The prepartum surge in fetal plasma cortisol is essential for the normal timing of parturition in sheep and may result from an increase in the ratio of ACTH to proopiomelanocortin (POMC) in the fetal circulation. In fetuses subjected to experimental induction of placental restriction, the prepartum surge in fetal cortisol is exaggerated, whereas pituitary POMC mRNA levels are decreased, and in vitro, unstimulated ACTH secretion is elevated in corticotrophs nonresponsive to CRH. We therefore investigated the changes in the relative proportions of cells expressing POMC, ACTH, and the CRH type 1 receptor (CRHR(1)) shortly before birth and during chronic placental insufficiency. Placental restriction (PR) was induced by removal of the majority of placental attachment sites in five ewes before mating. Pituitaries were collected from control and PR fetal sheep at 140 d (control, n = 4; PR, n = 4) and 144 d (control, n = 6; PR, n = 4). Pituitary sections were labeled with specific antisera raised against POMC, ACTH, and CRHR(1). Three major subpopulations of corticotrophs were identified that expressed POMC + ACTH + CRHR(1), ACTH + CRHR(1), or POMC only. The proportion of pituitary corticotrophs expressing POMC + ACTH + CRHR(1) decreased (P < 0.05) between 140 (control, 60 +/- 1%; PR, 66 +/- 4%) and 144 (control, 45 +/- 2%; PR, 56 +/- 6%) d. A significantly higher (P < 0.05) proportion of corticotrophs expressed POMC + ACTH + CRHR(1) in the pituitary of the PR group compared with controls. This study is the first to demonstrate subpopulations of corticotrophs in the fetal sheep pituitary that differentially express POMC, ACTH, and CRHR(1) and the separate effects of gestational age and placental restriction on these subpopulations of corticotrophs.

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To evaluate the hypothalamic contribution to the development of anterior pituitary (AP) cells we surgically disconnected the hypothalamus from the pituitary (hypothalamo-pituitary disconnection, HPD) in fetal sheep and collected pituitaries 31 days later. Pituitaries (n = 6 per group) were obtained from fetal sheep (term = 147 ± 3 days) at 110 days (unoperated group) of gestation and at 141 days from animals that had undergone HPD or sham surgery at 110 days. Cells were identified by labelling pituitary sections with antisera against the six AP hormones. Additionally, we investigated the colocalization of glycoprotein hormones. The proportions of somatotrophs and corticotrophs were unchanged by age or HPD. Lactotrophs increased 80% over time, but the proportion was unaffected by HPD. Thyrotrophs, which were unaffected by age, increased 70% following HPD. Gonadotrophs increased with gestational age (LH+ cells 55%; FSH+ cells 19-fold), but this was severely attenuated by HPD. We investigated the possible existence of a reciprocal effect of HPD on multipotential glycoprotein-expressing cells. Co-expression of LH and TSH was extremely rare (< 1%) and unchanged over the last month of gestation or HPD. The increase of gonadotrophs expressing FSH only or LH and FSH was attenuated by HPD. Therefore, the proportions of somatotrophs, lactotrophs and corticotrophs are regulated independently of hypothalamic input in the late gestation fetal pituitary. In marked contrast, the determination of the thyrotroph and gonadotroph lineages over the same time period is subject to complex mechanisms involving hypothalamic factors, which inhibit differentiation and/or proliferation of thyrotrophs, but stimulate gonadotrophs down the FSH lineage. Development of a distinct population of gonadotrophs, expressing only LH, appears to be subject to alternative mechanisms. The anterior pituitary is a highly heterogeneous organ composed of a variety of cells located throughout the gland. The process by which the individual secretory cells differentiate and proliferate during development is largely unknown. Considerable progress has been made in understanding the role played by transcription factors, but we still know relatively little about the input of the fetal hypothalamus in pituitary cell development. After birth, hypothalamic factors are critical in regulating anterior pituitary (AP) function, but the role played before birth remains largely unknown.Recent studies suggest that the timely expression of various transcription factors is wholly responsible for the composition of the AP, independent of the hypothalamus (Dasen & Rosenfeld, 1999Melmed, 2003). A number of experiments in genetically engineered mice are consistent with this view. For example, corticotrophin-releasing hormone (CRH) receptor knockout mice (Timpl et al. 1998) and thyrotrophin-releasing hormone (TRH) knockout mice have a normal complement of all pituitary cells (Shibusawa et al. 2000), clearly suggesting that these two hypothalamic factors are not required for normal...

During development, corticotrophs (ACTH‐secreting cells) undergo changes that alter the secretory products they release. In vivo, the ratio of plasma concentrations of ACTH to its biosynthetic precursors (which are ACTH antagonists at adrenals) increases over gestation, and the change is prevented by surgical hypothalamo‐pituitary disconnection (HPD). The present study aimed to determine whether the gestational changes observed in intact and HPD sheep fetuses are mirrored in changes in individual corticotrophs. Pituitaries were harvested from (1) adult sheep and fetuses at 110d gestation (2, term ~ 147d) or at 140d following sham (3) or HPD (4) surgery at 110d. Pituitary sections were simultaneously immunostained for full length POMC, the parent molecule of ACTH, and ACTH(1‐39). The fraction of pituitary cells containing only POMC decreased from 110d (8.8±1.45%) to adulthood (2.8±0.7%), with 140d sham pituitaries indistinguishable from adult, and 140d HPD indistinguishable from 110d. The fractions of cells containing only ACTH(1‐39) or ACTH+POMC were similar in all intact groups, but significantly higher in HPD pituitaries. These results suggest a linear progression over the final third of gestation into adulthood whereby more corticotrophs acquire the capacity to process POMC to ACTH(1‐39). This process is influenced in late gestation by signals from the hypothalamus.