Immunocytological determination of gonadotropic and thyrotropic cells in fetal rat anterior pituitary during normal development and under experimental conditions

Intrinsic regulation of thyroid function by thyroglobulin

BACKGROUND

The established paradigm for thyroglobulin (Tg) function is that of a high molecular weight precursor of the much smaller thyroid hormones, triiodothyronine (T3) and thyroxine (T4). However, speculation regarding the cause of the functional and morphologic heterogeneity of the follicles that make up the thyroid gland has given rise to the proposition that Tg is not only a precursor of thyroid hormones, but that it also functions as an important signal molecule in regulating thyroid hormone biosynthesis.

SUMMARY

Evidence supporting this alternative paradigm of Tg function, including the up- or downregulation by colloidal Tg of the transcription of Tg, iodide transporters, and enzymes employed in Tg iodination, and also the effects of Tg on the proliferation of thyroid and nonthyroid cells, is examined in the present review. Also discussed in detail are potential mechanisms of Tg signaling in follicular cells.

CONCLUSIONS

Finally, we propose a mechanism, based on experimental observations of Tg effects on thyroid cell behavior, that could account for the phenomenon of follicular heterogeneity as a highly regulated cycle of increasing and decreasing colloidal Tg concentration that functions to optimize thyroid hormone production through the transcriptional activation or suppression of specific genes.

Prenatal expression of interleukin 1beta and interleukin 6 in the rat pituitary gland

It is known that interleukin 1beta (IL-1beta) and interleukin 6 (IL-6) are expressed post-natally in normal and tumoral cells in the anterior pituitary, and that they play a role in both the liberation of different hormones and in the growth, proliferation and tumor formation of the pituitary gland. However, their expression and role during embryonic and fetal development remain unknown. We have performed an immunocytochemistry study of prenatal expression and distribution of IL-1beta and IL-6 in isolated embryonic rat Rathke's pouch prior to birth, more specifically between 13.5 and 19.5 days p.c. Western-blot analysis carried out on 19.5-day p.c. embryos showed positive immunolabelling for IL-1beta and IL-6. These interleukins were initially expressed simultaneously in the rostral and ventral portions of Rathke's pouch in 15.5-day p.c. embryos, and this expression progressed caudodorsally in later developmental stages, extending to most of the hypophysis before birth. The number of cells expressing these interleukins increased throughout this period: 48.22% of anterior pituitary cells expressed IL-6 in 19.5-day embryos, whilst IL-1beta was positive in 39.8% of the cells. Moreover, we have demonstrated that some adenohypophyseal cells co-express both interleukins. Such findings represent the first step towards an understanding of the physiological role of these interleukins in anterior pituitary development.

Hypothalamic input is required for development of normal numbers of thyrotrophs and gonadotrophs, but not other anterior pituitary cells in late gestation sheep

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.

Neuropeptides in anterior pituitary development

Recent studies using biotechnological methods have achieved significant advances in our knowledge of molecular mechanisms underlying pituitary gland development and the differentiation of pituitary cytotypes. A large number of neuropeptides have been reported in the adult pituitary gland as well as in the central and peripheral nervous system. The early presence of neuropeptides during pituitary development is reviewed here. Neuromedin U (NmU), galanin and the polypeptide 7B2 have been localised to different endocrine cells of the gland. Their expression seems to be manifold even though it is temporally and spatially regulated. There is now firm immunocytochemical evidence that neuropeptides are present during morphogenesis of the pituitary and can be present simultaneously with all pituitary hormones.

Presence of gonadotropin-releasing hormone (GnRH) mRNA in Rathke's pouch and effect of the GnRH-antagonist ORG 30276 on lactotroph development in vitro

