DISTRIBUTION OF CORTICOSPINAL MOTOR-NEURONS IN THE POSTNATAL RAT - QUANTITATIVE EVIDENCE FOR MASSIVE COLLATERAL ELIMINATION AND MODEST CELL-DEATH

The postnatal development of rat corticospinal motor neurons (CSMN) wa s studied by retrograde tracing with cholera toxin B subunit (CTB) inj ected into the upper cervical dorsal spinal cord on the first postnata l day (P0), P3, P10, P20, and at adulthood. CTB-labeled neurons were v isualized by immunocytochemistry and extensively quantified throughout the cortex. At P0, CSMN were found to an extent similar to that repor ted in P3 animals with other neuronal tracers, now permitting in vitro studies of neonatal CSMN. Between P0 and P3, the number of labeled ne urons increased by 30% to a total maximum of approximately 185,000 in both cortices. The increase occurred throughout the cortex. At P10, th e number of labeled CSMN had decreased to 60% of the number at P3. Few er CSMN were evident particularly in the perirhinal cortex. Between P1 0 and P20, the number of CSMN decreased further to 52% of the maximal number at P3. This decrease occurred predominantly in the cingulate an d parietal cortex. The number of labeled CSMN in rats injected at P0 a nd analyzed at P20 was 10% lower than the number in P0-injected litter mates that were analyzed at P3, which suggests that only a small porti on of the ''disappearing'' CSMN undergoes developmental neuronal death . Thus, the spinal projection of the remaining 38% is apparently elimi nated between P3 and P20. Detailed quantitative analysis of the CSMN d istribution demonstrated that neuronal death occurs predominantly in t he perirhinal cortex. In contrast, axonal elimination of corticospinal projections occurred throughout the CSMN field, i.e., primarily in th e frontal, occipital, and perirhinal cortex between P3-P10 and in the cingulate and parietal cortex between P10-P20. (C) 1994 Wiley-Liss, In c.