Defining spinal motor neuron subtypes across development: from embryonic specification to postnatal maturation.

Spinal motor neurons are essential for translating neural activity into coordinated muscle contraction, yet defining their functional subtypes across development remains a persistent challenge. While embryonic patterning establishes the initial positional and molecular framework of motor neuron identity, substantial refinement continues during early postnatal life as intrinsic electrophysiological properties, synaptic connectivity, and neuromuscular interactions mature. A major limitation in the field is the lack of temporally stable and functionally validated molecular markers that can reliably distinguish motor neuron subtypes across developmental stages, particularly during neonatal maturation when subtype-specific physiological features are emerging. In this review, we synthesize classical developmental studies with recent advances in single-cell transcriptomics, chromatin accessibility profiling, and multimodal approaches linking gene expression with electrophysiological and anatomical features. Focusing on lumbar spinal motor neurons that underlie locomotor behavior, we discuss how transcriptional programs, activity-dependent mechanisms, and non-cell-autonomous signals converge to shape subtype-specific maturation trajectories. We propose that motor neuron subtype identity is best understood as a dynamic molecular and physiological state shaped by developmental timing, circuit context, and activity-dependent mechanisms, rather than as a fixed category defined by a single marker. From this perspective, early postnatal life represents a sensitive window of identity consolidation during which molecular programs and functional properties become aligned. Establishing temporally robust subtype markers and integrating molecular and physiological datasets will be essential for resolving motor neuron diversity and for improving our understanding of subtype-selective vulnerability in neuromuscular diseases. While this review emphasizes embryonic and early postnatal development, understanding how molecular subtypes stabilize in the adult spinal cord, despite ongoing activity-dependent physiological plasticity, remains an essential reference point for defining temporally robust motor neuron identities.