivatives specialized as storage depots for lipids. They are found scattered singly or in groups in the loose connective tissue, especially along the blood vessels. When they accumulate in large numbers and crowd out the other cells, the tissue is transformed into adipose tissue.
Other blood-derived connective tissue cells Many wandering cell types originate in the bone marrow and are carried to connective tissue by the blood and lymph. Blood-derived cells found in connective tissues include the leukocytes (white blood cells, i.e., lymphocytes, monocytes, neutrophils, eosinophils, and basophils) which have roles in the immune response. When injury or inflammation damages tissue, leukocytes (e.g., monocytes), lymphocytes, and phagocytic granulocytes (e.g., neutrophils, eosinophils, basophils) leave the circulation and join fibroblasts and other connective tissue resident cells to repair damage and combat microorganisms that cause inflammation.
The fibers and ground substance constitute the extra cellular matrix. Connective tissues contain abundant matrix, which largely determines their mechanical properties. The fibers are of 3 types, collagen fibers, elastic fibers and reticular fibers. The ground substance, in which the fibers and cells are embedded, is composed mainly of glycosaminoglycans (GAGs) dissolved in tissue fluid. Matrix viscosity and rigidity are determined by the amount and types of GAGs, the association of GAGs with core proteins to form proteo- glycans, GAG-fiber associations, and GAG-GAG associations. Components of the fibers and ground substance are synthesized and secreted by connective tissue cells (mostly by fibroblasts), and the fibers are assembled in the extra cellular space.
1. Proteoglycans are composed of a core protein to which GAGs are attached. The GAGs of proteoglycans are straight-chain polymers of repeating sugar heterodimers made up of hexosamine (glucosamine or galactosamine) and uronic acid (glucuronic or iduronic acid). Five major classes of GAGs, differing in their sugars, exist in connective tissues: hyaluronic acid (which does not form proteoglycans), chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparan sulfate.
2. Glycoproteins are proteins to which shorter, branched oligosaccharide chains are co-valently bound. Glycoproteins of ground substance are much smaller than proteoglycans. Examples: fibronectin, which mediates the attachment of cells to the extracellular matrix; laminin, a component of basal lamina that mediates attachment of epithelial cells; and chondronectin, a component of cartilage matrix that mediates the attachment of chondrocytes to their matrix.
The characteristics of the amorphous ground substance are as varied as the connective tissues themselves.
a. In loose areolar connective tissue, it is sparse and watery.
b. The amorphous ground substance in cartilage contains proteoglycan, which provides structural rigidity and flexibility yet permits nutrients to diffuse from surrounding blood vessels.
c. In bones and teeth, the amorphous ground substance is calcified, providing these specialized connective tissues with increased tensile strength.
d. In blood, the amorphous ground substance is a complex mixture of glycoproteins dissolved in liquid.
Amorphous ground substance is a watery open mesh that allows water and metabolites to diffuse freely through connective tissues. It contains proteins that bind cells to fibers, and it provides connective tissues with tensile strength and flexibility appropriate for their functions.
Collagen and elastic fibers in loose connective tissue. H&E stain. Medium magnification.
The protein collagen is the most abundant in the body. There are many collagen types, some of which form fibers. Collagen fibers often collect to form fiber bundles. The main function of collagen fibers is the provision of tensile strenght.
1. Synthesis and assembly
a. Intracellular steps Free polysomes reading collagen mRNA attach to the rough en-doplasmic reticulum, and protocollagen polypeptides are deposited in the cisternae. Each protocollagen chain, or alpha chain, has a molecular weight of about 28,000 daltons and about 250 amino acids; every third amino acid is glycine. Proline and lysine residues within the chains are then hydroxylated by proline and lysine hy-droxylases (possibly in smooth endoplasmic reticulum) to form hydroxyproline and hydroxylysine, unusual amino acids present in relatively large amounts in collagen. Core sugars (galactose and glucose) attach to the hydroxylysine residues in the endoplasmic reticulum. With the aid of registration peptides at the ends of the alpha chains, 3 chains coil around one another to form a triple helical molecule called procollagen. Further glycosylation may occur in the Golgi complex, where procollagen is packaged for secretion. Golgi vesicles release procollagen into the extracellular space by exocytosis.
b. Extra cellular steps In the extra cellular space, the enzyme procollagen peptidase cleaves the registration peptides from procollagen, converting it to tropocollagen. Tro-pocollagen molecules become aligned in staggered fashion to form collagen fibrils, possibly under the control of the adjacent cell.
Collagen types Not all collagen types are well characterized. A few examples of collagens whose biochemical structure, function, and location have been studied in some detail are described here.
a. Type I collagen, the most abundant and widespread, forms large fibers and fiber bundles. It is found in tendons, ligaments, bone, dermis, organ capsules, and loose connective tissue.
b. Type II collagen is found in adults only in the cartilage matrix (some occurs in the embryonic notochord) and forms only thin fibrils.
c. Type III collagen is similar to type I, but it is more heavily glycosylated and stains with silver. Often found in association with type I, type III forms networks of thin fibrils that surround and support soft flexible tissues (adipocytes, smooth muscle cells, nerve fibers). It is the major fiber component (reticular fibers) of hematopoietic tissues (eg, bone marrow, spleen) and of the reticular laminae underlying epithelial basal laminae.
