Ontogeny of Prolactin Cells in Neonatal Rats: Initial Prolactin Secretors also Release Growth Hormone*

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Abstract
Reverse hemolytic plaque assays and immunocytochemistry were used to monitor the ontogeny of individual hormone-secreting and hormone-containing cells in rats. Monodispersed anterior pituitary cells from fetal rats (sex unspecified) and neonatal rats of each sex were cultured for 24 h and then subjected to immunocytochemistry or plaque assays for PRL or GH [growth hormone]. PRL secretors first appeared in appreciable numbers in cultures from 4 day old animals, and by day 5, they accounted for 8-12% of all cells in culture. The percentage of GH secretors rose to a peak on day 5 (comprising .apprx. 40% of all cells), when the values were slightly higher than those observed previously in adults. The percentage of cultured cells from 4-5 day olds that released PRL or GH was not influenced by the sex of the donor animal and was consistent with immunocytochemical estimates. Using a sequential plaque assay that enabled the detection of both GH and PRL release from the same cells, it was found that of every 100 pituitary cells from 5 day old males that released PRL and/or HG, 62.5 released GH only, 1.7 released PRL alone, and the remaining 35.8 released both hormones. Almost identical proportions were found for females. These findings were confirmed using an additional variation of the plaque assay and by double staining immunocytochemistry. Apparently, mammosomatotropes, cells that release both GH and PRL, appear early in the neonatal development of both sexes and raise the possibility that PRL-secreting cells arise from GH-secreting cells.