We and others have shown previously that the evolutionarily

We and others have shown previously that the evolutionarily conserved Notch signaling pathway plays an important role in early embryonic pituitary development (Kita et al., 2007; Raetzman et al., 2004, 2007; Zhu et al., 2006). Delta/Notch signaling, mediated by the critical transcription factor RBP-J, acts to prevent progenitor prostaglandin receptor in the RP from premature differentiation through Hes1, one of the downstream target genes of the Notch pathway. It also controls the competence of progenitor cells by maintaining expression of the Prop1 gene, which encodes a pituitary-specific, paired-like homeodomain transcription factor necessary for the commitment of the PIT1 lineage of three cell types—somatotropes, thyrotropes, and lactotropes. In the absence of canonical Notch signaling, resulting from deletion of the Rbp-J gene at embryonic day (E) 10.5 in the RP using Pitx1-Cre transgenic mice, the progenitors adopt an early-born corticotrope cell fate at the expense of the late-arising PIT1 lineage (Kita et al., 2007; Raetzman et al., 2007; Zhu et al., 2006). Interestingly, the proliferating progenitors, residing in the periluminal region, are still present at the end of embryonic development in the mutant pituitary gland (Zhu et al., 2006). However, the mutant animals died of cleft palate shortly after birth because of broad expression of Pitx1-Cre in the oral ectoderm (unpublished data), leaving an open question regarding whether continued Notch signaling is required to maintain these pituitary progenitors in the postnatal period. Recently, it has been suggested that Notch signaling is required for progenitor maintenance based on deletion of the Notch2 gene in the embryonic RP. However, despite a progressive decrease in the number of pituitary progenitors, these cells remain in the postnatal gland in this animal model, particularly in the anterior lobe (Nantie et al., 2014). An animal model with specific and complete depletion of Notch signaling is required to provide an unambiguous answer. At birth, all of the endocrine cell lineages are present in the mouse pituitary gland, but the gland continues to grow and mature substantially after birth, particularly during the first few postnatal weeks. It has been documented that this postnatal pituitary gland expansion in the rat is only partially brought about via proliferation of preexisting differentiated hormone-producing cells (Carbajo-Pérez and Watanabe, 1990; Taniguchi et al., 2000, 2001a, 2001b, 2002). Double immunolabeling of hormone and proliferation markers reveals that 10%–30% of the proliferating cells are differentiated endocrine cells, implying that some of the postnatal proliferation might take place in undifferentiated cells. On the other hand, the mature pituitary gland has a low turnover rate under basal conditions (Florio, 2011). However, one important feature of the pituitary gland is its plasticity. The cellular composition of the mature gland can change flexibly to adapt to the physiological or pathological demands of the organism (Levy, 2002). Recently, postnatal pituitary stem cells have been identified based on expression of a variety of stem cell-specific markers, including SOX2, SOX9, E-Cadherin, NES, and the pituitary-specific transcription factor LHX3 (Chen et al., 2009; Fauquier et al., 2008; Garcia-Lavandeira et al., 2009; Gleiberman et al., 2008; Rizzoti, 2010; Vankelecom and Chen, 2014). These cells are localized in the marginal region between the intermediate lobe and the anterior lobe, and, when cultured in vitro, they are capable of self-renewal and differentiation into diverse hormone-producing pituitary cell types, implying their “stemness.” In vivo characterizations of these SOX2+ cells have shown that they are most abundant in the neonatal pituitary gland (Gremeaux et al., 2012). Recent studies of cell ablation of terminally differentiated cells have suggested that these SOX2+ cells may contribute to pituitary regeneration (Fu et al., 2012; Fu and Vankelecom, 2012). In addition, lineage tracing of SOX2+, SOX9+ cells have provided genetic evidence that these cells can contribute to organ homeostasis and tissue adaptation (Andoniadou et al., 2013; Rizzoti et al., 2013). Expression of the Notch signaling component in postnatal stem cells has been described previously (Chen et al., 2006, 2009; Nantie et al., 2014; Tando et al., 2013; Vankelecom and Gremeaux, 2010). Interfering with Notch activation affects stem cell number in pituitary primary culture, implicating a critical role of Notch signaling in stem cell proliferation (Chen et al., 2006; Nantie et al., 2014; Tando et al., 2013). However, whether Notch signaling is essential for pituitary stem cell proliferation in vivo and whether these cells are necessary for any normal physiological pituitary functions remains unknown.