Epithelial Tissue PDF

Epithelial tissue (1) Dr. Fatma Meligy The organs of the human body are composed of four basic tissue types: epithelial, connective, Muscular and nervous. Epithelial tissue Epithelium Characteristic features of epithelial cells 1. Epithelial tissues are composed of closely aggregated cells, forming...

Epithelial tissue (1) Dr. Fatma Meligy The organs of the human body are composed of four basic tissue types: epithelial, connective, Muscular and nervous. Epithelial tissue Epithelium Characteristic features of epithelial cells 1. Epithelial tissues are composed of closely aggregated cells, forming cellular sheets that lines the cavities of organs, cover the body surface or form clusters of cells as in glands. 2. The shapes of epithelial cells are variable (squamous, cuboidal, columnar) with nuclear shape corresponding roughly to cell shape. 3. Adjacent epithelial cells are adhering strongly to one another by cellular junctions. 4. The basal surface of all epithelia rests on a thin extracellular sheet "the basement membrane". 5. Epithelia do not themselves normally contain blood vessels "avascular". 6. Most epithelia are adjacent to connective tissue containing blood vessels from which the epithelial cells receive nutrients. 7. The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory & urinary systems is called the "lamina propria". 8. Epithelia which are formed of one cell layer (simple epithelia) generally show “polarity”, with organelles and membrane proteins distributed unevenly within the cell. The region of the cell contacting the connective tissue is called the “basal pole” and the opposite end, is the “apical pole”, with the two poles differ significantly in both structure and function. 9. The area of contact between the epithelium and connective tissue may be increased by small invaginations called "papillae" projecting from the connective tissue into the epithelium. Papillae occur most frequently in epithelial tissue subject to friction such as the covering of the skin or tongue. 10. The principal functions of the epithelial tissues include: Protection (eg, epidermis). Absorption (eg, intestinal lining). Secretion (eg, glands). Contractile (eg, myoepithelium). Sensory (eg, taste buds). 11- They originate from all the embryonic layers, i.e. there are ectodermal, mesodermal and endodermal epithelia 12- They are traversed by numerous fine terminals of nerve fibers. Basement membrane Is a thin extracellular sheet of macromolecules between the basal surface of epithelia and the underlying connective tissue. Structure: L.M:the basement membrane appears as a thin layer and can be stained with PAS and silver. It separates the epithelium from underlying tissues. It is secreted from the epithelial cells. Structure: With the EM. Formed from three layers include: 1. The cell coat of epithelial cells which appears as a light zone just underneath the outer lamina of the plasma membrane..It is composed of glycoproteins and glycolipids 2. Basement membrane proper (basal lamina) which is chemically Composed of glycoproteins and collagen material.This layer is PAS+ve when seen with the LM. 3.A connective tissue layer highly rich in reticular fibers What is the cell coat? Oligosaccharide chains linked to lipid & protein molecules and extend outward from the plasma membrane forming the cell’s glycocalyx (cell coat) Functions of basement membrane: 1- Provides structural support for epithelial cells; attach epithelia to underlying connective tissue. 2- Acts as filter of diffusing macromolecules (e.g, in the kidney). 3- Maintains cell polarity. 4- Permits the flow of nutrients, as epithelium is avascular. 5- Controls epithelial growth & differentiation. It forms a barrier to downward epithelial growth. This is only breached in malignancy.