Postnatal development of cell types in the rat pineal gland.

The morphological development of the rat pineal gland has been studied from 1 to 60 days of age. During the first days, undifferentiated cells (pinealoblasts) with scanty cytoplasm and frequent mitotic figures were observed. The differentiation of cell types (Types I and II pinealocytes) began on the third day after birth and was completed by days 15-20. At 3 days of age, nerve fibres were first observed, both in the connective spaces and in the parenchyma. After 5 days, an important hypertrophy of pinealocytes began, mostly Type I, which continued until 60 days of age. After 45 days, all the ultrastructural features described in the adult pineal gland were already present. The findings are discussed.