Age-related changes of the somatotrophs of the domestic fowl Gallus gallus

Avian somatotrophs: differentiation, morphology, distribution, and regulation

Growth hormone (GH) is primarily synthesized, stored, and released by pituitary somatotrophs. These cells comprise a highly labile population that continuously undergoes proliferation, differentiation, and morphogenesis in response to changing physiological stimuli. They are also functionally and morphologically heterogeneous with distinct spatial and temporal distribution within the pituitary gland. The characteristics of these cells are discussed in this brief review.Key words: somatotroph, aves, growth hormone, adenohypophysis.

GH secretion in TRH-refractory and TRH-responsive chickens is independent of somatotroph abundance and morphometry

Chicken pituitary glands chronically exposed (for 2-4 h) to growth hormone (GH) secretagogues in vitro have increased GH secretion and increased numbers of GH-secreting cells. In contrast, thyrotropin-releasing hormone (TRH)-induced GH release in chickens in vivo is only transitory and cannot be maintained by constant infusion or repeated serial iv administration. The possibility that this reflects changes in somatotroph abundance, morphology, and GH content was therefore examined in chickens responsive or refractory to TRH in vivo. TRH-induced GH release was immediately (within 10-30 min) followed by a reduction in the size and number of immunoreactive pituitary somatotrophs and in the size of somatotroph clusters, resulting in a reduction in somatotroph area. The number and area of the immunoreactive GH-secreting cells was further reduced 60 min after the bolus administration of TRH, although control values were restored after 120 min. The decline in immunoreactive somatotroph number and size was attenuated by serial TRH injections, but this did not restore plasma GH responsiveness in TRH-refractory birds. These results demonstrate that somatotroph responses to GH secretagogues in vivo differ from those in vitro.

Growth hormone: a paracrine growth factor in embryonic development?

Although pituitary growth hormone is obligatory for normal postnatal growth and development, early embryonic and fetal growth is generally considered to be independent of pituitary GH. Indeed, in chickens, somatotrophs and serum GH are not detectable until late in embryogenesis, and neither partial decapitation nor pre-hatch GH administration greatly affects embryonic growth. However, since it is now known that GH can be produced and act in many extra-pituitary tissues, early embryonic growth may be independent of pituitary GH but dependent upon the paracrine actions of extra-pituitary GH. The possibility that growth hormone may be a paracrine growth factor during early development will therefore be considered in this brief review, which is based on the embryogenesis of the domestic fowl.

Quantitative studies of chicken somatotrophs during growth and development by morphometry, immunocytochemistry, and flow cytometry

Changes in the male chicken somatotroph during growth and maturation have been examined by morphometric and immunocytochemical (ICC) analysis of serial sections of the anterior pituitary gland and by flow cytometry of dispersed anterior pituitary cells. ICC showed that somatotrophs are confined to the middle and caudal thirds of the anterior pituitary gland at all ages from 5 to 26 weeks. At a given age somatotrophs are of equal size at all positions along the cephalocaudal axis of the anterior pituitary gland. However, there are age-related changes: from 5 to 11 weeks rises occur in both the mean total somatotroph volume per gland (64%) and the mean number of somatotrophs (78%), while the mean volume of the single somatotroph is unchanged. From 11 to 18 weeks the mean volume of the single somatotroph decreases 41%. From 18 to 26 weeks the mean volume of the somatotroph, the mean total somatotroph volume, and the mean number per gland do not change. Flow cytometry studies suggested that somatotrophs from adults have less growth hormone (GH) than somatotrophs from young birds. The increases in total somatotroph volume and number from 5 to 11 weeks are consistent with the rise in anterior pituitary GH reported previously. Basic quantitative morphological information about age-related changes in somatotrophs is reported here. When combined with additional facts from future work, they may explain the well-documented sharp decline in circulating GH from 5 to 11 weeks.

Immunocytochemical studies of chicken somatotrophs and somatotroph granules before and after hatching

Immunocytochemical methods were used to gain information about the embryonic development of chicken somatotrophs before and after hatching. To localize growth hormone, anterior pituitary sections were incubated with growth-hormone antibody, and then an indirect peroxidase method was used for light microscopy and an immunogold method for electron microscopy. The earliest evidence of embryonic somatotrophs was seen at 12 days. At this stage somatotrophs were sparse (0.2% of parenchymal cells) and their granules were pleomorphic with elongated ovoid and lozenge shapes predominating. Few of the immunogold-labeled somatotroph granules of the embryo were spherical until 15 days after fertilization. At 18 days, most of the granules were spherical (their shape in the adult chicken). During the six days between the 15-day-old embryo and the 1-day-old chick, the number of gold particles per granule section approximately doubled suggesting an increase in growth hormone content of the granules. This rise was the result of increases in the size of the granule sections and in the concentration of gold particles in the sections. During the embryonic period of 12-20 days, somatotrophs were not more than 3.6% of the anterior pituitary cell population. During the following two days, between the 20-day-old embryo and the 1-day-old chick, the percentage of somatotrophs in the pituitary parenchymal cell population rose rapidly from 3.6% to 20.7% and then increased slowly to 24.6% during the period of 1-5 days after hatching.(ABSTRACT TRUNCATED AT 250 WORDS)

"Paradoxical" growth hormone secretion in acromegaly: an avian model?

