GAP-43 expression in developing cutaneous and muscle nerves in the rat hindlimb

The expression of the growth associated protein, GAP-43, in developing rat hindlimb peripheral nerves has been studied using immunocytochemistry. GAP-43 is first detected in lumbar spinal nerves at embryonic day (E)12 as the axons grow to the base of the hindlimb. It is expressed along the whole length of the nerves as well as in the growth cones. GAP-43 staining becomes very intense over the next 36 h while the axons remain in the plexus region at the base of the limb bud before forming peripheral nerves at E14. It remains intense along the length of the growing peripheral nerves, the first of which are cutaneous, branching away from the plexus and growing specifically to the skin, their axon tips penetrating the epidermis of the proximal skin at E15 and the toes at E19. GAP-43-containing terminals form a dense plexus throughout the epidermis which subsequently withdraws subepidermally in the postnatal period. GAP-43 staining is also evident along the growing muscle nerves during muscle innervation, which follows behind that of skin. Axons branch over the surface of proximal muscles at E15 but do not form terminals until E17. As target innervation proceeds, GAP-43 staining declines in the proximal part of the nerve but remains intense in the distal portions. Overall GAP-43 expression in the hindlimb decreases in the second postnatal week as axon growth and peripheral terminal formation decline.