Ultrastructure and Transport Across Epithelial Membranes

For epithelia, all too frequently this or that ultrastructure is spoken of as “absorptive” or “(secretory.” Correlation between structure and function must relate to phenomena, such as the apparent active transport of weak electrolytes or water, that are unique for the epithelium. The macroscopic tubule of the gastric mucosa cannot sequester the active transport of hydrochloric acid from the solution bathing the mucosa. In the absence of both intracellular canaliculi and a microvillous border from the amphibian parietal cell, by exclusion, the endoplasmic reticulum or the plasma membrane proper must be considered as possible structures responsible for the sequestration.