Section of a muscular artery stained with picro-sirius and
observed with polarization optics. The upper tunica media (muscular layer)
contains reticular fibers consisting mainly of collagen type III. The lower
layer (tunica adventitia) contains thick fibers and bundles of collagen
d. Type IV collagen is the major collagen type in basal laminae. It does not form fibers or fibrils.
e. Type V collagen is present in placental basement membranes and blood vessels and in small amounts elsewhere. Its structure and function are poorly characterized.
f. Type X collagen is found in the matrix surrounding hypertrophic chondrocytes of degenerating growth plate cartilage in sites of future bone formation.
Electron micrograph of human collagen fibrils in cross and longitudinal sections. Each fibril consists of regular alternating dark and light bands that are further divided by cross-striations. Ground substance completely surrounds the fibrils. x100,000.
In the most abundant form of collagen, type I, each molecule (tropocollagen) is composed of two alpha1 and one alpha2 peptide chains, each with a molecular mass of approximately 100 kDa, intertwined in a right-handed helix and held together by hydrogen bonds and hydrophobic interactions. Each complete turn of the helix spans a distance of 8.6 nm. The length of each tropocollagen molecule is 280 nm, and its width is 1.5 nm.
Schematic drawing of an aggregate of collagen molecules (tropocollagen), fibrils, fibers, and bundles.
There is a stepwise overlapping arrangement of rodlike tropocollagen subunits, each measuring 280 nm (1).
This arrangement results in the production of alternating lacunar and overlapping regions (2)
that cause the cross-striations characteristic of collagen fibrils and confer a 64-nm periodicity of dark and light bands when the fibril is observed in the electron microscope (3).
Fibrils aggregate to form fibers (4),
which aggregate to form bundles (5)
routinely called collagen fibers. Collagen type III usually does not form bundles.
Collagen synthesis. The assembly of the triple helix and the hydroxylation and glycosylation of procollagen molecules are simultaneous processes that begin as soon as the 3 chains cross the membrane of the rough endoplasmic reticulum (RER). Because collagen synthesis depends on the expression of several genes and on several post-translation events, many collagen diseases have been described.
Light microscopy Collagen occurring in large or small bundles of fibrils or as individual fibrils exhibits acidophilic staining properties in H&E-stained sections. In sections stained with Masson's trichrome, collagen fibers stain green. Thin fibers (eg, type III) stain darkly with silver stains, but thicker bundles do not. Collagen molecules that do not form fibers or fibrils (eg, type IV) cannot be distinguished from the surrounding ground substance except by immunohistochemistry.
Electron microscopy All collagen fibrils and fibers have stripes at intervals of 64 nm along their length. This periodicity reflects the staggering of tropocollagen molecules.
Mechanical properties Collagen fibers' most important mechanical property is their tensile strength, which is (weight for weight) greater than that of steel.
Location Collagen fibers are found in all connective tissues and in the reticular laminae of certain basement membranes. In bone, its lacunar regions (spaces between overlapping tropocollagen units) may act as nucleation sites for the hydroxyapatite crystals of bone matrix.
Collagen Renewal: Collagen is a very stable protein, and its turnover is quite slow—slowest in tendons and other dense connective tissues, fastest in loose connective tissue. Macrophages and neutrophils release collagenase, which breaks down old collagen, and new collagen is synthesized by fibroblasts. As humans age, their extra cellular collagen becomes increasingly cross-linked, and its turnover slows in all connective tissues.
Elastic fibers consist of an amorphous albuminoid protein called elastin and numerous proteinaceous microfibrils that become embedded in the elastin.
Total preparation of young rat mesentery showing red picrosirius-stained nonanastomosing bundles of collagen fibers, while the elastic fibers appear as thin, dark anastomosing fibers stained by orcein. Collagen and elastic fibers provide structure and elasticity, respectively, to the mesentery. Medium magnification. B: The same preparation observed with polarizing microscopy. Collagen bundles of various thicknesses are observed. In the superimposed regions, the bundles of collagen are a dark color. Medium magnification.
Skin dermis, selectively stained for elastic fibers. Dark elastic fibers are interspersed with pale red collagen fibers. The elastic fibers are responsible for skin’s elasticity. Medium magnification.
1. Synthesis and assembly
a. Intracellular steps Microfibrillar proteins and proelastin are synthesized on ribo-somes of the RER and secreted separately. Proelastin contains large amounts of the hydrophobic amino acids glycine, proline, and valine, accounting for elastin's insolubility. Microfibrillar protein contains mostly hydrophilic amino acids.
b. Extra cellular steps Proelastin molecules polymerize extracellularly to form elastin chains. Lysyi oxidases then catalyze the conversion of certain lysine residues of elastin to aldehydes, 3 of which condense with a fourth, unaltered lysine residue to form desmosine and isodesmosine. These amino acids, very rare except in elastin, cross-link individual elastin chains. Elastin then associates with numerous microfibrils to form a branching and anastomosing network of elastic fibers.
