BASIC KNOWLEDGE OF TISSUES. EPITHELIAL TISSUES.
Using lectures (on the web-page of histology department), lecture presentations textbooks, additional literature and other resources, students should prepare such theoretical questions:
1. List the principal functions of epithelial tissues.
2. From which embryonic germ layer(s) epithelial tissues are derived? Give examples of epithelia that derived from each embryonic germ layer.
3. Structural and functional characteristics of epithelial tissues that distinguish them from other tissue types.
4. Classification of the epithelial tissues.
5. Describe the basal lamina in terms of its location, composition, staining properties. Compare basal lamina and basement membrane.
6. 4 types of epithelial cells junctions.
7. Types of simple epithelia. Give examples of body’s sites where each type can be found.
8. Types of stratified epithelia. Give examples of body’s sites where each type can be found.
9. Structural and functional characteristics of glandular epithelium.
10. Criteria used for classification of the glands. Classification of exocrine glands.
11. The types of glands commonly found in humans. Give examples of body’s sites where each type can be found.
12. Secretory cycle of the glandular cells.
13. Types of the glands, mode of secretion and describe each of them.
14. Regeneration of the glands.
Human’s body consists of billions of cells. A tissue is a collection of cells and noncellular structures, which have the similar origin, structure and functions. Organs are groups of tissues. Most organs are complex groups of different tissue types. An organism is composed of organs that are grouped together and functionally integrated.
Cells are bound together with varying amounts of intercellular substance to form tissues and organs. This applies also to the blood, the plasma of which constitutes the intercellular substance. Even in the earliest stages, when the fertilized egg divides repeatedly, the resulting cells stick together and pass through a series of complex developmental processes which take place in an orderly sequence. There is a phase when the early embryo consists only of simple epithelial layers. Later, the three primitive germ layers give rise to 4 primary tissues:
1. Epithelium – the cells are generally applied closely one to another with little cementing substance. The sheets of cells may develop into the covering of the outer and inner surfaces of the body and the glands and other structures derived from them.
2. Connective tissues – the cells are generally separated one from another to a greater or less degree by a rather rich amount of intercellular substance. Blood cells are derived from these cells and the blood cell-forming tissues, the connective tissue in its varieties, cartilage and bone. These derivates differ not only in their cell populations, but also in the nature and amount of the intercellular substance.
3. Muscle tissue – the cells are of several varieties, which are associated with movement of the skeleton and contractility in many organs, including those of the vascular system.
4. Nerve tissue - the cells are concerned primarily with rapid conduction of impulses in the integration of numerous functions. The brain, spinal cord, autonomic ganglia, peripheral nerves, and portions of sensory organs are composed of nerve tissue.
The epithelia are a diverse group of tissues, which, with rare exceptions, line all body surfaces, cavities and tubes. Epithelia thus function as interfaces between biological compartments. Epithelia interfaces are involved in a wide range of activities such as absorption, secretion and protection and all these major functions may be exhibited at a single epithelial surface. For example, the epithelial lining of the small intestine is primarily involved in absorption of the products of digestion, but the epithelium also protects itself from noxious intestinal contents by the secretion of a surface coating of mucus.
Epithelial tissues have morphofunctional features which may be changed in some pathologic states, for example inflammation, dystrophia, dysplasia and metaplasia.
Epithelial tissues usually occur as structurally minor but functionally important components of complex organs. Glands, derived from the invagination and ingrowths of lining epithelia into underlying connective tissue, are composed mainly of epithelial cells in adults and are considered a type of epithelial tissue.
Epithelial tissues are very diverse in form and function. They range from one to several cell layers in thickness, forming sheets, solid organs, or glands. Their functions range from protection to secretion and absorption.
Epithelial cells line surfaces. Epithelia line and protect virtually all-free surfaces in the human body except joint cavities and the anterior surface of the iris, which is a naked connective tissue domain.
a. The outer surface of the body is covered by the epithelial epidermis of skin. The hair follicles and glands in skin also have epithelial components.
b. Epithelium lines the digestive system and its diverticula, such as the respiratory system, liver, pancreas, and gallbladder.
c. The cardiovascular system is lined by an epithelium called endothelium.
d. Body cavities derived from the intraembryonic coelom (the pericardial cavity, thoracic cavity, and peritoneal cavity) are lined by an epithelium called mesothelium.
e. The urogenital system is lined by a layer of epithelial cells as well. Ectoderm, mesoderm, and endoderm can all give rises to epithelia.
Epithelial cells are polarized. The epithelial apical surface typically faces a free surface of the body or the lumen of an organ or blood vessel and may be covered by microvilli or cilia. The basal surface rests on an extra cellular layer of fibrils and glycoproteins called the basement membrane, or basal lamina, which is the boundary between the epithelium and the underlying connective tissue.
Epithelial cells have tight lateral adhesions. An epithelium is one or more layers of cells that are tightly joined together. The adhesions hold the epithelial cells together into a coherent barrier tissue. The apical junctions between cells in many epithelia have a sealing and adhesive structure called the junctional complex, which isolates the internal milieu of the organism and tightly joins epithelial cells together.
Epithelial tissues have very little intercellular substance. The cells are usually densely packed, closely apposed, and joined by specialized junctions.
Epithelia are continuously renewed and replaced. The epithelial cells closest to the basal lamina undergo continuous mitosis, and their progeny replace the surface cells.
When faced with a chronic change of environmental conditions, epithelia are capable of metaplasia, ie, they may change from one type to another.
Epithelia are avascular. In most organs, the connective tissue beneath or around the epithelium contains blood vessels and lymphatics, which nourish the epithelium by diffusion.
Classification of the epithelial tissues. Epithelia are classified and named according to the number of their cell layers and the shape of the cells in the surface layer.
a. Simple epithelia have one cell layer. All cells rest on the basement membrane and reach the apical surface.
b. Stratified epithelia have 2 or more cell layers, consequently not all cells rest on the basement membrane or reach the apical surface.
c. Pseudostratified epithelia have all their cells resting on the basal lamina, but not all the cells extend to the surface. The nuclei lie at different depths, giving the appearance of multiple cell layers.
Diagrams of simple epithelial tissue.
A: Simple squamous epithelium.
B: Simple cuboidal epithelium.
C: Simple ciliated columnar epithelium. All are separated from the subjacent connective tissue by a basement membrane.
In C, note the terminal bars that correspond in light microscopy to the zonula occludens and the zonula adherens of the junctional complex.