Reverse transcription-polymerase chain reaction (RT-PCR) with specific GnRH cDNA primers performed on RNA from Rathke's pouches removed from pregnant rats at day 12 of gestation (e12) generated an amplified DNA fragment of the expected length (357 bp). The fragment hybridized with a labeled GnRH cDNA probe in Southern blotting. DNA sequencing demonstrated identity with the known nucleotide sequence of the corresponding segment of rat GnRH cDNA. To determine whether GnRH mRNA was located in the Rathke's pouch cells or in remnants of surrounding tissue not completely removed during preparation, the pouches were treated with collagenase. Based on light and electron microscopic examination, this treatment disconnected virtually all contaminating tissue, allowing the 'pure' Rathke's pouches to be picked-up separately. Again, RT-PCR generated a DNA fragment of the expected length, the fragment hybridized with the GnRH cDNA probe and showed the nucleotide sequence of the corresponding region of rat GnRH cDNA. In Rathke's pouches established in explant culture on e12, lactotrophs were well developed when examined 9 days later by immunostaining of prolactin in paraffin-embedded sections of the tissue. Computerized image analysis showed prolactin immunoreactivity in 8.0+/-1.1% of the section area. Addition of the potent and long-acting GnRH antagonist ORG 30276 to the crude preparation of Rathke's pouches caused a significant decrease in the relative area staining for prolactin. The latter effect was abolished by concomitant addition of GnRH. In preparations of pure Rathke's pouches (collagenase-treated), ORG 30276 failed to affect the relative area of prolactin immunoreactivity. GnRH mRNA remained expressed in explants of both crude and pure Rathke's pouches until the end of the culture period. It is concluded that the GnRH gene is expressed in Rathke's pouch as early as e12 and that GnRH may be a physiological paracrine/autocrine peptide stimulating the development of lactotrophs. Mesenchymal and/or neural factors may be essential for the latter system to function.

Development and differentiation of pituitary cells

Classically, it was thought that the adenohypophyseal gland originated from the oral ectoderm. Its development has been the object of numerous studies over many years. However, several questions are still raised about its origin, differentiation, and commitment. The adenohypophyseal gland could originate from the anterior ridge of the neural plate. Glandular adenohypophyseal cells are committed very early in embryonic life. Interactions between adenohypophyseal presumptive territory and neighboring tissues can exist very soon, as early as at the open neural stage. The expression of a given phenotype by the committed cells seems to be controlled by a number of differentiation and/or transcription factors. In view of all these studies, performed with the use of different in vivo and in vitro models, classical concepts of the embryology of the adenohypophyseal gland need to be reevaluated. Indeed, many questions remain unanswered concerning the molecular mechanisms of known and unknown factors controlling development of the adenohypophyseal gland.

EGF release by rat gonadotroph cells: characteristics and effects of LHRH

The ability of anterior pituitary cells of immature female rats to secrete epidermal growth factor (EGF) was studied using the reverse hemolytic plaque assay. An average of 22% of cells (22.12 +/- 0.49%) spontaneously released low amount of EGF, as measured by the small area of plaques surrounding cells (1230 +/- 67 micron2). To determine the cellular origin of EGF from the rat anterior pituitary gland, a combination of immunocytochemistry and reverse hemolytic plaque assay was used. Most of the EGF-secreting cells were identified as luteinizing hormone (LH) containing cells (72.14 +/- 2.97%). Treatment with luteinizing hormone releasing hormone (LHRH; 10 nM) significantly increased the plaque area formed around EGF-secreting cells (2438 +/- 114 micron2) without altering the number of EGF-secreting cells. These results indicate that EGF is mostly secreted by gonadotrophs and that this EGF release is enhanced by LHRH.

Melatonin inhibits luteinizing hormone releasing hormone (LHRH) induction of LH release from fetal rat pituitary cells

The in vitro effect of melatonin on the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from fetal rat pituitary cells was investigated. A significant inhibition of LH release induced by 10(-9) M luteinizing hormone releasing hormone (LHRH) was seen when cells were incubated with 10(-9) M melatonin. FSH release was unaffected by either LHRH alone or LHRH in combination with melatonin. In addition, the significant inhibitory effect of melatonin was reduced by pretreatment of the pituitary cells with 10(-10) M melatonin. These findings indicate that melatonin can act directly on the fetal pituitary gland to suppress LHRH-induced release of LH perhaps by a mechanism which eventually involves down-regulation of the melatonin receptors.

