OUTER AND INNER MEDULLARY COLLECTING DUCT

The outer medullary collecting duct (OMCD) consists of principal cells and a smaller population of type A intercalated cells. Both appear taller and less dense than their equivalents in the cortical segment.

The inner medullary collecting duct (IMCD) is often subdivided into initial and terminal segments. The initial segment contains principal cells that appear similar to those in the OMCD. The terminal segment contains IMCD cells, which resemble principal cells but are taller and have denser microvilli, a lighter staining cytoplasm, and fewer basal invaginations. Both IMCD segments lack intercalated cells.