Acromegaly is a pathological human condition resulting from an excess of growth hormone (GH) secretion in adults. The regulation of GH secretion in acromegalics is characterised by "paradoxical" GH responses to dynamic tests of pituitary GH function. Many of these "paradoxical" responses appear to be normal, physiological GH responses in aves. Comparative studies on the control of GH secretion in immature birds may therefore provide an experimental model for testing the effects of therapeutic agents of GH secretion.

Thyrotrophin-releasing hormone-induced growth hormone secretion in ducks: independence of peripheral plasma somatostatin, insulin, and glucagon

In young, but not old, ducks the iv infusion of thyrotrophin-releasing hormone (TRH) markedly increased peripheral plasma growth hormone (GH) concentrations, which remained elevated throughout the 30-min period of infusion. This GH response to TRH was suppressed by the simultaneous infusion of somatostatin, which increased the level of circulating somatostatin-like immunoreactivity (SLI) to supraphysiological levels. Basal concentrations of plasma SLI in both young and old birds were suppressed by TRH infusion. Concentrations of glucagon-like immunoreactivity (GLI) were increased by the infusion of TRH in young birds but not in adults, whereas plasma immunoreactive insulin (IRI) was decreased in young birds and increased in adults following TRH infusion. These results indicate that TRH-induced GH secretion in ducks is unrelated to changes in peripheral plasma SLI, GLI, or IRI induced by TRH infusion.

Posthatch ontogeny and sex-related differences in somatotroph numbers in the chicken adenohypophysis

Adenohypophyseal total cell and somatotroph numbers were determined in male and female chicks over the early (12 wk) growth period. In a first experiment, adenohypophyses from male and female broiler-strain (Hubbard x Hubbard) chicks were harvested at 2 wk intervals from 2 to 12 wk of age. Glands were enzymatically dispersed to yield uniform cell suspensions that were fixed and stained (Herlant's Tetrachrome system) to distinguish somatotrophs. Percentages of somatotrophs in the total adenohypophyseal cell population were similar for both sexes (P greater than .05) and declined with age (P less than .01) from a high of 16.8 +/- .79% (least squares mean +/- SEM) at 2 wk to 6.3 +/- .88% at 12 wk. From 2 to 6 wk of age, total adenohypophyseal cell numbers increased over two-fold (P less than .01) but sexes did not differ significantly. From 8 to 12 wk of age, however, total cell numbers reached a plateau and were higher in males than females (P less than .01). Similarly, the absolute numbers of somatotrophs per adenohypophysis increased from 2 to 6 wk of age, where it reached a plateau. Somatotroph numbers in males were twice those in females over the 8 to 12-wk growth period. In a second experiment, the percentages of somatotrophs in the total adenohypophyseal cell population from males of a slower growing, layer-type breed (White Leghorn) were compared to those of Hubbard males at 4, 8, and 12 wk of age. Leghorn males were higher than Hubbard males over all ages in percentages of somatotrophs (14.9 +/- .87% vs. 10.8 +/- .71%; P less than .01), however, the absolute numbers of somatotrophs per pituitary of the two breeds did not differ.

Immunogold identification of the somatotrophs of domestic fowl of different ages

The somatotrophs of the pituitary gland of the male domestic fowl were identified by means of an immunoelectron-microscopic method based on gold as the electron-opaque label and an antibody to growth hormone. Gold particles indicating sites of growth hormone were restricted to cells in which virtually all of the granules were labelled. Little, if any, gold label was found outside the granules in these cells designated as somatotrophs, or at sites outside these cells. The size of these gold-labelled secretory granules presumed to contain growth hormone decreased with age, from a mean sectional diameter of 256 +/- 6.2 nm (SEM) at 4-6 weeks to 221 +/- 5.7 nm at 11-18 weeks and 205 +/- 8.6 nm at 24-30 weeks of age. On the basis of these values for mean sectional diameters the change between the first two periods represents a decrease in granule volume of about 36%. However, during the same period the growth hormone concentration of the granules increased. Accordingly, growth hormone content per granule changed little if at all. In contrast, from 11-18 weeks to 24-30 weeks of age there was a decrease of 31% in growth hormone content per granule. These data indicate that growth hormone packaging in the chicken somatotroph changes with age. The first change results in the production of smaller granules of higher growth hormone concentration. During this period growth hormone content per granule remains relatively constant. The later change results in the production of granules of lower growth hormone content than that of younger animals.

