A transitory population of substance P-like immunoreactive neurones in the developing cerebral cortex of the mouse

Immunocytochemical methods were used to investigate the developmental expression of substance P (SP) in mouse cerebral cortex. SP-like-immunoreactive cells were first detected at postnatal day 0 (P0), their numbers being notably increased by P2. Immunopositive cells were especially abundant in layer VIb and in the subjacent future white matter, although they were also present in layer V. Between P5 and P8 the number of SP-like-immunoreactive cells gradually decreased, being almost completely absent by P12. At these stages cells were only observed in the deepest cortical layers. From P16 onwards, the adult pattern of SP-like immunoreactivity emerged with a few immunopositive cells scattered throughout the cortical layers. The present data show a transitory population of SP-like-immunoreactive cells present in the mouse cerebral cortex during the first postnatal week. On the basis of close correlations of SP-like expression with the distribution or transitory populations and the timing of cell death in rodents, we propose that most of the SP-like-immunoreactive cells reported here would probably disappear by cell death.