Motor neurons and motor columns of the anterior spinal cord of salamanders: posthatching development and phylogenetic distribution.

The posthatching development of rostral (1st-4th) spinal motor neurons was studied in ten species of salamanders, using horseradish peroxidase and cobaltic lysine tracing techniques. Development of spinal motor neurons differs among species in association with differences in life history and general developmental patterns (i.e., between species with aquatic larvae versus those with direct development, with or without ontogenetic repatterning). In the plesiomorphic state, represented by species with aquatic larvae, five types of motor neurons are present: (1) large, multipolar neurons, believed to be primary motor neurons; (2) medial, pear-shaped neurons; (3) larger, spindle-shaped neurons, which increase in number during posthatching development; (4) cone-shaped neurons, and (5) bilaterally arborizing neurons (found only at the rostral pole of the first spinal nucleus). Direct-developing desmognathine salamanders have the plesiomorphic set of motor neurons, but appear to lack Mauthner neurons. Direct-developing plethodontine salamanders have cone-shaped, pear-shaped, and spindle-shaped neurons, but lack primary motor neurons and Mauthner neurons. Direct-developing bolitoglossine salamanders, which exhibit both pedomorphosis and ontogenetic repatterning, have only medial, pear-shaped neurons, and lack primary motor neurons, spindle-shaped neurons, cone-shaped and bilaterally arborizing neurons. At all developmental stages in all species studied, pear-shaped neurons are always found in medial positions and spindle-shaped neurons are always found in lateral positions. Spindle-shaped neurons are found more laterally as development proceeds. The medial and lateral motor columns of salamanders and amniotes differ in their connections with peripheral targets (i.e., axial muscles vs. limbs). This implies a lack of homology of neuron types in salamanders and amniotes, which has been obscured by the current terminology.