A quantitative electron microscopic study on the frequency of exocytosis in the anterior pituitary of perinatal rats

Granule exocytosis was quantitatively investigated in the perinatal rat anterior pituitary at the electron microscopic level. Both the number of cells in the process of exocytosis and the number of extruded granules per cell profile found in a standardized area of section were counted. The first distinct figure of exocytosis was detected in the anterior pituitary of fetal rats on day 18.5 of gestation, although occasional cells on day 17.5 had structures resembling granule extrusion. The frequency of cells showing granule discharge was very low on day 18.5 of gestation, but it sharply increased on day 19.5; a similar level was maintained up to day 21.5 of gestation. While the number of exocytosed granules per cell profile was almost unchanged during the fetal and neonatal period up to day 3 after birth. The frequency of cells undergoing exocytosis decreased near the time of birth, after which it transiently increased and dropped again to a minimum at 12 h after delivery. During days 1 to 3 of postnatal life, cells in the process of exocytosis were less frequent compared to fetuses between day 19.5 and 21.5. Both the number of cells undergoing exocytosis and the number of discharged granules per cell profile first exceeded the fetal values on the 6th postnatal day and were remarkably augmented between days 9-20 of the neonatal period. These data are discussed in relation to the hormone secreting activity of the anterior pituitary gland of perinatal rats.