Specific paracrystalline structures of rough endoplasmic reticulum in the follicular (stellate) cells of the dog adenohypophysis.

The fine structure of follicular cells of the adenohypophysis was examined in fetal, neonatal, and adult beagle dogs. Prior to birth, undifferentiated follicular cells are common. At birth mature cells that form follicles are routinely encountered. The fine structural appearance of follicular cells is unchanged between birth and adulthood. Follicular cells of puppies and adults are, however, distinguished by the presence of unusual complexes within distended cisternae of the rough endoplasmic reticulum. These complexes vary greatly in morphology, some appear as a maze of interconnecting tubules while others show a highly organized paracrystalline configuration. The presence of these paracrystalloid structures in follicular cells supports the view that they represent a distinct pituitary cell type.