Veterinary Histology and Embryology

EPITHELIUM

Definition

Epithelium is composed primarily of cells between which a very little amount of intercellular substance exists.   It is an unilaminar or multilaminar sheet of cells which covers the body surface and lines the body cavities or tubular passages.

Shape and arrangement of epithelial cell

• The shape of epithelial cells varies from a very flat type – squamous, a tall rod-like columnar cell and an intermediate between the two are cuboidal cells, which may be low or high.
• When an epithelium is lined by one layer of cells, it is called simple epithelium; when it is lined by two are more layers of cells it is stratified epithelium.
• In the stratified epithelium, the basal layer of cells rest on a basement membrane and only the uppermost cells have a free surface.
• In certain places the nuclei of epithelial cells lie at various levels giving a stratified appearance. But in these cases all cells reach the basement membrane though all do not have a free surface. This type of epithelium is called pseudostratified and is characteristic of ducts or passages of respiratory and reproductive systems.

Surface of the epithelial cell 

• Each epithelial cell has four surfaces.
  1. Basal surface rests upon the basement membrane,
  2. A free surface opposite to basal surface,
  3. lateral surfaces, with which it is in contact with the adjacent cells.
• Basement membrane is a condensation of the intercellular substance lying between the epithelial cell and the underlying connective tissue.

Attachment between cells 

• In some locations the cells of epithelium are attached to each other by adhesions of the cell membranes of adjacent or opposing cells.
• At these points of adhesions, the membrane of each cell is thick and under EM fine fibrils are seen in the adjacent areas of cytoplasm. These thickened areas of adhesion are known as desmosomes with light microscope.
• The adhesion present in stratified epithelium give the appearance by fixatives between the adjacent borders the cells because of shrinkage caused by fixatives between the adjacent borders of cells except at the sites of adhesions) and hence they were called intercellular bridges under EM it has been observed at the sites of desmosomes there is thickening of opposing membranes and in the adjacent areas of cytoplasm fine fibrils are seen to converge at the desmosomes but there is no protoplasmic continuity between the cells.

Modification of cell surface

• The cytoplasm of cells at the free surface may show modifications.
  • The cells of intestinal epithelium show a pronounced modification of free surface. They have a definite banded layer of uniform thickness known as the striated border / brush border in which are striations vertical to the surface. These striations have been found to be microvilli under EM. This is a particular adaptation for absorption.
  • The cells of kidney tubules (proximal convoluted part) show brush border .
  • The surface cells of epithelium lining respiratory passages and many cells of the epithelium lining uterine tubes show motile cilia (kino-cilia); parts of male reproductive tract have epithelial cell with non-motile cilia (stereo cilia).

Basement membrane (membrana propria)

• This is a sheet of variable thickness and is interposed between epithelium and subjacent connective tissue. It consists of a ground substance of the underlying connective tissue.
• Embedded in this condensed ground substance are delicate reticular fibres, which are continuous with those of underlying connective tissue Basement membranes in ordinary preparations are distinguishable in some locations (eg. trachea).
• In other locations beneath stratified squamous epithelium and columnar epithelium of the intestine it is not easily seen. Basement membrane serves for attachment of epithelium to the underlying connective tissue.
• Blood vessels with few exceptions, epithelia are avascular. Nutritive materials and oxygen enter by diffusion through cells and intercellular substance; Capillaries are present in the epithelium of the stria vascularis of internal ear.