Development of the hypothalamohypophysial unit in the chick

The chicken embryo as a model for developmental endocrinology: development of the thyrotropic, corticotropic, and somatotropic axes

The ease of in vivo experimental manipulation is one of the main factors that have made the chicken embryo an important animal model in developmental research, including developmental endocrinology. This review focuses on the development of the thyrotropic, corticotropic and somatotropic axes in the chicken, emphasizing the central role of the pituitary gland in these endocrine systems. Functional maturation of the endocrine axes entails the cellular differentiation and acquisition of cell function and responsiveness of the different glands involved, as well as the establishment of top-down and bottom-up anatomical and functional communication between the control levels. Extensive cross-talk between the above-mentioned axes accounts for the marked endocrine changes observed during the last third of embryonic development. In a final paragraph we shortly discuss how genomic resources and new transgenesis techniques can increase the power of the chicken embryo model in developmental endocrinology research.

Administration of adrenocorticotropic hormone during chicken embryonic development prematurely induces pituitary growth hormone cells

Treatment of fetal rats and embryonic chickens with exogenous glucocorticoids induces premature GH cell differentiation. However, it is unknown whether the developing adrenal gland is capable of mounting this response autonomously. The present study determined whether stimulation of the adrenal gland in developing chicken embryos through administration of ACTH could induce a premature increase in GH cells. We found that plasma corticosterone and ACTH levels increased between embryonic day (e) 11 and e17, consistent with GH cell (somatotroph) ontogeny. Injection of ACTH into eggs on e9, e10, or e11 increased somatotrophs on e14. In contrast, thyroid-stimulating hormone, CRH, alpha-MSH, GHRH, and TRH were ineffective. Culture of e11 pituitary cells with ACTH failed to induce somatotrophs, suggesting an indirect action of ACTH on GH cells in vivo. Intravenous administration of ACTH dramatically increased plasma levels of corticosterone within 1 h and increased the percentage of pituitary somatotrophs within 24 h. Although ACTH administration increased the relative abundance of pituitary GH cells, there was no effect on plasma levels of GH, IGF-I, or IGF-II, or in hepatic expression of IGF-I or IGF-II mRNA. We conclude that ACTH administration can increase the population of GH cells in the embryonic pituitary. However, this treatment alone does not lead to downstream activation of hepatic IGF production. These findings indicate that the embryonic adrenal gland, and ultimately anterior pituitary corticotrophs, may function to regulate pituitary GH cell differentiation during embryonic development.

Ontogeny of the hypothalamo-pituitary-adrenocortical axis in the chicken embryo: a review

The embryo of the domestic fowl (Gallus domesticus) tenders one distinctive advantage over general mammalian models for investigating the development of the hypothalamo-pituitary-adrenocortical (HPA) axis. This is the relative simplicity with which the embryonic endocrine environment can be influenced without confounding maternal influences. The ease of direct manipulation of the embryonic endocrine system has facilitated analysis of the development and function of the HPA axis in the chick embryo. As the chick embryo develops, functional activation of the adrenal gland is regulated at three different levels: the adrenal gland itself, the anterior pituitary, and the hypothalamus. The adrenal gland appears capable of independent secretion of glucocorticoids from day 8 until shortly after day 14 of embryonic development, at which point the pituitary influences adrenocortical activity. Around the same age, the hypothalamic level of control also begins. The information covered in this review will describe the major steps in the development of the HPA axis in the chicken embryo and show that the chicken has an emblematic HPA neuroendocrine axis.

Regulation of chicken embryonic growth hormone secretion by corticosterone and triiodothyronine: evidence for a negative synergistic response

We reported that growth hormone (GH)-secreting cells differentiated by d 16 of chick embryonic development and that these somatotrophs were responsive to GH-releasing hormone and thyrotropin-releasing hormone. The present experiments evaluated effects of corticosterone and triiodothyronine (T3) on embryonic GH secretion. Anterior pituitary cells from embryonic day (e) 16, e18, and e20 were subjected to reverse hemolytic plaque assays (RHPAs) for GH in the absence or presence of corticosterone or T3. Corticosterone increased GH secretion from embryonic somatotrophs, an effect particularly evident on e16 and e18. T3 decreased GH secretion on e16, while no effect of T3 was significant on e18 or e20. Next, pituitary cells were subjected to RHPAs with T3 and corticosterone alone or in combination. Combined treatment with these hormones suppressed GH secretion from e16, e18, and e20 somatotrophs to levels below those found under basal conditions. We conclude that corticosterone can stimulate GH secretion in vitro at all embryonic ages tested. Furthermore, T3 can suppress basal GH secretion on e16, and the combination of T3 and corticosterone can suppress GH secretion at all ages. These findings indicate that GH secretion during the end of chicken embryonic development may be regulated by the interactions of endogenous glucocorticoids and thyroid hormones that increase prior to hatching.