Histologic appearance Elastin occurs as short branching fibers which form an irregular network throughout the tissue. Elastin contains few charged amino acids, so it stains poorly with standard ionic dyes and elastin fibers are difficult to demonstrate in histological sections. Special stains, such as orcein, Verlhoeff's stain or Weigert's resorcin-fuchsin stain, are used in light microscopic preparations. In EM preparations, both amorphous elastin and microfibrils can be visualized.
Mechanical properties Elastic fibers are extremely pliable and elastic. They can be stretched to 150% of their length without breaking and then return to their original length.
Location Elastic fibers are found where their mechanical properties are necessary to allow tissues to stretch or expand and then return to their original shape, eg, in arterial walls, interalveolar septa, bronchi and bronchioles of the lungs, vocal ligaments, and ligamenta flava of the vertebral column.
Connectine tissue with special properties includes reticular connective tissue, mucous connective tissue, adipose tissue and pigmented tissue.
RETICULAR CONNECTIVE TISSUE
The reticular fibers (type III collagen) of this tissue form delicate 3-dimensional, netlike scaffolding upon which cells, the predominant element, are suspended.
There is very little ground substance. Reticular connective tissue functions in the support of motile cells and filtration of body fluids. It is found mainly in hematopoietic tissues such as bone marrow, spleen, and lymph nodes.
Usually reticular tissue has a lot of different macrophages (free, dendritic, interdigital), which promote protective function and control of blood cells maturation and interaction. Sinusoidal hemocapillaries are typical for this tissue. Their permeable wall allow mature blood formed elements to enter the blood stream, fixed macrophages in their wall control this process.
Reticular connective tissue showing only the attached cells and the fibers (free cells are not represented). Reticular fibers are enveloped by the cytoplasm of reticular cells; the fibers, however, are extracellular, being separated from the cytoplasm by the cell membrane. Within the sinuslike spaces, cells and tissue fluids of the organ are freely mobile.
Reticular cells are firmly attached to the fibers, which may be mostly covered by the long attenuated reticular cell processes. Reticular fibers are argyrophilic fibers. When treated with silver salts, they reduce the silver salts, creating silver metal deposits that stain the fibers black. Other cell types, such as lymphocytes, are suspended in the spaces of the network.
Reticular connective tissue showing reticular fibers of reticular cells. Silver impregnation. Medium magnification.
MUCOUS CONNECTIVE TISSUE
This tissue has small numbers of cells and fibers distributed randomly in the abundant ground substance, which has a syrupy to jellylike consistency and is composed chiefly of hyaluronic acid. Mucous tissue yields readily to pressure but can return to its original shape, so it is useful for protecting underlying structures from excess pressure. It is the predominant component (Wharton's jelly) of the umbilical cord, of the nucleus pulposis of the intervertebral disks, and of the pulp of young teeth. Mucous tissue has now fibers, it is rich with must cells and macrophages (Caschenko-Hofbayer cells).
Mucous tissue of an embryo showing fibroblasts immersed in a very loose extracellular matrix composed mainly of molecules of the ground substances. H&E stain. Medium magnification.
Mucous tissue of umbilical cord named “Wharton’s jelly”.
H&E stain. Low magnification.
Adipose tissue, or fat, is a connective tissue specialized to store fuel. If we were not equipped to store fuel, all of our time would have to be spent obtaining food.
Distribution of adipose tissue. In a human newborn, multilocular adipose tissue constitutes 2–5% of the body weight and is distributed as shown. The black areas indicate multilocular adipose tissue; shaded areas are a mixture of multilocular and unilocular adipose tissue.
The cytoplasm of fat cells, or adipocytes, contains large triglyceride deposits in the form of one or more lipid droplets with no limiting membranes. Together, the clusters of adipocytes scattered throughout the body constitute an extremely important metabolic organ that varies widely in size and distribution, depending on such factors as age, sex, and nutritional status.
Clusters of adipocytes are separated into lobes and lobules by septa of collagenous connective tissue of variable density. A network of reticular fibers surrounds individual cells. The ground substance is sparse.
There are 2 basic types of adipose tissue, termed white adipose tissue, or white fat, and brown adipose tissue, or brown fat. A white fat adipocyte has a single large lipid droplet; a brown fat adipocyte has many small droplets.
This type of adipose tissue comprises up to 20% of total body weight in normal, well-nourished male adults and up to 25% in females.
Distinguishing Features: White adipose tissue, the more abundant of the 2 types, is also termed unilocular adipose tissue, a reference to the single fat droplet in each of its cells. In mature adipocytes, the droplet is so large that it displaces the nucleus and remaining cytoplasm to the cellular periphery. Cell diameter varies from 50 to 150 nm. Adipocytes in histologic sections have a signet-ring appearance because most of the lipid is washed away during preparation, leaving only a flattened nucleus and a thin rim of cytoplasm. The cytoplasm near the nucleus contains a Golgi apparatus, mitochondria, a small amount of rough endoplasmic reticulum, and free ribosomes. The cytoplasm in the thin rim contains smooth endoplasmic reticulum and pinocytotic vesicles. This tissue is sometimes termed yellow adipose tissue or yellow fat; dietary carotenoids accumulate in the lipid droplets, making the tissue yellow. White fat is richly vascularized but not as richly as brown fat.
