Development of a motor nerve in the caudal fin of the medaka (Oryzias latipes).

The entire process of the development of the motor nerve to the middle interradial muscle (MIR muscle) in the tail region of the medaka (Oryzias latipes) was examined by light and electron microscopy. The motor nerve to the MIR muscle was observed to develop in a series of four steps. (1) Trunk-formation step: from stage 30 to stage 31, when the MIR muscle had not yet differentiated to the myotube stage, the motor axons starting from the caudal spinal cord, including spinal segment 27, formed a nerve trunk, the ventral caudalmost nerve (VCN). The VCN bypassed the place where the MIR muscle would differentiate and extended further caudad. (2) Wandering step: at stage 32, nerve processes of the motor axons (short branches) wandered from the course of the VCN. (3) Connection step: from stage 33/34 to stage 35, the mesenchyme cells differentiated into the MIR myotubes in the vicinity of the short branches, and the nerve and the muscle became connected. (4) Modification step: after stage 35, the morphological pattern of the motor nerve was modified by the growth and the shift of the short branches, and by the disappearance of the caudalmost part of the VCN. These results suggest that the complex pattern of the motor nerve in the adult fish is gradually constructed by multiple steps, each of which may have its own mechanism.