Hypothalamic regulation of the adenohypophyseal-testicular axis in the male chick embryo

An antibody against luteinizing hormone-releasing hormone (LHRH) as well as naloxone, an opioid antagonist, were added to the chorioallantoic membrane (CAM) of 11.5- and 14.5-day-old male chick embryos and plasma testosterone (T) concentrations were determined. This protocol was designed to demonstrate: (1) Whether LHRH is essential in the regulation of the adenohypophyseal-testicular axis in the male embryo and (2) if LHRH is operative in this unit's function, are opiatergic pathways involved in the secretion of LHRH by the hypothalamus. Both anti-LHRH and naloxone lowered plasma T levels in 14.5-day-old embryos, but not 11.5-day-old embryos. This indicates that the hypothalamus, via LHRH, begins to regulate the pituitary-testicular unit at some time between Days 11.5 and 14.5, i.e., the hypothalamo-adenohypophyseal-testicular axis is established. The results also strongly suggest that the normal secretory pattern of LHRH is dependent upon opiatergic innervation of the hypothalamus at the same embryonic time.

Plasma LH and gonadal LH-binding cells in normal and surgically decapitated chick embryos

Plasma luteinizing hormone (LH) concentrations and the numerical density (Nv) of gonadal LH-binding (LH-positive) cells were determined in intact male and female chick embryos from Days 10.5 through 18.5 of incubation for plasma LH and from Days 6.5 through 19.5 for LH-binding cells, as well as in Day 15.5 decapitated ("hypophysectomized") embryos with pituitary transplants for both plasma LH and LH-binding cells. The data demonstrate that LH is already present in the plasma of male and female embryos as early as Day 10.5, the first day examined. Plasma LH levels in hypophysectomized embryos were statistically significantly lower than those of intact embryos, while pituitary transplants to the chorioallantoic membrane of hypophysectomized embryos elevated plasma LH concentrations to levels not statistically different from those of intacts. The "role(s)" of plasma LH levels and changes in form and numerical density (Nv) of gonadal LH-binding cells in the maturation of the pituitary-gonadal axes of the chick embryo are discussed and evaluated.

Hypothalamo-adenohypophyseal-thyroid interrelationships in the developing chick embryo. VII. Immunocytochemical demonstration of thyrotrophin-releasing hormone

Thyrotrophin-releasing hormone (TRH) was demonstrated immunocytochemically in the infundibulum of the chick embryo as early as Day 4.5 of incubation. From Days 4.5 through 19.5 of embryonic development there is a gradual increase within the developing hypothalamus in the number of TRH-positive perikarya as well as the amount of immunoreactive-TRH (IR-TRH) per cell. There are no abrupt changes in either parameter during the critical time period (Days 10.5-13.5 of incubation) in the maturation of the pituitary-thyroid axis. Thus, although TRH is probably not directly responsible for the dramatic increase in the number of thyrotrophin-producing cells which occurs in the pars distalis of 10.5- to 11.5-day-old embryos (R. C. Thommes, J. B. Martens, W. E. Hopkins, J. Caliendo, M. J. Sorrentino, and J. E. Woods (1983). Gen. Comp. Endocrinol. 51, 434-443) the marked change in the activity of the pituitary-thyroid unit at this time may well reflect the response of these newly differentiated thyrothrophs to low levels of plasma TRH. This hypothesis is supported by the observations that between Days 10.5 and 11.5 the hypothalamic-adenohypophyseal-thyroid (HAT) axis is first responsive to cold (R. C. Thommes, J. B. Martens, J. B. Hopkins, D. A. Griesbach, D. J. Williams, M. J. Sorrentino, P. Wernke, and J. E. Woods. In "Proceedings, Ninth International Symposium on Comparative Endocrinology Hong Kong, 7-11 December 1981" (B. Lofts, ed.). Hong Kong Univ. Press, Hong Kong, in press) and also that the pituitary-thyroid unit exhibits a marked increase in its sensitivity to exogenous TRH (R. C. Thommes, D. J. Williams, and J. E. Woods (1984). Gen. Comp. Endocrinol. 55, 275-279).

