The longitudinal elongation of primary afferent axons (PAAs) in the dorsal
funiculus of chick embryo spinal cord was examined using a lipophilic trace
r, DiI and immunohistochemistry. The earliest developing PAAs in the brachi
al segments invaded the spinal cord around embryonic day (E) 3.5. Thereafte
r, they elongated both rostrally and caudally in the presumptive dorsal fun
iculus, with frequent contacts between pre-existing axons and later arrivin
g growth cones. By E3, the PAAs had elongated 3 segments both rostrally and
caudally. In the course of their longitudinal elongation, the PAAs shifted
their trajectory dorsally within the dorsal funiculus. By E6-6.5, the PAAs
had extended as far as 10 segments rostrally and 6 segments caudally in th
e dorsal funiculus, and collaterals began to enter the dorsal horn. By E9,
the PAAs extended up to 13 segments rostrally and 7 segments caudally, and
collaterals reached the ventral spinal cord. During their longitudinal cour
se, the PAAs shifted their trajectory medially within the dorsal funiculus.
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