MEDINA, C.L.; KOESTER, D.M.*; LUER, C.: Development of the electric organ of the skate, Raja eglanteria

The electric organ of Raja eglanteria consists of paired, longitudinal columns of electrocytes within the lateral musculature of the tail. Although it is known that electrocytes develop from embryonic hypaxial muscle fibers within the tail, little information exists on the stages of this differentiation process. Examination of H&E stained paraffin sections through the tail of skate embryos sampled at weekly intervals (hatching occurs around week 12) and at 3 months post-hatching, reveals that each electrocyte develops from a single skeletal muscle fiber. Prior to the 4th week of development the tail consists exclusively of skeletal muscle fibers oriented in a rostrocaudal direction. During the 4th week some of the skeletal muscle fibers begin to differentiate into electroblasts. Electroblasts are distinguished from adjacent skeletal muscle fibers by an increase in number of nuclei and a shortening of fiber length. As development proceeds (weeks 6-8), electroblasts continue to shorten as the rostral end expands to form a club-shaped cell. In addition, nerve fibers appear at the rostral end of each electroblast indicating formation of the motor end plate. Further expansion of the rostral end and regression and thinning of the caudal end result in the formation of a thick, transversely oriented disc-like cell by the 10th week. The 3 layers of a typical disc-shaped electrocyte, which include the electric plate, striated, and alveolar layers, are easily identified at this time. Further development of the electrocytes up to and after hatching involves an expansion and thinning of the disc in the transverse plane. Electrocytes of Raja eglanteria are fully differentiated by 3 months of age.