Effects of gonadotropin-releasing hormone (GnRH) on the cytodifferentiation of gonadotropes in rat adenohypophysial primordia in organ culture

The effects of gonadotropin-releasing hormone (GnRH) on the development of gonadotropes were investigated by the use of organ culture and by means of immunocytochemistry and radioimmunoassay. Pituitary primordia from rat fetuses were cultured in a medium with or without 10(-9) M GnRH during the first 24 h of culture. The ratio of the number of immunoreactive LH cells to the total number of cells in the explants derived from 13.5-day fetuses was increased by the GnRH treatment after 6 or 8 days of culture, while the total number of cells was not altered. LH released into the medium and LH content of explants were not affected by the GnRH treatment. Subsequent treatment with 10(-9) M GnRH for 4 h after 7 days of culture resulted in a marked release of LH, accompanying a significant decline in LH content, in both explants exposed or unexposed to the first GnRH treatment. However, the former explants contained a lower amount of LH than the latter explants. The present results indicate that pituitary primordia at 13.5 days of gestation are capable to respond to GnRH, and that GnRH is effective in stimulating the responsiveness of gonadotropes to GnRH during early pituitary cytodifferentiation.

Re-evaluation of gonadotropin-releasing hormone (GnRH) action on pituitary cell differentiation with special regard to its effect on LH and TSH cell types

We have previously demonstrated that TRH induces several hypophyseal cell type differentiation. In the present study, we re-evaluated the effect of GnRH on gonadotroph differentiation, and we determined whether gonadotropin is implicated in the appearance of other pituitary cell types. These effects were compared to the influence of TRH on LH and TSH cell differentiation. Pituitary primordia of 11 day-old rat embryos were cultured in a synthetic medium until the equivalent of term, and the LH and TSH cell differentiation was visualized using an immunohistochemical technique. The effect of GnRH on pituitary hormone cells was evaluated by measuring the percentage of immunoreactive area per section. GnRH, when added on the first day of culture, induced LH and TSH cell differentiation in a dose-dependent manner. GnRH was more effective than TRH in inducing gonadotropic phenotype expression. This suggests that GnRH, which evokes LH release, is the most effective factor inducing gonadotroph differentiation in early fetal life. However, GnRH was as effective as TRH in thyrotroph differentiation. Lactotrophs were detected in primordia cultured with GnRH. As lactotroph phenotype differentiation is controlled by the alpha-subunit of glycoproteic hormone, further studies are required to determine whether or not GnRH effect on lactotroph is direct. GnRH did not affect the corticotroph and somatotroph differentiation. These results confirm that hypothalamic factors such as TRH or GnRH are capable of inducing more than one pituitary cell phenotypes.

Immunohistological analysis in the distribution of cells in the fetal porcine adenohypophysis

Adenohypophyses of porcine fetuses from 25 to 110 days of gestation were studied by immunohistochemical staining to ascertain the ontogeny of specific cell types and their spatial distribution in the pars distalis. No hormone-containing cells were found before 30 days of gestation. ACTH cells were observed first at 40 days, while GH and LH cells appeared first at 60 days. PRL cells were initially detected at 105 days. ACTH immunoreactive cells were also observed in the pars intermedia at 40 days. Blood capillaries were interposed between cell cords of the pars distalis after 40 days of gestation. ACTH cells were evenly distribution in all areas of the pars distalis except the rostral area (sex zone). GH cells were densely distributed in lateral wings of the pars distalis and immediately anterior to Rathke's lumen. PRL cells resembled GH cells in their distribution pattern, but PRL cells were fewer in number. LH cells were scattered in the sex zone of the pars distalis from 60 to 80 days of gestation. After 90 days, they became scattered throughout the pars distalis but were more numerous in the sex zone than in other areas. The inductive elements of adenohypophysial cells from Rathke's pouch epithelia are discussed. We hypothesize that cell cords of specific areas facing Rathke's lumen may differentiate into specific cell types of the pars distalis during fetal life.

