Octreotide Effect on Growth Hormone and Somatostatin Subtype 2 Receptor mRNAs of the Human Pituitary Somatotroph Adenomas

Octreotide, a somatostatin analog used to treat acromegalic patients harboring a pituitary tumor, acts via somatostatin subtype 2 receptor (SSTR2) and causes significant decrease of circulating GH levels and sometimes mild to moderate tumor shrinkage. To further elucidate the mechanism of octreotide action, we studied GH and SSTR2 mRNAs by in situ hybridization in densely and sparsely granulated somatotroph adenomas removed by surgery from 14 treated and 14 untreated patients. Only in densely granulated adenomas were the GH and SSTR2 mRNA signals mildly decreased relative to untreated matched adenomas. The decrease of GH mRNA in densely granulated somatotroph adenomas suggests that they may have a more favorable response to octreotide therapy than sparsely granulated tumors.     

Histopathological analyses of silent pituitary somatotroph adenomas using immunohistochemistry, in situ hybridization and confocal laser scanning microscopic observation

To characterize the morphological and functional aspects of silent somatotroph adenomas with paradoxical responses of GH in TRH or GnRH provocation tests, which are considered to be a useful strategy for endocrinological identification of silent somatotroph adenomas, we examined three silent somatotroph adenomas histopathologically. The adenomas were investigated by immunohistochemistry, including the highly sensitive catalyzed signal amplification system, the non-radioisotopic in situ hybridization method, and confocal laser scanning microscopy. GH production and GH-immunopositive secretory granules in the adenoma cells were demonstrated histopathologically, and the adenomas were interpreted as being densely granulated somatotroph adenomas. Endocrinological identification of silent somatotroph adenomas in combination with paradoxical responses of GH in TRH or GnRH provocation tests may elucidate the increasing number of silent somatotroph adenomas that have been regarded as mammotroph or clinically inactive adenomas. One should be aware of the differences between the previously reported silent somatotroph adenomas, most of which are sparsely granulated somatotroph adenomas, a somatotroph adenomas with paradoxical and the silent somatotroph adenomas, most of which are sparsely granulated somatotroph adenomas, and the silent somatotroph adenomas with paradoxical responses of GH in TRH or GnRH provocation tests, which are densely granulated somatotroph adenomas.     

In vitro studies of human pituitary adenomas

The application of morphologic and tissue culture techniques to the study of human pituitary adenomas allows further investigation of structure-function correlations. Using these methods, we have documented morphometric differences between densely and sparsely granulated somatotroph adenomas but the release of growth hormone in vitro and responses to adenohypophysial hormones/drugs do not correlate with tumor type. The morphologic and functional alterations in somatotroph adenomas exposed to SMS 201-995 appear to be reversible in vitro. Incubation of lactotroph adenomas with bromocriptine for 3 days directly reduces tumor cell size, cytoplasmic volume and cytoplasmic volume densities of endoplasmic reticulum and Golgi regions; these changes are similar to the effects of longterm bromocriptine therapy in vivo. Tissue culture studies of gonadotroph adenomas of men confirm that gonadotropin-releasing hormone (GnRH) stimulates gonadotropin release by tumor cells and yields morphologic evidence of increased hormone synthesis whereas these tumors have variable sensitivity to gonadal steroids; structural changes in tumor cells correlate with hormone release after stimulation, suggesting that morphologic parameters may reflect the hormonal milieu of these adenomas. Null cell adenomas and oncocytomas release small quantities of glycoprotein hormones, predominantly gonadotropins in vitro and there are no functional differences between these 2 tumor types; gonadotropin release responds to GnRH stimulation and, paradoxically, to other adenohypophysiotropic hormones, but such stimulation does not result in secretion of other adenohypophysial hormones by these tumors.     

Growth hormone (GH) and prolactin (PRL) gene expression and immunoreactivity in GH- and PRL-producing human pituitary adenomas

