Changes in size and number of spinal motoneurons in relation to growth of the musculature in the eel, Anguilla.

Estimates of the numbers of spinal motoneurones in relation to growth in the eel, Anguilla, were made by counting the axons contained within the ventral roots. Motoneuronal size was determined as the surface area of cell somata labelled retrogradely from applications of tracer (horseradish peroxidase, cobalt lysine) to the musculature or spinal nerves. The sizes and numbers of muscle fibres from red and white portions of the myotomal musculature were determined from frozen sections. Measurements of motoneurone size and number in relation to the size and number of the muscle fibres were obtained from the same body region for eels of body lengths ranging from 65 to 930 mm and for 3 different stages of the eel's life history (glass, yellow and silver eels). The mean sizes and the numbers of red and white muscle fibres, and motoneuron size, increase substantially in relation to body length; the numbers of axons in the ventral roots of fish of different body lengths, however, are very similar (approx. 130 per half segment). Silver eels are indistinguishable from yellow eels of a similar size with respect to the size and number of motoneurone somata and axons and to the number of muscle fibres. In silver eels red muscle fibres are similar in size to those of yellow eels, but the white fibres are larger.