Diagrams of stratified and pseudostratified epithelial tissue.
A: Stratified squamous epithelium.
B: Transitional epithelium.
C: Ciliated pseudostratified epithelium. The goblet cells secrete mucus, which forms a continuous mucous layer over the ciliary layer.
2. Shape of the surface cells
a. Squamous cells are flat and platelike.
b. Cuboidal cells are polygonal and about as tall as they are wide.
c. Columnar cells are polygonal and taller than they are wide.
Type of Epithelium
Lining of peritoneal and pleural cavities; endo-thelial lining of all normal blood vessels
Proximal convoluted tubules; sweat glands; epithelium of small and large intestines
Pseudostratified ciliated columnar epithelium with goblet cells
Nasopharynx; trachea; bronchi
Pseudostratified columnar with stereocilia
Epididymis; ductus deferens
Vagina; parts of the oral cavity; pharynx; esophagus; anal canal
Stratified squamous keratinized
Epidermis; hard palate; gingiva
Sweat gland ducts
Renal pelvis; ureters; urinary bladder
Specific Epithelial Types:
1. Simple squamous epithelium is a single layer of flat, platelike cells that usually functions as a semipermeable barrier between compartments.
Simple squamous epithelium coats, or partially coats, the stomach, liver, gallbladder, and other visceral organs. It also is found in respiratory system alveoli and in the thin limbs of the loop of Henle in the kidney.
Mesothelium is the simple squamous epithelium that lines serous cavities (peritoneal, pleural, and pericardial cavities) and coats many of the organs in these cavities.
Simple squamous epithelium covering the peritoneum (mesothelium). Some blood capillaries are indicated by arrows. PT stain. Medium magnification
Simple squamous epithelium covering the peritoneum (mesothelium).Surface view. Silver impregnation.
Endothelium is the simple squamous epithelium that lines the lumen of the cardiovascular system. Cells of this type have three zones: central perinuclear, zone of organelles and peripheral thin part through which exchanges occur.
Electron micrograph of simple squamous epithelium (endothelium) x44,600.
Section of a vein. All blood vessels are lined with a simple squamous epithelium called endothelium (arrowheads). Smooth muscle cells in the vein wall are indicated by arrows. Pararosaniline–toluidine blue (PT) stain. Medium magnification
2. Simple cuboidal epithelium is a single layer of cells that are equal in height and width. It typically exists in areas where ion transport occurs (e.g., in kidney tubules, sweat glands, and some glandular ducts). It also covers the choroid plexus—the four clusters of capillaries in the walls of the ventricles of the brain that help produce cerebrospinal fluid (CSF).
1. Simple cuboidal epithelium is abundant in the kidney, in the proximal and distal tubules and parts of the collecting-duct system.
Simple cuboidal epithelium from kidney collecting tubules. Cells of these tubules are responsive to the antidiuretic hormone and control the resorption of water from the glomerular filtrate, thus affecting urine density and helping retain the water content of the body. PT stain. Low magnification
2. Many glands are composed of simple cuboidal epithelial cells assembled into round acini. In the configuration, the cuboidal cells are distorted into a rough pyramidal shape.
3. Follicular epithelial cells in the thyroid gland can assume a cuboidal shape, depending on their function.
4. Often, cuboidal epithelial cells have many apical microvilli and mitochondria, which facilitate ion pumping and fluid transport.
3. Simple columnar epithelium is a single layer of roughly cylindric cells whose apical (free) surfaces may be covered with cilia or microvilli. It functions in secretion, absorption, and propulsion of mucus (if ciliated) and often as a protective barrier. It lines the stomach, intestines, rectum, uterus, and oviducts. It is found in some glandular ducts and the gallbladder, and it lines papillary collecting ducts in the urinary system.
Simple columnar epithelium that covers the inner cavity of the uterus. Note that the epithelium rests on the loose connective tissue of the lamina propria. The epithelium and the lamina propria constitute the mucosa. H&E stain. Medium magnification
5. Pseudostratified columnar epithelium is a layer of cells, in which all of the cells rest on the basement membrane but only some extend to the apical surface of the epithelium. Cells reaching the surface are often ciliated. This epithelium forms a protective barrier and, when ciliated, moves surface mucus and trapped debris. Ciliated pseudostratified columnar epithelium, or respiratory epithelium, lines the larger diameter respiratory passageways. Pseudostratified columnar epithelium often covered with nonmotile stereocilia, also lines some parts of the male reproductive tract, where its apical surfaces are.
Pseudostratified ciliated columnar epithelium with goblet cells of trachea. H&E stain. Low magnification
6. Stratified squamous epithelium contains many layers of cells, including an apical layer of flat cells. These epithelia are classified according to the characteristics of the apical layer.
a. The keratinized (cornified) type is a multilayered sheet of cells. The superficial cells are squamous, dead, and filled with the scleroprotein keratin; they lack nuclei. Deeper layers have polygonal cells in progressive stages of keratinization. The deepest layer has cuboidal to columnar cells and lies on the basal lamina. Keratinized stratified squamous epithelium is found mainly in the skin and forms a highly specialized barrier against friction, abrasion, infection, and water loss.
The digestive system contains some stratified squamous epithelium. Stratified squamous epithelium on the tongue and lining the esophagus resists the abrasion of mastication, swallowing, and the passage of food from the oral cavity to the stomach. Stratified squamous epithelium in the anal canal resists the abrasion of passing semi-solid feces.
In the female reproductive system, stratified squamous epithelium is abundant in the vagina and covering the cervix, where it resists abrasion during sexual intercourse.
Stratified squamous keratinized epithelium of epidermis of the skin.
H&E stain. Low magnification
b. The nonkeratinized (noncornified) type is similar in structure but is thinner and lacks heavily keratinized cells. Its surface cells are flattened but nucleated. Nonkeratinized stratified squamous epithelium, also called mucous membrane, forms a protective barrier that is less resistant to water loss than the keratinized type. It lines wet cavities subject to abrasion (eg, mouth, esophagus, vagina, and anal canal, vocal folds).
Stratified squamous nonkeratinized (moist) epithelium of the esophagus. PT stain. Medium magnification.
Stratified squamous nonkeratinized epithelium of cornea. H&E stain. Low magnification
6. Stratified cuboidal epithelium usually has 2-3 layers of cuboidal cells. This relatively rare epithelium lines the ducts of epidermal glands (sweat) and salivary glands.
Stratified cuboidal epithelium that lined the ducts of salivary glands.