White adipose tissue
Photomicrograph of unilocular adipose tissue of a young mammal. Arrows show nuclei of adipocytes (fat cells) compressed against the cell membrane. Note that, although most cells are unilocular, there are several cells (asterisks) with small lipid droplets in their cytoplasm, an indication that their differentiation is not yet complete. Pararosaniline–toluidine blue (PT) stain. Medium magnification.
White adipose tissue
H&E stain. Medium magnification.
The process of lipid storage and release by the adipocyte. Triglycerides are transported in blood from the intestine and liver by lipoproteins known as chylomicrons (Chylo) and very low-density lipoproteins (VLDL). In adipose tissue capillaries, these lipoproteins are partly broken down by lipoprotein lipase, releasing free fatty acids and glycerol. The free fatty acids diffuse from the capillary into the adipocyte, where they are re-esterified to glycerol phosphate, forming triglycerides. These resulting triglycerides are stored in droplets until needed. Norepinephrine from nerve endings stimulates the cyclic AMP (cAMP) system, which activates hormone-sensitive lipase. Hormone-sensitive lipase hydrolyzes stored triglycerides to free fatty acids and glycerol. These substances diffuse into the capillary, where free fatty acids are bound to the hydrophobic moiety of albumin for transport to distant sites for use as an energy source.
Development of fat cells. Undifferentiated mesenchymal cells are transformed into lipoblasts that accumulate fat and thus give rise to mature fat cells. When a large amount of lipid is mobilized by the body, mature unilocular fat cells return to the lipoblast stage. Undifferentiated mesenchymal cells also give rise to a variety of other cell types, including fibroblasts. The mature fat cell is larger than that shown here in relation to the other cell types.
Subcutaneous fat Subcutaneous fat (hypodermis) is the layer of white adipose tissue found just beneath the skin except in the eyelids, penis, scrotum, and most of the external ear. (There is some fat in the earlobe.) In infants, it forms a continuous thermal insulating layer of uniform thickness covering the entire body and is termed the panniculus adiposus. In adults it becomes thicker or thinner in selected areas, depending upon the person's age, sex, and dietary habits. Where it becomes thinner, the tissue takes on the appearance of areolar tissue. In males, the fat layer thickens over the nape of the neck, deltoids (shoulders), triceps brachii (back of the upper arm), lumbosacral region (lower back), and buttocks. In females, additional fat is deposited in the breasts and buttocks and over the epitrochanteric region (hips) and anterior aspect of the thighs.
Intraabdominal fat Fat deposits of varying sizes surround blood and lymphatic vessels in the omentum and mesenteries suspended in the abdominal cavity. Additional accumulations occur in retroperitoneal areas, such as around the kidneys on the posterior abdominal wall.
Other locations Other prominent accumulations of fat are found within the eye orbits and surrounding major joints (e.g., knees). Such accumulations also form pads in the palms and soles.
Functional Characteristics: Adipocytes store fatty acids in triglycerides (esters of glycerol and 3 fatty acids). The triglycerides stored in both white and brown fat undergo continuous turnover. Their released fatty acids serve as a source of chemical energy for cells (the predominant source in resting muscle) and as a source of the raw materials for making phospholipids (the predominant component of biologic membranes). Turnover is regulated by several histophysiologic factors, which shift the equilibrium toward fat uptake or fat mobilization, depending on the body's level of, and need for, circulating fatty acids.
Histogenesis: The unilocular adipocytes of white fat derive from mesenchymal precursor cells that resemble fibroblasts. The appearance of numerous small lipid droplets in the cytoplasm signals the transformation of these cells into lipoblasts. As lipid accumulation continues, the small droplets fuse until a single large lipid droplet forms.
BROWN ADIPOSE TISSUE
Distinguishing Features: Brown fat is called multilocular adipose tissue because of the multiple small lipid droplets in its adipocytes. Brown adipocytes are smaller than white adipocytes and have a spheric, centrally located nucleus. They contain large numbers of mitochondria; the tan to reddish-brown tissue color is due chiefly to mitochondrial cytochromes. Loose connective tissue septa give brown adipose tissue a lobular appearance like that of a gland in histologic section. The vascular supply (partly responsible for the color) is very rich, as is the autonomic nerve supply. Many unmyelmated nerve fibers contact the adipocytes.
Photomicrograph of multilocular adipose tissue (lower portion) with its characteristic cells containing central spherical nuclei and multiple lipid droplets. For comparison, the upper part of the photomicrograph shows unilocular tissue. PT stain. Medium magnification.
Multilocular adipose tissue. Note the central nucleus, multiple fat droplets, and abundant mitochondria. A sympathetic nerve ending is shown at the lower right.
Brown adipose tissue
H&E stain. Low magnification.
Distribution: Brown fat is less abundant than white at all ages. Young and middle-aged adults have little or none, but fetuses, newborns, and the elderly have accumulations in the axilla, the posterior triangle of the neck (near the carotid artery and thyroid gland), and around the renal hilus.
