• Epithelial tissues are formed by closely apposed cells with little or no intercellular material and occur as membranes and as glands.
• The epithelial cells are supported by connective tissue containing vessels and nerves but are separated from subjacent connective tissue by a basal lamina.
• There are no blood vessels within epithelium itself and metabolism thus depends upon diffusion of oxygen and metabolites from blood vessels in the supporting connective tissue.
• Membranes are sheets of cells that cover an external surface or line an internal surface.
• Functionally are involved in protection, absorption, secretion, excretion, digestion and sensation.
• the membrane serving as selective barrier between the exterior (or interior cavity) and the connective tissue.
• Glands develop from epithelial surfaces by down growths or ingrowths into underlying connective tissue.
• The main function of glands is secretion.
• The connection to the surface remains as the duct of the gland and such glands are the exocrine glands.
• In some the surface connection is lost and the gland secretes into the vascular system, the secretion being a hormone. These are the endocrine glands.
• Cell aggregates which are closely apposed but lack a free surface are known as epithelioid tissue.
• Examples are;
– Interstitial cells of leydig (testis)
– Luteal cells (ovary)
– Parenchyma of adrenal glands
– Epithelioreticular cells (thymus)
Islets of Langerhaans in pancreas
– Pathologic responses to injury and tumors
CLASSIFICATION OF EPITHELIUM
• Based on the numbers of cell layers
• Based on the morphology of surface cells.
– Based on Numbers of Cell Layers
• Single layer of cells – simple
• Numerous cell layers – stratified
• In a stratified epithelium, the shape of the cells that form the surface layer is used in classifying the epithelium.
1. Squamous: Epithelium with flat cells or where the width of the cell is greater than its height.
• Special names are given to epithelium in certain locations:
– Endothelium is the epithelial lining of the vascular system.
– Mesothelium is the epithelium that lines the walls and covers the contents of the closed cavities of the body for example the abdominal, pericardial and plural cavities.
– Endothelium and mesothelium are almost always simple squamous epithelia.
– An exception is found in post capillary venules of certain lymphatic tissue where endothelium is cuboidal.
– Another exception is found in the spleen in which the endothelial cells of the venous sinusis are rod-shaped and arranged like the staves of a barrel.
2. Cuboidal: Epithelium with rounded surfaces cells or when the width, depth and height are approximately the same.
3. Columnar: Epithelium with tall surface cells, or when the height exceeds the width.
• Some simple columnar epithelium are classified as simple columnar ciliated when the apical surface domain possesses cilia.
• Fallopian tube
4. Psuedostratified epithelium is a simple epithelium but appears stratified although cells contact the basal lamina but not all reach the surface.
• In stratified squamous epithelium, the surface cells maybe keratinized or nonkeratinized.
1. STRATIFIED SQUAMOUS
• Multi layered
• Outer most layer- squamous cells
• Inner- cuboidal or columnar
• Lining of mouth, esophagus, skin
2. Stratified columnar
3. Stratified cuboidal
4. Urothelium is a term applied to the epithelium lining the lower urinary tract extending from the minor calyces of the kidney down to the proximal part of the urethra. It is a stratified epithelium that can accommodate to distention, the surface layer varying from squamous to cuboidal with the degree of distention.
SPECIAL TYPES OF EPITHELIUM
Sense organs like taste buds
Secretory acini of Mammary, Salivary glands
FUNCTIONS OF EPITHELIAL TISSUE
– as in the columnar epithelium of the stomach and the gastric glands.
– as in the columnar epithelium of the intestines and proximal convoluted tubules in the kidney.
– as in transport of materials or cells along the surface of an epithelium by motile cilia or in transport of materials across and epithelium to and from the connective tissue.
– as in the stratified squamous epithelium of the skin (epidermis) and the transitionary epithelium of the urinary bladder.
• Receptor function
– to receive and transduce external stimuli as in the taste buds of the tongue, olfactory epithelium of the nasal mucosa and the retina of the eye.