The size and number of neurons descending to the spinal cord in relation to body length in the European eel (Anguilla anguilla).

Cells of origin of descending spinal pathways in eels of different body lengths (15-59 cm) were identified by labelling them retrogradely with horseradish peroxidase from the rostral spinal cord. The aim was to determine whether changes in size, number and distribution of these neurons take place in line with changes in the size of the spinal targets that occur with increasing body length. The somal cross-sectional areas of the labelled cells were measured from horizontal frozen sections and counts were made of the axons within the ventral funiculus in 1 micron transverse sections of the pectoral spinal cord. The distributions and numbers of labelled cells in the brain, and the number of axons in the ventral funiculus in the pectoral spinal cord, were similar for all lengths of fish studied. The somata of most of the labelled cells in the brain increase in size in fish up to a length of 40 cm. Some reticulospinal neurons are individually identifiable and most of these show a distinct increase in size; however, the cells that have relatively large somata in small fish show no change in size as the fish lengthen.