Functional Characteristics: Brown fat has essentially the same functional capabilities as white, but its metabolic activity is more intense and can lead to generation of heat. Under conditions of excessive cold, autonomic stimulation can cause oxidative phosphorylation in the numerous mitochondria to be uncoupled from adenosine triphosphate (ATP) synthesis, and the released energy dissipates as heat. The numerous vessels supplying this tissue carry the heat to the body. Brown fat is important in hibernating animals and in human infants before other thermoregulatory mechanisms are well developed.
Histogenesis: The multilocular adipocytes of brown fat derive from mesenchymal precursors that assume an epithelial shape and arrangement. The multiple small fat droplets that appear during development do not coalesce during maturation.
Differences between brown and white adipose tissue
In the tunica suprachoroidea and in the lamina fucsa of the sclerae of the eye, the majority of the cells in the loose connective tissue are melanocytes. Such a tissue can be termed “pigment tissue”. The large cells of the pigmented epithelium are filled with coarse, dark brown melanin granules (endogenous pigmental protein inclusions), which obscure their outlines and their nuclei. Aggregations of pigmental cells could be found in the areola of mammary gland and around the anus.
Melanocytes of the skin without staining. High magnification.
The regenerative capacity of the connective tissue is clearly observed when tissues are destroyed by inflammation or traumatic injury. In these cases, the spaces left after injury to tissues whose cells do not divide (eg, cardiac muscle) are filled by connective tissue, which forms a scar. The healing of surgical incisions depends on the reparative capacity of connective tissue. The main cell type involved in repair is the fibroblast.
When it is adequately stimulated, such as during wound healing, the fibrocyte reverts to the fibroblast state, and its synthetic activities are reactivated. In such instances the cell reassumes the form and appearance of a fibroblast. The myofibroblast, a cell with features of both fibroblasts and smooth muscle cells, is also observed during wound healing. These cells have most of the morphological characteristics of fibroblasts but contain increased amounts of actin microfilaments and myosin and behave much like smooth muscle cells. Their activity is responsible for wound closure after tissue injury, a process called wound contraction.
When adequately stimulated, macrophages may increase in size and are arranged in clusters forming epithelioid cells (named for their vague resemblance to epithelial cells), or several may fuse to form multinuclear giant cells. Both cell types are usually found only in pathological conditions.
Macrophages act as defense elements. They phagocytose cell debris, abnormal extracellular matrix elements, neoplastic cells, bacteria, and inert elements that penetrate the organism. Macrophages are also antigen-presenting cells that participate in the processes of partial digestion and presentation of antigen to other cells (see Chapter 14). A typical example of an antigen-processing cell is the macrophage present in the skin epidermis, called the Langerhans cell (see Chapter 18). Although macrophages are the main antigen presenting cells, under certain circumstances many other cell types, such as fibroblasts, endothelial cells, astrocytes, and thyroid epithelial cells, are also able to perform this function. Macrophages also participate in cell-mediated resistance to infection by bacteria, viruses, protozoans, fungi, and metazoans (eg, parasitic worms); in cell-mediated resistance to tumors; and in extrahepatic bile production, iron and fat metabolism, and the destruction of aged erythrocytes.
When macrophages are stimulated (by injection of foreign substances or by infection), they change their morphological characteristics and metabolism. They are then called activated macrophages and acquire characteristics not present in their nonactivated state. These activated macrophages, in addition to showing an increase in their capacity for phagocytosis and intracellular digestion, exhibit enhanced metabolic and lysosomal enzyme activity. Macrophages also have an important role in removing cell debris and damaged extracellular components formed during the physiological involution process. For example, during pregnancy the uterus increases in size. Immediately after parturition, the uterus suffers an involution during which some of its tissues are destroyed by the action of macrophages. Macrophages are also secretory cells that produce an impressive array of substances, including enzymes (eg, collagenase) and cytokines that participate in defensive and reparative functions, and they exhibit increased tumor cell–killing capacity.
Collagen synthesis depends on the expression of several genes and several posttranslational events. It should not be surprising, therefore, that a large number of pathological conditions are directly attributable to insufficient or abnormal collagen synthesis.
Certain mutations in the 1 (I) or 2 (I) genes lead to osteogenesis imperfecta. Many cases of osteogenesis imperfecta are due to deletions of all or part of the 1 (I) gene. However, a single amino acid change is sufficient to cause certain forms of this disease, particularly mutations involving glycine. Glycine must be at every third position for the collagen triple helix to form.
In addition to these disorders, several diseases result from an over-accumulation of collagen. In progressive systemic sclerosis, almost all organs may present an excessive accumulation of collagen (fibrosis). This occurs mainly in the skin, digestive tract, muscles, and kidneys, causing hardening and functional impairment of the implicated organs.
Keloid is a local swelling caused by abnormal amounts of collagen that form in scars of the skin. Keloids, which occur most often in individuals of black African descent, can be a troublesome clinical problem to manage; not only can they be disfiguring, but excision is almost always followed by recurrence.
Vitamin C (ascorbic acid) deficiency leads to scurvy, a disease characterized by the degeneration of connective tissue. Without this vitamin, fibroblasts synthesize defective collagen, and the defective fibers are not replaced. This process leads to a general degeneration of connective tissue that becomes more pronounced in areas in which collagen renewal takes place at a faster rate. The periodontal ligament that holds teeth in their sockets has a relatively high collagen turnover; consequently, this ligament is markedly affected by scurvy, which leads to a loss of teeth. Ascorbic acid is a cofactor for proline hydroxylase, which is essential for the normal synthesis of collagen. Table 5–4 lists a few examples of the many disorders caused by failure of collagen biosynthesis.
