Epithelial Tissue PDF

EPITHELIAL TISSUE The human body is composed of four basic types of tissue: 1-Epithelial 2-Connective 3-Muscular 4- Nervous. These tissues are formed by cells and molecules of the extracellular matrix in variable proportions,...

EPITHELIAL TISSUE The human body is composed of four basic types of tissue: 1-Epithelial 2-Connective 3-Muscular 4- Nervous. These tissues are formed by cells and molecules of the extracellular matrix in variable proportions, forming different organs and systems of the body. The characteristics of epithelial tissue: Epithelial tissues are composed of closely aggregated cells with very little extracellular substance. These cells have strong adhesion to form cellular sheets that cover the surface of the body and line its cavities. Epithelium may be derived from ectoderm, mesoderm or endoderm. Epithelium lacks blood vessels of its own and depends on the diffusion of nutrients from the underlying connective tissue. Epithelium has high power of regeneration. Epithelium forms glands (glandular epithelium) or acts as receptors (neuroepithelium) or has a contractile function (myoepithelium). The functions of epithelial tissues: are the covering surfaces (e.g. skin), absorption (e.g. intestines), secretion (e.g. glands), sensation (e.g. neuroepithelium) and contractility (e.g. myoepithelial cells). The characteristic features of epithelial Cells The shape of epithelial cells range from high columnar to cuboidal to low squamous cells. Epithelial cell nuclei have different shapes. The form of the nuclei of epithelial cells corresponds to the cell shape; so, cuboidal cells have rounded nuclei and squamous cells have flattened nuclei. All epithelial cells rest on a connective tissue (C.T). It supports the epithelium, binds it to neighboring structures and provides nutrition. Types of Epithelial tissues Surface epithelium Glandular epithelium Neuroepithelium Myoepithelium Surface Epithelium Its cells cover the external surface or line the cavities of the body. They can be classified according to the number of cell layers and the shape of the cells in the surface layer. 1- Simple epithelium contains only one layer of cells. 2- Stratified epithelium contains more than one layer. Common Types of Surface Epithelia in the Human Body. Type Cell shape Examples Function 1-Simple Squamous Lining of vessels & heart Provide smooth surface for (endothelium). easy passage, Facilitates the movement Serous lining of cavities; of the viscera. pericardium, pleura, Allow gas exchange and peritoneum facilitate filtration (mesothelium). Lung alveoli and Bowman,s capsule of kidney. Cuboidal Covering the ovary, thyroid, Covering, secretion and tubules in glands and reabsorption kidney. Columnar Lining of intestine, stomach, Lubrication, absorption, gallbladder and uterus. secretion. Pseudo- Lining of trachea, bronchi, Protection, cilia-for transport stratified nasal cavity. of particles trapped in mucus. columnar 2- squamous Epidermis. Protection; prevents water Stratified keratinized (dry) loss. squamous Mouth, esophagus, larynx, Protection, secretion; nonkerat-inized vagina, anal canal. prevents water loss. (moist) Cuboidal Sweat glands, developing Protection, secretion. ovarian follicles. Transitional: Bladder, ureters, renal Protection, distensibility. domelike to calyces. flattened, depending on the functional state of the organ Columnar Conjunctiva. Protection.  Based on cell shape, simple epithelium can be squamous, cuboidal, columnar or pseudostratified. Columnar epithelium may be ciliated as in lung bronchioles, uterus and fallopian tube or non ciliated as in stomach, intestine and gall bladder. Pseudostratified epithelium, so called because the nuclei appear to lie in various layers, all cells are attached to the basal lamina, although some do not reach the surface. The example of this tissue is the ciliated pseudostratified columnar epithelium in the respiratory passages. Stratified epithelium is classified according to the cell shape of its superficial layer: squamous, cuboidal, columnar, and transitional. Stratified squamous epithelium is found in places subject to friction (skin, mouth, esophagus, and vagina). Its cells form many layers, and the basal cells are usually columnar. The superficial cells become irregular in shape, flatten and squamous. Stratified squamous keratinized epithelium covers dry surfaces such as the skin. Non keratinized epithelium covers wet surfaces. The most superficial cells of keratinized epithelia are transformed into dead scales of protein without nuclei (keratin). Transitional epithelium: a special form of epithelium, in which the cells can alter their shape. It lines the urinary bladder, the ureter and the upper part of the urethra where number of layers is changeable during filling or emptying. In empty bladder: It is formed of 6-8 layers, lies on non-clear basement membrane. Basal cells are high cubical with rounded nuclei.Cells of intermediate layers are polygonal with rounded nuclei. Intercellular spaces contain mucous-like substance, allow gliding of cells. Superficial cells are large cuboidal cells with upper convex Surface, known as dome cells and may be bi-nucleated. In Full (expanded) bladder: Cells glide over each other, helped by intercellular mucous like substances. Epithelium compressed and stretch without damage, becomes thinner & formed of 3-4 layers, allowing distention of bladder.