Fine structural features of secretion in adenomas of human pituitary gland

Ultrastructural investigation of 274 human pituitary adenomas and 39 nontumorous adenohypophyses revealed two distinct types of secretion. Exocytosis was characteristic of prolactin cell adenomas, mixed growth hormone-prolactin cell adenomas, acidophil stem cell adenomas, and nontumorous prolactin cells. The second type, termed "transmembrane effusion," was noted in corticotroph, thyrotroph, gonadotroph, undifferentiated cell adenomas, and oncocytomas, as well as nontumorous corticotrophs, thyrotrophs, and gonadotrophs. It differed from that of prolactin cells and resembled diacrine secretion of gastrointestinal gastrin cells and membrane release in the neurohypophysis. In adenomatous and nontumorous growth hormone cells, neither exocytosis nor transmembrane effusion were apparent, hence a third type of release is suggested. Electron microscopic study of release mechanisms is helpful in the differential diagnosis of pituitary adenomas, since discharge of secretory products is not identical in the various tumor types.     

Glucocorticoid receptor expression in nontumorous human pituitaries and pituitary adenomas

Glucocorticoids have multiple actions, including a suppressive feedback effect on pituitary corticotrophs via the glucocorticoid receptor (GR). By immunocytochemistry, we studied GR expression in 86 surgically removed various pituitary adenoma types. Ten cases contained nontumorous pituitary fragments, which were suitable for immunocytochemical investigation. In addition, 30 autopsy-obtained pituitaries, 10 of them containing incidental microadenomas, were examined as well. Using a polyclonal GR antibody, the streptavidin-biotin-peroxidase complex method revealed nuclear and/or cytoplasmic GR immunoreactivity in many nontumorous corticotrophs and other adenohypophysial cell types and in S-100 protein immunopositive stellate cells. Cellular localization was confirmed by double immunostaining. Pars intermedia corticotrophs, posterior lobe axons, Herring bodies, and pituicytes as well as several endothelial cells lining the capillaries were also immunopositive. GR immunoreactivity was also demonstrated in many GH, PRL, ACTH, TSH, FSH, LH α-subunit producing adenomas, null cell adenomas, and oncocytomas. The extent and degree of immunostaining varied considerably from case to case. Suppressed corticotrophs showing the Crooke’s hyaline change due to glucocorticoid excess were present in the nontumorous pituitaries of patients with Cushing’s disease and in those treated with pharmacologic doses of glucocorticoids. Many suppressed nontumorous corticotrophs exhibited only weak or no GR immunopositivity, indicating GR downregulation accompanied by cellular injury. Study of autopsy obtained pituitaries for GR yielded inconclusive results indicating that autopsy obtained adenohypophyses are not suitable for the immunocytochemical investigation of GR.     

In situ hybridization study of pro-opiomelanocortin (POMC) gene expression in human pituitary corticotrophs and their adenomas

Summary Pro-opiomelanocortin (POMC) mRNA was detected on paraffin sections by in situ hybridization (ISH) in corticotrophs of 12 nontumorous pituitaries, 11 functioning corticotroph, and 11 silent pituitary adenomas. ISH combined with immunocytochemistry for adrenocorticotrophic hormone (ACTH), a POMC-derived peptide, was also performed. ACTH immunoreactive cells of the anterior lobes and those invading the posterior lobe showed a high or moderate level of POMC mRNA that was not correlated with the intensity of ACTH immunoreactivity. Variable levels of POMC gene expression were present in Crooke's cells, corticotrophs suppressed by glucocorticoid excess. Most functioning corticotroph adenomas and silent subtype 1 adenomas had an intense hybridization signal and ACTH immunoreactivity. In silent subtype 2 and 3 adenomas, POMC mRNA had a diffuse low level or was absent; in these adenomas ACTH immunoreactivity was diffuse, restricted to some cells, or negative. The results indicate that POMC gene is expressed in both normal and suppressed nontumorous corticotrophs. Intense signals for POMC mRNA are found in most functioning corticotroph adenomas. The difference between POMC gene expression in silent 1 and silent 2 and 3 adenomas suggests that different mechanisms are responsible for the lack of endocrine activity.     

