Immunohistochemical distribution of regulatory peptides in the human fetal adenohypophysis

We have studied here the cellular distribution of several regulatory peptides in hormone-producing cells of the human pituitary during the fetal period. Immunohistochemistry was used to show the expression of several regulatory peptides, namely Angiotensin-II, Neurotensin and Galanin, at successive gestational stages and their co-localization with hormones in the human fetal adenohypophysis. Somatotrophs, gonadotrophs and thyrotrophs were differentiated earliest. At gestational week 9, Angiotensin-II immunoreactivity was co-localized only with growth hormone immunoreactivity in somatotrophs, one of the first hormone-producing cells to differentiate. This co-localization remained until week 37. Neurotensin immunoreactivity was present in gonadotrophs and thyrotrophs in week 23, after FSH and TSH hormone differentiation. Galanin immunoreactivity was present in all hormone-producing cell types except corticotrophs. The different pro-opiomelanocortin-derived peptides were detected at different stages of gestation and adrenocorticotrophic hormone immunoreaction was the last to be detected. Our results show an interesting relationship between regulatory peptides and hormones during human fetal development, which could imply that these peptides play a regulatory role in the development of pituitary function.     

Ultrastructure of the fetal rat anterior pituitary gland as term and during prolonged gestation.

The ultrastructure of the fetal rat anterior pituitary gland is described at term (day 22) and during experimentally prolonged gestation (days 23, 24, 25). The latter was achieved by daily subcutaneous injections of five mg progesterone to gravid females from the twentieth through the twenty-fourth day. Using morphological criteria for classifying pituitary cells (Moriarty, '73), six different types were observed: thyrotrophs, gonadotrophs, corticotrophs, mammotrophs, somatotrophs and non-granulated cells. During prolonged gestation (days 24 and 25 only), cells designated as corticotrophs revealed changes suggesting increased activity, i.e.,proliferation and dilation of the endoplasmic reticulum, dilated Golgi membranes and a redction of secretory granules. In addition, meconium staining , which is indicative of fetal distress, was also observed on days 24 and 25. The ultrastructural changes noted suggest an increased in corticotroph activity due to fetal hypoglycemia and hypoxia which are known to occur as a result of placental insufficiency during prolonged gestation (Roux et al., '64; Vorherr, '75; Thliveris, '76.     

The pars tuberalis of the human pituitary

Summary Forty autopsy pituitaries were studied to elucidate the histology, immunocytology and ultrastructure of pars tuberalis in subjects with normal and abnormal endocrine homeostasis. Pars tuberalis consisted mainly of gonadotrophs interspersed with few corticotrophs and thyrotrophs, histologically resembling those of pars distalis. Somatotrophs and lactotrophs were not identified. There were no histologic differences attributable to age or sex. In cases of glucocorticoid excess, pars tuberalis corticotrophs showed Crooke's hyalinization. Following castration or hypophysectomy, pars tuberalis gonadotrophs exhibited more intense immunostaining for FSH¹ and LH than did normals. Ultrastructural analysis revealed gonadotrophs and corticotrophs showing no evidence of active secretion; immunoelectron microscopy demonstrated FSH, LH and ACTH in secretory granules. By light microscopy, squamous nests, often identified in pars tuberalis, were positive for immunoreactive keratin; cells at their periphery contained FSH, LH or ACTH, indicating derivation of nests by squamous metaplasia from gonadotrophs and corticotrophs. By electron microscopy, clusters of epithelial cells containing desmosomes and tonofilaments were surrounded by granulated gonadotrophs.Human pars tuberalis cells represent mainly a subpopulation of gonadotrophs possessing all organelles required for synthesis and storage of hormones but showing ultrastructural features of functional inactivity; the reasons for this inactivity and for the formation of squamous nests remain unexplained.     

