Membrane specialization in the rat epididymis. I. Rod-shaped intramembrane particles in the apical (mitochondria-rich) cell.

Following freeze-fracture of the rat epididymis, a population of cells whose apical, but not baso-lateral, plasma membrane contains elongated intramembrane particles was found in the initial segment. From their size, shape and distribution, these cells are assumed to be the so-called apical cells of this segment. Apical cells also contain the enzyme, carbonic anhydrase which, together with the presence of rod-shaped particle on their membranes, permits their classification as 'mitochondria-rich' cells, a specialized cell type found in some transporting epithelia. On the basis of these observations, we suggest that the apical cells may play a role in fluid and/or electrolyte transport in the initial segment of the epididymis.