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Journal of Neuroscience, Vol 5, 388-407, Copyright © 1985 by Society for Neuroscience
Developmental neural kinship groups in the leech
AP Kramer and DA Weisblat
We have traced the developmental origins of various CNS neurons and glial
cells of a leech to 10 clonally related groups of cells, the bilaterally
paired M, N, O, P, and Q kinship groups. Each kinship group is descended
from one of 10 identifiable blastomeres of the early embryo, the
teloblasts. Of the approximately 200 neurons in each side of a segmental
ganglion, 130 to 160 are in the ipsilateral N, 20 to 50 in the O, 8 to 12
in the P, 6 to 9 in the Q, and 3 to 6 in the M kinship group. A given
identified neuron or glial cell was invariably found to belong to a
particular kinship group, indicating that in leech development neuronal
lineage is highly stereotyped. But cells of related function and morphology
do not necessarily belong to the same neuronal kinship group: of the
mechanosensory neurons, the T and N neurons belong to the N, the Pv neuron
belongs to the P and the PD neuron belongs to the O kinship group.
Similarly, glial cells arise from all four ectodermal teloblasts.
Conversely, neurons within a kinship group are not obviously related in
structure or function: the N kinship group includes sensory, motor, and
effector neurons and interneurons: the O and P kinship groups each include
sensory neurons and interneurons; both the P and Q groups contain
representatives of three distinct morphological classes of interneurons.
Consequently, in early development, the determinants of neuronal identity
in the leech CNS are not segregated in any obvious thematic way in the
cleavages that give rise to the five bilateral pairs of teloblasts. Rather,
the neural kinship groups may be merely the evolutionary vestige of a
primordial distributed nervous system, each quadrant of which was derived
from one teloblast.
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