Lateral cell surface folds (plicae) create interdigitating cytoplasmic processes of adjoining cells
The lateral surfaces of certain epithelial cells show a tortuous boundary due to infoldings or plicae along the border of each cell with its neighbor (Fig. 4.15). These infoldings increase the lateral surface area of the cell and are particularly prominent in epithelia that are engaged in fluid and electrolyte transport, such as the intestinal and gallbladder epithelium. In active fluid transport, sodium ions are pumped out of the cytoplasm at the lateral plasma membrane by Na+/K+-ATPase localized in that membrane. Anions then diffuse across the membrane to maintain electrical neutrality, and water diffuses from the cytoplasm into the intercellular space, driven by the os motic gradient between the salt concentration in the intercellular space and the concentration in the cytoplasm. The intercellular space distends because of the accumulating fluid moving across the epithelium, but it can distend only to a limited degree because of junctional attachments in the apical and basal portions of the cell. Hydrostatic pressure gradually builds up in the intercellular space and drives an essentially isotonic fluid from the space into the underlying connective tissue. The occluding junction at the apical end of the intercellular space prevents fluid from moving in the opposite direction. As the action of the sodium pump depletes the cytoplasm of salt and water, these are replaced by diffusion across the apical plasma membrane, whose surface area is greatly increased by the presence of microvilli, thus allowing the continuous movement of fluid from the lumen to the connective tissue as long as the Na ' /K+-ATPase is active.
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