Epithelial Tissue PDF

Epithelial Tissue: Classification, structural features, specialization, function. Dr Abayomi T.A Dept of Anatomy UNIOSUN Introduction Epithelial tissues are composed of closely aggregated polyhedral cells with very little extracellular substance. These cel...

Epithelial Tissue: Classification, structural features, specialization, function. Dr Abayomi T.A Dept of Anatomy UNIOSUN Introduction Epithelial tissues are composed of closely aggregated polyhedral cells with very little extracellular substance. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities. Functions of epithelium The epithelial tissue serves many purposes, including ⚫ Covering, lining, and protecting surfaces ⚫ ⚫ (eg, skin) Absorption (eg, the intestines) ⚫ ⚫ Secretion (eg, the epithelial cells of glands) Contractility (eg, myoepithelial cells). ⚫ Sensory Filtration, and reception - most epithelium are extremely sensitive to stimulation due to large sensory nerve supply present e.g taste buds or the olfactory epithelium CHARACTERISTICS OF EPITHELIUM ⚫ Polarity- Epithelium is arranged so there is one free surface (apical surface) and one attached surface (basal surface) ⚫ Cellular nature- Cells in epithelium fit closely together side by side and sometimes atop each other to form sheets of cells. These sheets are held together by specialized junctions. CHARACTERISTICS CONTD. ⚫ Supported by connective tissue- Attachment to a layer of connective tissue at the basal surface forms a layer called the basement membrane, an adhesive layer formed by secretions from the epithelial cells and the connective tissue cells. ⚫ Avascular- Epithelium typically lacks its own blood supply. ⚫ Regeneration- Epithelium cells can regenerate if proper nourished. An extracellular basal lamina (membrane) always lies at the interface of epithelial cells and connective tissue. This section of kidney shows the typical basement membranes (arrows) of several tubules and of structures within the single glomerulus included here. In renal glomeruli the basement membrane, besides having a supporting function, has an important role as a filter. X100. Classification of Epithelium ⚫ 1) By cell shape, epithelia are classified as a) squamous - flat and scale-like ⚫ b) cuboidal - as tall as they are wide ⚫ c) columnar - tall, column-shaped ⚫ d) Pseudostratified- a single layer of cells that appears to be multiple layers due to variance in height and location of the nuclei in the cells. ⚫ e) Transitional- cells are rounded and can slide across one another to allow stretching Classification contd 2) By cell arrangement ⚫ a) simple epithelium - single layer of cells (usually for absorption and filtration) ⚫ b) stratified epithelium - stacked up call layers (protection from abrasion (rubbing) - mouth, skin.) and are named according to the cell shape of the superficial layer(s): squamous, cuboidal, columnar, and transitional. Classification contd Epithelia can be divided into two main groups according to their function to: ⚫ Covering/Surface epithelia - layers of cells that cover internal or external surfaces e.g ⚫ Glands – structures attached to or derived from epithelium that produce fluid secretions Simple Epithelia ⚫ Based on cell shape, simple epithelia are classified as ⚫ squamous (thin cells), ⚫ cuboidal (cells roughly as thick as they are wide) ⚫ or columnar (cells taller than they are wide) Simple squamous epithelia ⚫ In simple squamous epithelium, cells of the single layer are flat and usually very thin, with only the thicker cell nucleus appearing as a bulge to denote the cell. ⚫ Simple epithelia are typically specialized as lining of vessels and cavities and regulate substances which can enter underlying tissue from the vessel or cavity. ⚫ Examples are endothelium and mesothelium Simple cuboidal epithelium. ⚫ Cells of simple cuboidal epithelia vary in their height but are roughly as tall as they are wide. ⚫ Examples of simple cuboidal epithelia are from a renal collecting tubule (a), a pancreatic duct (b ), and the mesothelium covering an ovary. Simple columnar epithelium. ⚫ Cells of simple columnar epithelia are taller than they are wide. ⚫ They are usually highly specialized for absorption, with microvilli, and often have interspersed secretory cells or ciliated cells. ⚫ Such epithelial cells always have tight and adherent junctional complexes at their apical ends, but are often loosely associated in more basolateral areas. ⚫ Examples are from a renal collecting duct (a), the oviduct lining, with both secretory and ciliated cells (b), and the lining of the gall bladder. Stratified epithelia ⚫ Stratified epithelia are classified according to the cell shape of the superficial layer(s) ⚫ It can be squamous, cuboidal, columnar, and transitional. ⚫ The very thin surface cells of stratified squamous epithelia can b e "keratinized" (rich in keratin intermediate filaments) o r "nonkeratinized" (with relatively sparse amounts of keratin).  