An immunoperoxidase study of a human pituitary adenoma associated with Cushing's syndrome

A large endocrine active pituitary adenoma causing a Cushing's syndrome was investigated for the presence of subunits of the corticotropin-lipotropin precursor by immunohistology. A quantitative study revealed immunoreactivity (ir) for ACTH in 87.1% of the adenoma cells, beta-lipotropin-ir in 77.1%, beta-endorphin-ir in 75.3%, alpha-MSH-ir in 22.9% and methionine-enkephalin-ir in 7.8%. The adjacent distal pituitary gland showed a six-fold increase of prolactin-ir cells indicating the release of biologically active endogenous opiates. The strong alpha-MSH-ir within the adenoma cells in contrast to those of the distal pituitary may signify that alpha-MSH was being secreted by the adenoma. The proportions of other endocrine cell types within the anterior pituitary were normal (3.1% corticotrophs, 49.4% somatotrophs, 8.2% gonadotrophs and 7.1% thyrotrophs). 0.5% of the cells of the distal pituitary and 0.1% within the adenoma were VIP-ergic, which may be due to a vasoregularitory system and/or be involved in prolactin release.     

Direct actions of adrenergic agents on rat anterior pituitary cells

We studied the effects of adrenergic agents on the five main cell types of the rat anterior pituitary by monitoring the changes of the cytosolic free [Ca2+] ([Ca2+]i) in single cells that were identified by multiple sequential primary immunocytochemistry at the end of the Ca2+ measurements. Adrenaline (100 nM) increased [Ca2+]i in 30% of the cells. Responses were most prominent in somatotrophs and corticotrophs (40-65% of the cells responded) whereas the other three cell types, lactotrophs, thyrotrophs and gonadotrophs, gave poorer responses. Selective agonists and antagonists revealed the presence of both alpha1- and beta-adrenergic receptors. Alpha1-receptors dominated in corticotrophs, beta-receptors in somatotrophs. The alpha1-adrenergic responses increased with culture of the cells. The beta-adrenergic responses were mediated by cAMP and consisted of stimulation of Ca2+ entry through L-type voltage-gated channels. Stimulation of alpha1-receptors released Ca2+ from intracellular stores in corticotrophs and induced cAMP-independent Ca2+ entry in somatotrophs. The effects of alpha1-agonists were additive with those of the releasing factors growth hormone-releasing hormone (GHRH) and corticotropin releasing factor (CRF) whereas those of the beta-agonists were not. Our results suggest that direct effects of plasma catecholamines on AP cells may contribute to the hormonal response to stress.     

Coexpression of galanin and adrenocorticotropic hormone in human pituitary and pituitary adenomas.

Galanin is a neuropeptide that regulates the secretion of several pituitary hormones, including prolactin (PRL) and growth hormone (GH). Galaninlike immunoreactivity (Gal-IR) and galanin mRNA in the rat anterior pituitary is cell lineage specific, with predominant expression in lactotrophs and somatotrophs. The authors examined the cellular distribution of human Gal-IR in seven normal postmortem pituitaries and 62 pituitary tumors by immunoperoxidase staining. In contrast to the rat, Gal-IR in human anterior pituitaries was present in corticotrophs scattered throughout the gland, but not in lactotrophs, somatotrophs, thyrotrophs, or gonadotrophs. Distinct Gal-IR also was present in hyperplastic and neoplastic corticotrophs in 19 of 22 patients with Cushing's disease. In noncorticotroph cell tumors, unequivocal Gal-IR was present in 5 of 11 GH-secreting tumors associated with clinical acromegaly, 9 of 18 nonfunctioning pituitary adenomas, and 2 of 14 prolactinomas. Of these galanin-positive tumors, four of the five GH-secreting adenomas, six of the nine nonfunctioning adenomas, and both of the prolactinomas also contained adrenocorticotropic hormone immunoreactivity (ACTH-IR). Immunostaining and in situ hybridization on adjacent sections using an 35S-labeled probe complementary to human galanin mRNA demonstrated predominant galanin expression in normal corticotrophs. Immunoelectron microscopy confirmed the presence of Gal-IR in pituitary cells characteristic of corticotrophs in both normal and neoplastic pituitaries. Thus, as in the rat, galanin gene expression in the human pituitary is cell-type specific. Unlike the rat, however, human galanin gene expression is restricted to the corticotroph lineage. Studies of tumors confirmed the observed coexpression of galanin and adrenocorticotropic hormone. The divergent cell type specificity of galanin production in human and rat pituitaries reflects different patterns of gene activation in these two species. In addition, these results suggest that galanin in the human pituitary may participate locally in the regulation of the hypothalamic-pituitary-adrenal axis.     

