Restricted fiber growth from substance P-immunoreactive brain neurons grafted to the anterior eye chamber of the rat.

Substance P (SP)-immunoreactive cell groups from fetal rat brain were homografted to the anterior eye chamber and left there for 2 months to mature, to elucidate their capacity to form nerve fibers in this ectopic environment. All grafts survived well, were rapidly revascularized from the host iris and proliferated to final sizes 2- to 4-fold the volume at transplantation. In spite of a rich SP fiber innervation of the transplant neuropil, no or almost no SP-positive fibers reached out from the transplant on to the iris. Monoamine histochemical analysis of the same locus coeruleus transplants revealed abundant fluorescent locus-derived nerve fibers of the host irides. Thus, a marked difference in growth behavior between noradrenergic and SP-immunoreactive neurons from the same brainstem region was found.