The fine structure of the S-100 protein positive cells in the rat pituitary gland: an immunoelectron microscopic study

Folliculostellate cells in pituitary pars distalis of male viscacha: immunohistochemical, morphometric and ultrastructural study

Folliculostellate cells (FSC) have been reported in pituitary of several mammalian species. FSC morphology and secreted substances have been instrumental to the understanding of their function. The purpose of this work was to perform an immunohistochemical, morphometric and ultrastructural study of the pituitary pars distalis FSC in adult male viscacha and to analyze their relation with hormone secreting cells. Immunohistoche-mistry and image analysis were carried out in different sectors of the gland, from the middle (sector 1) to the glandular periphery (sector 5). Transmission electron microscopy with lanthanum as electrodense tracer was used. FSC formed follicles with PAS-positive colloid inside. They expressed S-100 protein mainly in both nucleus and cytoplasm. FSC were stellate-like in shape and exhibited short cytoplasmic processes that contacted with blood vessels and endocrine cells. In addition, some follicular colloids were immunostained with anti-S-100 protein. A few FSC were immunostained with anti-glial fibrillary acidic protein (GFAP) and anti-vimentin. The morphometric parameters analyzed (percentages of S-100-positive total, cellular and colloidal areas) increased from sector 1 to sector 3 and then decreased to sector 5. Hormone secreting cells, mainly lactotrophs, gonadotrophs and corticotrophs were associated with FSC and follicles. The ultrastructural study demonstrated that FSC developed junctional complexes and desmosomes between their lateral membranes. Lanthanum freely penetrated the spaces between granulated cells and FSC, but did not penetrate into the follicular lumen.

IN CONCLUSION

1) the differential expression of S-100 protein, GFAP and vimentin may indicate different physiological stages of FSC; 2) the expression of these proteins suggests a neuroectodermic origin of these cells; 3) FSC spatial distribution, association with endocrine cells, and the generation of an intercellular communication network suggest that FSC are involved in the pituitary pars distalis paracrine regulation of the viscacha.

Nestin-immunoreactive cells in rat pituitary are neither hormonal nor typical folliculo-stellate cells

Nestin is an intermediate filament protein that has originally been identified as a marker of neuroepithelial stem/progenitor cells. The present study explored whether nestin immunoreactivity (nestin-ir) is present in the rat pituitary and in which cell type(s). Nestin-ir was observed in scattered cells in the anterior, intermediate, and neural lobes. Nestin-ir cells were predominantly of stellate shape and were more numerous in immature than in adult animals. Nestin-ir did not colocalize with any pituitary hormone, and did not colocalize or only very sporadically with the folliculo-stellate cell marker S100. In the intermediate lobe, nestin-ir cells contained glial fibrillary acidic protein in an age-dependent manner. Nestin-ir cells were closely associated with endothelial and fibronectin-ir cells, but did mostly not coincide. Nestin-ir was not found in alpha-smooth muscle actin-ir myofibroblasts or in microglial cells. Regardless of age, nestin-ir was detected in some unidentifiable cells that border the pituitary cleft. Nestin-ir remained present in pituitary cultured as three-dimensional aggregates. Treatment with basic fibroblast growth factor or leukemia inhibitory factor increased the number of nestin-ir cells. Starting from anterior lobe cell monolayer cultures, nestin-ir cells could be selected and propagated to a virtually pure population. These nestin-ir cells displayed remarkable motility and proliferative activity, and did not express hormones, glial fibrillary acidic protein, or S100, but contained vimentin-, fibronectin-, and alpha-smooth muscle actin-ir. In conclusion, nestin-ir is present in the pituitary in cells that are neither hormonal nor typical folliculo-stellate. The expression pattern depends on age and lobe examined. Pericapillar localization suggests a pericyte phenotype for some of them. Whether the heterogeneous nestin-ir population also contains pituitary progenitor cells remains to be explored.

Immunohistochemical Localization of S-100 Protein in the Pig Pituitary Gland

Immunohistochemical localization of S-100 protein was studied in anterior, intermediate and posterior lobe of the pig pituitary gland. Two immunopositive cells for S-100 protein were identified: the folliculo-stellate cells (FSc) in the glandular lobes and the pituicytes in the neural lobe. In the anterior lobe, immunoreactive folliculo-stellate cells were scattered among secretory cells. In the area where the secretory cells form strands and follicle-like groups the positive cells were concentrated in groups. In the intermediate lobe, S-100 protein-positive cells were located sparsely among secretory cells and next to secretory follicles and the pituitary cleft. These FSc were more voluminous and displayed fewer cytoplasmic processes. In the neurohypophysis, positive reaction for S-100 protein was seen in the pituicytes. These cells were distributed singly or concentrated in groups. The distribution, and morphologic characteristics of the FSc in the glandular lobes and the pituicytes in the neural lobe in the pig indicate different origin of both S-100 protein-positive cells.