Proliferation and differentiation of pituitary somatotrophs and mammotrophs during late fetal and postnatal periods

Proliferation of somatotrophs and mammotrophs in the rat pituitary during late fetal and postnatal periods up to 4 weeks after birth was quantitatively studied with the double immunostaining of bromodeoxyuridine and the hormones produced by them. Somatotrophs were first detected in 18.5-day fetuses and rapidly increased in number throughout the periods studied. The cells labeled with both anti-BrdU and anti-GH were few in number until shortly before birth and then increased conspicuously during the first 10 days after birth. Mammotrophs were detected at gestational day 19.5 but they were few until the second week after birth, when their number began to increase rapidly. The percentage of the number of the cells double-labeled with both anti-BrdU and anti-GH to all somatotrophs was 8.3% at the most. This was about the same as that of corticotrophs during the late fetal period and that of thyrotrophs in the early postnatal period. In contrast, the percentage of double-labeled cells to all mammotrophs was 3.8% as a maximum, which is lower than the values for somatotrophs, corticotrophs, or thyrotrophs, indicating a smaller contribution of mitosis to mammotroph proliferation. It is possible that this smaller contribution is compensated for by transdifferentiation of cells committed to become the somatotroph lineage. However, coexistence of GH and PRL was not observed in the present material.     

Characterization of pituitary cell populations in rats with induced polycystic ovaries

Several hypotheses have been proposed about the pathogenesis of polycystic ovarian syndrome (PCOS), however, the fundamental physiological interactions that initiate the development of follicular cysts have not yet been elucidated. Hence, in this study the proliferation, density and population of gonadotrophs, mammotrophs, somatotrophs and corticotrophs of the pituitary glands of rats with induced follicular cysts have been investigated by 2 experimental models (continuous light exposition and estradiol valerate-treated rats). Specific immunoreactivity associated with follicle-stimulating hormone, luteinizing hormone, prolactin, growth hormone and adrenocorticotropic hormone was evaluated by immunohistochemistry with specific hormone antibodies and proliferation of secretory cells by their colocalization (double-labeling) with proliferating cell nuclear antigen. The results indicate a reduction in the density and proliferation of gonadotrophs in both experimental groups. A reduction in the average density, proliferation and population of lactotrophs and corticotrophs was also observed in estradiol valerate-treated animals. However, no significant differences were found in somatotrophs. The present study contributes to the information about alterations of some cell populations that occur in the pituitary gland of rats with polycystic ovaries, and will enhance our understanding of the pathogenesis of this disease.     

Intracerebroventricular infusion of neuropeptide Y up-regulates synthesis and accumulation of luteinizing hormone but not follicle stimulating hormone in the pituitary cells of prepubertal female lambs