Summary Growth hormone(GH)-producing pituitary adenomas are morphologically heterogeneous and frequently contain not only GH immunoreactivity but also variable numbers of prolactin (PRL) immunopositive cells. Paraffin sections of 59 surgically removed GH- and/or PRL-producing adenomas classified by histology, immunocytochemistry (ICC) and electron microscopy were studied using in situ hybridization (ISH) for GH and PRL mRNA and combined with ICC for the coded hormones. Somatotroph adenomas (10 densely and 10 sparsely granulated tumours) and mammosomatotroph adenomas (10 cases) contained both GH mRNA and GH immunoreactivity. In 4 densely and 4 sparsely granulated somatotroph adenomas and 4 mammosomatotroph adenomas, only GH mRNA and its product were found. In 28 cases (6 densely and 6 sparsely granulated somatotroph adenomas, 10 mixed somatotrophlactotroph adenomas and 6 mammosomatotroph adenomas) both GH and PRL mRNA were present, although no PRL immunoreactivity was not in 2 densely granulated somatotroph adenomas. In these cases, ISH for PRL mRNA combined with GH immunostaining revealed the presence of variable numbers of mammosomatotrophs. In 9 acidophil stem cell adenomas only PRL mRNA and its product were found; one tumour expressed both GH and PRL mRNA and their products. Nine lactotroph adenomas contained only PRL mRNA and PRL immunoreactivity. The results show that GH and/or PRL mRNA content could not be correlated with ICC for coded proteins and ultrastructural features. The mammosomatotrophs were more numerous using ISH when compared with ICC. Somatotroph, mammosomatotroph and mixed adenomas are closely related and they can be considered to represent one basic tumour type originating in a cell committed to GH production. This may undergo clonal differentiation towards a mammosomatotroph and further to the lactotroph line. The results also indicate that lactotroph adenomas arise in a cell committed to PRL production. Acidophil stem cell adenomas seem to be more closely related to lactotroph cells than somatotroph.     

Interphase Analysis of Chromosome 11 in Human Pituitary Somatotroph Adenomas by Direct Fluorescence In Situ Hybridization

Chromosome 11 numerical abnormalities were detected by fluorescencein situ hybridization (FISH) technique in four surgically removed pituitary somatotroph adenomas from patients clinically associated with acromegaly. The tumors were diagnosed by histology, immunocytochemistry, and electron microscopy, and included two densely granulated somatotroph (DG-SM) and two sparsely granulated somatotroph (SG-SM) adenomas. For demonstration of chromosome 11, the direct FISH technique was applied on imprints from fresh adenoma tissue fixed in acetone using an α-satellite specific centromeric probe. The slides were studied with a fluorescence microscope and the percentages of positive nuclei with aberrant fluorescent signals were counted. All tumors exhibited numerical chromosomal abnormalities in 8–23% of their cell population and included nuclei containing 1–3 extra chromosome 11 copies. The SG-SM adenomas exhibited more prominent abnormalities compared with the DG-SM adenomas. We conclude that numerical chromosome 11 abnormalities represent a frequent event among somatotroph adenomas with a tendency for higher frequency in SG-SM adenomas.     

Pituitary adenomas in acromegaly: Comparison of different adenoma types with clinical data

Adenoma tissues from 309 patients with active acromegaly was examined by routine light microscopy and immunohistochemistry, and selectively by electron microscopy. All adenomas were immunoreactive for growth hormone. Eighty-seven adenomas were monohormonal (28%), 58 were bihormonal (immunoreactive for growth hormone and prolactin) (19%), and 157 adenomas were plurihormonal (51%), with positivity for glyco-proteins and/or their α-subunit as well. The mean tumor size was significantly greater in monohormonal adenomas than in other adenoma types. There was no difference in invasiveness among the various adenoma types. Younger patients showed invasive tumor growth more often. Patients with densely granulated GH cell adenomas had a significantly longer duration of symptoms compared to patients with other adenoma types. More than half of the patients with sparsely granulated GH cell adenomas had a duration of less than 5 yr. There was no correlation between duration of symptoms and tumor size. The preoperative mean GH level was significantly higher in patients with sparsely granulated GH cell adenomas than in those with mixed GH/PRL cell adenomas. The preoperative mean PRL level was significantly higher in patients with bihormonal adenomas than in those with plurihormonal adenomas. There was an inverse correlation between age and preoperative GH and PRL levels. No linear correlation was found between preoperative basal GH and PRL levels. Monohormonal adenomas presented more often with suprasellar and/or parasellar extension than other adenoma types. Our data suggest a positive correlation between tumor extension and preoperative GH and PRL levels. Patients with plurihormonal adenomas were significantly older than patients with sparsely granulated GH cell adenomas and mixed GH/PRL cell adenomas. No significant difference was found between the various adenoma types and the extent of surgical removal, which depends on the degree of invasiveness, tumor size, and extrasellar tumor extension.     