Lipocortin 1 (annexin 1): a candidate paracrine agent localized in pituitary folliculo-stellate cells

It is now well established that lipocortin 1 (LC1) plays an important role as a mediator of early delayed glucocorticoid feedback action in the hypothalamo-hypophysial system. In both the hypothalamus and anterior pituitary gland, LC1 mimics some of the actions of glucocorticoids; moreover, glucocorticoids stimulate the synthesis of LC1 and cause the translocation of intracellular LC1 to the outer cell surface. The mechanism by which LC1 acts in these tissues is only partially understood, but may involve paracrine and/or autocrine actions. To address these possibilities we have investigated the localization of LC1 in the rat pituitary gland, using double labeling immunohistochemistry to identify the pituitary cell types that express LC1. At the light microscopic level LC1 was not detected in the endocrine cells in cryosections of the pituitary, but it was found in abundance in the surrounding folliculo-stellate (FS) cells. In the anterior and interme diate pituitary lobes, there was a near total colocalization of LC1 and S100, a specific marker of FS cells. By contrast, in the posterior pituitary gland, LC1 immunoreactivity was not colocalized with S100 which labeled most pituicytes, or with OX-42 monoclonal antibody, a marker of the microglial cells. Immunogold electron microscopy confirmed that LC1 is present in the nongranulated FS cells. LC1 im munoreactivity was also present in a mouse pituitary FS-like cell line (TtT/GF), particularly in the periphery of the cytoplasm. The localization of LC1 in the FS cells of the anterior pituitary gland defines LC1 as a new marker of the FS cell population. These results support our hypothesis that LC1 acts as one of the paracrine agents liberated by FS cells that modulate the release of pituitary hormones.

Transient expression of S-100 by melanotropes of the rat pituitary intermediate lobe during development

S-100, an acidic calcium-binding protein, is co-localized with vimentin in glial-like cells in the adult rat pituitary intermediate lobe. S-100 and melanotrope markers were not co-localized in the adult. During development, S-100 and vimentin were not co-localized but appeared in cells with different morphological characteristics. S-100 was co-localized with POMC mRNA and beta-endorphin during prenatal time and the first three postnatal weeks. This was demonstrated by double-label immunohistochemistry, using combinations of antisera against S-100, vimentin and beta-endorphin, and in situ hybridization histochemistry for POMC mRNA combined with immunohistochemistry for S-100. In the second and third weeks of postnatal development, S-100 was observed in fewer melanotropes and more frequently in stellate cells, which also expressed vimentin. Thus, S-100 appeared to be transiently expressed in melanotropes during prenatal and early postnatal development. S-100 serves as a neurotrophic and glial maturation factor in the CNS. Since S-100 expression in melanotropes coincides with the onset of dopaminergic innervation and morphological changes in glial-like cells of the lobe, it could have similar functions in the rat pituitary intermediate lobe.

Regulation of GFAP expression in glial-like cells of the rat pituitary intermediate lobe by lactation, salt-loading, and adrenalectomy

Glial-like cells in rat pituitary intermediate lobe were localized and characterized by immunohistochemistry with antisera against glial fibrillary acidic protein (GFAP) and S-100. Individual GFAP immunoreactive (IR) cells possessed several processes that often branched into secondary and tertiary processes, terminating with end-feet. The GFAP-immunopositive cell population was distributed in specific rostrocaudal and dorsoventral patterns. The distribution and numbers of cells differed between male and female rats. Examination of altered physiological states, e.g., adrenalectomy, lactation, and salt-loading, revealed state-specific changes in the appearance and distribution of GFAP-IR cells. Adrenalectomy and lactation increased GFAP-IR glial-like cell numbers, whereas salt-loading decreased their numbers and the typical pattern of distribution. By contrast, S-100-expressing cells were evenly distributed in male and female rats, and its expression was not affected by the experimental conditions. Double-label immunocytochemistry indicated that GFAP-IR cells are a subpopulation of S-100-IR cells. These results suggest that cells normally expressing only S-100 may be induced to express GFAP under altered physiologic conditions.

Post-embedding tem signal-to-noise ratio of S-100

We assessed the reactivity of purified S-100 antiserum in immuno-electron microscopy by counting the number of gold particles per microns 2 over inner ear tissues embedded in different media. Sections containing predominantly Schwann's cell cytoplasm and nucleus, afferent fiber axoplasm and myelin sheath of chick cochleae were reacted with anti-S-100 IgG, an antibody to a calcium binding protein of neuronal tissues, then labeled with anti-IgG-gold conjugate. This investigation was conducted because previously published procedures, unmodified, did not yield acceptable results. Preparation of all specimens was identical. Only the medium (PolyBed 812, Araldite or Spurr epoxies; and LR White, LR Gold or Lowicryl plastics) was changed. The medium was made the changing variable because antigens available in post-embedding immuno-electron microscopy are decreased by heat, either used and/or released during polymerization of the embedding medium. The results indicate that: (a) none of the embedding media above provided optimal signal-to-noise ratio for all parts of the nerve stained in the same section; (b) aggregation of gold particles over cells was highest in embedding media with high background labeling over areas devoid of tissue (noise); (c) aggregation occurred randomly throughout both cellular and acellular regions; and (d) particles aggregated less and were distributed more evenly in tissues from media yielding good ultrastructural integrity.