Fetal development and regulation of pituitary cell types

Abstract

The ontogenesis of the pituitary gland is considered from anatomical and functional points of view. Embryogenesis of the hypothalamo-pituitary unit involving development of the hypothalamo-hypophyseal portal system is complete during early life as shown in several mammalian species. The ultrastructural characteristics of the different cell types during development are described according to observations made by using immunochemical techniques. The patterns of differentiation of the cell types are reviewed according to studies of pituitary glands from human anencephalic fetuses and encephalectomized rat fetuses as well as in vitro studies of cultured pituitary primordia in synthetic media. The maturation of the neuroendocrine mechanisms controlling the secretion of fetal hormones is also analyzed. During fetal life, the factors implicated in the regulation of pituitary hormone secretion are generally the same as in adults, but the intensity of the response of pituitary cells to their action is variable according to the species, thus reflecting an immaturity in the functioning of certain cell types.

Experimental Evidence for the Early Commitment of the Presumptive Adenohypophysis

“…However, the spe cific mechanisms and tissue interactions underlying this commitment are still unknown. An important role of the hypothalamus in the development of the adenohypophy sis has been suggested in several species such as chick [22][23][24][25][26], urodele [27], anuran [28], and rat [6,[29][30][31]. How ever, it should be noted that these studies have been per formed at later stages, when Rathke's pouch has already taken place.…”

“…in ref. 6], However, whether the pre sumptive territory of nasal cavity ectoderm and/or that of the hypothalamus participate in the commitment of the adenohypophysis remains to be elucidated.…”

“…Moreover, the fate of Rathke's pouch cells in vitro has been amply documented in various animal species. Several authors have suggested that the adenohypophysis could be already committed before its morphological appearance [6,[13][14][15][16], but they were unable to determine precisely the time when this commitment could occur. To our knowledge, no data are available to date about this phenomenon prior to neural tube closure.…”

Sexually dimorphic effects of maternal alcohol intake and adrenalectomy on left ventricular hypertrophy in rat offspring

“…However, decreased levels of maternal plasma corticosterone (Cort) by maternal adrenalectomy also result in reduced birth weight, and very low levels of Cort are sufficient to normalize birth weight and increase fetal Cort levels (51). As fetal adrenals start functioning during the last week of gestation (16), maternal glucocorticoid levels are the primary regulators of fetal development during the first 2 wk of gestation. Subsequently, maternal Cort might affect fetal development indirectly through regulating fetal adrenal function during the last week of gestation.…”

Pituitary Morphology in Anencephalic Human Fetuses

“…It has been suggested that they also are important for the differentiation of the various cell types of the adenohypophysis [4], Several transcription-activat ing proteins have been implicated in the definition of ade nohypophysial cell phenotypes: Pit-1, a homeobox pro tein, is selectively expressed in cells responsible for growth hormone (GH), prolactin (PRL) and (i-thyrotropin ((3-TSH) synthesis [5], is expressed during gestation immediately prior to the onset of these hormones [6][7][8] and is necessary for the differentiation of these cells [9], The nuclear receptor steroidogenic factor-1 (SF-1), a member of the steroid receptor superfamily [10,11], is necessary for the differentiation of pituitary gonadotropes [12]. Anencephalic fetuses provide an excellent model to study the development of adenohypophysial cells in the absence of hypothalamic regulation; despite the absence of a hypothalamus, anencephalic fetuses develop pituitaries composed of adenohypophysial cells [13,14].…”