Counts of axons in electron microscopic sections of ventral roots in lampreys

The number of motoneurons in the lamprey spinal cord was estimated by counting axons in electron microscopic sections of ventral roots. Physiologically identified and marked motoneurons had one axon each in an ipsilateral ventral root. The corresponding axons in sections were unmyelinated, individually ensheathed, and 2-20 micron in diameter. Lampreys lack an organized sympathetic nervous system, but a minor and variable population of small axons with dense-cored vesicles was also present in ventral and dorsal roots. The estimated numbers of motor axons per ventral root in adult Ichthyomyzon unicupis and I. castaneus were 70.6 +/- 9.3 (mean +/- SD), in adult landlocked Petromyzon marinus 100.1 +/- 15.0, and in large ammocoetes of anadromous Petromyzon marinus 106.8 +/- 14.3. The total estimated numbers of myotomal and fin motoneurons were as many as 13,000 in the spinal cord of adult Ichthymyzon and 24,000 in adult Petromyzon.