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Development and Markers Expressed
Rinehart and Farquhar first discovered follicullostellate (FS) cells through electron microscopy of the anterior pituitary gland. Initially these cells were thought to be corticotrophs, but are now described as adrenocorticotroph-like cells without the secretory element. Due to their follicle formation and stellate-like appearance, Vila-Porcile named these non-endocrine cells folliculo-stellate cells in 1972.

These non-hormone secreting cells are thought to act as supporting cells of the pituitary endocrine cells. It is clear that they do not just have a structural role though, because they have similar properties to dendritic cells and macrophages, implying a phagocytic role. FS cells also produce multiple different growth factors and cytokines, such as basic fibroblast growth factor (bFGF), follistatin, leukemia inhibitory factor (LIF) and interleukin 6.

Due to the unclear role of these non-hormone secreting cells, experiments have been carried out to assess the markers they express, in order to determine their cell-type and thus exact function in the pituitary. Due to their non-endocrine properties, studying these cells is much harder because simple immunohistochemistry using an antibody against a particular hormone cannot be used. The first marker protein discovered in these FS cells was S-100b, which is a calcium-binding protein expressed by glial cells. Discovering the S-100b protein marker made it much easier to visualise these cells under a light microscope.

Although the main marker is S100 protein, some populations of FS cells have also been found to express different cell markers, including GFAP (glial fibrillary acidic protein), cytokeratins, vimentin and fibronectin. S-100 protein and GFAP expression seem to be strongest in early, newly – formed FS cells, thus could be important in early FS cell development. GFAP expression implies these cells could be of a neuroectodermal origin, whereas keratin-positive FS cells express epithelial – like characteristics. The study of fibronectin expression in these cells suggests that FS cells may help regulate pituitary function, by interacting with hormone secreting cells through fibronectin. Furthermore, as FS cells express vimentin, an intermediate filament protein marker, this supports the theory that FS cells may be derived from glial neuroectodermic cells.

Due to the different array of markers expressed in these cells, it is difficult to specify their exact cell-type and function. Multiple FS cell lines have been developed to try to observe the location and function of these cells. Recently the mRNA levels of FS cells can now be observed via laser capture microdissection and RT-PCR, so progress is being made in terms of understanding the expression and function of these non-endocrine cells of the pituitary. As they have multiple markers, it is plausible that these cells are a hybrid of several different cell types.