Neuropeptide Y (NPY) is a putative neuroregulator of the reproductive axis in the central nervous system. In this study we evaluated the effects of central infusion of exogenous NPY on the secretory activity of pituitary gonadotrophic cells in prepubertal lambs. Immature female Merino sheep (n=12) were infused of Ringer solution (control) or 50 microg of NPY to the third ventricle for 5 min and then slaughtered 3 h later. Immunoreactive luteinizing hormone (LH) and follicle stimulating hormone (FSH) cells were localised by immunohistochemistry using antibody raised against LHbeta and FSHbeta. Messenger RNA analyses were performed by in situ hybridisation using sense and antisense riboprobes produced from beta subunits of LH and FSH cDNA clones. The results were generated by computer image analysis to determine the area fraction occupied by immunoreactive and/or hybridising cells and optical density for immunostaining and hybridisation signal. LH in the blood plasma was determined by radioimmunoassay. It was found, that in the lambs infused with NPY the area fraction and optical density for immunoreactive LH cells and mRNA LHbeta-expressing cells increased significantly (P<0.001), compared to the vehicle-infused animals. The concentration of LH in the blood plasma did not differ between control and treated groups. The NPY infusions had no effect on the immunoreactivity of FSH cells or on expression of mRNA for FSHbeta. In conclusion we suggest that NPY may be an important component of mechanisms stimulating the synthesis and storage but not the release of LH in the pituitary gonadotrophs from prepubertal female sheep. In addition, this effect is specific for LH, no such effect was apparent on FSH.     

The pituitary in turner syndrome

Although Turner syndrome is not uncommon, studies of the pituitary in this condition are few. We undertook a histochemical and immunohistochemical study of four cases. As expected, “gonadal failure cells” were seen, but without recognizable gonadotroph hyperplasia. No gonadotroph adenomas were encountered. Instead, three silent corticotroph microadenomas were seen; their etiology remains unexplained. The question of whether the simultaneous occurrence of Turner syndrome and silent corticotroph adenoma is causal or incidental cannot be answered on the basis of the study of our material. Because these two diseases are rare, an etiologic association has to be considered. For example, it is possible that (a) protracted stimulation of gonadotrophs leads to transdifferentiation to corticotrophs, a hypothesis supported by the fact that normal and neoplastic gonadotrophs can contain ACTH and that some corticotroph adenomas produce LH and/or alpha subunit, (b) corticotrophs develop gonadotropin-releasing hormone (GnRH) receptors and undergo neoplastic transformation when exposed to continuous elevation of GnRH, FSH, and/or LH levels, and (c) the genetic defect in Turner syndrome promotes the formation of corticotroph adenomas.     

Exposure to dexamethasone reduces pituitary volume and gonadotropic cell number in rat fetuses

Overexposure to glucocorticoids during the fetal period induces changes in developmental processes in various fetal tissues. The aim of this study was to investigate the effects of the synthetic glucocorticoid, dexamethasone (Dx), on pituitary volume and gonadotropic cells during a critical period of pituitary development. The effects of Dx on stereological parameters of the pituitary gland and FSH and LH cells were investigated in 19 and 21-day old fetuses. On day 16 of pregnancy, the experimental dams received 1.0 mg Dx/kg b.w. subcutaneously, followed by 0.5 mg Dx/kg b.w./day on days 17 and 18 of gestation. The control gravid females received the same volume of saline. FSH and LH cells were stained immunohistochemically by the peroxidase–antiperoxidase method (PAP). In 19-day old fetuses, exposure to Dx caused a significant decrease of pituitary volume, estimated by Cavalieri's principle. Also, the total number of FSH and LH cells per pituitary, determined by physical fractionator counting technique, was significantly reduced. These changes persisted until fetal day 21. Volume densities and numerical densities of FSH and LH cells after exposure to Dx in 19 and 21-day old fetuses remained unaffected. Our results suggest that altered stereological parameters in pituitary gland after exposure to dexamethasone in fetal period could be long-lasting.     

