Oxygen-sensitive neuroendocrine (NE) cells (yellow) associated with blood vessels (red) serve as sensors for internal oxygen levels (top). These include catecholaminergic cells in an ancestral structure in non-amniotes like fish (left), and the glomus cells in the carotid body of amniotes like humans (right). Other neuroendocrine cells act as sensors for external oxygen (bottom). These include cells in the gills of fish (left) and the airways of amniotes (right). Hockman et al. propose that the clusters of catecholaminergic cells near the blood vessels in non-amniotes evolved into the carotid bodies of amniotes. The neuroendocrine cells in these internal sensors are all derived from the neural crest. By contrast, the external oxygen sensors in gills and airways are derived from the endoderm in both non-amniotes and amniotes. Neurons are shown in blue; accessory cells are shown in gray. NEB: neuroendocrine body.