Topographic organization of the peripheral projections of the trigeminal ganglion in the fetal rat.

Retrograde tracing with true blue (TB) and diamidino yellow (DY) was used to determine the topography of the peripheral projections of the trigeminal (V) ganglion in rats on embryonic day 16 (E-16; E-0 was the day of conception). On E-16, the earliest age at which we were able to accomplish retrograde tracing successfully, the topographic organization of the V ganglionic projection to the periphery was quite adult-like. Cells projecting to the vibrissa pad were restricted to the ophthalmic-maxillary portion of the ganglion, with those innervating dorsal row follicles located medially and those supplying ventral row follicles located laterally. Injections of tracer into ophthalmic skin and/or the cornea labeled cells that were tightly clustered in the most dorsal and anteromedial portion of the ophthalmic-maxillary region. Injections of tracer into the lower jaw or the skin just rostral to the ear labeled cells that were restricted to the lateral, mandibular part of the ganglion. None of the combinations of injections we carried out resulted in large numbers of double-labeled V ganglion cells. Injection of TB into the vibrissa pad and DY into the upper lip produced a small number of double-labeled ganglion cells. This was also the case for paired injections of TB and DY into the lower jaw and lip, respectively. No more than 15 such cells were observed in a ganglion. These findings suggest that the substantial cell death that has been reported to occur in prenatal V ganglion development (Davies and Lumsden, 1984) is probably not involved in the correction of major peripheral targeting errors by the axons of V ganglion cells.