H&E stain. Low magnification
7. Stratified columnar epithelium is similar to stratified cuboidal, but its superficial cells are columnar and may be ciliated. Also rare, it lines the larger ducts of some large glands, forms the conjunctiva, and occurs in small, isolated patches in some mucous membranes. It sometimes covers the respiratory surface of the epiglottis.
Stratified columnar epithelium. H&E stain. Low magnification
8. Transitional epithelium is a stratified epithelium found exclusively in the urinary passages of the urinary system. It contains many layers of polyhedral cells and an outer apical layer of round pillow-shaped cells. It is found in the minor calyces, major calyces, renal pelvis, ureters, urinary bladder, and proximal urethra. Transitional epithelium in the bladder undergoes a reversible morphologic change during bladder distension and evacuation. When the bladder is empty, the surface cells appear domelike, giving the epithelium a "cobblestone" appearance. When the bladder is full, the surface cells stretch and flatten.
Stratified transitional epithelium of the urinary bladder. PT stain. Medium magnification
Stratified transitional epithelium of the urethra. The basement membrane between the epithelium and the underlying loose connective tissue is indicated by arrows. PSH stain. Medium magnification.
Some epithelia are highly specialized and, therefore, defy simple classification or description.
1. chorionic villi in the placenta consist of a syncytial epithelial layer (syncytiotrophoblast) lying on a proliferative cuboidal epithelial layer (cytotrophoblast), which, in turn rests on a basement membrane.
2. Seminiferous epithelium in the male is stratified, highly proliferative, and, after puberty, sheds live haploid gametes from its apical surface.
3. Follicular epithelial cells in the ovary are surrounded by a basement membrane and make intimate contact with the developing oocyte.
4. Some epithelia, such as those in the thyroid follicular epithelium, are in a constant state of flux. They may be squamous, cuboidal, or columnar, depending on the amount of thyroglobulin contained in the follicle lumen.
5. Many sensory epithelia (e.g., organ of Corti in the cochlear duct; retina) are highly complex epithelia.
POLARITY & SPECIALIZATIONS OF EPITHELIAL CELLS
Polarity (structural and functional asymmetry) is characteristic of most epithelial cells. It is best seen in simple epithelia, where each cell has 3 types of surfaces: an apical (free) surface, a variable number of lateral surfaces that abut neighboring cells, and a basal surface attached to the basal lamina.
Specializations of the Apical Surface. The cell's apical surface is on the organ's external or internal (lumen) surface. It is specialized to carry out functions that occur at these interfaces, including secretion, absorption, and movement of luminal contents.
An unusual, but functionally crucial, feature of most epithelial apices is that they are not adhesive. For example, when you clap your hands they do not adhere, no matter how hard you clap or how long you hold your hands together.
Similarly, the apices of the mesothelium lining the thoracic cavity, pericardial cavity, and peritoneal cavity do not adhere to one another, and organs in these cavities move past one another without impediment or pain. However, if infection or surgery destroys the mesothelium, the organs will stick together, resulting in painful adhesions.
The apex of luminal epithelial cells also exhibits this nonadhesive property, even in the smallest tubules in the body. Consequently, a lumen is maintained in all tubules, including the ductus deferens and capillaries, because walls cannot collapse and adhere to one another.
In many instances, especially in the small intestines, the apical portion of the cell is coated with a thick, glycoconjugate-rich layer external to the outer leaflet of the plasma membrane called the glycocalyx, which makes apices nonadhesive.
Apical protrusions. The epithelial apex often contains protrusions from the cell surface. The protrusions may be scattered, as in mesothelium and endothelium, and may take the form of microvilli.
a. Microvilli are cylindrical, cell-surface projections, 80 nm wide and 1—2 (mm long, which increase the cell surface area for absorbing materials from the lumen. Microvilli can be sparse (as in mesothelium), numerous (as in the syncytiotrophoblast), or very dense (as in the brush border of kidney tubules and intestinal epithelium). Microvilli are motile. They have many actin-containing microfilaments and large amounts of myosin.
Electron micrograph of the apical region of an intestinal epithelial cell. Note the terminal web composed of a horizontal network that contains mainly actin microfilaments. The vertical microfilaments that constitute the core of the microvilli are clearly seen. An extracellular cell coat (glycocalyx) is bound to the plasmalemma of the microvilli. x45,000.
Electron micrograph of a section from the apical region of a cell from the intestinal lining showing cross-sectioned microvilli. In their interiors, note the microfilaments in a cross section. The surrounding unit membrane can be clearly discerned and is covered by a layer of glycocalyx, or cell coat. x100,000.
b. Stereocilia are long microvilli present in the male reproductive tract and in the membranous labyrinth of the inner ear. They are similar to microvilli except that they are longer and constricted at the point where they join the cell apex. Little is known about their movements in vivo. Sensory stereocilia contain many actin-containing microfilaments and large amounts of myosin. In epididimus and ductus deferens they have an absorptive function, and in the internal ear (hair cells of the maculae and organ of Corti), they have a sensory function.
c. Cilia. Many epithelia apices are ciliated in some locations :
(1) In the trachea, certain columnar epithelial cells have many apical cilia that move mucus along the apical surface of the trachea! lumen (airway).
(2) Epithelial cells in the uterine tubes have apical cilia that help move the fertilized ova into the reproductive tract.
(3) The apical portions of many sensory epithelial cells have modified cilia or cilia-like structures involved in the energy transduction function of the cell. However, these cilia are not motile like the cilia in the respiratory and female reproductive tracts.
Electron micrograph of the apical portion of a ciliated epithelial cell. Cilia are seen in longitudinal section. At the left, arrowheads point to the central and peripheral microtubules of the axoneme. The arrowhead at right indicates the plasma membrane surrounding the cilium. Each cilium has a basal body (B) from which it grows. Microvilli (MV) are shown. x59,000. Inset: Cilia in cross section. The 9 + 2 array of microtubules in each cilium is evident. x80,000.
d. Flagella are also concerned with movement. Spermatozoa, derived from seminiferons epithelia, are the only human flagellated cells.
Specializations of the Lateral Surfaces:
Epithelial cells are usually tightly attached to one another by specialized intercellular junctions composed of opposing membranes and some intra– and extracellular material. Junctions occur in 3 major forms: Zonulae are bandlike and completely encircle the cell; maculae are disklike and attach 2 cells at a single spot; gap junctions are similar in shape to maculae but differ in composition and function. A junctional complex (formerly called a terminal bar) is any combination of intercellular junctions close to the cell apex that looks dark in the light microscope.