Thyrotrophin-releasing hormone induces growth hormone secretion in adult hypothyroid fowl

While thyrotrophin-releasing hormone (TRH) stimulated growth hormone (GH) secretion in adult anesthetized cockerels, the GH response was blocked in anesthetized birds pretreated with thyroxine (T4) or triiodothyronine (T3). Moreover, whereas GH secretion in conscious adult birds was poorly responsive to TRH stimulation, conscious birds made hypothyroid by goitrogen pretreatment (with propylthiouracil, methimazole, or thiourea) were responsive to TRH challenge. Basal circulating GH concentrations in the goitrogen-pretreated birds were also higher than in the vehicle-injected controls. Surgical thyroidectomy similarly increased the basal GH concentration in adult birds and promoted TRH-induced GH secretion. These results demonstrate inhibitory effects of the thyroid hormones on basal and stimulated GH secretion in adult domestic fowl and suggest that GH release in adults is partly under tonic thyroidal inhibition.

Ontogenesis of immunocytochemically demonstrable somatotrophs in the adenohypophyseal pars distalis of the developing chick embryo

Growth hormone (GH)-containing cells in the hypophyseal pars distalis of the chick embryo were identified immunohistochemically using anti-chicken GH sera. GH cells are first demonstrable in Rathke's Pouch as early as Day 4.5 of incubation. By Day 9.5, when the two lobes (rostral and caudal) of the pars distalis are easily recognized, GH cells are confined exclusively to the caudal lobe. Although the number of GH cells increases gradually during embryonic development, there is a statistically significant difference between Days 10.5 and 12.5 in both the cellular density and the percentage of somatotrophs. GH cells, which contain coarse granules evenly distributed throughout the cytoplasm, varied in diameter from 9.3 micron on Day 4.5 to 11.8 micron on Day 20.5, while the nuclear diameter of these cells increased from 2.8 micron on Day 4.5 to 4.9 micron on Day 20.5. There was a statistically significant difference in the nuclear/cytoplasmic ratio between Days 4.5 and 13.5 and between Days 13.5 and 20.5 of incubation. Aspects of the regulation of growth hormone synthesis and secretion in the chick embryo are discussed.

Effects of neonatal gonadectomy on the differentiation of prolactin and growth hormone cells in the chicken adenohypophysis: a quantitative electron microscopic study

The ultrastructural stereological analysis of prolactin, growth hormone, and gonadotroph cells in the hypophysis of neonatally gonadectomized, 3-month-old male and female chickens was performed. The observed ultrastructural changes (especially of granular endoplasmic reticulum and content of secretory granules) demonstrated reduced biosynthetic activity of prolactin cells in neonatally ovariectomized females and increased activity of prolactin cells in neonatally orchidectomized male chickens.

Growth hormone secretion from chicken adenohypophyseal cells in primary culture: effects of human pancreatic growth hormone-releasing factor, thyrotropin-releasing hormone, and somatostatin on growth hormone release

A primary culture of chicken adenohypophyseal cells has been developed to study the regulation of growth hormone (GH) secretion. Following collagenase dispersion, cells were exposed for 2 hr to vehicle (control) or test agents. Human pancreatic (tumor) growth hormone-releasing factor (hpGRF) and rat hypothalamic growth hormone-releasing factor stimulated GH release to similar levels. GH release was increased by the presence of dibutyryl cyclic AMP. Thyrotropin-releasing hormone (TRH) alone did not influence GH release; however, TRH plus hpGRF together exerted a synergistic (greater than additive) effect, increasing GH release by 100 to 300% over the sum of the values for each secretagogue acting alone. These relationships between TRH and hpGRF were further examined in cultured cells exposed to secretagogues for two consecutive 2-hr incubations. TRH pretreatment enhanced subsequent hpGRF-stimulated GH release by about 80% over that obtained if no secretagogue was present during the first incubation. In other experiments, somatostatin (SRIF) alone did not alter GH secretion. However, SRIF reduced hpGRF-stimulated GH release to levels found in controls. Furthermore, GH release stimulated by the presence of both TRH and hpGRF was lowered to control values by SRIF. The results of these studies demonstrate that a primary culture of chicken adenohypophyseal cells is a useful model for the study of GH secretion. Indeed, these results suggest that TRH and hpGRF regulate GH secretion by mechanisms which are not identical.