Developmental Neurobiology of the Leech
The leech CNS consists of a ventral chain of 32 segmentally iterated ganglia. The first four and last seven segmental ganglia are fused, constituting a rostral and caudal ganglionic mass, respectively. The rostral ganglionic mass, or subesophageal ganglion, is connected at its anterior end to a dorsally situated supraesophageal ganglion. Each segmental ganglion contains about 400 bilaterally paired neurons, as well as a few unpaired neurons. Their somata form a cortex around the outer surface of the ganglion. The neurons are monopolar; their processes project into a central neuropil, where they make synaptic contacts. From there, the processes of some neurons project to other ganglia via a connective nerve. Sensory and effector neurons project their processes to targets outside the CNS via segmental nerves, whose roots emerge from the lateral edge of the ganglion. In each ganglion, the neuronal somata are distributed over six cell packets, of which two form an anterior and two a posterior pair of lateral packets. The remaining two packets are unpaired, one lying anterior and the other posterior on the ventromedial aspect. Each cell packet contains one giant glial cell, whereas two giant glial cells are associated with the ganglionic neuropil. Additional giant glia are present in the interganglionic connective nerves.
KeywordsNerve Cord Segmental Ganglion Cell Lineage Analysis Developmental Neurobiology Embryonic Nervous System
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