Stratified squamous keratinized epithelium ⚫ is found mainly in the epidermis of skin. ⚫ Its cells form many layers, and the cells closer to the underlying connective tissue are usually cuboidal or low columnar. ⚫ The cells become irregular in shape and flatten as they accumulate keratin in the process of keratinization and are moved progressively closer to the surface ⚫ This surface layer of cells helps protect against water loss across this epithelium. Stratified squamous nonkeratinized epithelium ⚫ Epithelia lining many internal surfaces such as the esophagus (b), mouth, and vagina or covering the cornea (c) are considered nonkeratinized because the differentiating cells accumulate much less keratin and retain their nuclei. ⚫ Such epithelia provide protection against microorganisms, but do not fill with keratin because water loss is less of an issue. Stratified cuboidal and stratified columnar epithelia ⚫ Stratified cuboidal and stratified columnar epithelia are rare. ⚫ Stratified columnar epithelium can be found in the conjunctiva lining the eyelids, where it is both protective and mucus secreting. ⚫ Stratified cuboidal epithelium is restricted to large excretory ducts of sweat and salivary glands, where it apparently provides a lining more robust than that of a simple epithelium. Transitional epithelium or urothelium ⚫ Lines only the urinary bladder, the ureter, and the upper part of the urethra, ⚫ It is characterized by a superficial layer of domelike cells that are neither squamous nor columnar. ⚫ These cells, sometimes called umbrella cells, are essentially protective against the hypertonic and potentially cytotoxic effects of urine. ⚫ Importantly, the form of the surface cells changes according to the degree of distention of the bladder wall. Pseudostratified columnar epithelium ⚫ These are so called because all cells are attached to the basal lamina even though their nuclei lie at different levels in the epithelium and the height of some cells does not extend to the surface. ⚫ The best-known example of pseudostratified columnar epithelium is that lining the passages of the upper respiratory tract. ⚫ The columnar cells of this epithelium are also heavily ciliated. Glandular epithelia ⚫ Glandular epithelia are formed by cells specialized to secrete which contains small membrane-bound vesicles called secretory granules. ⚫ The epithelia that form glands can be classified according to various criteria. ⚫ Unicellular glands consist of large isolated secretory cells and multicellular glands have clusters of cells. Types of glands Depending on whether the glands retain connection with the surface epithelium from which they originate, glands can be divided into; ⚫ Exocrine glands and ⚫ Endocrine glands Exocrine glands retain their connection with the surface epithelium, in the form of tubular ducts lined with epithelial cells through which the secretions pass to the surface. Endocrine glands lost their connection to the surface epithelium and are therefore ductless with their secretions being transported to their sites of action by the bloodstream rather than by a duct system. Exocrine glands structure ⚫ Exocrine glands have a secretory portion, which contains the cells specialized for secretion, and ducts, which transport the secretion out of the gland. ⚫ Variations are seen in the morphology of exocrine glands as follows; ◦ Ducts can be simple (unbranched) or compound (with two or more branches ◦ Secretory portions can be tubular (either short or long and coiled) or acinar (round or globular). ◦ Either type of secretory portion may be branched. ◦ Compound glands can have tubular, acinar, or tubuloacinar secretory portions ⚫ ⚫ Hence, the glands to be classified as ⚫ Simple glands have unbranched ducts, although the ducts may be short or long and coiled. The secretory portions attached to these ducts may themselves be branched. The secretory portions are either tubular, if more or less cylindrical in shape, or acinar, if bulbous or saclike. ⚫ Compound glands: If the ducts branch to serve multiple secretory units. On compound glands, the secretory units may be all tubular, all acinar, or a combination of the two shapes (tubuloacinar) Mode of secretion in Exocrine glands Exocrine glands are also classified functionally according to the way the secretory products leave the cell. These are; ⚫ Merocrine secretion (sometimes called eccrine) involves typical exocytosis of proteins or glycoproteins. This is the most common mode of secretion. Example is the mammary gland ⚫ Holocrine secretion involves the cell filling with secretory product and then the whole cell being disrupted and shed. This is best seen in the sebaceous glands of skin ⚫ In an intermediate type, apocrine secretion, the secretory product is typically a large lipid droplet and is discharged together with some of the apical cytoplasm and plasmalemma eg in mammary gland. Types of secretion ⚫ Exocrine glands with merocrine secretion can be further categorized as either serous or mucous according to the nature of the proteins or glycoproteins secreted and the resulting staining properties of the secretory cells. ⚫ Serous acinar cells of the exocrine pancreas are arranged in small acini of 5-10 cells with a very small central lumen. Each acinar cell is roughly pyramid- shaped, with its apex at the lumen. ⚫ The parotid salivary glands is also an example of the serous type which secrete digestive enzymes. ⚫ Mucous cells are typically larger than serous cells, with more flattened basal nuclei. They contain strongly hydrophilic glycoproteins called mucins which when released from the cell, become hydrated and form mucus, a viscous, elastic, protective lubricant material. The lumens (small arrows) of mucous tubules are larger than those of serous acini. ⚫ Other types of mucous cells are found in the stomach, the various salivary glands, the respiratory tract, and the genital tract. Serous gland Mucous gland Endocrine glands ⚫ Endocrine glands are the producers of hormones, which are generally polypeptide or lipid-derived factors that are released into the interstitial fluid. ⚫ Hormones diffuse into the blood for circulation and bind specific receptors on target cells elsewhere in the body, often within other endocrine glands.