A population of interstitial cells in the anterior pituitary with a hematopoietic origin and a rapid turnover: A relationship with folliculo- stellate cells?

The non-hormone secreting folliculo-stellate (FS) cells in the anterior pituitary (AP) appear heterogeneous. Some of these cells have been described as having a neuroectodermal origin and being glial, while some others have been suggested to be monocytic or dendritic cells (DC). We have analyzed here the hematopoietic origin of interstitial cell populations in the AP. In the rat AP, the relative densities of S100+ FS cells and major histocompatibility complex (MHC) class II-expressing DC-like cells show a parallel increase in the postnatal period between the age of 3 weeks to 2 months. We first looked for the presence of donor derived cells in the AP of lethally irradiated bone marrow (BM)-transplanted rats. Donor derived myeloid cells carrying the n haplotype of the MHC class I antigen (RTI.A(n)) reacting with the OX27 moAb, could not be detected in the AP three months after transplantation. It appeared, however, that OX27+ DC-like cells a-priori were virtually absent from the rat AP. Therefore this transplantation model was not suitable for our studies. We then turned to a model of transgenic mice expressing a suicide gene in subpopulations of dendritic cells. Mice were lethally irradiated and received a BM transplant from the transgenic animals, with or without a treatment with ganciclovir (GCV) that specifically kills the dividing cells expressing the suicide gene. This model has already been used to identify and delete mainly dendritic cell populations, viz N418+ and ER-BMDM1+ dendritic cells in the marginal zones of the spleen and in the thymic medulla. We observed in the AP a 30% reduction of the ER- BMDM1+ FS-like cells and a 50-100% reduction of interstitial cells expressing the F4/80, Mac-1 and MOMA-1 markers in the mice receiving the transgenic BM and treated with GCV, compared to control mice that were not treated with GCV or that received non-transgenic BM. When a treatment with granulocyte-macrophage colony-stimulating factor (GM-CSF) was initiated during the GCV treatment, we observed an even stronger reduction of the above-mentioned interstitial cell populations. These data indicate that in the mouse AP a population of stellate cells exists with a hematopoietic origin, that expresses markers of myeloid cells, and that has a rapid turnover.