The development and postnatal organization of motor nuclei in the rat thoracic spinal cord.

The topographic organization of the motor nuclei supplying individual thoracic muscles in postnatal rats was investigated by horseradish peroxidase (HRP) labelling from the periphery. To determine whether aspects of the same organization are apparent at early developmental stages, we used HRP to label neurons contributing axons to the two primary rami of thoracic nerves in rat fetuses ranging in age from the 13th day of gestation to birth. This developmental period includes stages during which peripheral nerves and muscles are forming. The results show that motoneurons in the appropriate positions in the ventral horn have axons in each primary ramus even at stages prior to the innervation of muscle. However, in fetuses at certain developmental stages, there are also neurons located outside the motor nuclei which send axons into the peripheral nerves.