1. Zonula occludens. Zonulae occludentes (tight junctions, occluding junctions) are located near the cell apex and seal off the intercellular space, allowing the epithelium to isolate certain body compartments (eg, they help keep intestinal bacteria and toxins out of the bloodstream). Their structure, best seen in freeze-fracture preparations, results from the fusion of 2 trilaminar membranes to form a pentalaminar structure (as seen in transmission electron microscopy [TEM]); this fusion may require specific "tight-junction proteins." In some tissues, tight junctions can be disrupted by removing calcium ions or treating with protease.
2. Zonula adherens. Zonulae adherentes (sometimes called belt desmosomes) are usually just basal to the tight junctions. The membranes of the adhering cells are typically 20-90 nm apart at a zonula adherens; the gap may be wider there than in nonjunctional areas. An electron-dense plaque containing myosin, tropomyosin, alpha actinin, and vinculin is found on the cytoplasmic surface of each of the membranes participating in the junction. Actin-containing microfilaments arising from each cell's terminal web insert into the plaques and appear to stabilize the junction.
3. Macula adherens. A macula adherens, or desmosome, consists of 2 dense, granular attachment plaques composed of several proteins and borne on the cytoplasmic surfaces of the opposing cell membranes. Transverse thin electron microscopic (EM) sections show dense arrays of tonofilaments (cytokeratin intermediate filaments) that insert into the plaques or make hairpin turns and return to the cytoplasm. The gap between the attached membranes is often over 30 nm. Sometimes fibrillar or granular material (probably glycoprotein) is seen as a dense central line in the intercellular space. Desmosomes, distributed in patches along the lateral membranes of most epithelial cells, form particularly stable attachments but do not hamper the flow of substances between the cells.
4. Gap junction. A gap
junction (nexus) is a disk- or patch-shaped structure, best appreciated by
viewing both freeze-fracture and transverse thin
Specializations of the Basal Surface. The basal surface contacts the basal lamina. Because it is the surface closest to the underlying blood supply, it often contains receptors for blood-borne factors such as hormones.
1. Basal lamina underlies all true epithelial tissues.
Structure. The basal lamina is a sheetlike structure, usually composed of type IV collagen, proteoglycan, and laminin, a glycoprotein that aids in adhesion of the basal lamina to the cells. The basal lamina exhibits electron-lucent and electron-dense regions termed the lamina lucida (lamina rara) and the lamina densa, respectively. The lamina densa is composed of a 20- to 100-nm-thick fibrillar network; the amount of lamina lucida is variable. Basal lamina components are contributed by the epithelial cells, the underlying connective tissue cells, and (in some locations) muscle, adipose, and Schwann cells. In some sites, a layer of type III collagen fibers (reticular fibers), produced by the connective tissue cells and termed the reticular lamina, underlies the basal lamina. Basal laminae accompanied by reticular laminae are often thick enough to be seen with the light microscope as PAS-positive layers and are sometimes termed basement membranes.
Functions. The basal lamina forms a sieve like barrier between the epithelium and connective tissue. It aids in tissue organization and cell adhesion and (through trans-membrane linkages with cytoskeletal components) helps maintain cell shape. It has a role in maintaining specific cell functions, probably through its effect on shape. Muscle basal laminae are critical in establishing neuromuscular junctions.
2. Hemidesmosomes are located on the inner surface of basal plasma membranes in contact with the basal lamina. Essentially half desmosomes, they help to attach epithelial cells to the basal lamina. The best examples are found in the basal layers of stratified squamous epithelium.
3. Sodium-potassium ATPase is a plasma membrane-bound enzyme localized preferentially in the basal andbasolateral regions of epithelial cells. It transports sodium out of and potassium into the cell.
Intracellular Polarity. The nucleus and organelles are often found in characteristic regions of epithelial cells, a feature particularly important to glandular cells. For example, in protein-secreting cells, the rough endoplasmic reticulum is preferentially located in the basal cytoplasm, the nucleus in the basal to middle region just above the rough endoplasmic reticulum, and the Golgi complex just above the nucleus. Mature secretory vesicles collect in the apical cytoplasm.
In some organs the epithelium persists as such or as special structures called glands. When glands secrete, they usually produce an aqueous fluid which differs from bllod plasma or tissue fluid. This product of cellular activity is called the secretion. This difference in composition of the secretion and the tissue fluid may manifest itself in the production of new substances present only in the tissue fluid (insulin and other hormones, trypsin and other enzymes, mucin, milk).
Two types of glands exist: endocrine glands and exocrine glands. Glands are composed of single cells or groups of cells specialized for secretion.
All glands arise in early development from lining or covering epithelia. Exocrine glands are those that keep their connection with the epithelium in the form of a duct. Endocrine glands (ductless glands) lose their connection with the surface and release their secretions into the bloodstream.
Classification of Exocrine Glands.
Exocrine glands may be classified according to their structure, secretory product, or mode of secretion.
1. By structure.
Structural classification is based on the number of cells, the type of duct system, and the shape of the secretory portion of the gland.
2. By number of cells. Unicellular glands are single secretory cells scattered among other cell types in epithelia (e.g., mucus-secreting goblet cells). Multicellular glands occur in 2 forms: Sheet glands, which empty their secretions directly into the lumen of a hollow organ (e.g., glands of the trachea), and solid glands, whose secretions are carried by ducts to the body surface (e.g., sweat glands) or to a lumen (e.g., salivary glands).
Section of large intestine showing goblet cells secreting mucus to the extracellular space. The mucus precursor stored in the cytoplasm of the goblet cells is also stained in a dark color. PAS-PT stain. Medium magnification.
Goblet cell in respiratory epithelim, mucous stained badly with H&E
Electron micrograph of a goblet cell from the small intestine. The rough endoplasmic reticulum is present mainly in the basal portion of the cell (R), while the cell apex is filled with light secretory vesicles or granules (SG) some of which are being discharged. The Golgi complex (G) lies just above the nucleus. Typical columnar absorptive cells with microvillar borders (M) lie adjacent to the goblet cell. x7000.
Diagram of a mucus-secreting intestinal goblet cell showing a typically constricted base, where the mitochondria and rough endoplasmic reticulum (RER) are located. The protein part of the glycoprotein complex is synthesized in the endoplasmic reticulum. A well-developed Golgi complex is present in the supranuclear region.