垂体生长激素细胞腺瘤的电镜及免疫电镜观察

24 cases of growth hormone(GH)-producing pituitary adenomas were studied with electron microscopy and immunoelectron microscopy by protein A-gold complex, 6 cases were identified as densely granulated GH adenoma and 15 cases as sparsely granulated GH adenoma, among which 4 cases were proved by immunoelectron microscopy to be containing granules with prolactin(PRL) activity simultaneously. Intracytoplasmic fibrous bodies were often seen in the sparsely granulated cells anyhow, not all those cells with fibrous bodies possess the secretory granules with GH activity, and fibrous bodies were also detected in some PRL cells of certain mixed type adenoma. This suggests that fibrous bodies might not be the specific morphological feature of pituitary growth hormone cell adenomas.     

Pituitary Adenomas that Show a Faint GH-Immunoreactivity but Lack Fibrous Body: Pit-1 Adenoma with Endocrinologically Low Activity

Growth hormone (GH)-producing pituitary adenomas have been classified into densely and sparsely granulated adenomas. The latter are chromophobic with weak GH-positivity and characteristically possess fibrous body (FB), aggregation of cytokeratin filaments. We report eight cases of unusual chromophobic adenomas. GH-immunoreactivity was detected in most adenoma cells in five cases and scattered in three cases. However, it appeared much weaker than that seen in ordinary GH-producing adenomas because of spotty immunoreactivity. Although intracytoplasmic organelles were well-developed, secretory granules were small and sparse. FB was not identified in any cases. Thyroid-stimulating hormone was positive in four cases. Pit-1 protein was positive in all eight cases. A weak labeling with GH probe was detected in two of two cases examined by in situ hybridization. Acromegalic features were evident in four cases, while mild or absent in four cases. GH levels were below 5 μg/l in four cases and 5–10 μg/l in the remaining cases. Macroadenomas and invasive adenomas were seen in seven and six cases, respectively.     

Pituitary adenomas producing growth hormone in acromegalic patients.

Growth hormone was shown in histological sections of 25 pituitary adenomas from acromegalic patients by means of the unlabelled peroxidase-antiperoxidase (PAP) technique. On the basis of the numbers of cytoplasmic granules, the cells of the adenomas were of two types: densely granulated and sparsely granulated. The densely granulated cells had abundant cytoplasm containing numerous granules, whereas the sparsely granulated cells had little cytoplasm with scanty granules. Depending on the predominant cell type the adenomas were also classified as densely granulated or sparsely granulated: 21 of the 25 adenomas (84%) were densely granulated and four (16%) sparsely granulated. There was some variation, however, in the relative numbers of the two types of cell from one part of an adenoma to another, a feature consistent with one type of cell in different phases of activity. There was no significant difference in mean serum growth hormone concentrations between the two groups, and granularity of the adenomas in histological sections did not therefore correlate with secretory activity. Nine adenomas showed extrasellar extension. The mean serum growth hormone concentration in these cases was lower than the mean of the adenomas confined to the sella turcica. Thus the size of the tumour did not correlate with the serum growth hormone concentration. Three of the four adenomas in the sparsely granulated group showed extrasellar extension, compared with 6 of 21 classified as densely granulated. This suggests that sparsely granulated adenomas have a more aggressive pattern of behaviour, but histological evidence for this was lacking.     

Effect of a long-acting somatostatin analogue (SMS 201-995) on a growth hormone and thyroid stimulating hormone-producing pituitary tumor

A 46-year-old woman with acromegaly and hyperthyroidism due to a pituitary adenoma. She had high serum thyroid-stimulating hormone (TSH) levels and very high serum growth hormone (GH) levels. Transsphenoidal removal of the tumor, post-operative irradiation, frontal craniotomy for removal of residual tumor and large-dose bromocriptine therapy were carried out consecutively. After therapy, serum GH levels gradually decreased, but not to the normal range, and serum TSH levels remained at inappropriately normal levels. Using immunoperoxidase techniques, GH-, TSH- and follicle-stimulating hormone (FSH)-containing cells were demonstrated in the adenoma. A long-acting somatostatin analogue (SMS 201-995, 600 micrograms/day) suppressed the serum GH level to the normal range with a concomitant suppression of TSH. Furthermore, the paradoxical serum GH responses to TRH and LH-RH were slightly improved. No important subjective side-effects were noted. Therefore, SMS 201-995 appeared to be a very effective drug in this patient with a GH- and TSH-producing pituitary tumor.     