Differentiation of the melanotrophic cells of rat pituitary primordium in organotypic culture in defined medium

Organotypic cultures, in defined medium, of pituitary primordia obtained from 15-day-old rat fetuses were performed in order to study the in vitro differentiation of melanotrophic cells. The morphological and ultrastructural features of the transplants resembled those of the gland developing in vivo. In situ hybridization on semi-thin sections, using a 35S-labelled oligonucleotide probe, revealed pro-opiomelanocortin-mRNA-containing cells on the first day of culture in the anterior lobe and after 2-3 days in the intermediate lobe. Immunoperoxidase labelling of adjacent sections showed that the same cells reacted with antibodies against alpha-melanocyte-stimulating hormone (alpha MSH), gamma 3MSH and adrenocorticotropic hormone in both lobes. The pro-opiomelanocortin-mRNA-containing cells formed progressively conspicuous areas in the intermediate lobe, which was almost uniformly labelled after 6 days. In the anterior lobe, these cells remained scattered in small cell groups, and colloidal gold immunolabelling showed the progressive disappearance of alpha MSH labelling from the secretory vesicles in cells exhibiting morphological features of adult corticotrophic cells. Both the alpha MSH content of the explants and alpha MSH release into the culture medium increased with time. Treatment with the dopamine agonist bromocriptine induced a strong dose-dependent decrease in alpha MSH secretion, which was significant after 3 days in culture, indicating that dopamine D2 receptors are able to regulate hormonal release of melanotrophic cells at early stages. This system constitutes a suitable model for further studies of factors controlling cell differentiation and cellular interactions involved in histogenesis.

Fetal development and regulation of pituitary cell types

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.

Morphological, developmental and immunohistochemical observations on the opossum pituitary with emphasis on the pars intermedia

Development of the pituitary in Didelphis follows the general pattern of that described for most mammalian species. The dorsal region of a multichambered pituitary vesicle, which forms from Rathke's pouch, comes to lie adjacent to the presumptive infundibulum by the 10 1/2 d of gestation. The epithelial wall of this vesicle consists of spindle-shaped cells. The dorsal wall of the upper chamber of the pituitary vesicle forms the pars intermedia; the ventral wall of this chamber gives rise to cells of the pars distalis. Corticotropes, somatotropes, and lactotropes are seen in the presumptive adenohypophysis of the 11 1/2 d embryo. Gonadotropes and thyrotropes appear about 1 d later, shortly after birth. By the 2 postnatal week, the adult distribution of all 5 cell types within the pars distalis appears to have been established. The wall bounding the pituitary cleft in the adult represents an epithelial continuum limited by a basal lamina and corresponds to the upper chamber of the original pituitary vesicle. Ultrastructurally, the limiting walls of the pituitary cleft consists of stellate (marginal) cells, large, bulbous cells, and granulated cells. The latter correspond to the various endocrine cell types normally associated with the pars distalis. Non-granular folliculo-stellate cells also are observed within the epithelial cords of the pars distalis.

Behavioral changes and structural defects in rats irradiated in utero

Pregnant rats were irradiated with whole-body doses (0.25-1.25 Gy) of Cs-137 gamma-rays on gestational day 15, or with 1.0 Gy on gestational days 11, 13, 15, or 17. Postnatal growth (body weight) and several preweaning behaviors (righting reflex, negative geotaxis, reflex suspension, modified open field activity, spatial maze exploration, continuous corridor activity, and gait) of the offspring were monitored prior to sacrifice on post-parturition day 28. Brain (sensorimotor cortex) and pituitary tissues were processed for histological evaluation and morphometric analysis. For most behavioral endpoints, there were dose-dependent changes produced by irradiation on gestational day 15, with one endpoint (continuous corridor activity) demonstrating changes after 0.25 Gy that were significantly different from control values. The data indicate that the most sensitive organ showing radiation-induced alterations changes from the pituitary at gestational day 11 to the primitive cortex of the brain at days 13 to 17 with a peak of sensitivity at day 15. These results demonstrate that a spectrum of related functional and morphological deficits can be produced by even low-dose in utero irradiation, with the specific endpoint showing the greatest change being determined by the specific day of gestation on which irradiation occurs. Extrapolating from these experimental data with rats to the human situation, it is recommended that care be taken, when possible, to avoid exposure of the fetus, even after the early stages of organogenesis.

