The topogenesis of the hindlimb nerves of Splotch homozygous mutant mouse embryos was studied using light and electron microscopy. Homozygous mutants show multiple defects of neural crest-derived tissues. The defects increase along a rostro-caudal gradient. The cervical and upper thoracic segments have small spinal ganglia, and Schwann cells are associated with the spinal nerves. In the lumbo-sacral region neurulation is not complete, and the derivatives of the neural crest are missing. The lumbo-sacral nerve trunks are formed by ventral roots only. They are occasionally associated with presumptive glial cells that have migrated from the spinal cord for a short distance. Beyond the vertebral primordia, the spinal nerves are not accompanied by Schwann cells. No compartmentalization of the axons within the lumbo-sacral nerves was visible, whereas Schwann cells did segment the nerve into the fascicles in brachial nerves. The lumbo-sacral plexus develops, and its branches grow into the hindlimb despite the absence of Schwann cells. On day 13.5 of gestation, the lumbo-sacral nerve trunks extend well into the distal calf. They are topographically correctly positioned. Their branches enter the muscle primordia and form contacts with their mesenchymal cells though the cutaneous branches are missing. Generally, the outgrowth of lumbo-sacral nerves is slower than in phenotypically normal littermates, whose nerves reach the foot plate at corresponding stages of development. These results demonstrate that the lumbo-sacral plexus and the topographically correct position of lumbo-sacral nerve trunks develop despite the absence of Schwann cells. Therefore Schwann cells are not necessary for the outgrowth and guidance of axons within the limb.
