Mesencephalic innervation of the vibrissal follicle–sinus complex in the mouse embryo

Peripheral projections of neurones whose cell bodies lie in the mesencephalic nucleus of the fifth cranial nerve, situated between the central grey and mesencephalic reticular formation, were studied in mouse embryos aged between day 9 and 15 and in postnatal day 1 mice. Nonspecific neural antibody staining allowed visualisation of the developing cranial nerves, in particular the descending mesencephalic tract. This facilitated successful dissection of the descending mesencephalic tract and trigeminal ganglion in the heads of fresh mouse embryos and postnatal mice. The fluorescent dye, 1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI), was injected into the descending mesencephalic tract in mouse embryos aged 12.5, 13.5 and 15 days of gestation and also into postnatal day 1 mice. Following a period of incubation, 100 μm sections were viewed under visible light and episcopic fluorescence. Mesencephalic neurones were observed to pass superiorly over the trigeminal ganglion and enter the maxillary division to innervate vibrissal follicle–sinus complexes, whilst none was observed innervating mandibular and maxillary intraoral structures. There was no fluorescent labelling in non‐DiI injected control specimens. Using a highly specific neuronal tracer, this study shows that mesencephalic neurones in the periphery project exclusively to follicle–sinus complexes in the developing mouse embryo and remain at least until postnatal day 1. These observations, contrary to those made in other animals, indicate a species specificity of mesencephalic peripheral projections.