Heterogeneity of the MtTw15 mammosomatotropic tumor. II. Characterization of parenchymal cells by superimposition immunocytochemistry and electron microscopy

Ultrastructure of spontaneous and transplanted pituitary tumors in laboratory animals

The present review examines ultrastructural studies of pituitary tumors in laboratory animals. Such studies have been facilitated by the development of newer techniques such as immunocytochemistry and in situ hybridization. Using these methods it has been possible to correlate morphological features with function by characterizing specific pituitary hormones in secretory granules and analyzing the messenger ribonucleic acids produced by specific cell types. Analyses of spontaneous and experimentally induced tumors have provided a great deal of insight into the function and ultrastructure of these neoplasms. The description of tumor cells producing more than one hormone has been facilitated by immunocytochemical analyses. Studies of transgenic models of pituitary hyperplasia and tumor development are expanding our knowledge about the ultrastructure and about other aspects of pituitary tumors in a more controlled experimental setting. The production of various hormones by the same pituitary tumors and even the same tumor cells indicates that complex regulatory mechanisms must be analyzed by a combination of ultrastructural and other techniques to learn more about growth and hormone secretion in pituitary neoplasms.

Double-labeling immunogold electron-microscopic study of hormonal colocalization in nontumorous and adenomatous rat pituitaries

According to the one cell, one hormone theory, the pituitary gland is composed of 5 cell types which secrete 6 hormones. Recent investigations indicate that this theory must be modified, as there are some bihormonal cells containing 2 hormones, i.e., mammosomatotrophs prolactin-growth hormone (PRL-GH). This study was undertaken in order to elucidate whether other adenohypophysial cells are capable of producing 2 hormones and to demonstrate the presence of cells coexpressing PRL-GH, PRL-thyrotropin (TSH), or TSH-GH. Sixteen nontumorous and 16 adenomatous male and female Sprague-Dawley and Long Evans rat pituitaries were removed immediately after the animals were killed and processed for transmission electron microscopy and the immunogold double-labeling technique. Coexpression of PRL-GH, PRL-TSH, and TSH-GH was found in both nontumorous and adenomatous pituitaries. Double labeling was present not only in the same cell cytoplasm but also in the same secretory granules. The question of whether these double-labeled cells represent different cell populations, transitional cell types, or precursor cells requires further investigation.

Regulation of dopamine receptors in the MtT/W15 transplantable pituitary tumor by estrogen

We investigated the effects of estrogens on the regulation of dopamine receptors in the MtT/W15 transplantable rat pituitary tumor. Diethylstilbestrol (DES) and 17beta-estradiol treatment in female rats significantly decreased the number of dopamine binding sites (B max) from 85 +/- 3.9 fmol/mg protein in untreated rats to 9.2 +/- 1.2 and 8.2 +/- 2.8 fmol/mg protein in DES and 17beta-estradiol-treated rats, respectively, while the binding affinities (Kd) did not change significantly. Testosterone treatment did not change the B max, while ovariectomy resulted in a significant increase in the B max (146.3 +/- 6.7 fmol/mg protein). The effects of DES on the B max were reversible, since removal of the DES for one week before sacrificing the animals led to a marked increase in the B max (54.9 +/- 3.1 fmol/mg protein). Pituitaries from normal female rats treated with DES for 6 and 9 weeks had a significant decrease in the B max. These results show that the number of dopamine binding sites in the membranes of MtT/W15 tumors is decreased by estrogen treatment and that this effect is reversible after removal of the estrogenic stimulus.

Fine structure of somatotrophs and mammotrophs in the pituitary pars distalis of the little (lit) mutant mouse

The cellular pathology and postnatal differentiation of somatotrophs and mammotrophs in the pars distalis of little (lit/lit) and normal (+/+; lit/+) mice were studied by means of electron microscopy. The results indicate that in little the pituitary at birth contains recognizable somatotrophs and mammotrophs; however, between 14 and 24 days of age a contrast between little and normal becomes conspicuous with respect to the somatotrophs. Little somatotrophs are less heavily populated with granules, have granules of smaller size, and show less developed organelles than do normal somatotrophs at comparable stages. Beyond 24 days the little somatotrophs become more difficult to locate, and those that do occur show a minimal increase in complexity from that present at 14 days. In contrast, the mammotrophs in little are similar in appearance to normal mammotrophs and show increasing complexity as development proceeds, often sending forth cellular processes between neighboring cells, as do normal mammotrophs.

Functional classification of cell types in the growth hormone- and prolactin-secreting rat MtTW mammosomatotropic tumor with ultrastructural immunocytochemistry

The aim of this study was to identify the neoplastic endocrine cells which contain growth hormone (GH) and prolactin (PRL), in the MtTW15 mammosomatotropic tumor, with ultrastructural immunocytochemistry. We used tumors recovered after 5 to 11 weeks of tumor development, from normal (untreated) rats and from rats treated with the progestin medroxyprogesterone acetate (MPA)--a stimulator of GH secretion in these tumors. Immunocytochemical staining was done with the peroxidase-antiperoxidase technique on ultrathin sections of tumor that had been fixed in glutaraldehyde and postfixed on osmium tetroxide. Immunospecific staining for PRL was found over small (150 nm) secretion granules, whereas staining for GH was deposited on the larger secretion granules (250 nm). Tumors from MPA-treated rats contained profuse numbers of neoplastic cells with large, GH-positive granules. Immunocytochemical staining for GH and PRL was also found in crinophagic, lysosome-like inclusions, particularly in cells that contained many secretion granules. The results support the hypothesis that GH and PRL are produced by separate neoplastic endocrine cell types in the MtTW15 mammosomatotropic tumor, and demonstrate the value of ultrastructural immunocytochemical analysis for functional classification of cell types in chromophobic pituitary adenomas.