Formation of glands from covering epithelia. Epithelial cells proliferate and penetrate connective tissue. They may—or may not—maintain contact with the surface. When contact is maintained, exocrine glands are formed; without contact, endocrine glands are formed. The cells of endocrine glands can be arranged in cords or in follicles. The lumens of the follicles accumulate large quantities of secretions; cells of the cords store only small quantities of secretions in their cytoplasm.
Duct system. The duct system may be simple (unbranched), or compound (branched). Simple ducts may be straight or coiled.
Principal types of exocrine glands. The part of the gland formed by secretory cells is shown in black; the remainder shows the ducts. The compound glands have branching ducts.
Secretory portion The secretory portion of the gland may be tubular (test tube-shaped); alveolar, or acinar (flask-shaped); or tubuloacinar (with acini branching off the straight tubular portion).
Examples of Structural Classifications of Multicellular Exocrine Glands
Intestinal crypts of Lieberkuhn
Eccrine sweat glands of the skin
Fundic glands of the stomach
Sebaceous glands of the skin
Cardiac glands of the stomach
Submandibular salivary glands
Esophageal mucous secretory gland with characteristic irregular, clear cytoplasm and basal nuclei. Loose connective tissue surrounds a secretory duct.
3. By secretory product.
Mucous secretion, or mucus, is a thick secretion containing proteins, chiefly highly glycosylated glycoproteins called mucins or mucin precursors called mucinogens. Other glycoproteins (eg, membrane glycoproteins) commonly have short, N-linked oligosaccharides attached to asparagine. Mucous glycoproteins have longer, 0-linked oligosaccharide chains attached to hydroxyl groups of serine or threonine. This attachment is mediated by special glycosyltransferases in the Golgi complex of the mucus-secreting cells. Examples of mucus-secreting glands include goblet cells and the sublingual salivary glands.
Serous secretion is a watery secretion containing proteins and glycoproteins. The ex-ocrine pancreas and parotid salivary glands produce serous secretions.
Seromucous secretion is a mixed secretion of intermediate thickness. The submandib-ular salivary gland contains both serous and mucous secretory cells and produces sero-mucous secretions.
Submandibular salivary gland showing 2 types of secretory epithelial cells in a compound tubuloacinar gland. The light cells are mucous and the dark cells are serous. PT stain. Medium magnification.
4. By mode of secretion
In merocrine secretion (eccrine secretion), the secretory product exits by exocytosis, with no loss of cytoplasm or membrane. Most secretory cells release their products in this manner. Specific examples include the pancreas and thyroid gland.
In apocrine secretion, the secretory product collects in the cell apex, and the entire apex is released, with some loss of cytoplasm and membrane. Apocrine sweat glands of the skin and mammary glands both employ this type of secretion.
Section of the secreting portion of a mammary gland; apocrine secretion is characterized by the discharge of the secretion product with part of the cytoplasm (arrows). PSH stain. Medium magnification.
In holocrine secretion, storage of large amounts of secretory products in the cytoplasm is followed by cell lysis. The entire cell is released into the duct. The skin's sebaceous glands are the classic examples for this mode of secretion.
Sebaceous gland. This is a holocrine gland, because its product is secreted with the remnants of a dead cell. Stem cells (arrows) in the base of the gland proliferate to replace the lost cells. Collagen fibers are stained in red. PSP stain. Medium magnification.
Some Comparisons of Exocrine and Endocrine Glands
Typically by ducts.
Typically by the bloodstream
May be unicellular
(eg, goblet cells)
(eg, salivary glands)
May be unicellular (eg, DNES cells)
or multicellular (eg, thyroid gland).
(eg, digestive enzy-mes), glycoproteins (e.g., mucus),
and some mixtures
(e.g., sebum, bile,
apocrine sweat, and milk)
Hormones of 2 main types
peptide hormones (eg, insulin)
and steroid hormones (eg, adrenocorticoids).
Plasma proteins produced by the liver
(e.g., serum albumin and clotting factors)
are also considered endocrine secretory products.
Merocrine (by exocytosis, no loss of cytoplasm), apocrine (loss of apical cytoplasm); holocrine (entire cell released into duct).
Diagram of a serous (pancreatic acinar) cell. Note its evident polarity, with abundant basal rough endoplasmic reticulum. The Golgi complex and zymogen granules are in the apical region. To the right is a scale indicating the approximate time necessary for each step.
Steps of secretory process.
1. Accumulation of necessary precursors.
2. Synthesis and deposition.
3. Extrusion of secretory products.
4. Regeneration of glandular cell.
Electron micrograph of a pancreatic cell. Note the nucleus, mitochondria, Golgi complex, secretory (zymogen) granules in various stages of condensation, and rough endoplasmic reticulum. x13,000.
As the endocrine glands develop in the embryo,
their connection with the surface epithilium is lost. In some
cases the gland consists of sacs lined with epithilium and surrounded by
connective tissue. In most cases the the invagination of the epithilium loses
Other endocrine glands, which are not entirely dissociated from the excretory duct system, are called mixed glands. In the liver, for example, the hepatic cells that secrete bile into the duct system also eliminate internal secretions directly into the blood vessels. On the other hand, in the testis and pancreas, one group of cells secretes into the external duct system, while another group passes its internal secretion into the blood.
The endocrine glands secrete their specific products, called hormones, directly into the blood stream. The endocrine glands are all circumscribed, with minor exceptions. They are thus set aside from numerous other structures believed to produce internal secretions and also important in coordination and integration within the organism. The circumscribed endocrine glands of man are the adrenal, hypophysis, thyroid, parathyroid, islets of Langerhans, and portions of the testis and ovary. Other glands, which resemble these morphologically in some respects, but do not produce any known secretion, are the pineal body and the paraganglia.
Endocrine glands are subject to control by the central nervous system, by other endocrine glands, by certain metabolites, or by a combination of these factors. There is, then, a complicated series of endocrine interrelationships, which are highly important.
Electron micrograph of a cell of the diffuse neuroendocrine system. Note the accumulation of secretory granules in the basal region of the cell. The Golgi complex seen in the upper part of the micrograph shows some secretory granules; it is here that these granules first appear. The arrow indicates the basal lamina.
