Immunolabeling of cryo-sections of human anterior pituitaries obtained at autopsy, and of cryo-sections of freshly prepared rat anterior pituitaries, with a panel of monoclonal antibodies against markers of the monocyte/dendritic cell/macrophage lineage, reveals in both species a characteristic pattern of immunopositive cells, among which many cells with dendritic phenotype are found. Cells characterized by marker expression of MHC-class II determinants and a dendritic morphology are present in both human and rat anterior pituitary. Markers characteristic of dendritic cells such as the L25 antigen and the OX62 antigen were present in anterior pituitaries from human and rat respectively. The population of MHC-class II expressing dendritic cells of the rat anterior pituitary is compared at the ultrastructural level with the folliculo-stellate cell population, which cell type has been previously characterized by its distinctive ultrastructure and immunopositivity for the S100 protein. Using immune-electron microscopy of rat anterior pituitaries fixed with periodate-lysine-paraformaldehyde, we were able to distinguish non-granulated cells expressing MHC-class II determinants, whereas no MHC-class II expression was found in the granulated endocrine cells. Using double immunolabeling of cryo-sections of these rat AP with 25 nm and 15 nm gold labels, we demonstrated an overlap between the populations of MHC-class II-expressing and S100 protein-expressing cells. Furthermore, MHC-class II-expressing and S100-positive cells showed ultrastructural characteristics that have been previously ascribed to folliculo-stellate cells. At the light microscopical level in the rat AP, a proportion of 10 to 20% of the S100-positive cells was found immunopositive for the MHC-class II marker OX6. In the human AP, S100-positive folliculo-stellate cells and cells expressing the leukocyte common antigen CD45 were found to occupy predominantly different tissue compartments in the human anterior pituitary, namely the epithelial parenchyme cords and perivascular compartments respectively. A proportion of CD45+ cells was found in the parenchyme compartment and, vice versa, indicating an overlap of the tissue compartments in which both cell types occur. However, at the light microscopical level we could not find cells expressing both the S100 and CD45 marker. The present finding of a proportion of S100-positive pituitary cells with ultrastructural and immunohistochemical characteristics of both dendritic cells and folliculo-stellate cells, confirms the suggested heterogeneity of the latter cell group with respect to their ultrastructural phenotype and putative function. The possibility of a myeloid origin of part of the folliculo-stellate cell group in the AP, is discussed and might elucidate some of the discrepancies in the literature concerning the embryological origin of this cell group.

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Journal of Neuroendocrinology
Department of Pathology