Innervation of the early pelvic fin bud of the trout embryo, Salmo gairdneri.

Innervation of the early pelvic fin bud in the trout embryo involves four nerves. Electron microscopy discloses axons in the mesodermal mesenchyme and in the epidermis of the bud as early as stage I of the development of the pelvic fins. Sensory axons alone penetrate the epidermis. Unmyelinated axons invade the pelvic fin territory before the bud is obvious on the abdomen. Schwann cells occur in the vicinity of the ventral edge of the myotomes and later in the core of the bud and in subepidermal regions. Consequently, the nerve fibers are present early in the development of the pelvic fin bud of the trout embryo. Although the role of these axons is unknown, it is speculated that they play a role in development. Our results are discussed in the light of data available in the literature dealing with the development of tetrapod appendages.