A quantitative analysis of the spinal motor pool and its target muscle during growth in the dogfish, Scyliorhinus canicula

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Abstract
In order to relate the phenomenon of continuous growth in fishes to the development of the neuromuscular system, we established the numbers and sizes of spinal motoneurons and red and white axial muscle fibers in late‐and post‐embryonic dogfish within the size range 6–71 cm. We found that (1) motoneuron somata, ventral root axons, and red and white muscle fibers increase their size throughout the life of the animal; (2) there is an initial increase in the number of motoneurons that ceases as the fish reaches a length of about 40 cm; (3) white muscle fibers initially decrease in number during post‐embryonic life; and (4) red muscle fibers increase in number, but this increase levels off at a fish length of about 40 cm. Spinal motoneurons innervating red myotomal muscle fibers or centrally located white muscle fibers were identified after retrograde labeling with horseradish peroxidase, which was injected in small amounts into the respective muscle areas. The motoneurons supplying the red muscle are smaller and occupy a more lateral position in the ventral horn than the white muscle motoneurons. The number of motoneurons in the ventral horn per unit area increases from medial to lateral and this is associated with a reduction in the sizes of the somata. Values for the ratio of number of muscle fibers to number of supplying motoneurons in the various muscle areas were based on axon counts within the peripheral nerves. This ratio was higher for the centrally located white muscle fibers than for the ventral or dorsal white fibers, but was highest for the red fibers.