Epithelial Tissue

•         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

•         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

•         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.

EPITHELIOID TISSUE

•         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.

SIMPLE  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

•         Uterus

4.  Psuedostratified epithelium is a simple epithelium but       appears stratified although cells contact the basal lamina but not all reach the surface.

STRATIFIED EPITHELIUM

•         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

1.NEURO-EPITHELIAL CELLS

Examples:

Sense organs like taste buds

2.MYO-EPITHELIAL CELLS

Examples

Secretory acini of Mammary, Salivary glands

FUNCTIONS OF EPITHELIAL TISSUE

•         Secretion

–        as in the columnar epithelium of the stomach and the gastric glands.

•         Absorption

–        as in the columnar epithelium of the intestines and proximal convoluted tubules in the kidney.

•         Transport

–        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.

•         Protection

–         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.

Want a clearer concept? Also see

Histological slides of epithelium

Special features of epithelial tissue

Glands