Utility of Pit-1 Immunostaining in Distinguishing Pituitary Adenomas of Primitive Differentiation from Null Cell Adenomas

Pit-1 immunostaining is not routinely used in the characterization of pituitary adenomas, and its utility in distinguishing adenomas dedicated towards the lactotroph, somatotroph, and thyrotroph lineage from null cell adenomas warrants further evaluation. Pituitary adenomas that were negative for expression of a basic panel of hormonal markers (ACTH, prolactin, and growth hormone) were further evaluated for TSH, SF-1, and Pit-1 expression using a tissue microarray. Among the 147 identified pituitary adenomas that were negative for ACTH, prolactin, growth hormone, and TSH, expression of SF-1 was present in 68 cases (46%). Of the remaining 72 cases with sufficient tissue for further analysis, four were Pit-1 positive (6% of the adenomas negative for ACTH, prolactin, growth hormone, TSH, and SF-1); the remaining 68 were potentially null cell adenomas. Two of the Pit-1-positive adenomas displayed a paranuclear CAM 5.2 staining pattern suggestive of a sparsely granulated somatotroph adenoma; however, only one case contained fibrous bodies within a majority of the adenoma cells. Our data suggests that Pit-1 can be utilized as a second tier immunostain in cases of clinically non-functioning adenomas that are immunonegative for ACTH, prolactin, growth hormone, TSH, and SF-1 in order to further segregate rare cases of Pit-1-positive adenomas from null cell adenomas. Pit-1 immunostaining can recognize rare cases of sparsely granulated somatotroph adenomas that appear immunonegative for growth hormone, as well as rare cases of other Pit-1-positive adenomas that are negative for Pit-1 lineage hormones. Overall, pituitary adenomas of the Pit-1 lineage that do not produce prolactin, growth hormone, or TSH are rare, with only four cases identified in the current study.     

Localization of insulin-like growth factor-II mRNA in human pituitary adenomas

Insulin-like growth factors (IGFs) have been reported to promote cell proliferation in many tumours, but their contribution to pituitary adenoma development and growth has not been characterized. We report the presence of insulin-like growth factor II (IGF-II) mRNA in pituitary adenomas using in situ hybridization (ISH). The intensity of IGF-II hybridization signal was correlated with adenoma type, and the presence of Ki-67. Among the 109 adenomas examined, 55 (50.4%) were positive for IGF-II mRNA. All acidophil stem cell, functioning corticotrophic and plurihormonal adenomas contained the message; a high incidence of signal was found among sparsely (7/8) and densely (4/6) granulated growth hormone (GH) cell adenomas, mixed GH cell–prolactin (PRL) cell adenomas (6/7), thyrotrophic (4/6) and null-cell (6/7) adenomas. Less frequently, IGF-II mRNA was localized in mammosomatotrophic, silent subtype 3, gonadotrophic, and oncocytic adenomas, whereas all sparsely granulated PRL cell adenomas and silent corticotrophic adenomas of subtypes 1 and 2 were negative. The MIB-1 labelling index was significantly higher in adenomas with a moderate to intense IGF-II signal than in adenomas with weak or no signal. The results suggest that IGF-II, when highly expressed, may have a role in pituitary adenoma proliferation.     

A composite somatotroph-corticotroph pituitary adenoma

A 76-year-old woman presented with enlargement and weakness of her hands and feet coarsening of facial features, proximal muscle weakness, and worsening of her noninsulindependent diabetes mellitus. Serum growth hormone, somatomedin-C, and prolactin levels were elevated. Thyroid function test results and serum cortisol and adrenocorticotropic hormone levels were within normal limits. Luteinizing and follicle-stimulating hormone levels were both low, suggesting possible partial hypopituitarism. Magnetic resonance imaging of the sella demonstrated a pituitary lesion that measured 2.2 x 1 x 0.5 cm; it partially obliterated the suprasellar cistern and it distorted the optic chiasm. Light microscopic and ultrastructural examination of the trans-sphenoidally resected tissues identified characteristic features of 2 discrete pituitary adenomas that were in close apposition, but they were sharply demarcated. The 2 components were a corticotroph adenoma and a sparsely granulated somatotroph adenoma. Multiple adenomas of the pituitary are not rare; however, the majority are endocrinologically “nonfunctional.” We report a patient with clinical features of acromegaly whose tumor was a composite lesion: one area exhibited morphological characteristics of a corticotroph adenoma and another distinct area exhibited features of a somatotroph adenoma. The possible histogenesis is discussed.     