Pituitary and extrapituitary actions of gonadotrophin-releasing hormone and its analogues

The hypothalamic decapeptide gonadotrophin-releasing hormone (GnRH) binds to high affinity receptors on pituitary gonadotrophs. These receptors mediate the effects of GnRH on secretion and synthesis of gonadotrophins. The GnRH receptor is coupled to Gq/G11, which activates phospholipase C. This enzyme leads to the generation of several second messenger molecules. Among these, diacylglycerol (DG) and inositol 1,4,5-tris-phosphate (IP3) are critically important. DG leads to activation of protein kinase C and IP3 releases Ca2+ from intracellular pools. Both events result in secretion and synthesis of luteinizing hormone (LH) and follicle stimulating hormone (FSH). In addition, other components of the GnRH signal transduction pathway are involved in cellular responses to GnRH. GnRH receptors and their functions are regulated by GnRH itself or other hormones such as ovarian steroids. The prolonged exposure of pituitary gonadotrophs to GnRH leads to desensitization and consequently to suppressed LH and FSH secretion. This mechanism is employed for the clinical use of GnRH agonists. GnRH antagonists act by competitive binding to the pituitary GnRH receptors. Apart from the well-established pituitary actions of GnRH, receptors for the decapeptide have been demonstrated in a variety of extrapituitary tissues. Here we report on the ovarian actions of GnRH which are predominantly inhibitory in the rat ovary. In the human ovary the existence of GnRH receptors is controversial. Recent reports have demonstrated the mRNA for the GnRH receptor in the human ovary. However, to date there is no consensus on the ovarian actions of GnRH or its analogues.     

Proliferation and differentiation of pituitary corticotrophs during the fetal and postnatal period: a quantitative immunocytochemical study

 To study the proliferation and differentiation of pituitary corticotrophs, we administered bromodeoxyuridine (BrdU) to pregnant rats at 15.5–21.5 days of gestation and to rat pups at 3, 7, and 28 days after birth. The pituitary sections of fetuses and pups were consecutively immunostained with anti-BrdU and anti-adrenocorticotropic hormone (ACTH) to detect proliferating cells and corticotrophs, respectively. The number of cells labeled with BrdU, ACTH, or both were counted. The diameters of their nuclei and the volume of the pituitary were measured. The BrdU-positive cells were around 76,000–96,000/mm³ during the period studied. The corticotrophs were first detected in the fetus at 15.5 days and they increased during the fetal and postnatal periods. The double-labeled cells were first detected in the 17.5-day fetus. They increased markedly at 19.5 days and comprised about one-quarter of the corticotrophs that increased in 24 h at this stage. These results indicate that: (1) at 15.5–18.5 days the corticotrophs were derived almost exclusively from undifferentiated cells; (2) during the later fetal and early postnatal periods, the proliferation of existing corticotrophs contributed, at least in part, to their increase; (3) About 1/20 of proliferating cells differentiated to corticotrophs when their increase was required. Accepted: 5 October 1999     

Quantitative morphological analysis of the pituitary gland in protein-calorie malnourished rats

A quantitative analysis of the pituitary gland was conducted to ascertain the effects of protein-calorie malnutrition on the morphology of the somatotrophs, gonadotrophs, thyrotrophs, and corticotrophs. Male rats were fed a low, 8% protein diet from 20 to 50 days of age, while their age-matched controls were given a diet containing 27% protein. The hypophyses were then processed for light microscopic immunocytochemical staining using antibodies to growth hormone, the β subunits of luteinizing hormone and thyroid stimulating hormone, and adrenal corticotrophic hormone. The number of each cell type along with an evaluation of the cell, cytoplasmic, and nuclear areas was conducted using a computerized image analyzer. All of these parameters were reduced significantly in the somatotrophs as a result of the low protein diet, while in the gonadotrophs, the cell, cytoplasmic, and nuclear areas were similarly affected. Smaller cell number, cell area, and nuclear area were noted in the corticotrophs of the malnourished animals, while in the thyrotrophs, only the cell and nuclear areas were reduced. The data demonstrate that each pituitary cell type responds in a unique manner to undernutrition. © 1993 Wiley-Liss, Inc.     