Corticotroph (Basophil) invasion of the pars nervosa in the human pituitary: Localization of proopiomelanocortin peptides,galanin and peptidylglycine α-amidating monooxygenase-like immunoreactivities

Corticotroph (basophil) invasion or the migration of corticotroph cells into the pars nervosa of the human pituitary gland was found in 35 of 767 (4.4%) consecutive pituitaries obtained at autopsy. The degree of invasion increased with patient age and extensive invasion was more common in men than in women. Immunoreactive ACTH, β-MSH, α-MSH, and galanin were detected both in the anterior lobe and invading corticotroph cells in approximately equal frequency. Fewer cells stained positively for α-MSH than for the three other peptides in both the anterior lobe and invading corticotrophs. Twelve corticotropic pituitary adenomas obtained surgically from patients with Cushing’s disease were also examined and expressed varying degrees of immunoreactivity for ACTH, α MSH, β-MSH and galanin. Staining for all major pituitary hormones revealed only ACTH in the invading basophil cells. Peptidylglycine α-amidating monooxygenase (PAM) was present in the anterior pituitary, in invading corticotroph cells, and in some cells lining the cysts of the pars intermedia zone. PAM immunoreactivity was also detected in 4/12 corticotroph adenomas. These results indicate that corticotroph cells invading the pars nervosa are immunohistochemically similar to anterior lobe corticotrophs and have the ability to amidate various peptides such as proopiomelanocortin cleavage products and galanin with PAM.     

Localization of Epidermal Growth Factor (EGF) and Epidermal Growth Factor Receptor (EGFr) in Human Pituitary Adenomas and Nontumorous Pituitaries: An Immunocytochemical Study

Epidermal growth factor (EGF) and epidermal growth factor receptor (EGFr) were investigated by immunocytochemistry (ICH) in 57 human pituitary adenomas and 10 nontumorous autopsy pituitaries. EGF immunoreactivity was demonstrated in 24 adenomas (42%), representing 23 functioning tumors and 1 nonfunctioning tumor of oncocytic type, and in all nontumorous pituitaries. Among 40 tumors, EGFr was found positive in 15 functioning adenomas (37.5%), representing 50% of them. The presence of both EGF and EGFr was found mainly in corticotroph adenomas (60%) and less frequently in somatotroph and lactotroph adenomas (20%). ICH on serial sections with EGF or EGFr and adrenocorticotrophic hormone (ACTH) or S-100 protein revealed that EGF and EGFr are localized specifically in corticotrophs and EGFr in stellate cells of nontumorous adenohypophysis. These results confirm the presence of EGF and EGFr in human pituitary adenomas and nontumorous pituitaries and highlight their frequent occurrence in hormone-producing adenomas. Further work is required to explore the possibility that EGF and EGFr play a role in hormone production, release, and tumor progression.     

Galanin immunoreactivity in neuroendocrine tumors.

We investigated immunoreactivity for galanin, a 29-amino acid peptide, in formalin-fixed, paraffin-embedded sections of 123 neuroendocrine tumors. Galanin-immunoreactive cells were found in one of 12 hypothalamic gangliocytomas, nine of 18 adrenal pheochromocytomas, nine of 14 pituitary corticotroph adenomas, and one of two thymic endocrine tumors. In pheochromocytomas, galanin-immunoreactive cells were seen either singly or in clusters. In corticotroph adenomas, many tumor cells were positive for galanin, indicating colocalization of corticotropin and galanin in the same tumor cells. No galanin-immunoreactive cells were noted in four extra-adrenal paragangliomas; 10 medullary carcinomas of the thyroid; 35 endocrine tumors arising in the lung, pancreas, and gastrointestinal tract; and 28 pituitary adenomas composed of cells other than corticotrophs. In nontumorous counterparts of these neuroendocrine tumors, galanin immunoreactivity was observed in nerve cells of the hypothalamus, nerve fibers of the duodenum, and adenohypophyseal cells corresponding to corticotrophs. These findings indicate that galanin expression in neuroendocrine tumors is uncommon and restricted to some tumor types.     

