A) Adrenal development from fetal adrenal to definitive adrenal. Note colors of respective zones: capsular cell, green; fetal zone, red; medulla, yellow; subcapsular/progenitor cell, black; definitive zone, gray. B) Definitive adrenal cells arise from fetal adrenal cells. Model based on work by Zubair, et al. (2006, 2008). C) Definitive adrenal cells arise from capsular stem/progenitor cells (blue) or subcapsular stem/progenitor cells (black), based on work by King, et al. (2009), Huang, et al. (2010), and Ching, et al. (2009). D) Proposed unifying model of adrenal stem cells in which fetal adrenal cells give rise to a transition cell in the capsule which in turn gives rise to the definitive adrenal cells.