MAJOR TYPES OF EPITHELIAL CELLS
Epithelial Cells Specialized for Transport:
1. Ion-transporting cells. Some epithelial cells are specialized for transcellular transport, ie, they can pump ions across their entire thickness, apex to base. Sheets of such cells form active barriers that control ion and water concentrations in body compartments. Tight junctions are often found between the cells and appear to prevent backflow. Ion-transporting cells typically have highly infolded basal plasma membranes that inter-digitate with numerous mitochondria. Commonly, the ion pump is specific for sodium (ie, it is Na+/K+-ATPase), and chloride ions and water follow the sodium-ion flow passively. Some ion-transporting epithelia exploit this mechanism to concentrate other solutes by moving water from one compartment to another. Important ion-transporting epithelia are found in the kidney tubules, the striated ducts of the salivary glands, the gallbladder, the choroid plexus and the ciliary body of the eye.
2. Cells that transport by pinocytosis. Epithelial cells specialized for pinocytosis have tight junctions and abundant pinocytotic vesicles. The vesicles transport substances across the cell from the luminal surface to the basal surface, where the vesicle contents are released. The best example is the endothelial cells lining the blood vessels, where transcellular transport is rapid (2-3 minutes).
Epithelial Cells Specialized for Absorption:
Specialized absorptive cells lining the digestive tract (especially the small intestine) have numerous microvilli on their apical surfaces to increase the exposed area. Small nutrient molecules diffuse into the microvilli, and contraction of the microfilaments shortens the microvilli, bringing the nutrients into the cytoplasm. Other nutrients are pinocytosed between microvilli. Absorptive cells with similarspecializations occur in the proximal tubules of the kidney.
Epithelial Cells Specialized for Secretion:
1. Protein-secreting cells. Cells that synthesize proteins for segregation and secretion have abundant basophilic rough endoplasmic reticulum, a well-developed Golgi complex, and, frequently, an accumulation of secretory granules in the cell apex. Proteins secreted by epithelial cells include the digestive enzymes, produced by pancreatic acinar cells and the chief cells of the stomach; serum albumin, produced by liver hepatocytes; and protein hormones, eg, parathyroid hormone produced by the chief cells of the parathyroid gland.
2. Polypeptide-secreting cells. Secreted polypeptides have fewer amino acids than the secreted proteins just mentioned. Polypeptide-secreting cells have a small amount of rough endoplasmic reticulum, a supranuclear Golgi complex, and an accumulation of 100- to 400-nm secretory granules in their bases. These APUD cells (amine precursor uptake and decarboxylation) characteristically concentrate important bioactive amines such as epinephrine, norepinephrine, and serotonin in their cytoplasm. They may absorb these amines from the bloodstream or synthesize them from amino acid precursors by means of amino acid decarboxylases, also found in high concentrations in these cells. Most APUD cells are unicellular glands scattered among other epithelial cells. The number, variety, and wide distribution of cells with these characteristics has generated the concept of the diffuse neuroffndocrine system (DNES). DNES is becoming the preferred designation, but DNES and APUD refer to the same polypeptide-secreting cells. Some APUD polypeptide hormone cells have paracrine effects on neighboring cells; others are released into the bloodstream and have endocrine effects on distant cells. Some important APUD polypeptides are glucagon, from pancreatic islet A cells; insulin, from pancreatic islet B cells; gastrin, from the stomach, small intestine, and pancreatic islet G cells; and somatostatin, from the stomach, small intestine, and pancreatic islet D cells. Tumors composed of APUD cells are called apudomas.
3. Mucous cells. Mucus-secreting cells occur as unicellular, sheet, or solid glands. His-tologic features of these cells include a light-staining, foamy appearance caused by numerous large mucus-containing vesicles concentrated near the cell apex; PAS-positive staining from an abundance of oligosaccharide residues; predominantly acidophilic staining with H&E; a large supranuclear Golgi complex with distinctive glycosyltransferases; and nuclei and sparse rough endoplasmic reticulum in the base of the cell. Goblet cells and mucous cells of the sublingual salivary glands are examples of unicellular and multicellular mucous glands, respectively.
4. Serous cells. Cells that produce serous secretions have characteristics of protein-secreting cells. They are usually smaller, darker-staining, and more basophilic than mucus-secreting cells. Serous cells include pancreatic acinar cells and secretory cells of the parotid salivary glands.
5. Steroid-secreting cells. Endocrine cells specialized to secrete steroid hormones are polygonal or rounded, with a central nucleus and pale-staining, acidophilic cytoplasm that often contains numerous lipid droplets. Their abundant smooth endoplasmic reticulum contains enzymes for cholesterol synthesis and for converting steroid hormone precursors (eg, pregnenolone) into specific hormones (eg, androgens, estrogens, and progesterone). Their numerous mitochondria typically have tubular rather than shelflike cristae and contain enzymes that convert cholesterol to pregnenolone. Steroid hormones include testosterone, produced by interstitial cells of the testes; estrogen, from follicle cells of the ovaries; progesterone, from granulosa lutein cells of the corpus luteum; and cortisone and aldosterone, from cells of the adrenal cortex.
Diagram of the ultrastructure of a hypothetical steroid-secreting cell. Note the abundance of the smooth endoplasmic reticulum (SER), lipid droplets, Golgi complex, and lysosomes. The numerous mitochondria have mainly tubular cristae. They not only produce the energy necessary for the activity of the cell but are also involved in steroid hormone synthesis. Rough endoplasmic reticulum (RER) is also shown.
Contractile Epithelial Cells:
Contractile epithelial cells, or myoepithelial cells, are located between the basal lamina and the bases of the epithelial cells of secretory acini and ducts. They are stellate or spindle-shaped, flattened epithelial cells, with fingerlike processes that embrace the acinus or duct. Their cytoplasm contains abundant actin microfilaments, myosin, tropomyosin, and intermediate filaments (cytokeratin). Several myoepithelial cells may surround a single acinus or duct, and their contraction helps expel the products of exocrine glands. Gap junctions between myoepithelial and other cells facilitate synchronous contraction. Myoepithelial cells are found in lacrimal, salivary, mammary, and sweat glands and around the seminiferous tubules in the testis.
Stem cells of epithelial tissue. Like all mitotic cells, stem cells divide into two cells. One daughter cell remains as a stem cell while the other differentiates and moves toward the apex. Proliferative stem cells are undifferentiated and usually rest on the basement membrane. The cell cycle varies tremendously among stem cell populations in different epithelia, ranging from days to years. For example, the epidermis turns over every 27 days, while seminiferous epithelium turns over every 72 days.