Clinicopathological Features of Growth Hormone-producing Pituitary Adenomas: Difference among Various Types Defined by Cytokeratin Distribution Pattern Including a Transitional Form

Pituitary adenomas producing almost exclusively growth hormones (GH) have been ultrastructurally classified into two distinct types: densely granulated somatotroph (DG) adenomas and sparsely granulated (SG) adenomas. Fibrous body (FB), an intracytoplasmic globular aggregation of cytokeratin (CK) filaments, is a hallmark of SG adenomas. Under light microscope, FB could be identified by CK immunohistochemistry as a dot-pattern immunoreaction versus a perinuclear pattern for cells without FB. However, it has been noted that numerous adenomas contain mixed populations of the two patterns. To clarify clinicopathological characteristics of the adenomas with mixed populations ("intermediate type" adenomas) and to confirm clinicopathological differences between strictly defined DG-type and SG-type adenomas, we performed this study on 104 GH cell adenomas. Having segregated "intermediate-type" adenomas (26 cases), we found significant differences between typical DG-type (47 cases) and SG-type adenomas (31 cases); SG-type adenomas had younger ages (44 vs. 50), higher frequency of macroadenomas (86% vs. 58%), invasiveness (65% vs. 38%), advanced grades (3 or 4) in Knosp's classification (50% vs. 24%), and weaker immunoreaction for GH, beta-TSH, alpha-subunit, E-cadherin, and beta-catenin. Clinicopathological characteristics of "intermediate-type" adenomas were identical to those of DG-type adenomas. These findings confirm that SG-type adenoma is a distinct section of GH cell adenomas with special properties and biological behavior, and suggest that intermediate-phenotype adenomas are enrolled in DG-type adenomas. Special properties and biological behavior of SG-type adenomas may appear after the majority of tumor cells possess a fully developed fibrous body.     

垂体生长激素分泌瘤细胞有关生长抑素调节的受体和受体后过程的某些变化

为了解生长抑素（ＳＲＩＦ）在人垂体生长激素（ＧＨ）分泌瘤发病中的作用，在２５例肢端肥大症患者体外地养的垂体瘤细胞观察了ＳＲＩＦ激动剂（ＳＭＳ），抑制性鸟苷酸调节蛋白（Ｇｉ）拮抗剂百日咳毒素（ＰＴ）和钙离子载体Ａ２３１８７对ＧＨ分泌的影响．以及ＳＭＳ对细胞内ｃＡＭＰ水平的抑制作用。发现：ＳＭＳ可以使８４．０％（２１／１２５例）的ＧＨ瘤细胞ＧＨ分泌量明显抑制，揭示ＳＲＩＦ受体及受体后过程的功能基本正常；在３１．６％（６／１９例）患者的ＧＨ瘤细胞ＰＴ不能阻断ＳＭＳ对ＧＨ分泌的抑制作用，提示这些瘤细胞膜上Ｇｉ的功能异常；在３５．０％（７／２０例）用者的ＧＨ瘤细胞Ａ２３１８７未能拮抗ＳＭＳ对ＧＨ分泌的抑制作用，说明ＳＲＩＦ对Ｃａ￣（２＋）通道功能的调节有异常。在部分瘤细胞ＳＭＳ对ＧＨ分泌和细胞内ｃＡＭＰ的作用不一致。上述结果表明ＳＲＩＦ对垂体ＧＨ瘤细胞ＧＨ分泌的抑制作用由第二信使介导，但部分瘤细胞存在着受体后Ｇｉ和／或Ｃａ￣（２＋）通道功能的缺陷。     

Silent somatotroph adenomas of the human pituitary. A morphologic study of three cases including immunocytochemistry, electron microscopy, in vitro examination, and in situ hybridization.

Pituitary adenomas, removed surgically from three women with normal or slightly elevated serum growth hormone levels and no evidence of acromegaly, were studied. The tumor cells were shown by electron microscopy to correspond to sparsely granulated somatotrophs but immunocytochemistry showed that they contained no, moderate, or little growth hormone. Two tumors examined in vitro secreted small amounts of growth hormone in the tissue culture medium initially with a spontaneous rise after several days, and responded to growth hormone-releasing hormone stimulation with increased growth hormone release. In situ hybridization demonstrated growth hormone mRNA expression in adenoma cells. Clinically silent somatotroph adenomas represent a hitherto undescribed entity; electron microscopy shows that they consist of somatotrophs, and express growth hormone mRNA but do not secrete growth hormone in amounts needed to raise substantially serum growth hormone levels and cause acromegaly. Further work is required to clarify the mechanisms accounting for the lack of clinical and biochemical evidence of hormone excess associated with these tumors.     

