Combinatorial Developmental Controls on Striatonigral Circuits.

Cortical pyramidal cells are generated locally, from pre-programmed progenitors, to form functionally distinct areas. By contrast, striatal projection neurons (SPNs) are generated remotely from a common source, undergo migration to form mosaics of striosomes and matrix, and become incorporated into functionally distinct sectors. Striatal circuits might thus have a unique logic of developmental organization, distinct from those of the neocortex. We explore this possibility in mice by mapping one set of SPNs, those in striosomes, with striatonigral projections to the dopamine-containing substantia nigra pars compacta (SNpc). Same-age SPNs exhibit topographic striatonigral projections, according to their resident sector. However, the different birth dates of resident SPNs within a given sector specify the destination of their axons within the SNpc. These findings highlight a logic intercalating birth date-dependent and birth date-independent factors in determining the trajectories of SPN axons and organizing specialized units of striatonigral circuitry that could influence behavioral expression and vulnerabilities to disease.