Examples of clinical disorders resulting from defects in collagen synthesis.
Collagen turnover and renewal in normal connective tissue is generally a very slow process. In some organs, such as tendons and ligaments, the collagen is very stable, whereas in others, as in the periodontal ligament surrounding teeth, the collagen turnover rate is very high. To be renewed, the collagen must first be degraded. Degradation is initiated by specific enzymes called collagenases, which are members of an enzyme class called matrix metalloproteinases or MMPs. Collagenases clip collagen molecules in such a way that they are then susceptible to further degradation by nonspecific proteases.
Fibrillin is a family of proteins related to the scaffolding necessary for the deposition of elastin. Mutations in the fibrillin gene result in Marfan syndrome, a disease characterized by a lack of resistance in the tissues rich in elastic fibers. Because the large arteries are rich in components of the elastic system and because the blood pressure is high in the aorta, patients with this disease often experience aortic swellings called aneurysms, a life-threatening condition.
The degradation of proteoglycans is carried out by several cell types and depends on the presence of several lysosomal enzymes. Several disorders have been described in which a deficiency in lysosomal enzymes causes glycosaminoglycan degradation to be blocked, with the consequent accumulation of these compounds in tissues. The lack of specific hydrolases in the lysosomes has been found to be the cause of several disorders in humans, including Hurler, Hunter, sanfilippo, and Morquio syndromes.
Because of their high viscosity, intercellular substances act as a barrier to the penetration of bacteria and other microorganisms. Bacteria that produce hyaluronidase, an enzyme that hydrolyzes hyaluronic acid and other glycosaminoglycans, have greater invasive power because they reduce the viscosity of the connective tissue ground substance.
Multiadhesive glycoproteins have carbohydrates attached, but in contrast to proteoglycans the protein moiety usually predominates. The carbohydrate moiety of glycoproteins is frequently a branched structure. Several such glycoproteins have important roles in the adhesion of cells to their substrate. Fibronectin (L. fibra, fiber, + nexus, interconnection) is an important example synthesized by fibroblasts and some epithelial cells. This dimeric molecule, with a molecular mass of 222–240 kDa, has binding sites for collagens, certain GAGs, and integrins of cell membranes, ie, it is multiadhesive. Interactions at these sites help to mediate normal cell adhesion and migration and cause fibronection to be distributed as a network in the intercellular spaces of many tissues (Figure 5–18a). Another multiadhesive glycoprotein, laminin is a larger, trimeric, cross-shaped glycoprotein that participates in the adhesion of epithelial cells to the basal lamina, with binding sites for type IV collagen, GAGs, and integrins. All basal laminae are rich in laminin (Figure 5–18b).
The participation of fibronectin and laminin in both embryonic development and the increased ability of cancer cells to invade other tissues has been well-studied. The importance of fibronectin is shown by the fact that mice whose fibronectin gene has been inactivated die during early embryogenesis.
In connective tissue, in addition to the hydrated ground substance, there is a small quantity of free fluid—called interstitial or tissue fluid—that is similar to blood plasma in its content of ions and diffusible substances. Tissue fluid contains a small percentage of plasma proteins of low molecular weight that pass through the capillary walls as a result of the hydrostatic pressure of the blood. Although only a small proportion of connective tissue proteins are plasma proteins, it is estimated that because of its volume and wide distribution, as much as one third of the plasma proteins of the body are stored in the intercellular connective tissue matrix.
Edema is promoted by the accumulation of water in the extracellular spaces. Water in the extracellular compartment of connective tissue comes from the blood, passing through the capillary walls into the extracellular compartment of the tissue. The capillary wall is only slightly permeable to macromolecules but permits the passage of water and small molecules, including low-molecular-weight proteins. In several pathologic conditions, the quantity of tissue fluid may increase considerably, causing edema.
Edema may result from venous or lymphatic obstruction or from a decrease in venous blood flow (eg, congestive heart failure). It may also be caused by the obstruction of lymphatic vessels due to parasitic plugs or tumor cells and chronic starvation; protein deficiency results in a lack of plasma proteins and a decrease in colloid osmotic pressure. Water therefore accumulates in the connective tissue and is not drawn back into the capillaries.
Another possible cause of edema is increased permeability of the blood capillary endothelium resulting from chemical or mechanical injury or the release of certain substances produced in the body (eg, histamine).
Blood vessels bring to connective tissue the various nutrients required by its cells and carry away metabolic waste products to the detoxifying and excretory organs, the liver and kidneys.
Two forces act on the water contained in the capillaries: the hydrostatic pressure of the blood caused by the pumping action of the heart, which forces water out across the capillary wall; and the colloid osmotic pressure of the blood plasma, which draws water back into the capillaries (Figure 5–20). Osmotic pressure is due mainly to plasma proteins. (Because the ions and low-molecular-weight compounds that pass easily through the capillary walls have approximately the same concentration inside and outside these blood vessels, the osmotic pressures they exert are approximately equal on either side of the capillaries and cancel each other.) The colloid osmotic pressure exerted by the blood proteins—which are unable to pass through the capillary walls—is not counterbalanced by outside pressure and tends to bring water back into the blood vessel (Figure 5–20).
