We recently reported that fetal sheep corticotrophs (ACTH-producing cells) at 108 ± 5 d (days) of gestation are relatively more responsive to CRH than to AVP, whereas those at 139 ± 0 d (term = 145 d) and in the adult are more responsive to AVP. To further characterize these developmental changes, we used immunocytochemical, RIA, and cell immunoblotting techniques to examine populations of corticotrophs and individual cells. Immunocytochemical studies revealed that corticotroph frequency decreased from 22 ± 1% of all pituitary cells at 100 d of gestation to 14 ± 1% at 135 d and 9 ± 0% in the adult. RIA measurements of ACTH secretion by cell populations showed that the response of corticotrophs to CRH diminished, whereas that to AVP increased during gestation and into adulthood.

Cell blot analysis of individual corticotrophs identified two types of secretory responses (increases in the number of secreting cells and average amount of ACTH released per cell) to CRH or AVP that changed during fetal development. At 100 d of gestation, CRH increased the proportion of secreting cells from 65 ± 3% (no test agent) to approximately 90%; AVP exerted a negligible effect on the relative abundance of secreting cells. At 120 d of gestation, both secretagogues, alone or in combination, increased the proportion of secreting corticotrophs from 49 ± 6% to about 85%. At 135 d of gestation and in the adult, AVP, alone or in combination with CRH, increased secreting corticotrophs from about 53 ± 6% to about 80%. CRH alone exerted a nominal effect on the proportion of secreting cells. Additional analyses showed that, at 100 or 120 d of gestation, the average amount of ACTH secreted by individual corticotrophs did not change in response to CRH or AVP. However, near term and into adulthood, the average quantity of ACTH released from individual cells increased in response to these agents.

Our findings suggest that maturational changes in fetal corticotrophs dictate whether their secretory response to CRH or AVP results from an increase in the proportion of cells secreting ACTH and (or) an increase in the average amount of hormone secreted by individual cells.