Journal of Neuroscience, Vol 9, 175-182, Copyright © 1989 by Society for Neuroscience
Expression of nerve-muscle topography during development
MB Laskowski and JA High
Department of Physiology, St. Louis University School of Medicine, Missouri 63104.
Previous studies have indicated that in 2 muscles of the adult rat, the
anterior serratus and the diaphragm, the rostrocaudal axis of the
motoneuron pool projects topographically onto the rostrocaudal axis of the
muscle. In the present work we have asked whether this orderly topography
emerges as a function of postnatal synaptic rearrangement or whether this
pattern is already established at birth. The anterior serratus muscle was
studied over the period ranging from embryonic day 17 through postnatal day
30. Using 2 criteria of topography, average segmental innervation and
average target field of cervical roots C6 and C7, we found that a
topographic distribution of the motoneuron pool is already present prior to
birth and maintained throughout the postnatal period. Moreover, both C6 and
C7 form an orderly map over the surface of the serratus in the embryo, and
the topography is sharpened during postnatal periods. The diaphragm also is
topographically innervated at birth and undergoes a comparable sharpening
of the projection map postnatally. We conclude that the topographic
projection of motoneurons is established prior to birth in these muscles,
and postnatal synaptic rearrangement serves to sharpen the topographic map
toward the adult pattern. These results also suggest that the pursuit of
basic mechanisms underlying topography should be directed toward initial
embryonic nerve-muscle contacts.
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