In addition to leptin, white adipose tissue secretes numerous other cytokines and other factors with paracrine and autocrine activity, including many proinflammatory cytokines. It is not clear whether these are produced by adipocytes or other cells of the tissue such as macrophages or fibroblasts. With its increased amounts of white adipose tissue, obesity is characterized by a state of chronic mild inflammation. Cytokines and other factors released from visceral fat are being investigated for links to the inflammation-related disorders associated with obesity such as diabetes and heart disease.
Humans are one of the few mammals born with fat stores, which begin to accumulate at week 30 of gestation and are well-developed by birth in both the visceral and subcutaneous compartments. After birth, the development of new adipocytes is common around small blood vessels, where undifferentiated mesenchymal cells are fairly abundant.
Excessive formation of adipose tissue, or obesity, occurs when energy intake exceeds energy expenditure. Although fat cells can differentiate from mesenchymal stem cells throughout life, adult-onset obesity is generally believed to involve largely increased size or hypertrophy in existing adipocytes (hypertrophic obesity). Childhood obesity can involve both increased adipocyte size and formation of new adipocytes by differentiation and hyperplasia of preadipocytes from mesenchymal cells. This early increase in the number of adipocytes may predispose an individual to hyperplastic obesity in later life.
Connective tissue has a large variety of functions in the body, and can be as different as blood and bone! However every connective tissue is made up of two basic elements - cells and which of the following?
Matrix. The matrix is the substance that exists between the cells in the connective tissue. The main constituents of the matrix are fibers, and ground substance, which is the material existing between the cells and fibers. In contrast to epithelial tissues, connective tissues normally do not cover surfaces (although areolar connective tissue is an exception: it lines joint cavities). Instead, connective tissues have a wide variety of functions, including support (e.g. the skeleton), energy reserves (e.g. fat), and in nutrition (e.g. blood).
The suffix "-blast" (e.g. osteoblast) refers to the mature form of the cell, while "-cyte" (e.g. osteocyte) refers to the immature form.
It is actually the other way around. The suffix "-blast" means "to bud" or "to sprout". The blasts actually produce the matrix, and matures into cytes when the matrix is sufficient. They retain the ability for cell division. Cytes, on the other hand, have a much reduced capacity for cell division. Their main role is to maintain the matrix. From the question: Osteoblasts and osteocytes are cells found in bone.
Connective tissue contains various types of cells, one of which is mast cells. What are these cells principally involved in?
Mediating allergic reactions. Mast cells are large, and have a granulated cytoplasm. They are important in allergic reactions; they secrete histamine, which dilates small blood vessels in an immune response. This allows blood to reach the site of damage, and causes the redness seen with inflammation. You may have heard of anti-histamines being used to treat allergies, especially hay fever; these help to decrease the response of mast cells. Mast cells also release heparin, which is thought to defend against invading pathogens such as bacteria. Connective tissues also contain fibroblasts/-cytes, adipocytes, and macrophages.
Three types of fibers are found in connective tissue matrix. Which of these is NOT one of those?
Muscle fibers. Muscle is in fact another tissue type altogether, which will be covered in the next quiz of the series: Human Tissue Types III. Collagen fibers are very strong but flexible fibers that make up around 30% of your dry body weight! There are different types of collagen (at least 15), which can each confer different properties to the tissue it is present in. For example, cartilage contains Type II collagen, which is particularly good at retaining water, which makes cartilage quite spongy compared to bone. Reticular fibers are found primarily in the spleen, liver, and lymph nodes. They also contain collagen protein (mainly Type III) as collagen fibers do. They provide a more delicate branched network, and provide support and strength to the tissue. They also help to form the basement membrane, which underlies epithelia. Elastic fibers contain elastin as the main protein, and their main property is that of stretching - they can stretch up to about 1.5 times their length. They form irregular branched networks, and are found primarily in the lung and aorta.
What is the abbreviation commonly used to describe glycosaminoglycans, a component found in the ground substance of the matrix?
GAG & GAGs. Glycosaminoglycans are polysaccharides that trap water, forming hydrated gels. Most exist as proteoglycans, which means they are associated with proteins. The proteins form a spine, and the GAGs project from the spine like bristles of a brush - this conformation is very good at holding water. GAGs include chondroitin sulfate (found in cartilage and bone) and dermatan sulfate (found in skin, tendons, and blood vessels). As well as making the connective tissue more spongy, GAGs provide strength under compression.
Which of these diseases is a disorder of the elastic fibers found in connective tissues? Sufferers tend to be tall and have long limbs and digits.
Marfan's syndrome. Marfan's syndrome is inherited from generation to generation, and is caused by a mutation in the gene for the protein fibrillin, found in elastic fibers. Tissues that have abundant elastic fibers are thus affected; these include the covering layer of bone (periosteum), the ligament that supports the lens in the eye, and the walls of arteries. Symptoms can therefore include blurred vision (due to the displacement of the lens) and, more seriously, the aorta can weaken, which can lead to sudden bursting and subsequent death.
