The development of intersegmental connections in embryonic spinal cord: an anatomic substrate for early embryonic motility

This ultrastructural study of the embryonic chick spinal cord was designed to determine whether intersegmental inputs are formed before the closure of the reflex arc. Lesions were made in the thoracic cord of chick embryos at five days of incubation, when synapses are present in the ventral half of the cord, but before dorsal root afferents have made contacts in the central nervous system. Axons degenerating were observed in the upper brachial and lower lumbar regions of embryos sacrificed two days later. Synapses exhibiting degenerative changes were present up to six segments above and below the lesion. These observations were interpreted to indicate that: (1) intersegmental fibers connect the thoracic cord to brachial and lumbar areas by the fifth day of incubation; (2) early synapses contain both segmental and intersegmental connections; and (3) some intersegmental connections occur prior to the closure of the reflex arc. These morphologic studies, in conjunction with behavioral and electrophysiological data, strongly suggest that the early intersegmental connections are part of the propriospinal system. The development of these intersegmental tracts coincides with the onset of movement and thus provides an anatomical substrate for understanding embryonic motility.