Postnatal development of acinar cells in rat submandibular gland as revealed by electron microscopic staining for carbohydrates.

The postnatal differentiation of acinar cells in rat submandibular gland was studied by staining with periodic acid-thiosemicarbazide-silver proteinate to identify carbohydrate-containing macromolecules in the electron microscope. This method revealed glycogen particles and internal substructure in the secretory granules of developing acinar cells. On the basis of morphologic and histochemical criteria three phases of acinar cell development were defined. In the pro-acinar phase, during the first week after birth, pro-acinar cells and terminal tubular cells were the main components of the terminal tubules in the rudimentary gland. The secretory granules of the pro-acinar cells contained speckled or rod-like substructures which stained intensively for carbohydrates and were digested by proteolytic enzymes. During the second to third week after birth, which is the immature-acinar-cell phase, thread-like substructures were seen in the secretory granules. These structures, which were not digested by proteolytic enzymes, disappeared gradually. The acinar cells of 4-week-old or older rats displayed no particular substructure in the secretion granules and represented the final, mature phase of development.