In this article we will discuss about the structural features of epithelial cell with the help of a suitable diagram.
In many cell types, particularly in epithelial cells (Fig. 1.78) and other certain cells (cardiac and smooth muscle cells) show some modifications which are mostly related to their functions. These specialised structures are generally seen under electron microscope. In some apical cells, microvilli which are the cytoplasmic projection of the plasma membrane are generally seen under electron microscope.
Under light microscope these microvilli were generally seen like brush or striated borders. The principal function of microvilli is absorption. These microvilli are concentrated profusely in cells whose functions are absorption. These are present abundantly in the intestinal epithelium and also in proximal convoluted tubules. There is also a surface specialisation in the epithelial lining of the male reproductive tract (epididymis) where an unusual and long pyriform tuft projection is seen to some one from each cell.
These projections are known as steriocilia. In electron microscope it reveals that these have got no resemblance to true cilia but are composed of long and slender microvilli. Glycoprotein surface coat covers the luminal surface of microvilli in the proximal renal tubule and sometimes in the intestine. The surface glycoprotein coat is also known as fuzz (by Ito & Winchester) or flluffy coat (by Farquhar & Palade).
Another type of motile cell processes, larger than microvilli, is seen in certain epithelial cells. They are numerous and arranged in rows in a single cell. Cilia have got resemblance to flagella. It is larger than microvilli. Its principal function is to aid in transporting material in one direction.
There are also specialisations for cell attachment and cohesion particularly between the adjacent epithelial cells and also in certain other cell types (intercalated disc in cardiac muscle and nexus in smooth muscles). These special types of cell attachment are generally called as junctional complex.
These junctional complexes are of three types:
i. Zonula Occludens (Tight Junction):
In which the plasma membrane is fused together so as to obliterate the intercellular space. This component of the junctional complex is present immediately below the free surface and surrounds the entire perimeter of the cell extending deeply for about half a micron.
ii. Zonula Adhaerens (Intermediate Junction):
Next to zonula occludens is the zonula adhaerens where the plasma membrane is again placed 150 to 200 Å apart from the adjoining cells. Here the inner aspects of the plasma membrane contain some dense filamentous material forming a continuous band parallel to the zonula occludens.
Other than functioning as attachment and cohesion, this component has got a special function. It is probably the passage of fluid between the lumen and the intercellular spaces so that the material passing through lumens may pass through the apical cytoplasm to come into the intercellular space. Zonula occludens and zonula adhaerens are collectively known as terminal bar.
iii. Macula Adhaerens or Desmosome:
Next to zonula adhaerens is the mucula adhaerens or desmosome. It is a small discrete bipartite elipsoidal disc. At the desmosome the cytoplasmic surfaces of the plasmalemma of opposing cells appear dense and thickened. The thickening is due to the presence of thicker layer of fine cytoplasmic fibrils or filaments arranged in a beltwork and these filaments are known as tonofilaments.
In some epithelia hemidesmosomes are sometimes found on the basal plasma membrane adjacent to the basal lamina. The lateral surfaces of the cells are not arranged in parallel to each other but show interdigitation forming a jigsaw or zipper interlocking in association with desmosome.