Prenatal development of rat primary afferent fibers: I. Peripheral projection

Abstract

Development of the peripheral innervation patterns of the L1-S1 lumbosacral ganglia and motor segments in embryonic day 12-17 (E12-17) rat embryos was examined using carbocyanine dyes. Individual dorsal root ganglia (DRGs) and/or isolated ventral horn (VH) segments, or individual peripheral nerves, were isolated in rat embryos fixed at different stages and filled with one of three carbocyanine dyes; DiI, DiA, and DiO. Individual experimental preparations included labeling of 1) single DRGs; 2) multiple DRGs with alternating dyes, DiO, DiI, and DiA; 3) single isolated VH segments; 4) multiple VH segments with alternating dyes; 5) single VH segments and the corresponding segmental DRGs with different dyes; and 6) two or more individual peripheral nerves labeled with different dyes. Results from these preparations have shown that the first fibers exited the lumbar ventral horn and DRGs at E12. At E13 major nerve trunks (e.g., femoral and sciatic) were visible as they exited the plexus region. By E14 afferent fibers were present in the epidermis of the proximal hindlimb, and the major nerve trunks extended into the leg. Fibers originating from L3 to L5 (DRG and VH) reached the paw by E14.5-E15, and the epidermis of the most distal toes was innervated by E16-E16.5. While afferent fibers and motor axons of the same segmental origin mixed extensively in the spinal nerve, fibers of different segmental origin combined in the plexus and major nerve trunks with little or no interfascicular mixing. Dermatomes observed at E14 were in general spotty and non-contiguous. However, by E16 the dermatomes resembled mature forms with substantial overlap only between adjacent ones. Thus the adult pattern of spatial relationships between cutaneous afferent fibers in the periphery is established early in development.

The Generation of Neuronal Heterogeneity in a Rat Sensory Ganglion

“…CGRP mRNA was detected at E18.5 and P0, times at which the peptide can also be detected immunocytochemically and when functional central and peripheral connections are present in vivo. son et al, 1974;Mirnics and Koerber, 1995). A subpopulation of DRG neurons /neuron precursors from E12.5 DRG developed CGRP expression at 9 d in culture (Fig.…”

“…DiI injection and growthassociated protein-43 immunolabeling indicates some sensory afferents in the epidermis of the proximal hindlimb quite early, around E14, and some fibers reach the paw by E14.5-E15 (Reynolds et al, 1991;Coggeshall et al, 1994;Mirnics and Koerber, 1995), but it is not possible to quantify their prevalence. Physiological assays indicate that the first afferent terminals are formed later, at E17-E18 in hindlimb muscles (Kudo and Yamada, 1985;Kucera et al, 1988Kucera et al, , 1989, and cutaneous plantar reflexes appear at E17.5 (Kudo and Yamada, 1985).…”

“…Prespecified neurons could match with appropriate target through the growth of exuberant projections, followed by pruning of processes that contacted inappropriate targets. When one peripheral nerve is labeled in embryonic hindlimb, however, little label is detected in another nerve, suggesting that there is little branching of DRG afferents in the chick and rat (Honig, 1982;Payne et al, 1991;Mirnics and Koerber, 1995). Alternatively, neurons could send processes to various target tissues, and those that contacted inappropriate targets might be eliminated during the period of neuronal cell death.…”

“…This hypothesis would predict that CGRP-IR axons that contact muscle and CGRP-and substance P-IR axons that contact skin would be segregated within appropriate nerves. Indeed, embryonic spinal nerves include bundles that remain fasciculated into the distal parts of the hindlimb, with a high degree of physical segregation (Honig, 1982;Mirnics and Koerber, 1995). Such segregated projection patterns do not necessarily mean that the sensory neurons are specified as to function, and additional studies are necessary to evaluate the phenotype of segregated axons.…”

“…Cutaneous afferents appear in rat proximal hindlimb on embryonic days 14 -15 (E14 -E15) and in skin of distal toes at E16 -E17 (Reynolds et al, 1991;Mirnics and Koerber, 1995). These fibers are functional slightly later, beginning at E17 (Saito, 1979;Kudo and Yamada, 1985;Fitzgerald, 1987;Kucera et al, 1988;Fitzgerald, 1991).…”