Hypothalamo-adenohypophyseal-thyroid interrelationships in the chick embryo. VI. Midgestational adenohypophyseal sensitivity to thyrotrophin-releasing hormone

The functional abilities of 9.5-, 10.5-, 11.5-, and 12.5-day-old chick embryos to respond to exogenous thyrotrophin-releasing hormone (TRH) were evaluated by means of changes in plasma total thyroxine (T4). T4 concentrations were determined 3.0, 6.0, 12.0, and 24.0 hr after TRH treatment. The data of the present investigation show that chick embryo adenohypophyseal sensitivity to exogenous TRH, as evidenced by changes in plasma T4 levels, increase during the 10.5- to 12.5-day incubation interval; however, the pattern and magnitude of plasma T4 response to administered TRH change markedly between Days 10.5 and 11.5 of development. This modification of the adenohypophyseal response pattern corresponds in embryonic time with the previously reported increases in immunocytochemically demonstrable TSH cells within the adenohypophysis(R. C. Thommes, J. B. Martens, W. E. Hopkins, D. A. Griesbach, D. J. Williams, M. J. Sorrentino, P. Wernke, and J. E. Woods, 1981, In 'Ninth International Symposium of Comparative Endocrinology"; R. C. Thommes, J. B. Martens, W. E. Hopkins, M. J. Sorrentino, J. C. Caliendo, and J. E. Woods, 1983, Gen. Comp. Endocrinol. 51, 434-443).

Hypothalamo-adenohypophyseal-thyroid and gonadal interrelationship in the chick embryo. I. Differential effect of ectopic pituitary grafts on plasma total T4 and testosterone levels

Plasma total thyroxine (T4) and testosterone concentrations of Day 15.5 intact, surgically partially decapitated (hypothalamo-hypophyseoprivic) and partially decapitated chick embryos with adenohypophyseal transplants were determined by radioimmunoassay of the same plasma sample. Decapitation in both male and female chick embryos at 33-38 h of incubation results in plasma thyroxine and testosterone levels which are statistically significantly lower than those of intact embryos. The addition of a single adenohypophyseal transplant to the chorioallantoic membrane (CAM) of male and female decapitated embryos only partially restores plasma thyroxine levels to intact levels. However, the same procedure results in plasma testosterone levels which are higher than those of both decapitated and intact individuals. The data suggest that in the developing chick embryo there are differences in dependence of the thyroid and the gonads upon a functional hypothalamo-adenohypophyseal unit. The possible differential regulatory effects of the hypothalamus on the adenohypophyseal-thyroid and adenohypophyseal-gonadal components of the hypothalamic-adenohypophyseal-thyroid and hypothalamic-adenohypophyseal-gonadal axes are discussed.

Hypothalamo-adenohypophyseal-thyroid interrelationships in the chick embryo. IV. Immunocytochemical demonstration of TSH in the hypophyseal pars distalis

The cells that synthesize thyroid-stimulating hormone (TSH) in the pars distalis of the chick embryo were identified immunocytochemically (immunoperoxidase and immunofluorescence) using anti-bovine TSH-beta and anti-human TSH-beta sera. TSH cells are first demonstrable on Day 6.5 of incubation. By Day 11.5, when the two lobes (rostral and caudal) of the pars distalis are easily recognized, TSH cells are confined exclusively to the rostral lobe. TSH cells identified by means of immunofluorescence were stained with the periodic acid-Schiff component of the performic acid-Alcian blue periodic acid-Schiff's Orange G stain. Immunoreactive TSH cells in the pares distales of Day 13.5 chick embryos, injected at 5.5 days of incubation with thiourea, were more intensively stained than their normal counterparts. The marked change in immunocytochemically demonstrable TSH on Day 11.5 corresponds with physiological and morphological events occurring within the hypothalamus, adenohypophysis, and the thyroid gland of the developing chick during this midincubational (midgestational) period. The data suggest that not only is hypophyseal TSH present in greater quantities after Day 10.5, but that adenohypophyseal synthesis and secretion of TSH may be stimulated by another factor (hypothalamic TRH) at this time, signaling functional maturation of the hypothalamo-adenohypophyseal-thyroid axis.

Functional development of the hypothalamic-adenohypophyseal-testicular (HAT) axis in the chick embryo

Plasma testosterone concentrations in intact and surgically partially decapitated (hypothalamo-hypophyseoprivic) chick embryos were determined by radioimmunoassay for the incubation interval, Days 7.5-17.5. "Hypophysectomy" of male chick embryos at 33-38 hr of incubation resulted in plasma testosterone levels which were significantly lower than those of intact embryos on Days 13.5, 15.5, and 17.5. The addition of luteinizing hormone (LH) to the chorioallantoic membrane (CAM) of decapitated embryos increased plasma testosterone concentrations to normal levels by Day 13.5, while LH and pituitary transplants to the CAM elevated plasma testosterone to above normal values by Day 15.5. These observations, together with other findings, are interpreted as demonstrating that in the chick embryo the adenohypophyseal-testicular axis is functional by Day 13.5 of incubation. There are also indications that the hypothalamic-adenohypophyseal-testicular complex is functional at this same embryonic time.