Ontogeny of gonadotropic and thyrotropic cells in fetal mouse anterior pituitary. Comparison between two species C57 BL6 and Balb/C

The fetal period constitutes a determinant stage in the ontogenesis of the hypothalamo-hypophysial-gonadal system. This work mainly concerns gonadotropic and thyrotropic functions and compares their different aspects in two strains of fetal mice. Balb/c and C57 BL6 fetal mice were studied at 16, 17, 18 and 19 days of gestation. The appearance and distribution of immunoreactive gonadotropic and thyrotropic cells of the anterior pituitary were observed by immunocytology using an indirect method with anti-porcine luteinizing hormone beta serum, anti-bovine thyrotrope hormone serum after saturation with bovine luteinizing hormone and anti-rat luteinizing hormone serum. In the two strains and in both sexes, LH gonadotropes appeared at 17 days of gestation and preferentially localized in the ventral part of the anterior lobe; a similar distribution was noted at 18 days and there was an increase in the number and staining intensity of labeled cells. By 19 days of gestation the gonadotrophs seemed more numerous, more generally distributed throughout the gland and often abutted to sinusoidal capillaries. An account of immunoreactive cells with anti-porcine luteinizing hormone serum and statistical evaluation of the results performed by variance analysis showed significant differences between the two strains. LH gonadotropic cells were always more numerous at each day of gestation in Balb/c fetuses especially in female fetuses. The possibility of a different evolution and/or differentiation for this cell population is discussed. Comparison of gonadotropic function between rat fetuses, mouse fetuses and human fetuses lead us to conclude that mouse fetuses appeared as an experimental model more closely related to human fetuses.

Correlation of behavior with brain damage after in utero exposure to toxic agents

Early postnatal behaviors involving sensorimotor integration were measured along with thickness of the sensorimotor cortex in rats irradiated with 1.0 Gy on gestational day 11 or 17. Body weight and morphology of anterior pituitary cells were recorded. Irradiation on day 17 was more effective in reducing cortical thickness and body weight and performance on behavioral tests and less effective in altering pituitary cells than irradiation on day 11. Prediction of behavioral effects, using cortical layers, body weight and pituitary morphology as predictors in stepwise multiple regression, was measured in both irradiated and control rats. Cortical Layer V more than I more than IV and VI as significant predictors of behavior. The best predictions accounted for about half of the variance in the data. When behavioral data were used to predict brain damage, the best predictor was negative geotaxis. Significant association of behavior with Layers V and VI was found. These experiments show the difficulties in correlating complex behaviors with specific brain areas and, at the same time, implicate especially Layer V of the sensorimotor cortex in these behaviors.

Differentiation of embryonic mouse GH cells and ACTH cells in vitro

The ability of cells that produce growth hormone (GH) and cells that produce adrenocorticotropic hormone (ACTH) to differentiate in various culture media was analyzed by means of ultrastructural immunocytochemistry on 13-day embryonic mouse pituitaries that were maintained in organ culture for 3-11 days. At the time of culture, relatively undifferentiated nongranulated or poorly granulated cells that were unreactive with anti-growth-hormone serum (anti-GH) and anti-adrenocorticotropic-hormone serum (anti-ACTH) were present in the pituitary. After 10-11 days in culture, immunoreactive GH cells were obtained only in media that were supplemented with cortisol, whereas ACTH cells were obtained in all media tested, including Medium 199 alone. In cortisol-supplemented media, the GH cells showed ultrastructural features typical of those that occur in vivo, and anti-GH immunoreactivity was obtained after as little as 3 days in culture, i.e., at a stage comparable to that which occurs in vivo. The results indicate that mouse GH cells are capable of differentiating in Medium 199 supplemented only with cortisol, without the addition of fetal calf serum or insulin; cortisol therefore appears to be an essential component of the embryonic milieu for the production of GH-secretory granules.

Induction of pituitary lactotrope differentiation by luteinizing hormone alpha subunit

Addition of gonadotropin releasing hormone to cultures of fetal rat pituitary induced differentiation of lactotropes as revealed by immunocytochemistry. Antiserum to luteinizing hormone (LH) (recognizing native LH), but not antiserum to LH-beta (recognizing both native LH and its beta subunit), inhibited this induction. Further addition of highly purified LH-alpha subunit in culture medium also induced lactotrope differentiation. Thus, the alpha subunit may have a specific biological activity of its own with probable practical use in clinical investigations.

