The distribution and incidence of Type III secretory cells in the terminal portion of rat submandibular gland were examined under light and transmission electron microscopes. The incidence of Type III secretory cells was higher at the hilus of the gland and decreased with aging. Terminal portions which contain Type III secretory cells were sphere in shape and same in size as proper terminal portions which contain only seromucous cells. Most of these terminal portions were consisted of only Type III secretory cells, and some of them consisted of both Trpe III secretory cells and proper seromucous cells. Type IV secretory cells were found in some terminal portions which had Type III secretory cells, and their morphological features were between those of Type III secretory cell and proper seromucous cell. From the observation on serial semithin sections, it was clarified that each intercalated duct of these terminal portions joined together and finally reached one granular duct. These findings suggest that Type III secretory cells differentiate to proper seromucous cells via Type IV secretory cell and that the growth center in rat submandibular gland is near hilus of the gland.