Immunohistochemical study of p53 protein in human and animal pituitary tumors

In many human cancers, p53 gene mutations are frequently occurring genetic abnormalities, which may be detected by immunohistochemical staining for p53 protein. In the present study, p53 immunoreactivity was investigated in formalin-fixed, paraffin-embedded tissues from human and animal pituitary tumors, using the avidin-biotin-peroxidase complex technique. No p53 was detected in 3 nontumorous human adenohypophyses or in 40 human pituitary tumors including 5 GH cell adenomas, 10 PRL cell adenomas, 2 mixed GH cell-PRL cell adenomas, 2 acidophil stem cell adenomas, 8 ACTH cell adenomas, 1 TSH cell adenoma, 1 FSH/LH cell adenoma, 5 null cell adenomas, 5 oncocytomas, and 1 plurihormonal adenoma. Twenty nontumorous and hyperplastic pituitaries of hGRH transgenic mice and 8 tumors in these transgenic animals were immunonegative for p53. All pituitary tumors found in AVP/SV40 transgenic mice contained p53 immunoreactivity in the nuclei, while the nontumorous adenohypophysis of one such transgenic mouse was negative. It can be concluded that p53 mutations are apparently not involved in the pathogenesis of human pituitary adenomas or of the pituitary tumors which develop in hGRH transgenic mice. However, pituitary tumors in AVP/ SV40 transgenic mice are accompanied by p53 expression.     

Vascular Endothelial Growth Factor (VEGF) Expression in Human Pituitary Adenomas and Carcinomas

Vascular endothelial growth factor (VEGF) is a key mediator of endothelial cell proliferation, angiogenesis, and vascular permeability. Little is known about its expression in human pituitary adenomas. We examined 148 human pituitary adenomas for VEGF protein expression by immunohistochemistry. The strongest immunoreactivity was present in GH adenomas, corticotroph, silent corticotroph, silent subtype 3, and nononcocytic null cell adenomas. GH adenomas treated with octreotide strained less intensely than did untreated tumors. Relatively weak staining was present in PRL, gonadotroph, thyrotroph, and oncocytic null cell adenomas in the same sections showed evidence of down-regulation of VEGF protein expression in adenomas. Pituitary carcinomas usually had stronger staining than adenomas. In situ hybridization studies with oligonucleotide probes showed positive staining in all groups with stronger staining in GH, ACTH, TSH, and gonadotroph adenomas and in pituitary carcinomas. These results indicate that VEGF expression is more prominent in certain adenoma subtypes, that decreased expression occurs in adenomas as compared to nontumorous pituitary and that carcinomas show increased VEGF expression relative to adenomas suggesting up-regulation of VEGF during pituitary tumor progression.     

Unusual configurations of endoplasmic reticulum in human pituitary adenomas

Electron microscopic study of 435 surgically-removed human pituitary adenomas and 43 nontumorous adenohypophyses revealed unusual configurations of endoplasmic reticulum in 102 adenohypophysial tumors (23%) and in 12 nontumorous adenohypophyses (28%). These configurations classified as paired reticulum, annulate lamellae and ribosome-lamellar complexes were noted in various adenomatous and nontumorous adenohypophysial cells, indicating that they could not be used as specific markers for pituitary adenomas or for a particular adenohypophysial cell type. Paired reticulum was a common finding, whereas annulate lamellae and ribosome-lamellar complexes were rarely encountered. Whether or not these endoplasmic reticulum configurations could be considered as normal constituents of adenohypophysial cells was difficult to assess, since nontumorous cells studied were from patients who had various diseases and who had been treated with different hormones. The presence of endoplasmic reticulum configurations was neither related to age, sex of the patients nor degree of differentiation or endocrine activity of pituitary adenomas.     

Argyrophil organizer region proteins (AgNORs) in adenohypophysial cells and adenomas of the human pituitary

Nucleolar organizer regions (NORs), claimed to represent valuable markers of tumor proliferation, were visualized on paraffin sections by an argyrophil method and counted in 18 nontumorous adenohypophyses and 132 pituitary tumors. The AgNOR counts could not be correlated with the hormonal activity of nontumorous and adenoma cells. In pituitary adenomas, the mean AgNOR values were higher than in their corresponding nontumorous cell types. Some adenomas, especially growth hormone (GH), and prolactin (PRL) cell adenomas, however, had AgNOR readings in the range of nontumorous cells. Long-acting somatostatin analog and bromocriptine treatment decreased AgNOR counts in GR- and PRL-producing tumors. Most, but not all invasive and/or recurrent adenomas had high AgNOR counts. In a corticotroph carcinoma, AgNORs were not higher than in the adenomas. These inconsistent results limit, at the present time, the use of AgNORs as reliable markers of cell proliferation in pituitary tumors. Further studies may help to establish the value of this promising method in pituitary pathology.     

