Proliferation and differentiation of pituitary somatotrophs and mammotrophs during late fetal and postnatal periods.

Proliferation of somatotrophs and mammotrophs in the rat pituitary during late fetal and postnatal periods up to 4 weeks after birth was quantitatively studied with the double immunostaining of bromodeoxyuridine and the hormones produced by them. Somatotrophs were first detected in 18.5-day fetuses and rapidly increased in number throughout the periods studied. The cells labeled with both anti-BrdU and anti-GH were few in number until shortly before birth and then increased conspicuously during the first 10 days after birth. Mammotrophs were detected at gestational day 19.5 but they were few until the second week after birth, when their number began to increase rapidly. The percentage of the number of the cells double-labeled with both anti-BrdU and anti-GH to all somatotrophs was 8.3% at the most. This was about the same as that of corticotrophs during the late fetal period and that of thyrotrophs in the early postnatal period. In contrast, the percentage of double-labeled cells to all mammotrophs was 3.8% as a maximum, which is lower than the values for somatotrophs, corticotrophs, or thyrotrophs, indicating a smaller contribution of mitosis to mammotroph proliferation. It is possible that this smaller contribution is compensated for by transdifferentiation of cells committed to become the somatotroph lineage. However, coexistence of GH and PRL was not observed in the present material.