Retention of peptide hormones during partial secretion in pituitary somatotrophs and corticotrophs

The secretion of peptide hormones during exocytosis of an individual vesicle can result in either complete discharge of vesicle content or can occur in a partial manner in which some hormone is retained during transient fusion. In anterior pituitary lactotrophs, the retained hormone prolactin was internalized and recycled into a pool of vesicles that underwent preferential use during subsequent exocytic stimulations [Bauer et al., (2004) J Cell Sci. 117:2193–2202]. The aim of the present study was to determine whether retention and preferential recycling of retained hormones occurred in other anterior pituitary cells. Stimulation of somatotrophs with high K⁺ resulted in 50 discrete puncta per cell that were positive for growth hormone immunoreactivity. Identical stimulation of corticotrophs resulted in 150 puncta per cell that were anti-adrenocorticotrophic hormone (ACTH) positive. However, unlike what was observed for lactotrophs, the number of structures containing retained growth hormone and ACTH decreased to less than 10% of the initial value in 80 min in somatotrophs and in less than 10 min in corticotrophs. Our results indicate that functional recycling of retained hormones is not shared by all anterior pituitary cell types.     

Ontogeny of androgen receptor expression in the ovine fetal central nervous system and pituitary

In fetal sheep, circulating androgens influence fetal stress responsiveness and the timing of parturition. Nevertheless, little is known about the presence and development of androgen receptors (ARs) in the fetal brain. The present study was undertaken to test the hypothesis that expression of androgen receptor occurs in fetal brain and pituitary, and that the abundance of the AR is ontogenetically regulated. We isolated mRNA from pituitary, hypothalamus, hippocampus, and brainstem in fetal sheep that were 80, 100, 120, 130, and 145-day gestation, and 1 and 7 days postnatal (n=4-5 per group). Using real-time RT-PCR, we measured mRNA expression levels of the receptor in these brain regions and pituitary. In a separate study, we isolated protein from the same brain regions in fetal sheep that were 80 (n=3), 120 (n=4), and 145 (n=4) days. AR mRNA expression in hypothalamus increased in late gestation, starting at 145 days, and increasing progressively after birth. A trend of increasing AR protein in hypothalamus was not significant. AR mRNA expression in pituitary was elevated after 80 days gestation, but with no further increases or decreases in late gestation, while AR protein increased significantly at the end of gestation. In hippocampus and brainstem AR mRNA was constant throughout the latter half of gestation, and AR protein was below the sensitivity of our Western blot assay. We conclude that the fetal brain and pituitary are target sites for circulating androgens or androgen precursors in fetal plasma, and we speculate that the increase in hypothalamic action of androgens immediately prior to birth might be integral to the timing of parturition.     

Immunoelectron microscopic studies of gonadotrophs in the male and female rat anterior pituitaries, with special reference to their changes with aging.

Gonadotrophs immunocytochemically identified with an antibody to LH beta were studied in both male and female rats of various ages, viz., 2, 14 and 23 months after birth. The rat gonadotrophs are classified into two cell types, i.e., Type I containing large (300-700 nm) and small (150-200 nm) secretory granules, and Type II containing only small (100-200 nm) secretory granules. In normal young adults, the male rat pituitary contains a large number of Type I cells (more than 90% of all gonadotrophs), whereas the female pituitary contains much more Type II cells (more than 70%) than Type I cells. With age, Type I gonadotrophs in the male rat pituitary decrease remarkably to less than 25%, while Type II cells constitute more than 70% at the age of 23 months; the ratio of Type I to Type II comes to be the same as in the female. The main cause of this change is the decrease in the number of large secretory granules as the animal ages. FSH and LH levels in both the pituitary and serum were determined by radioimmunoassay. Pituitary FSH content decreased gradually in the male with age, while that in the female showed no change. LH content in the male decreased slowly, but LH in the female increased with aging. The large secretory granules indicating the presence of FSH markedly decreased with aging. The Type II cells contain only small secretory granules which probably contain LH. Thus the results of morphological counting of each type of gonadotroph and those of radioimmunoassay of gonadotropins during aging were essentially parallel. Some signs of degeneration were observed in the gonadotrophs in the aged rat pituitaries.     