Characterization of C-Kit (CD117) Expression in Human Normal Pituitary Cells and Pituitary Adenomas

c-kit (CD117) is a tyrosine kinase receptor involved in the proliferation, differentiation, and secretory functions of various cells. In experimental animal models, c-kit has been detected in the pars intermedia of the normal pituitary gland and in alpha-melanocyte-stimulating-hormone-positive adenomas and it has been suggested that it plays a role in regulating adrenocorticotropic hormone (ACTH) secretion. To the best of our knowledge, the expression of c-kit in normal human pituitary cells and in pituitary adenomas has never been reported, so the possible biological role of this receptor in the control of pituitary hormone secretion remains unclear. The aim of this study was to evaluate the immunohistochemical expression of c-kit in normal human pituitary glands and in a series of 62 well-characterized pituitary adenomas. In normal adenohypophyses, several cells, mainly located in the central mucoid wedge, showed a c-kit immunoreactivity (IR). Double label immunostaining procedures showed that the c-kit-IR cells corresponded to ACTH cells. Out of 62 adenomas, 15 (24%) were c-kit-IR, including 7/16 (44%) ACTH cell, 3/7 (42%) null cell, 4/11 (36%) alpha-subunit cell, and 1/11 (10%) follicle-stimulating hormone-luteinizing hormone cell adenomas. By contrast, all ten prolactin cell and seven growth hormone cell adenomas were c-kit negative. These data suggest that, in normal conditions, c-kit may be involved in the pituitary-adrenal axis regulation.     

Immunohistochemical localization of chromogranin in human hypophyses and pituitary adenomas

Chromogranin was demonstrated by immunohistochemistry in the cytoplasm of human beta-thyrotropin, human beta-follicle-stimulating hormone-, human beta-luteinizing hormone-, and human alpha-subunit-containing cells of the non tumorous human adenohypophyses. Some surgically removed human beta-thyrotropin-, human beta-follicle-stimulating hormone-, human beta-luteinzing hormone-, and human alpha-subunit-producing pituitary adenomas, as well as some null cell-adenomas, exhibited chromogranin immunoreactivity, whereas adenomas storing human growth hormone, human prolactin, or corticotropin were negative. Chromogranin immunopositivity was variable in extent and intensity; not every glycoprotein-producing cell could be immunostained in the nontumorous adenohypophysis and the majority of chromogranin-containing adenomas showed only focal positivity. No explanation can be offered for this variability. The demonstration of chromogranin by the avidin-biotin-peroxidase technique may be helpful in the immunohistochemical characterization of some glycoprotein hormone-producing pituitary adenomas, as well as null-cell adenomas of the human pituitary.     

Crooke's hyalinization in silent corticotroph adenoma: report of two cases

Corticotroph adenomas rarely show Crooke’s hyaline change in neoplastic cells, a feature similar to that of normal corticotroph cells exposed to excess cortisol. Crooke’s cell adenomas are usually associated with Cushing’s disease. Nonfunctioning examples are uncommon. We report two clinically silent corticotroph adenomas featuring extensive Crooke’s hyalinization in neoplastic cells. The two patients were 49 and 59 yr of age and neither had Cushing’s disease. Serum and urinary cortisol were normal. One patient had elevated serum adrenocorticotropic hormone. In our experience, the two patients accounted for 0.4% of pituitary adenomas operated on from January 1992 to December 2001 and 3.5% of all corticotroph adenomas. The two lesions had features of the subtype 1 silent corticotroph adenoma. Cytogenetic analysis performed on one lesion showed a normal karyotype (46;XY). Hyalinization in clinically silent Crooke’s cell adenoma indicates that hyaline changes do not always relate to excess cortisol. It is known that neoplastic Crooke’s cells show immunoreactivity for glucocorticoid receptors stronger than nontumorous Crooke’s corticotrophs. This fact suggests that receptor overexpression or lack of receptor downregulation may result in hypersensitivity of neoplastic Crooke’s cells to physiologic cortisol plasma levels.     

