The pineal gland of the mole (Talpa europaea L.).

The ultrastructure of the pinealocytes in the fetal mole (Talpa europaea) was examined and compared with that in the adult mole. The parenchyma of the fetal pineal gland consists primarily of pinealocytes with very few dispersed "glial" cells. Three different morphological types of pinealocytes (I, II and III) were observed. Pinealocytes of types I and II, homologous to the rudimentary photoreceptor cells of lacertilians and birds, were commonly found, especially around the pineal lumen, which is still present in the fetus. These results support the concept of the phylogenetic origin of mammalian pinealocytes from the pineal photoreceptor cells of the nonmammalian vertebrates. Considering their synthetic/secretory activity, the pinealocytes of the mole fetus are characterized by the presence of accumulations of proteinaceous material (APM) in the cisternae of the granular endoplasmic reticulum (GER). Two types of APM were also found. One type shows a paracrystalline organization as described in the adult mole. In the fetus, however, the paracrystalline-organized APM occur only infrequently. A progressive transformation, via an increase in size or fusion of vacuoles containing material originating from the cisternae of the GER in APM, free of a paracrystalline structure, has been observed. Granular vesicles (GV) originating from the Golgi saccules were rarely observed. Sometimes GV and APM were found to be present in the same cell.