The development of cutaneous innervation in the rat and the mouse has been studied by silver techniques. In particular, the innervation patterns of adult and of perinatal skin have been compared. From the second prenatal day until the sixth postnatal day, the relatively thick epidermis is freely penetrated by nerve fibers; thereafter, the nerve endings appear to be confined to the dermis. The intraepidermal axans are directed obliquely, with a rough (and not exact) correspondence to the plane of future emergence of the hair shafts. The appearance of a transitory intra-epidermal innervation in rodent hairy skin is interpreted in the light of the development of intra-epidermal nerves elsewhere.In the perinatal period the surface epithelium of mouse and rat skin undergoes a pronounced but transient architectural change. The germinal cells which lie beneath the fetal periderm multiply some five days before birth to form a deeply stratified epidermis with basal, spinous, granular and horny elements. Within two weeks after birth the epidermis assumes its adult form by reversion to a double layer of cells, thinly covered by keratin (Glucksmann, '45; Butcher, '51; Slee, '62). This paper reports and discusses the associated changes in epidermal innervation.
MATERIAL AND METHODSForty-two albino mice of BALB or Schneider strains, and 40 Wistar rats, were used. Thirty mice were sacrificed between the sixteenth day of gestation and the eleventh postnatal day, inclusively, to show the daily development of cutaneous innervation. Thirty-two rats provided daily material for the same period. The remaining 20 animals supplied adult skin for comparison.Following an overdose of chloroform, skin pieces from the mid-dorsal trunk of all the younger animals were mounted flat on filter paper to prevent curling, and fixed in alcoholic Bouin's solution. The older skin did not retract and the pieces were fixed without mounting. The nerves were studied i n serial 15 CI paraffin sections after staining with a double-impregnation protargol method (FitzGerald, '64). In addi-J. COMP. NEUR., 126: 37-42.