No Evidence for WT1 Involvement in a Beta-Catenin-Independent Activation of the Wnt Signaling Pathway in Pituitary Adenomas

The overexpression of Wilms' tumor gene product WT1, which acts as a tumor suppressor or oncogene, has been reported in various malignancies. Recent studies have shown that the interaction partner Wnt-4 is upregulated in pituitary adenomas dependent on the Pit-1 lineage (somatotrophs, lactotrophs, and thyrotrophs). However, no data on WT1 expression in nontumorous pituitary tissue or pituitary adenomas is available to date. We investigated WT1 expression in 90 paraffin-embedded pituitary adenomas, including eight atypical adenomas, and in 28 nontumorous pituitary glands by immunohistochemistry. WT1 is absent in epithelial cells of all nontumorous pituitary glands and in 87 out of 90 pituitary adenomas. Only two GHomas (including one atypical adenoma) and one gonadotropin-producing adenoma expressed WT1 in the cytoplasm of single tumor cells without nuclear staining. There is no evidence that WT1 does regulate the Wnt-4/beta-catenin-independent pathway which is activated in the Pit-1-expressing subset of pituitary adenomas.     

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.     

Immunohistochemical characterization of an anti-epithelial monoclonal antibody (mAB lu-5)

Summary A mouse monoclonal antibody (mAB lu-5) was prepared using a lung cancer cell line as an antigen. The selected clone produces an IgG with a gamma-1 heavy chain and a kappa-light-chain. Immunohistochemical testing of mAB lu-5 on 117 normal tissue biopsies and 474 tumours revealed reactivity with an intracytoplasmic, formaldehyderesistant antigen present in most epithelial and mesothelial cells, but absent in mesenchymal cells. The antibody can therefore be used as a first order, pan-epithelial marker. It proved also useful for fast tumour diagnosis on frozen sections.     

S-100 protein immunopositivity in human nontumorous hypophyses and pituitary adenomas

The presence, distribution, and morphological appearance of S-100 protein-immunoreactive cells in the human hypophysis were studied by immunocytochemistry. One hundred and twelve nonadenomatous pituitaries from fetuses to adults and pituitaries affected by several lesions including metastases, acute infarcts, and lymphocytic hypophysitis, as well as 115 pituitary adenomas were examined.S-100 protein immunoreactivity was detected in neurohy-pophyseal pituicytes and stellate cells of the pars distalis from 5 months following birth. In adults, S-100 protein-immunopositive cells displayed a preferential topographical association with growth hormone-, follicle-stimulating hormone-, luteinizing hormone-, and alpha-sub-unit-immunoreactive cells and with capillary walls. Colloid-containing follicles were mainly lined by hormone-containing cells, although scattered S-100 protein-immunoreactive processes or cell bodies were also observed forming their walls.No major changes in S-100 protein-immunoreactive cells were observed in the pituitary parenchyma bordering metastatic, inflammatory, necrotic, or adenomatous tissues. Eighteen of 115 pituitary adenomas contained a variable number of S-100 protein-immunoreactive cells. No preferential association of these cells with any type of pituitary adenoma was found.We propose that S-100 protein expression in the nontumorous adenohypophysis and pituitary adenomas may constitute a dynamic process and that S-100 protein-positive cells may constitute a heterogeneous cell population composed of pure, fully differentiated stellate cells and of transdifferentiated follicular cells.     