Both benign and malignant tumors can arise from most types of epithelial cells. A carcinoma (Gr. karkinos, cancer, + oma, tumor) is a malignant tumor of epithelial cell origin. Malignant tumors derived from glandular epithelial tissue are usually called adenocarcinomas (Gr. adenos, gland, + karkinos); these are by far the most common tumors in adults. In children up to age 10 years, most tumors develop (in decreasing order) from hematopoietic organs, nerve tissues, connective tissues, and epithelial tissues. This proportion gradually changes, and after age 45 years, more than 90% of all tumors are of epithelial origin.
Carcinomas composed of differentiated cells reflect cell-specific morphologic features and behaviors (eg, the production of keratins, mucins, and hormones). Undifferentiated carcinomas are often difficult to diagnose by morphologic analysis alone. Since these carcinomas usually contain keratins, the detection of keratins by immunocytochemistry often helps to determine the diagnosis and treatment of these tumors.
Epithelia are normally capable of rapid repair and replacement of apoptotic or damaged cells. In some large glands, most notably the liver, mitotic activity is normally rare but is actively renewed following major damage to the organ. When a portion of liver tissue is removed surgically or lost by the acute effects of toxic substances, cells of undamaged regions quickly begin active proliferation and normal functional mass of liver tissue is soon regenerated.
Under certain abnormal conditions, one type of epithelial tissue may undergo transformation into another type in another reversible process called metaplasia, which is illustrated by the following examples.
A tissue is defined as a group of similar cells that work together to carry out a certain specialized function. There are four basic types; muscle, nervous, connective, and of course epithelial. Which of these options defines (vaguely) epithelial tissue?
Cells arranged in sheets of single or multiple layers. The cells in epithelia (the plural of epithelium) are closely packed together in sheets, which can have either a single layer, or multiple. They are held together by many cell junctions, which can be of many types, including gap junctions which are formed by connexin protein tunnels between the cells. Because epithelial cells are so densely packed, there is little intracellular space between the plasma membrane of one cell and its neighbor. The option "Groups of elongated cells that use ATP to produce energy" actually defines muscle cells.
Epithelial cells have different surfaces which have different functions. Which of these surfaces do you think would contain the intraepithelial cell junctions?
Lateral surfaces. As to the locations of these surfaces, their names actually tell you a lot, as you would expect: the apical, or free, surface is the surface of the epithelial cell which faces the lumen (interior space) of an organ, or the body surface, or a body cavity, depending on where in the body it is. If the epithelium is multi-layered, the apical surface is simply the most superficial surface. These surfaces can contain cilia (little hairs) or microvilli (membrane folds or protrusions which increase surface area). The lateral surface is on the side of the cell("latus" in Latin means "side"), and is the surface which joins each cell to its epithelial neighbor, and thus must contain the intraepithelial cell junctions. The basal surface is the bottom surface of the epithelial cell and joins the cell to extracellular components, and the basement membrane anchors the epithelium to connective tissues.
Cell junctions are between epithelial cell neighbors, and are points of contact between their plasma membranes. Which of these junctions do not consist of a protein called plaque?
Tight junctions. Tight junctions contain strands of transmembrane protein which cinch the plasma membranes together at several points, leaving small gaps between them. They are found in abundance in the epithelia of the stomach and intestines; their function is to hinder the passage of materials between cells, which prevents the cell contents leaking out. Adherens junctions have a block of plaque protein on the inside of the plasma membranes, and transmembrane proteins called cadherins stretch across the junction, "plugging in" to the plaque on the other cell, thus binding the two cells together. Desmosomes have the same basic structure as that of adherens junctions, and have in addition intermediate keratin filaments which extend from the desmosome to other desmosomes on the other side of the cell, giving the junction more stability. They are common in skin cells, particularly in the outermost layer, the epidermis. They also prevent cardiac muscle cells from tearing apart during contraction of the heart. Hemidesmosomes are basically half-desmosomes: instead of attaching a cell to another cell, they attach cells to the basement membrane. Instead of cadherins, they have proteins called integrins, which bind to proteins in the basement membrane called laminin.
The basement membrane consists of two layers: which of them is closest to the epithelium, the basal lamina, or the reticular lamina?
basal lamina & basal. It is the basal lamina. This lamina (thin layer) joins onto the basal surface of the epithelial cells and is actually secreted by them. It contains proteins which attach the cells to the basement membrane. The reticular lamina is closer to the connective tissue underneath the epithelium, and contains proteins which are secreted by this connective tissue called fibroblasts. Basement membranes function to support the epithelium, and also provide a route through which epithelial cells can move, for example during growth, or healing of wounds.
Epithelial tissue is avascular, i.e. it does not have its own blood supply.
t. This is true; although the epithelia have a nerve supply, their blood supply actually comes from the adjacent connective tissues. Substances are exchanged by diffusion, e.g. gases, or nutrients coming in and out of the cells. Epithelial tissue can be sub-divided into two further groups: Covering and lining epithelium, which as the name suggests forms the inner lining of structures like blood vessels, ducts, and also structures of various body systems, e.g. respiratory, urinary. This type also forms the outside layer of your skin. The second sub-type is Glandular epithelium, which forms the secreting sections of glands in the body, such as adrenal or sweat glands.
The second type of classification for covering and lining epithelium [the arrangement of cells within their layer(s)] has three types: simple, stratified, and pseudo-stratified. Which of them contain(s) only one layer?
Simple and pseudo-stratified. Both simple and pseudo-stratified are one-layered. Simple epithelium is a single layer which functions for diffusion, secretion and absorption. An example is blood vessels which are simple squamous epithelia. Pseudo-stratified epithelia look like they have multiple layers, because the cells are not all at the same level, but they do actually only contain one layer. Cells that don't extend up to the apical surface either normally have cilia, or they secrete mucus. Examples are the epithelia in the upper respiratory tract and part of the male urethra, both of which are pseudo-stratified columnar epithelia. Stratified epithelia have simply two or more layers, which helps to protect the layers underneath that would otherwise be prone to abrasion (wear-and-tear).
Can you take a guess at which of these body parts would have stratified squamous epithelia?
Vagina. I hope you can relate the function of the body parts to their possible structure: Cells in the eardrum, air sacs, and lymphatic vessels all rely on transfer of materials (or sound waves in the case of the eardrum) through them for their function, and in previous questions, we decided that a simple squamous epithelium is best for this kind of function. This leaves the vagina, and indeed the vagina contains stratified squamous epithelium: a multi-layered tissue of flat, thin cells which helps to protect the lower layers against abrasion. The vagina is prone to abrasion due to the passage of mucus, and also sexual intercourse, and when apical cells are lost, they are replaced from the basal cells of the epithelium. This is the basis of the Papanicolaou (Pap) test, or Pap smear, which checks for any abnormalities in the vagina and cervix by analyzing the epithelial cells that have naturally come off. It is very important in catching pre-cancerous cells early and thus women can be treated more effectively if they do go on to develop cancer, so it is vital that women get one performed regularly.
