The developmental role of type III and type IV cells in the rat submandibular gland.

In addition to seromucous cells and a few mucous cells (Type II cells), submandibular glands of 2-8 month old rats contain two additional cell types: type III and type IV. Type III cells contain serous-type secretory granules that sometimes have a complex substructure; type IV cells appear to be seromucous, but their granules clearly are different from those in conventional endpiece seromucous cells. Both type III and IV cells are involved in histogenesis of new endpieces in a process that differs markedly from that occurring in perinatal glands. In this process, intercalated ducts bud and give rise to immature endpieces that consist entirely of type III cells. These differentiate into type IV cells, which in turn differentiate into standard seromucous cells. Concurrently, the intercalated ducts become shorter as their most distal cells differentiate into granular duct cells. This type of developmental process begins approximately 2 months postnatally, when histogenesis of endpieces by means of terminal tubules has ended, and continues until 6 months, when its frequency sharply declines.