Gonadotropin receptor complexes and free receptors in porcine Leydig cell cultures during recovery from hCG stimulation

Free and occupied gonadotropin receptors were studied in vitro in porcine Leydig cells culture maintained in chemically defined medium. Free receptors were evaluated by the binding capacity for 125I-hCG. hCG bound molecules (or hCG receptor complexes) were evaluated using immunocytochemical visualization on fixed cells. Exposure to hCG for 16 hours (.5 to 50 ng/ml) induced the disappearance of free receptors. After removal of the hormone, the return to control levels was observed at 48 and 72 hours. Visualization of hCG bound at the cell surface indicates that, following continuous exposure to gonadotropins for 48 hours, hCG molecules are still present on the cell. Following short-time exposure (1 h) to hCG and 48 hrs washing the number of stained cells is very close to the initial value suggesting that the occupied sites (at 48 hours) represent the initial hormone receptor complexes. These results indicate that, during prolonged incubation, hCG binding is not reversible, that the half-life of some of the complexes at the cell surface is very long and that the receptors recovery is slow and is probably the result of a de novo synthesis.

Ontogenetic development of TRH-like immunoreactive nerve terminals in the median eminence of the rat

Ontogenetic development of TRH-like immunoreactive nerve terminals in the median eminence of the rat was studied immunocytochemically. By light microscopy, TRH-like immunoreactivities were first detected on the 1st day after birth in the external layer of the median eminence. By electron microscopy, TRH-like immunoreactive nerve fibers and terminals were visible on the 0.5th day after birth. The nerve terminals were first found in direct contact with the perivascular basal lamina of the portal vessel on the 2nd day. TRH-like immunoreactivities were only localized on dense granular vesicles about 105 nm in diameter in the axoplasm throughout the developmental stages. The immunoreactive nerve fibers with TRH-like immunoreactive granular vesicles gradually increased in number with development. The physiological significance of TRH as a hormone is discussed in relation to the presence of TRH-like immunoreactive nerve terminals in the median eminence of the developing rat.

Structure of the pars distalis in pouch-young tammar wallabies (Macropus eugenii)

The pars distalis of pouch-young wallabies (Macropus eugenii) aged 1 to 50 days was studied by means of light-microscopic immunohistochemistry and electron microscopy. In the pars distalis of these pouch-young presumptive somatotrops, which constituted up to 70% of the gland, and nongranulated cells were the most numerous cell types. Small numbers (together representing less than 30% of the pars distalis) of immunoreactive mammotrops, thyrotrops, gonadotrops and corticotrops were also found. The presumptive mammotrops, gonadotrops and thyrotrops increased in number and apparent activity between 1 and 50 days postpartum. Presumptive corticotrop cells in 25 to 30 day-old animals were relatively most numerous, and apparently more active than at any other stage of pouch life; these cells decreased in apparent activity and relative number in older animals. The changes in number and activity of cell types in the pars distalis correlated well with major developmental events such as the onset of adrenal activity, the rapid growth phase in the first 100 days postpartum, and the generally low thyroid activity in pouch-young of less than 50 days of age.

Proliferation of Sertoli cells in fetal and postnatal rats: a quantitative autoradiographic study

Proliferation of Sertoli cells during fetal and postnatal development of the rat was examined and quantified with light microscope autoradiography. Fetuses in utero were injected subcutaneously with 3H-thymidine. The percentages of Sertoli nuclei that had incorporated label were determined in autoradiographs from fetuses aged 16 through 21 days of gestation. To compare the degree of Sertoli cell proliferation during fetal development with that occurring after birth, pups were also studied at intervals between the day of birth and 3 weeks of age. For each fetus or pup, at least 500 Sertoli cell nuclei in each of three sections were scored as labeled or unlabeled. These data were subjected to analysis of variance and the Newman-Keuls test. The percentage of Sertoli cells incorporating 3H-thymidine increased progressively from day 16 of gestation onward, to a maximum of 26.8% on day 20, two days before birth. Thereafter, this percentage dropped steadily until, in pups 21 days after birth, no labeled Sertoli cells were detected. These findings highlight the fetal period as the time of greatest expansion of the Sertoli cell population and indicate that, at birth, proliferation of these cells is already on the decline.