Transcriptional and cellular maturation of the chick spinal cord in the context of distinct neuromuscular circuits.

The chicken spinal cord is a classic model system to study the early specification of neuronal cell types along its anterior-posterior axis. Here, we follow the ensuing maturation dynamics at limb levels with single-cell resolution and contrast neuronal populations innervating appendages of distinct form and function. We use gene co-expression modules to identify rare cell populations with specific biological functions, and show that appendages with different motor outputs - wings and legs - rely on largely similar spinal cord cell type repertoires. Challenging the system with experimental alterations to the peripheral limb musculature reveals limited transcriptional changes, but spatially restricted plasticity in spinal cord motor neuron numbers. Collectively, our results provide a resource to investigate the molecular and cellular basis of neuronal maturation in the avian spinal cord and highlight the plastic nature of embryonic cells to adapt to changes in the limb periphery at both developmental and evolutionary timescales.