Glandular epithelium, as the name suggests, forms the secreting portions of both endocrine and exocrine glands. Which of these is a type of unicellular exocrine gland, which, instead of having a duct like multicellular exocrine glands, secretes mucus directly onto the apical surface of the epithelium?
Goblet cells. Goblet cells, as stated in the question, secrete mucus, and are mainly found in the respiratory and digestive tracts. Salivary glands are multicellular exocrine glands and adrenal glands are endocrine glands, and alveolar cells are not glands at all! More info on glands: The secretions of endocrine glands are transported in the bloodstream, and are called hormones. Endocrine glands include the adrenal glands and thyroid gland. Exocrine glands, on the other hand, secrete products into ducts which carry them to the site of function - examples include the salivary glands, where the product of saliva is transported to the mouth.
Bone is an epithelial tissue.
f. This statement is false. Bone is classed as a connective tissue, which you can learn about in the next part of this series, Human Tissue Types II: Connective Tissue. As has been covered in this quiz, epithelial tissue can be of two main types: covering and lining epithelium, and glandular epithelium. I hope you can see why bone comes under neither of these headings.
Student’s Practical Activities
Task No 1. Students must know and illustrate such histologic specimens.
Specimen 1. Simple squamous epithelium (mesothelium of the peritoneum).
Silver impregnation method.
In this prepapation, the mesothelial lining of the peritoneal cavity has been stripped from the underlying connective tissue. This lining is supported by a thin basement membrane. The intercellular substance has been stained with silver salts thereby outlining the closely interdigitating cell boundaries. Epithelium is composed of flattened, irregularly-shaped cells. The nuclei have been stained with the dye, neutral red.
Illustrate and indicate:
Specimen 2. Simple cuboidal epithelium of the kidney.
Stained with haematoxylin and eosin.
This specimen of the cells that line a small collecting tubule in the kidney shows simple cuboidal epithelium in section. Although the boundaries between individual cells are indistinct, the nuclear shape provides an approximate indication of the cell size and shape. Epithelium is supported by the underlying basement membrane.
Illustrate and indicate:
2.Cuboidal epithelial cells.
3.Lumen of the tubule.
Specimen 3. Stratified squamous nonkeratinized epithelium of the human cornea.
Stained with haematoxylin and eosin.
The epithelium of the cornea demonstrated in specimen is a typical example of stratified squamous epithelium. Note the highly cellular basal layer and the transformation through the large polygonal cells of the intermediate layers to the superficial squamous cells. Intercellular substance between cells is absence. The junction between epithelium and underlying connective tissue is regular, and, as in all epithelia, blood vessels do not extend beyong the basement membrane.
Illustrate and indicate:
1. Basement membrane.
2. Stratum basale.
3. Stratum spinosum.
4. Squamous cells.
Specimen 4. Stratified squamous keratinized epithelium (fingertip skin).
Stained with haematoxylin and eosin.
Stratified squamous keratinized epithelium (epidermis) lies on the basement membrane and is supported by loose connective tissue with blood vessels. The junctions between epithelium and loose connective tissue are irregular and form papillae. Note the five morphological layers of this epithelium: 1. Stratum basale – the cells are cuboidal and form a single layer separated from the loose connective tissue by a thin basement membrane; 2. Stratum spinosum – consists of 3-4 layers of prickle cells; 3. Stratum granulosum consists of layers of cells, cytoplasm contains dense basophilic granules; 4. Stratum lucidum appears as a homogenous layer between stratum granulosum and stratum corneum; 5. Stratum corneum consists of layers of the flattened, fused cells devoid of organelles.
Illustrate and indicate:
1. Connective tissue.
2. Basement membrane.
a) stratum basale;
b) stratum spinosum;
c) stratum granulosum;
d) stratum lucidum;
e) stratum corneum.
Specimen 5. Unicellular endoepithelial gland - goblet cell (small intestine).
Stained with iron haematoxylin
The intestinal villi are lined by a simple columnar epithelium. Among these cells there are a lot of glands to named goblet cells. They are modified columnar epithelium cells which synthesise and secrete mucus. The apical cytoplasm contains a dense aggregation of mucous granules which, when released by exocytosis, combine with water to form the viscid secretion called mucous. Cytoplasm is packed with rough endoplasmic reticulum and a few mitochondria. A prominent Golgi apparatus is found in the supranuclear region. The 'stem' of the goblet cell is occupied by a condensed, basal nucleus and is crammed with other organelles in mucous synthesis. Note in this example the lining of the small intestine, the tall columnar nature of the surrounding absorptive cells. All cells are lying on the basement membrane.
Ilustrate and indicate:
1. Columnar epithelium.
2. Goblet cell:
Specimen 6. Simple tubular gland (uterus).
Stained with haematoxylin and eosin
This example of simple tubular gland is taken from uterus. This type of gland has a single, straight tubular lumen with secretory products are discharged.
Illustrate and indicate:
1. Fundus of the gland.
2. Body of the gland.
3. Neck of the gland.
Specimen 7. Simple branched alveolar gland (sebaceous gland of the skin).
Stained with haematoxylin and eosin.
Sebaceous glands provide a good example of simple branched acinar glands. Each gland consists of several secretory acini, which opened into a single excretory duct; the excretory duct is formed by the stratified epithelium surrounding the hair shaft. Each acinus consist of a mass of rounded cells which are packed with lipid-filled vacuoles; during tissue preparation the lipid is largely removed, thus the cytoplasm of these cells is poorly stained. Towards the ducts, the lipid content of the acinar cells increases greatly and the distended cells degenerate, so releasing their contents, sebum, into the duct. The mode of secretion of sebaceous glands is holocrine. Cells destroyed by holocrine secretion and replaced by mitosis in the basal layer in the acinus.
Illustrate and indicate:
1. Excretory duct.
2. Secretory portion:
a) secretory cells;
b) stem cells;
c) degenerate cells.
3. Hair root.
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 33-48.
4. Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 82 – 101.
5. Inderbir Singh Textbook of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002. – P. 43-52.
6. Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 105 – 158.
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