Dimensions and branching patterns of triceps surae alpha-motor axons and their recurrent axon collaterals in the spinal cord during the postnatal development of the cat.

Triceps surae alpha motoneurons in the cat were stained intracellularly with horseradish peroxidase (HRP) at different postnatal ages from birth to the adult stage. The motor axons and axon collaterals were studied with regard to length, diameter and branching pattern. The postnatal increase of internodal length, measured as the distance between two subsequent axon collateral origins, was about 100% which paralleled the total length increase of the main axon in the grey matter. The axon collaterals were unmyelinated at birth and branched exclusively dichotomously until after 3 weeks of age when a substantial fraction of the branching points gave off 3-5 daughter branches. This was interpreted as signs of a fusion between neighboring branching points during the period of myelination of the axon collaterals. The length analysis of the collaterals indicated that the postnatal elimination of collateral branches described previously is preferentially located in the distal parts of the collateral tree.