Histology Theory 2 PDF

Lecture (2) Histology Dr. Reshna kamal Albarzanji [email protected] Specializations of epithelial cells surfaces. — Each epithelial cell has 3 surfaces (an apical , lateral and a basal surface). A- specialization of the apical surface. — The apical surface of many types of epith...

Lecture (2) Histology Dr. Reshna kamal Albarzanji [email protected] Specializations of epithelial cells surfaces. — Each epithelial cell has 3 surfaces (an apical , lateral and a basal surface). A- specialization of the apical surface. — The apical surface of many types of epithelial cells contain specialized structures that increase the surface area for absorption or move substances. 1- Cilia Are cylindrical motile surface evagination covered by plasma membrane, each about (5-10)µm long & (0.2) µm in diameter. They contain a central pair of isolated microtubules surrounded by nine pairs, cilia are beating in waves to moving the surface coat mucus & trapped materials. — (as in ciliated epithelium of trachea). — 2- Stereocilia: — Are long non-motile extensions of cells of epididymis, genital ducts of male reproductive system, are long and branched microvilli, it increase the surface area, facilitating the movement of molecules into and out of the cell. — 3- Flagella : — A flagellum is similar to cilium but is longer & there is usually only one or two per a cell. flagella are concerned with movement as in the spermatozoa (the male gamete). — 4- Microvilli : — Are finger like extensions covered cell surface they increase the apical surface area of absorptive cells and called brush border. Within the microvilli are clusters of actin filaments that are cross-linked to each other and to the surrounding plasma membrane its found lining the small intestine and also in proximal convoluted tubules. Microvilli- Absorption B- Specialization of the lateral surfaces. — The lateral membrane of many epithelial cells often exhibit several types of membrane modifications the intracellular junctions, these junctions from the apex toward the base of the cells are: — 1- Zonula occludens (tight junction): — Is one of the major hallmarks of epithelia, it contributes to the trans epithelial permeability barrier by controlling the passage of molecules through the Para-cellular pathway. Located near the cell apex forms permeability seal prevent the flow of materials between epithelial cells in either direction, their structure results from the fusion of outer leaflets of plasma membranes of two adjacent cells, because that tight junctions act as a barrier to the lateral diffusion it allow the epithelium to isolate certain bodies compartments (they help to keep the intestinal bacteria and toxins out of the blood stream). — 2- Zonula adherens= adherens junction: — This junction encircles the cell and are usually located at basal of the tight junction. composed of 2 electron-dense plaques on opposing cell membranes, containing thin actin and intermediate filaments to strong the joining between adjacent cells. — 3- Macula adherens (Desmosomes ) — Is a complex disk-shaped structure (plaque) at the surface of one cell that is match with an identical structure at the surface of the adjacent cell. — The cell membrane in this region are very straight, and a group of intermediate filaments are inserted into the attachment plagues, these filaments are very strong. — These adhesive elements stably connect cells together playing an essential role in tissue organization and function. They are most abundant in tissues that are subject to mechanical stress, such as skin. — 4- Gap junctions — The individual unit of gap junction is called a connexon each connexon is formed by six gap junction proteins called connexins which join together leaving hydrophilic pore about 1.5nm in diameter in the center, the connexus of adjacent cells form channels between the two cells, each gap junction composed of tens to hundreds of aligned pairs of connexus. This type is widely Distributed in body tissues & best example found in cardiac & smooth muscles. C-Basal lamina & Basement membrane- — Most epithelial cells are separated from the connective tissue by a sheet of extracellular material called the basal lamina. — This structure is visible only with the electron microscope, it appear as a dense layer of about 20-100 nm in thick, consisting of a delicate network of very fine fibrils (lamina densa). In addition, basal lamina may have an electron-lucent layer on one or both sides of the lamina densa, called lamina lucida. — The main components of basal laminae are type IV collagen, the glycoproteins (laminin and entactin), and proteoglycans. — Basal lamina have many functions ,in addition to simple structural functions supporting the cells, they provide a barrier that limit material exchanges. — The term Basement membrane is usually formed by the association of either two basal lamina and a reticular lamina such in renal tubules. — Hemidesmosome: These structures take the form of half a desmosome and bind the epithelial cell to the subjacent basal lamina.  Major types of epithelial cells according to their functions 1-Epithelial cells specialized for transport: a- Ion transporting cells: Have highly enfolding basal membrane that interdigitate with numerous mitochondria. Found in the kidney tubules, the striated ducts of the salivary glands.  b-Cells that transport by pinocytosis: Have tight junctions & abundant pinocytotic vesicles. Transport substances across the cell from luminal surface to basal surface & vice versa. For example the endothelial cells lining the blood vessels  2-Epithelial cells specialized for absorption: Have numerous microvilli on their apical surface to increase the exposed area. As absorptive cells lining the digestive tracts & in the kidneys proximal tubules. 3-Epithelial cells specialized for secretion A-Protein secreting cells: Have abundant RER, well developed Golgi apparatus & accumulation of secretory granules in the cell apex. Pancreatic acinar cells, the stomach’s chief cells, liver hepatocytes & parathyroid glands chief cells  B-Polypeptide secreting cells Are smaller than protein secreting cells Accumulation of secretory granules in their base. These cells called Amine Precursor Uptake Decarboxylate cells (APUD) Most APUD cells are unicellular glands scattered among other epithelial cells. C-Mucous cells Unicellular sheet glands light staining, caused by large mucous containing vesicles concentrated near the apex The nuclei found in the base of the cell. D-Serous cells Have characteristic of protein secreting cells Smaller, darker staining & more basophilic than mucous- secreting cells. Pancreatic acinar cells & the secretory cells of the parotid salivary glands. E-Steroid secreting cells Polygonal or rounded with a central nucleus & the cytoplasm that contain lipid droplets. Testosterone produces by teste’s interstitial cells. Estrogen from ovarian follicle cells. Progesterone from granulose lutein cells of the corpus luteum. Cortisone & aldosterone from cells of the adrenal cortex. 4-Contractile epithelial cells or myoepithelial cells spindle shaped, Flattened with cytoplasmic processes that embrace the acinus or duct. Lie between the epithelial cells & their basal lamina. Found in lacrimal glands & around the testes seminiferous tubules.