Pituitary progenitor cells tracked down by side population dissection.

The pituitary gland represents the endocrine core, governing the body's hormonal landscape by adapting its cellular composition to changing demands. It is assumed that stem/progenitor cells are involved in this remodeling. Recently, we uncovered a candidate stem/progenitor cell population in the anterior pituitary. Here, we scrutinized this "side population" (SP) and show that, unexpectedly, not the subset expressing high levels of "stem cell antigen-1" (Sca1(high)) but the remainder non-Sca1(high) fraction clusters the pituitary progenitor cells. Transcriptomal interrogation revealed in the non-Sca1(high) SP upregulated expression of the pituitary stem/progenitor cell markers Sox2 and Sox9, and of multiple factors critically involved in pituitary embryogenesis. The non-Sca1(high) SP encloses the cells that generate spheres and display multipotent hormone differentiation capacity. In culture conditions selecting for the non-Sca1(high) subset within the SP, stem cell growth factors that induce SP expansion, affect transcription of embryonic factors, suggesting impact on a developmental program that unfolds within this SP compartment. Non-Sca1(high) SP cells, revealed by Sox2 expression, are observed in the postulated periluminal stem/progenitor cell niche, but also in small groups scattered over the gland, thereby advocating the existence of multiple niches. In early postnatal mice undergoing a pituitary growth wave, Sox2(+) cells are more abundant than in adults, concordant with a larger SP and higher non-Sca1(high) proportion. Together, we tracked down pituitary progenitor cells by SP phenotype, and thus provide a straightforward method to isolate and scrutinize these cells from the plastic pituitary ex vivo, as well as a culture system for in-depth exploration of their regulatory network.