The development of the segmental pattern of skin sensory innervation in embryonic chick hind limb

1. The development of dermatomes in the chick hind limb was investigated with both electrophysiological recording from and horseradish peroxidase (HRP) labelling of neurones in lumbosacral dorsal root ganglia (d.r.g.s). The embryonic stages studied spanned the period before and after cell death.2. In mature embryos, after the bulk of cell death, physiological mapping showed that the location of the dermatome of each d.r.g. is consistent from embryo to embryo. HRP studies showed that axons from each d.r.g. project through the limb to the skin via a characteristic set of nerve trunks. Both the dermatomes and axonal projection pathways of adjacent d.r.g.s partially overlap one another, producing an orderly progression in the location of dermatomes and projection pathways along and within the limb, respectively.3. In younger embryos, before cell death, the location and amount of overlap of dermatomes, as well as the axonal projection pathways, are similar to the mature pattern. D.r.g.s that innervate distal skin on the shank and foot in mature embryos do not project out cutaneous nerve trunks in the thigh or contact nearby skin on the thigh at earlier stages.4. Axons from a single d.r.g. initially contact the skin at one or more characteristic spots; the dermatomes then enlarge, adding fine axonal branches.5. Carbon-marking experiments showed that there are no large distal migrations of skin on the limb during the stages studied.6. Together these findings show that dermatomes on the chick hind limb do not develop by skin sensory axons simply growing to the nearest available skin, nor are axons towed to their final location by skin movements. Moreover, dermatomes are not shaped by cell death and the elimination of random or excessive axonal projections in the limb or the skin. It appears that skin sensory axons from each d.r.g. grow directly to their target skin along a defined set of pathways and establish their dermatome precisely at its characteristic location.