Connective tissue can be classified according to the relative amount of ground substance and the orientation of fibers. The classifications are loose (or areolar), dense irregular, and dense regular. Which of the following has a dense regular classification?
Tendons and ligaments. In dense regular tissues there is very little ground substance and few cells (generally fibrocytes are present). There are many fibers (mainly collagen) arranged in regular, parallel bundles, which gives a high tensile strength to the tissue - the tissue can withstand pulling along the axis of the fibers. This tissue is normally silvery white. Dense irregular tissue also has little ground substance and high proportions of fibers. However, here the bundles are in all directions, and this means that stress can be loaded on the tissue in a non-specific direction. Examples of where it is found are in the periosteum and perichondrium. Finally, loose connective tissue is mostly made up of ground substance, and has a high water content. It has relatively few fibers, but has some cells. This can be found in blood vessels and nerves.
Student’s Practical Activities
Task No 1. Students must know and illustrate such histologic specimens.
Specimen 1. Loose connective tissue (surface view). Iron haematoxylin
This specimen demonstrates the typical histological appearance of mature fibroblasts, macrophages, mast cells, plasma cells in loose connective tissue. The fibroblast nuclei are condensed and elongated in the direction of the extracellular fibers. The cytoplasm is reduced and spindle-shaped, with long cytoplasmic processes extending into the matrix to meet up with those of other fibroblasts; the cytoplasmic extensions are usually difficult to see with the light microscope.
Macrophages are large, stellate cells, cytoplasm of which contain many lysosomes and well developed Golgi complex. Mast cells are found in connective tissue, particularly in association with blood vessels. The characteristic feature of mast cells is an extensive cytoplasm packed with large granules, which are similar in composition to basophil granules, containing histamine and heparin. Plasma cells are large and ovoid with eccentrically nuclei and well developed rough endoplasmatic reticulum. The characteristic “clock face” of the nucleus results from a large, central nucleolus and several large heterochromatin clumps regularly spaced inside the nuclear envelope.
Collagen fibers are stained dark in this specimen, they are thicker and nonbranched in opposite to elastic fibers, which are thin and branhed.
Illustrate and indicate: 1. Fibroblasts. 2. Collagen fibers. 3. Elastic fibers. 4.Macrophages (histiocytes). 5.Mast cells. 6.Plasma cells. 7.Ground substance.
Specimen 2. Accumulation of dye by macrophages. Methylene blue
On the large magnification in the cytoplasm of macrophages you can see a material with stain, which was phagocyzed by cells after injection into the human organism.
Illustrate and indicate: 1.Nucleus. 2. Vacuoles with methylene blue
Specimen 3. Dense regular connective tissue (tendon).
Stained with haematoxylin and eosin.
This specimen of the tendon shows the typical dense arrangement of collagen fibers, where mechanical support is the primary function. Tendon is the densest form of connective tissue proper, consisting of bundles of regullary arranged collagen fibers among which are scattered rows of fibroblasts with elongated nuclei. Each tendon is composed of small bundles of such dense tissue bound together by a small amount of loose connective tissue, which contains the scanty blood supply and tiny nerve fibers. The connective tissue of the tendon surface is smooth and condensed with minimal connections with the surrounding tissue, so as to allow relatively unimpeded movement of the tendon. In some sites, tendons are invested in a connective tissue sheath lined by synovium.
Illustrate and indicate: 1. Collagen fibers. 2. Nuclei of fibrocytes. 3. Bundle of tendon fibers. 4.Interfascicular connective tissue. 5.Vessels.
Specimen 4. Dense irregular connective tissue.
Stained with haematoxylin and eosin.
This specimen demonstrate the reticular layer of the skin, where the collagen fibers of the dense irregular connective tissue are arranged in coarse irregular interwoven bundles which confer great tensile streight. Collagen is acidophilic (pink-stained) due to its positively-charged side groups. The fibroblasts are inactive with highly condensed nuclei and minimal cytoplasm.
Illustrate and indicate: 1.Collagen fibers. 2.Cells. 3. Ground substance.
Specimen 5. White adipose tissue.
The typical appearance of white adipose tissue is illustrated in this specimen. Fat stored in adipocytes accumulates as lipid droplets, which fuse to form a single large droplet which distends and occupies most of the cytoplasm. The adipocyte nucleus is compressed and displaced to one side of the stored lipid droplet and the cytoplasm is reduced to a small rim around the periphery. Note the minute dimensions of capillaries compared with the size of the surrounding adipocytes.
Illustrate and indicate: 1.Adipose cells. 2.Lipid droplet. 3.Nucleus.
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 49-62.
4. Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 65 – 82.
5. Inderbir Singh Textbook of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002. – P. 54-69.
6. Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 158 – 198, 254 – 268.
1. Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. – Lippincott Williams and Wilkins, 2008. – P. 73-85.
Junqueira L. Basic
Histology / L. Junqueira, J. Carneiro, R. Kelley.
– [seventh edition]. –
Volkov K. S. Ultrastructure of cells
and tissues / K.
Created by Violetta Kulbitska