Pituitary stem cells drop their mask

The pituitary gland represents the organism's endocrine hub, integrating central and peripheral inputs to generate the appropriate hormonal signals that govern key physiological processes. To meet the changing endocrine demands, the gland has to flexibly remodel its hormone-producing cell compartment. Mechanisms underlying pituitary cellular plasticity, as well as homeostatic turnover, are poorly understood. Similar to other tissues, resident stem cells may participate in the generation of newborn cells. Although in the past recurrently postulated to exist, pituitary stem cells remained obscure until the quest recently regained momentum, resulting in a surge of studies that designated very strong candidates for the stem/progenitor cell position. The cells identified express stem cell-associated markers and signaling factors, as well as transcriptional regulators that play essential roles during pituitary embryogenesis. They exhibit the stem cell properties of multilineage differentiation and prominent efflux capacity ("side population" phenotype), and display a topographical pattern reminiscent of niche-like configurations. Yet, the stem cell tenet of long-term self-renewal remains to be unequivocally demonstrated. Taken together, pituitary stem cells commence to drop their mask. While their "face gradually becomes visible, the "character" they play in the pituitary awaits further disclosure. The aim of this review is to highlight the recent progress in pituitary stem/progenitor cell identification by sketching the historical context, describing the new findings with inclusion of critical and cautionary reflections, proposing a tentative stem/progenitor cell model, and pointing out remaining gaps and challenges. The recent acceleration in pituitary stem cell research may announce an exciting era in this endocrine field.