Figure 8. Model for the evolution of the hypoxia-sensitive cells involved in amniote respiratory reflexes.
(a) Schematic ancestral vertebrate with internal gills. Neural crest-derived (magenta) chromaffin cells are associated with large blood vessels in the pharyngeal arches, while NECs differentiate within the endoderm-derived (green) epithelium lining the gills and orobranchial cavity. (b) During the transition to terrestrial life, the glomus cells of the carotid body evolved via the aggregation of neural crest-derived chromaffin cells (which must also have acquired serotonergic properties), while (c) NECs persisted as PNECs in lung airway epithelia. Yellow arrows indicate shared ancestry. Bv, blood vessel; NEC, neuroepithelial cell; PNEC, pulmonary neuroendocrine cell.