Hypophysentumoren

Pituitary adenomas must be clearly differentiated from other tumors of the sellar region (especially meningiomas, granular cell tumors, chordomas and germinomas), which may look very similar. The sub-classification of adenomas depends on the methods used, in particular the immunostaining for pituitary hormones. This sub-classification is not necessary in every case, but must be performed if unusual findings are observed during surgery or if surgery is unsuccessful and radiation or drug-therapy is planned. Special structures and non-immunohistochemical stainings are very helpful for typing adenomas. We differentiated monohormonal densely or sparsely granulated GH-cell adenomas, monohormonal sparsely or very rarely densely granulated prolactin cell adenomas, monohormonal densely or sparsely ACTH-cell adenomas, monohormonal TSH-cell adenomas and FSH/LH cell adenomas from bihormonal adenomas of mammosomatotroph or GH/prolactin cell type or of the acidophil stem cell adenoma type. The number of plurihormonal adenomas decreased with the use of improved monoclonal antibodies. Clinically inactive adenomas are classified as null cell adenomas, oncocytic adenomas or FSH/LH-cell adenomas. These appear as subtypes of one entity deriving from the gonadotroph cell type. Craniopharyngiomas are classified into adamantinous and papillary types, which are not only structurally but also clinically different. If adamantinous craniopharyngiomas show very strongly regressive changes, immunostaining for keratin may be necessary to identify the squamous epithelia for the demonstration of craniopharyngioma.     

Ultrastructural Morphology of Folliculo-Stellate Cells in Human Pituitary Adenomas

Fifty-six pituitary adenomas were studied by electron microscopy in a search for the presence of folliculo-stellate cells (FSCs) with the aim of evaluating their prevalence and ultrastructural morphology. FSCs were scattered in two adenomas (one oncocytoma and one densely granulated GH cell adenoma) and were numerous in a sparsely granulated GH cell adenoma; their overall prevalence was 5.4%. Ultrastructural examination of the three neoplasms revealed that FSCs were hypertrophic element with abundant cytoplasm and organelles (in contrast to FSCs of the normal pituitary) and no obvious signs of neoplastic transformation. Junctional complexes between FSCs were similar to those described in the normal gland. Numerous follicular structures were lined by FSCs. FSCs in pituitary adenomas are probably nonneoplastic, reactive cells showing signs of hyperactivity, similar to FSCs found during pituitary hypersecretion and in estrogen-induced tumor.     

Evidence for growth hormone (GH) autoregulation in pituitary somatotrophs in GH antagonist-transgenic mice and GH receptor-deficient mice

Growth hormone (GH) modulates the hypothalamic release of somatostatin and GH-releasing hormone; however, there has been no evidence of GH autoregulation on the pituitary somatotroph. To determine the effects of GH on its own regulation, we examined the pituitaries of giant transgenic mice expressing a GH agonist (E117L), dwarf transgenic mice expressing a GH antagonist (G119K), and dwarf mice devoid of the GH receptor/binding protein (GHR/BP). In the E117L transgenic mice, the number and distribution of pituitary GH-immunoreactive cells were unchanged from nontransgenic littermate controls; an ultrastructural examination revealed typical, densely granulated somatotrophs. In contrast, the pituitaries of the G119K mice contained both moderately granulated somatotrophs and a sparsely granulated (SG) population with well-developed synthetic organelles and a distinct juxtanuclear globular GH-staining pattern. GHR/BP-deficient mice exhibited a marked reduction in the intensity of cytoplasmic GH immunoreactivity; however, prominent GH staining in the juxtanuclear Golgi was seen. GH-immunoreactive cells were increased in number, and the reticulin network pattern was distorted; stains for proliferating cell nuclear antigen confirmed mild hyperplasia. Electron microscopy showed that the somatotrophs were hyperactive SG cells with prominent endoplasmic reticulum membranes, large Golgi complexes, and numerous mitochondria. These findings are consistent with synthetic and secretory hyperactivity in pituitary somatotrophs due to the reduced GH feedback regulation. The changes are most striking in animals that are devoid of GHR/BP and less marked in animals expressing a GH antagonist; both models had reduced insulin-like growth factor-I levels, but the more dramatic change in the GHR/BP animals can be explained by abrogated GH signaling. This represents the first evidence of direct GH feedback inhibition on pituitary somatotrophs, which may have implications for the use of GH analogs in different clinical settings.