Constantinescu, A. M.; Fabrizi, L.; Koch, S. C.

Corticospinal projections from the primary somatosensory cortex (S1) form a distinct descending pathway that engages spinal dorsal horn circuits and modulates sensory processing. Despite advances in our understanding of the role of this pathway in the adult, the postnatal maturation of somatosensory corticospinal projections remains poorly defined. Here, we provide a quantitative anatomical analysis of the postnatal development of corticospinal projections from the hindlimb representation of S1 (S1hl) to the lumbar dorsal horn in mice. Using retrograde tracing, we show that lumbar-projecting S1hl corticospinal neurons are first detected at postnatal day (P) 9 and reach adult-like numbers in S1hl by P12. Using anterograde tracing we then show that S1hl CST axonal projections are initially confined to the dorsolateral funiculus when they reach the lumbar cord at P9, but then rapidly invade the lumbar dorsal horn, reaching peak grey matter terminal density at P14. During this early innervation period, projections transiently extend beyond their mature termination zones in laminae III-V before retracting and becoming confined to superficial laminae I-II by P17. Together, these findings define three developmental phases of somatosensory corticospinal dorsal horn connections: an initial arrival phase, followed by grey matter ingrowth, and finally laminar refinement of the terminal projections. These results provide an anatomical framework for understanding how descending corticospinal somatosensory control becomes integrated into spinal circuits during the early postnatal period.