Proliferative activity of cells of the intermediate lobe of the rat pituitary during the postnatal period.

Cellular proliferation was studied in the intermediate lobe (IL) of the pituitary gland of developing rats by labelling cells at the S-phase of the cell cycle with bromodeoxyuridine (BrdU). The number of BrdU-labelled cells in the IL decreased from birth until the 14th postnatal day and was low from that day until the end of the first month after birth. Throughout the postnatal period a large proportion of BrdU-labelled cells was found in the marginal layer (ML) of the IL, suggesting for the ML a role as a germinative layer of the IL during postnatal growth. Double immunostaining with anti-BrdU and anti-MSH showed that MSH cells actively proliferate as from the day of birth. Cells doubly immunostained with anti-BrdU and anti-S100 protein were first seen on the 14th postnatal day. From then onwards, most proliferating cells were labelled with either anti MSH or anti S-100 protein. This, together with the high proportion of proliferating cells found in the ML marks a clear difference with the pattern of cellular proliferation previously reported during a similar period in the anterior lobe of the rat pituitary.