Neonatally wounded skin induces NGF-independent sensory neurite outgrowth in vitro.

An in vitro model was established to investigate factors underlying the sensory hyperinnervation of neonatal rat skin wounds that has been observed in vivo (Reynolds and Fitzgerald, J. Comp. Neurol. 358 (1995) 487-489). Explants of normal and wounded rat dorsal foot skin were co-cultured with explants of embryonic chick or newborn rat dorsal root ganglia for 24 h and the number of sensory neurites counted. Explants of skin surrounding a wound made at birth were taken 3 (P3) or 10 (P10) days later and compared with normal skin of the same age. In addition, explants were taken from adult skin wounded 3 and 10 days earlier. At P3, normal skin induced weak neurite outgrowth (mean 13.1 +/- 2.1 neurites per ganglion explant) but skin that had been wounded 3 days earlier, at birth, induced three times more neurite outgrowth (37.8 +/- 3.3). Ten days after wounding at birth, neurite outgrowth was still substantial (40.9 +/- 3.3) although at that age (P10), even normal skin stimulates substantial growth (37.4 +/- 2.9). Normal adult skin also stimulated neurite outgrowth (28.7 +/- 0.45) but this was not increased by wounding 3 or 10 days earlier, and this was enhanced 3 days but not 10 days after wounding. Anti-NGF (nerve growth factor) added to the culture medium blocked the constitutive neurite stimulating activity from normal P10 and adult skin but was ineffective in blocking the neurite stimulating activity produced by neonatal wounding. It is concluded that skin wounding at birth results in release of one or more sensory neurotrophic factors that stimulate rat and chick dorsal root ganglia neurite outgrowth for at least 10 days, but which do not include NGF.