Cytokeratin CK20 is a sensitive marker for Crooke's cells and the early cytoskeletal changes associated with hypercortisolism within pituitary corticotrophs

Crooke's cells are nonneoplastic corticotroph cells found in the adenohypophysis of patients who have an endogenous or exogenous excess of glucocorticoids. Classic Crooke's cells have a prominent hyaline cytoplasmic ring that displaces the basophilic granules of the normal cell. This characteristic appearance is produced by a perinuclear accumulation of cytokeratin filaments. Immunohistochemistry for cytokeratins is a sensitive way to identify Crooke's cells, but a keratin antibody specific for Crooke's hyaline change has not been reported. Normal pituitary epithelial cells are variably reactive for many keratin antibodies but are negative for cytokeratin 20 (CK20) expression. We evaluated the use of CK20 immunohistochemistry as a marker for Crooke's cells. We examined sections from 25 pituitary glands resected from 15 patients who had undergone exogenous glucocorticoid administration and from 10 patients with an endogenous source of hypercortisolism; sections from 10 normal pituitary glands obtained at autopsy were used as controls. CK20 immunoreactivity was observed only in corticotrophs. A staining pattern consistent with classic Crooke's cells was seen in pituitary gland sections from 15 of the cases. Cells with less intense CK20 positivity were present in sections from all 25 cases. We found CK20 to be a sensitive and specific marker for Crooke's cells and also for the previously unrecognized, subtle, cytoskeletal changes that occur in corticotrophs in response to hypercortisolism.     

Presence of neurophysins in the human pituitary corticotrophs, Cushing''s adenomas, and growth hormone-producing adenomas detected by immunohistochemical study.

Neurophysins have been recognized as the carrier proteins of vasopressin and oxytocin. The distribution of neurophysins is immunohistochemically confirmed in the hypothalamus, median eminence, and posterior lobe of the pituitary gland. The authors detected neurophysins in the human corticotrophs and pituitary adenomas with the use of the immunohistochemical method with antiserum to human neurophysins, which did not cross-react with adrenocorticotropic hormone (ACTH), beta-endorphin, and corticotropin-releasing factor. All of ten pituitary glands obtained by autopsy revealed the presence of neurophysin-positive cells in the anterior, intermediate, and the posterior lobes. The neurophysin-positive cells were similar to the corticotrophs in shape and distribution. Simultaneous staining for ACTH and neurophysins in the serial sections revealed that neurophysin-positive cells were also ACTH-positive. One hundred twenty-four cases of pituitary adenoma operated upon were investigated. All of 7 Cushing's adenomas were composed of neurophysin-positive cells. Six tumors with giantism showed sparsely distributed neurophysin-positive cells. No neurophysin-positive cells were observed in any other cases. This study is the first reported evidence of the presence of neurophysins in the human corticotrophs and pituitary adenomas.     

Clinicopathologic correlations of silent corticotroph adenomas of the pituitary: report of four cases and literature review

Silent corticotroph adenomas (SCA) are rare pituitary tumors with histologic hallmarks of corticotroph differentiation, including ACTH immunoreactivity, but lacking clinical evidence of Cushing's syndrome. We report on four female patients, aged 19-66 years, each presenting with a nonfunctional macroadenoma. Leading symptoms were headache in two cases and visual field deficits in one. One patient was incidentally diagnosed while undergoing cranial MRI for an unrelated condition. Three patients had marked obesity; none of them presented constitutional signs of Cushing's syndrome. Serum cortisol levels were moderately elevated in the two patients systematically tested in this respect. Marginal to moderate hyperprolactinemia was present in two cases. Two patients also were shown to be deficient in either gonadotroph or thyrotroph axis, while a third had a combined insufficiency of both gonadotroph and thyrotroph axis. MRI scans revealed intratumoral hemorrhage and/or cystic change in three cases, as well as tumor-related occlusive hydrocephalus in one. The latter patient was biopsied only, while the remaining underwent gross total resection. Histologically, all four lesions were diagnosed as SCA subtype I displaying intense immunoreactivity for ACTH. In three tumors, scattered cells coexpressed PRL as well. In addition, Crooke's hyaline change was noted in a significant number of tumor cells and in residual non-neoplastic corticotrophs in one case each. With MIB-1 labeling indices of 1-3%, none of the tumors qualified as atypical adenoma. We conclude that SCAs are more likely to be discovered as expansile tumors, whose advanced local space-occupying character at surgery rather than an inherently aggressive growth potential may negatively influence the clinical outcome. Subtle morphologic evidence of corticotroph suppression in residual pituitary adjacent to tumor lends further support to literature data indicating minimal or intermittent functional activity in this tumor type.