Electroreceptor development in the electric fish Eigenmannia: a histological and ultrastructural study.

Developmental patterns of electroreceptors in the weakly electric fish Eigenmannia were investigated by histological, histochemical, immunocytochemical, cell kinetic, ultrastructural, and computer-assisted three-dimensional reconstruction methods. The first cell of an electroreceptor primordium is embedded in the stratum germinativum of the epidermis. An unmyelinated, afferent nerve fiber ends near this cell below the basal lamina. Protrusions and vacuole-like inclusions at the basal lamina above the nerve fiber ending suggest a mechanism of nervous induction. The receptor primordium cell subsequently divides into a single cell layer. Within 48 hours, a second apical cell layer forms from the first, and, thus, the primordium differentiates into an apical layer of presumptive receptor cells and a basal layer of presumptive supporting cells. While the two layers further differentiate into mature receptor and supporting cells, the afferent fiber penetrates the basal lamina, sprouts, and forms a synapse with each receptor cell. Transitory fibers also project along the receptor cells to the top of the developing electroreceptor but degenerate during development. Synapses are smaller in early developmental stages compared to older stages, and pre- and postsynaptic vesicles are more abundant and widely distributed in younger stages. Moreover, presynaptic ribbons are longer and are interconnected at their apical ends. Supporting cells continue to divide during further maturation and form new receptor cells. The number of receptor cells per tuberous organ increases during the first 4 days of electroreceptor development and plateaus when the fish are 9 days old. It declines again when organs begin to divide into clusters.