Phox2a Defines a Developmental Origin of the Anterolateral System in Mice and Humans

Anterolateral system neurons relay pain, itch, and temperature information from the spinal cord to painrelated brain regions, but the differentiation of these neurons and their specific contribution to pain perception remain poorly defined. Here, we show that most mouse spinal neurons that embryonically express the autonomic-system-associated Paired-like homeobox 2A (Phox2a) transcription factor innervate nociceptive brain targets, including the parabrachial nucleus and the thalamus. We define the Phox2a anterolateral system neuron birth order, migration, and differentiation and uncover an essential role for Phox2a in the development of relay of nociceptive signals from the spinal cord to the brain. Finally, we also demonstrate that the molecular identity of Phox2a neurons is conserved in the human fetal spinal cord, arguing that the developmental expression of Phox2a is a prominent feature of anterolateral system neurons.