OBJECTIVE 1: TO DETERMINE THE GENERAL FEATURES OF A TRANSPORTING EPITHELIUM.

1. Around each cell near its luminal or apical face is a circumferential belt, the zonula occludens (tight junction) which connects it with neighboring cells. The basal surface of each cell is embedded in a basement membrane. The lateral surfaces of neighboring cells are separated by a fluid filled space. This paracellular space is separated from the lumen by the zonula occludens (Fig 3-1).

Fig. 3-1. The basic structure of epithelial cell layers. The width of the zonula occludens and the paracellular space have been greatly exaggerated for purposes of emphasis.

2. The microanatomy of the apical membrane may differ dramatically from that of the basolateral membrane, for instance, the brush border of the apical face and the extensive folding of the basolateral membrane of the proximal tubular cells.

3. The protein or molecular complexes within the apical and basolateral membranes differ in type and quantity, for instance, Na-K-ATPase is present in basolateral membranes of tubular cells but is not found in apical membranes. Receptors for antidiuretic hormone (ADH) are located in the basolateral but not the apical membranes of collecting tubular cells.

B. The functional features of the epithelium are determined by the molecular complexes found in the apical and basolateral membranes and by the permeability of the zonula occludens. The asymmetrical features of the apical and basolateral membranes determine the direction of transport across the cell and, in part, the rate of transport. For instance, Na⁺ channels exist in the apical membrane of principal cells in the collecting tubule that allow Na⁺ to enter the cell from the tubular fluid. Na-K-ATPase in the basolateral membrane pumps Na out into the extracellular fluid. Thus there is a net reabsorptive transport of Na⁺ across the cell. The zonula occludens surrounding these cells has a low permeability to Na⁺ so there is little passive diffusion back into the lumen and the cells can reduce the urine Na⁺ concentration to very low levels.

C. The paracellular space constitutes a micro environment which may differ in composition with the interstitial fluid. Its composition is affected by transport across the surrounding basolateral membranes and diffusion across the zonula occludens. In turn its composition affects the rate of that transport.

D. Solutes may cross the epithelium via the cellular pathway or via the paracellular pathway. Transport through the cellular pathway usually involves specific carriers and channels. Transport through the paracellular pathway occurs by diffusion driven by electrical and chemical gradients between the tubular fluid and the interstitial fluid. The permeability or conductance of the paracellular pathway for passive movement is higher than the path through the cells. But active transport rates through the cell usually exceed the passive transport rates through the paracellular path.

QUESTIONS:  
1. What are the major anatomical features of an epithelial cell layer?

2. What is the essential feature of epithelial cells that allows net transport of solutes and water in one direction across the cell layer?

3. What paths may solutes take across the epithelial cell layer?

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