Ultrastructure of the submandibular gland in the African multimammate rodent, Praomys natalensis.

Praomys natalensis, an African rodent that is phenotypically and cytogenetically intermediate to rats and mice, possesses a submandibular gland that is histologically similar to that in both of these near relatives, but is ultrastructurally unique. Acinar cells, which are seromucous in nature, contain secretory granules that often contain a perfect "bull's eye" inclusion (or some variant of this configuration) suspended in a dense matrix. The Golgi apparatus in these cells has an unusual structure, with the Golgi saccules often being doubled over, so that the outermost saccule also is the innermost. This peculiar architecture apparently arises fairly late in the secretory process, i.e., a Golgi apparatus of conventional structure gives rise to a nascent granule (condensing vacuole), then its saccules secondarily fold over. Intercalated ducts are preceded by a ring of specialized cells that have a number of serous-type granules, the duct cells themselves being devoid of such granules. Granular convoluted tubules (GCT) contain large dense granules that appear to be spontaneously involved in chain exocytosis. These GCT granules probably are the repositories of nerve growth factor, which is particularly abundant in Praomys. Striated ducts for the most part are typical in appearance, but they and, to a lesser extent, GCTs contain prominent, membrane-bound crystalloids with a periodicity of about 15 nm.