Storage of Anterior Lobe Adrenocorticotropin in Corticotropes and a Subpopulation of Gonadotropes during the Stress-Nonresponsive Period in the Neonatal Male Rat*

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Abstract
The neonatal rat exhibits a stress-nonresponsive period during days 4–11 of postnatal development. The role and response of the anterior lobe corticotrope was studied with immunocytochemical stains for ACTH on fixed embedded pituitaries from male rats 2–21 days of age. Serially sectioned fields were stained for β-chains of LH or FSH to test for joint storage of ACTH and one or both gonadotropins. Cell counts on semithin sections were used to determine the frequency of stained corticotropes in the developing pituitary. The 2-day-old rats had twice as many corticotropes (17.6%) as the adults (8.1%). During the stress-nonresponsive period, the frequency of corticotropes declined to 6.4% of the population. This was followed by a recovery to 16.9% at 15 days of age. The serial fields showed that stellate cells containing only ACTH declined sharply (by 95%) to less than 1% of the pituitary cell population during the first week of postnatal life. Cells containing ACTH and both gonadotropins predominated in the corticotrope population and were 4–6% of the pituitary cell population during this time period. In the 15-day-old rats, the corticotropes included cells storing ACTH alone (5%) and cells storing ACTH and both gonadotropins (9%). Throughout development, cells storing ACTH alone were distinguished by their intense staining, stellate shape, and peripheral granules. Cells storing ACTH and gonadotropins were stellate or ovoid and often resembled maturing gonadotropes. We hypothesize that this second group of cells serves a function related to adrenal-gonadal maturation, or they may be stem cells, abundant during development and present in relatively low percentages (1–3%) in adult rats.