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The ultrastructure of adult male rat pinealocytes was studied after orchidectomy and orchidectomy followed by LH-RH administration. Castration causes an increased development of the rough endoplasmic reticulum and the Golgi apparatus as well as an increase in the number of lipid droplets and lysosomes. The changes after orchidectomy followed by LH-RH administration were more marked than after castration alone. The ultrastructural features observed after castration can be interpreted as a morphological equivalent of increased protein synthesis. The observations reported appear to indicate mutual interaction between the pineal organ and the hypothalamo-hypophyseo-gonadal system. A working hypothesis is put forward implying the presence of a negative feedback mechanism between the pineal and the adenohypophysis.

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The ultrastructure of the pinealocytes of noctule bats, mammals which live most of the time in darkness or very low light intensities, was examined and compared with the pinealocytes of other mammals. Two different populations of pinealocytes (I and II) were observed. They differ in general aspect, in location and especially in their content of cell organelles involved in synthetic processes. Mitochondria, ribosomes, lysosomes and lipid inclusions were present in the perikaryon of pinealocytes of both populations. In the pinealocytes of population I some granular vesicles, of presumed Golgi origin, and some other structures were observed. Pinealocytes of population II are characterized by many glycogen granules, more or less associated with a large vacuolar system. Moreover, some small vacuoles originating from cisterns of the granular endoplasmic reticulum and containing flocculent material of a moderate electron density are described. The possibility is discussed that these small vacuoles are involved in one of the secretory processes of the pineal gland while the granular vesicles of the pinealocyte of the population I are the products of another.

The ultrastructure of pinealocytes of the mole-rat (Spalax ehrenbergi), a blind subterranean mammal living in complete darkness, was examined and compared with pinealocytes of other mammals. Two different populations of pinealocytes (I and II) were observed. They differed in general aspect. In location and especially in their content of cell organelles involved in synthetic processes. Mitochondria, ribosomes, granular endoplasmic reticulum, lysosomes, lipid inclusions and glycogen granules were present in the perikarya of pinealocytes of both populations. In the pinealocyte of population I some granular vesicles were occasionally observed in the cell body. Their presumed origin from the Golgi apparatus could not be clearly demonstrated. In the perikaryon of this pinealocyte, concentrations of ribosomes and of cisternae of the granular endoplasmic reticulum were constantly observed. These concentrations may indicate an intensive synthetic activity. Pinealocytes of population II were characterized by accumulations of proteinaceous material in some cisternae of the granular endoplasmic reticulum and between the two layers of the nuclear membrane. The origin of these peculiar elements is discussed.

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