The neuromuscular system in the trunk of larval and adult zebrafishes was studied by means of light and electronmicroscopical methods. Spinal motoneurons were identified with the horseradish peroxidase retrograde transport method. We correlated the differentiation and growth of the myotomal muscle with the number of motoneurons per spinal cord segment and the size of the motoneuron somata. The adult number of motoneurons is reached in an early larval stage, before the muscle fiber type differentiation in the myotomes is completed. The mean motoneuron size does not bear a clear correlation with the size of the myotomal muscle. In adult zebrafishes we could distinguish the motoneurons which innervate the superficial slow red and the deep fast white muscle fibers on the basis of soma size and position in the motor column. The motoneurons of the red muscle part are small; they are located in the ventrolateral part of the motor column. The motoneurons of the deep fast white fibers are large; they lie near the central canal.

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 motoneuron somata, ventral root axons, and red and white muscle fibers increase their size throughout the life of the animal; there is an initial increase in the number of motoneurons that ceases as the fish reaches a length of about 40 cm; white muscle fibers initially decrease in number during post-embryonic life; and 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.

No abstract

As a working hypothesis it is supposed that in the teleost Brachydanio rerio, the muscle contractions, the growth, and probably some other factors, cause the first changes in the shape of the somites. Furthermore, the movements of the embryo could yield the forces by which the somites are brought to their theoretically optimal shape.In order to test this hypothesis, we investigated the somite shape and structure of spontaneously immobile embryos. Although the results are difficult to interpret, they certainly do not contradict the hypothesis.For further analysis we applied two kinds of lesions in order to immobilize early embryos: removal of the brain, and damage to the midbody somites. The results of these experiments indicate that both the development of the shape of the somite and the arrangement of the muscle fibres are dependent on movements of the embryo.