Folliculo-stellate cells of human pituitary adenomas: Immunohistochemical study of the monocyte/macrophage phenotype expression

Folliculo-stellate cells (FS) represent a small percentage of anterior pituitary elements of still undetermined embryological origin. They are sparse among endocrine pituitary cells and are characterized by the lack of secretory granules and by the presence of few branching processes inserted between hormone-secreting cells. Although FS cell role is still under discussion, recent reports showed that they produce monocyte-derived cytokines able to influence the hormone production and modulate the immunoendocrine connections. In this study we applied three monocyte-macrophage markers (HAM56, KP1, HLA-DR) to 15 pituitary adenomas in order to ascertain whether FS cells belong to the macrophage lineage. In this case FS cells could be considered the resident macrophages of the pituitary. FS cells were identified according to the reactivity to S-100, GFAP and vimentin. We confirm that S-100 represents the most useful marker for these cells that were detected scattered between tumor cells in more than half of the adenomas. GFAP stained only a percentage of FS cells, while vimentin recognized in addition to stellate cells endothelia, perivascular and infiltrating macrophages. We were unable to detect the expression of the macrophage markers on S-100 and GFAP reactive cells. Indeed, HAM56, KP1 and HLA-DR-positive cells were mostly round, small size and located in the perivascular and septal positions where FS cells were never detected. Lack of expression of monocyte-macrophage lineage markers by FS cells in pituitary adenomas suggests their preferential neuroectodermal origin. However, further studies on normal human pituitary will be needed before ruling out a possible role for FS cells as resident pituitary macrophages.