Localizing the corticospinal neurons in neonatal, developing and mature albino rat

Following the administration of horseradish peroxidase (HRP) in the middle or lower cervical cord segments of neonatal (1-10 days), developing (15-20 days) and mature (2-4 months) albino rats, labeled pyramidal neurons were found in layer V of the cerebral cortex. They formed one continuous band of labeled neurons in the frontal and parietal isocortex in neonatal animals. The band began close to the frontal pole and extended distally for about 3600 microns. Mediolaterally the lateral limit of the band was at the dorsal lip of the rhinal sulcus. Neurons in the 'shoulder' region along the medial surface of the cerebral hemisphere were also labeled. As the animals increased in age it became increasingly more difficult to label the pyramidal neurons in the parietal band. The band appeared broken and discontinuous in the parietal region in the 15-day-old rats. In the 20-day-old and mature rats the labeled neurons appeared in two bands, a dorsolateral one and a parietal one, the two being separated by a variable gap of cortex where practically no or very few neurons were labeled. Labeled neurons in the 'shoulder' region were distinctly seen only in the rostral region of the frontal isocortex in older animals. Following the administration of HRP in lumbosacral cord segments, no labeled layer V pyramidal neurons were seen in the 2-day-old rats, but they were seen to form one continuous band in the dorsolateral part of the cerebral cortex in rats aged 5 days postnatally and older. They appeared in a caudo-rostral sequence, with neurons in the caudal part of the cerebral cortex projecting to the more caudal cord segments and those in the frontal part to the more rostral cord segments. Mediolaterally the band was about 2 mm wide. Neurons in the 'shoulder' region were infrequently seen in neonatal and developing animals.