Appearance of LH-immunoreactive cells in the Rathke's pouch of the chicken embryo

The differentiation of the pituitary of the chicken embryo was studied by means of an immunohistochemical technique using antisera to turkey and chicken pituitary hormones. Immunoreactive LH-cells are detected in 4-day embryos (stage 23 of Hamburger and Hamilton) when the primordium of the anterior pituitary, the Rathke's pouch is only composed of a single-layer epithelium lined with an undifferentiated mesenchyme. A few immunoreactive cells are observed grouped on the posterior aspect of the pouch. As development proceeds, a strip of positive cells is detected encircling the Rathke's pouch. Prolactin-, growth hormone-, and ACTH-immunoreactive cells are detected in 6- and 7-day embryos, only after the pituitary has acquired its characteristic structure with cords in which different cell types become progressively recognizable. The early appearance of immunoreactive LH-cells following a precise distribution shows that secretory properties and differentiation capacities are acquired simultaneously in the epithelium of the Rathke's pouch and may be induced by the same stimulus.

Preferential accumulation of [3H] corticosterone in chick brain during embryonic development

In the present study, we examined the distribution of [3H]corticosterone ([3H]B) in chick embryonic brain during development using two different routes of administration: intracerebral and intraocular. After injection of 1 microCi into the brain of 8-day embryos, [3H]B was preferentially accumulated in the retinas, whereas regions such as cerebral hemispheres, optic tecta, and midbrain showed lower amounts of [3H]B. In 14-day embryos, a slightly higher amount of [3H]B was found in retinas and midbrain in comparison with other regions of the brain. After injection into the eye, [3H]B seemed to easily diffuse to brain regions and toe preferentially accumulate in the opposite eye and very slowly diffused to other brain areas. The accumulation of the hormone in the retina parallels the presence of hormone receptors reported by others. A correlation between the preferential accumulation of hormone and its action is proposed.

Ultrastructural study of the adenohypophysis of the male Chinese quail

The ultrastructural analysis of the adenohypophysis in the male Chinese quail reveals seven different types of granular cells, and agranular folliculo-stellate cells. The cell types are assumed to be endocrine cells and are classified as: Type I cells (presumptive LH-gonadotrophs), with dilated endoplasmic reticulum, perinuclear spaces, and granules of 150--260 nm; Type II cells (presumptive FSH-gonadotrophs), with regularly-shaped cytoplasmic cisterns and small granules (80--150 nm); Type III cells (presumptive thyrotrophs), very close in appearance to the type II cells of normal birds; Type IV cells (presumptive prolactin cells), with very large secretory granules (up to 400 nm), Type V cells (presumptive corticotrophs), with abundant and electron-dense granules (160--300 nm); Type VI cells, with irregularly-shaped granules; Type VII cells (presumptive somatotrophs), with abundant granules (130--220 nm) and less cytoplasmic structures. Cytological characteristics of the nucleus, and more particularly the presence of a Feulgen-postive nucleolus with a very particular ultrastructure are here reported. It is proposed that heterospecific associations of Chinese quail cells with chick cells can be used in embryological work for the study of cellular interactions.

Quail Rathke's pouch differentiation an electron microscopic study

Adenohypophyseal region of quail embryo has been examined by electron microscopy from stage 12 to stage 21 of Zacchei (1961). The Seessel's pouch develops prior to the early stages of adenohypophysis formation, then regresses while Rathke's pouch proliferates and differentiates. From Rathke's pouch formation by stage 12 (48 h of incubation) until appearance of the first secretory granules by stage 21 (6 days of incubation), there are no major ultrastructural modifications in adenohypophyseal cells. Mitochondria, Golgi vesicles, polysomic ribosomes, pinocytotic vesicles, and mitotic figures become more numerous while nucleocytoplasmic ratio and the number of ribosomes and lipid droplets decreases. The major change is the appearance of secretory granules by day 6 of incubation. This phenomenon occurs at the same time as in chick embryo, despite an incubation period shorter for quail than for chick. Mitotic figures are mainly distributed near the pouch lumen, while secretory granules are first located in the peripheral cells of the cephalic part of pars distalis primordium. The hypothetical role of mesenchyme and vascularization is discussed.