Glycoconjugate localization with lectin and PA-TCH-SP cytochemistry in rat hypophysis

Glycoconjugates were localized by light microscopy with lectin-peroxidase conjugates and by electron microscopy with the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) sequence in immunocyto-chemically or morphologically identified cell types in rat pituitary. Lectin histochemistry demonstrated sialic acid and glycoconjugates with N-glycosidically linked oligosaccharides in gonadotrophs, thyrotrophs, and corticotrophs. Galactose penultimate to sialic acid was observed mostly in gonadotrophs. The terminal galactose-N-acetylgalactosamine disaccharide was detected in a few gonadotrophs and in a moderate number of mammotrophs. Fucose was localized in only corticotrophs with two fucose-binding lectins and in thyrotrophs with another. Several different monosaccharides were seen in glycoconjugates in melanotrophs and in Herring bodies. Melanotrophs displayed heterogeneous staining with fucose-binding lectins. A small number of nonsecretory cells were also visualized in the pars distalis by virtue of their glycogen content. PA-TCH-SP staining revealed complex carbohydrates in secretory granules and some Golgi cisternae in all types of hormone-producing cells in the pars distalis except for the somatotrophs. Melanotrophs of pars intermedia exhibited stained secretory granules and irregular dense bodies containing a stained meshwork. Corticotrophs of the pars distalis lacked the latter bodies, although they from the same glycoprotein precursor hormone as melanotrophs. Lectin conjugates and the PA-TCH-SP sequence stained some groups of secretion granules in Herring bodies, possibly representing vasopressin-containing granules as well as other cell types in the pars nervosa.     

Rathke's pouch grafts in adult brain sites

Donor tissue containing Rathke's pouch (RP) with its associated mesenchyme and neural lobe was isolated from 15-day fetal rats and stereotaxically transplanted either to hypothalamic hypophysiotropic sites or to cerebral cortex of adult females for 30 days. Hosts either were intact or had been hypophysectomized 2–4 weeks prior to transplantation of Rathke's pouch. Grafts in the hypothalamus of either intact or hypophysectomized hosts were pleomorphic and large, often as wide as 1–2 mm, and occasionally larger. Grafts in the cortex of either intact or hypophysectomized hosts were nodular and occasionally projected upward in association with the meninges (cortex/meninges grafts). Certain features were characteristic of the grafts in all experimental groups, i.e., development of histotypic pars distalis with cell cords and fenestrated capillaries. In all experimental groups gonadotrophs and somatotrophs, when present, were localized at the graft margin adjacent to the connective-tissue interface; mammotrophs, when present, were distributed throughout the graft. Features specific to each experimental group also were apparent. Grafts in the hypothalamus of both intact and hypophysectomized hosts typically were encapsulated by a labyrinthine meshwork of cell processes, whereas cortex/meninges grafts directly abutted dense connective tissue or neural tissue. In hypothalamic grafts in intact hosts, moderately differentiated mammotrophs, gonadotrophs, and somatotrophs could be identified by their cytological features and immunopositivity for prolactin, luteinizing hormone, and growth hormone, respectively. In hypothalamic grafts in hypophysectomized hosts, mammotrophs were absent, and gonadotrophs and somatotrophs were poorly granulated and not abundant. Grafts in the cortex of intact hosts contained numerous, well-differentiated mammotrophs, gonadotrophs, and somatotrophs. Many of the mammotrophs in these grafts were hypertrophied, and profiles of exocytosis were common. In grafts in the cortex of hypophysectomized hosts, mammotrophs were either absent or very few, whereas gonadotrophs and somatotrophs were numerous. Gonadotrophs in these grafts were dramatically hypertrophied, although exocytosis was rare. The results indicate that development of histotypic pars distalis may occur in hypophysiotropic and nonhypophysiotropic brain sites and that the hormonal state of the host as well as implantation site modulate cytodifferentiation of specific pars distalis cell types.