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The development of intersegmental connections in embryonic spinal cord: an anatomic substrate for early embryonic
Insights
Early chick spinal cord development shows intersegmental connections form before reflex arcs close. These connections, part of the propriospinal system, are crucial for embryonic movement.
Area of Science:
- Neuroscience
- Developmental Biology
- Embryology
Background:
- Synapses are present in the embryonic chick spinal cord by five days of incubation.
- Dorsal root afferents have not yet made central nervous system contacts at this stage.
Purpose of the Study:
- To investigate the formation of intersegmental inputs in the embryonic chick spinal cord.
- To determine if these inputs develop before the closure of the reflex arc.
Main Methods:
- Ultrastructural analysis of the embryonic chick spinal cord.
- Surgical lesions were introduced in the thoracic cord of five-day-old chick embryos.
- Degenerative changes in axons and synapses were examined two days post-lesion.
Main Results:
- Degenerating axons were observed in brachial and lumbar regions, indicating intersegmental connections.
- Synapses with degenerative changes were found up to six segments away from the lesion.
- Intersegmental fibers connect thoracic cord to brachial and lumbar areas by incubation day five.
Conclusions:
- Early synapses in the chick spinal cord integrate both segmental and intersegmental connections.
- Intersegmental connections are established prior to the completion of the reflex arc.
- These early connections are part of the propriospinal system and coincide with the onset of embryonic movement.
Abstract:
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.