Fig. 3.

Stem cells during embryogenesis and adulthood. Pituitary stem cells differentiate during embryogenesis and form the pituitary lineages (14). These cells evolve from SOX2 to SOX9 positivity. No other marker has been evaluated during embryogenesis. Nestin does not seem to play a role at this time point because nestin⁺ cells begin to proliferate after birth (16). In adults, rare cells express only SOX2. The majority of adult “progenitors” are SOX2⁺, SOX9⁺ (14). It is likely that the same cells express GFRa2 (15), AMCA, and angiotensin-converting enzyme (ACE) (55) and have a little or no SCA1 expression. These cells are probably transit-amplifying cells and are capable of limited self-renewal. Nestin may be expressed in a subset of these cells, although it is not a specific marker of pituitary cells (16). Final differentiation requires spatiotemporal regulation by several transcription factors and signaling pathways (review in Ref. 40). Diff. cell, Differentiated cell.