The adult pituitary contains a cell population displaying stem/progenitor cell and early-embryonic characteristics

Abstract

A "side population" (SP) has been identified in a number of tissues where it typically represents a small population enriched in stem/progenitor cells. Here, we show that the adult mouse anterior pituitary (AP) also contains a characteristic SP displaying verapamil-sensitive Hoechst dye-efflux capacity. A majority of the SP cells express "stem cell antigen 1" at a high level (Sca1hi). Using (semi-)quantitative RT-PCR and immunofluorescence, we characterized the Sca1hi SP as a population enriched in cells expressing stem/progenitor cell-associated factors, and components of the Notch, Wnt and Shh signaling pathways, functional in stem cell homeostasis as well as in early pituitary embryogenesis. Lhx4, a transcription factor pivotal for early-embryonic development of the AP, was only detected in the Sca1hi SP whereas Lhx3, in contrast to Lhx4 not downregulated after AP development, was only found in the "main population" (MP). The Sca1hi SP was depleted from cells expressing phenotypic markers of differentiated AP cells (hormones) but contained a small proportion of folliculo-stellate cells. Stem cells of many tissues can clonally expand to non-adherent spheres in culture. Clonal spheres also developed in AP cell cultures. Spheres showed an expression pattern resembling that of Sca1hi SP cells. Moreover, the sphere-initiating cells of the pituitary segregated to the SP and not to the MP. In conclusion, we show that the adult pituitary contains a hitherto undescribed population of cells with SP phenotype and clonal expansion capacity. These cells express (signaling) molecules generally found in stem/progenitor cells and/or operative during pituitary early-embryonic development. These characteristics altogether are supportive of a stem/progenitor cell phenotype. Copyright © 2005 by The Endocrine Society.