Variation and symmetry in the lumbar and thoracic dorsal root ganglion cell populations of newly metamorphosed xenopus laevis

Abstract

The sizes of the lumbar and thoracic dorsal root ganglion cell populations in normally developing newly metamorphosed Xenopus laevis were measured in order to determine whether these neuron populations have the same characteristics as the hindlimb motoneuron population (i.e., large individual as well as sibling group differences, striking bilateral symmetry, and a rough correspondence between neuron number and body size that suggests some peripheral control of cell number during normal development (Sperry, J. Comp. Neurol. 264:250-267). Among animals from three sibling groups, the total numbers of thoracic and lumbar ganglion cells are highly variable and symmetrical, although symmetry is not uniformly present at the level of individual ganglion pairs. Significant sibling group differences in neuron number are also present. Metamorphic body size and cell number in the thoracic but not in the lumbar ganglia are significantly correlated. The motoneurons innervating the hindlimbs were also counted and measured in the same animals. While variable as well as symmetrical, motoneuron number and metamorphic body size are correlated in only two of the three sibling groups. Interestingly, the numbers of motoneurons and lumbar ganglion cells, two populations of neurons whose sizes one might predict would be significantly correlated in normally developing animals, are not correlated. The relationship between these observations and currently held views concerning how neuron numbers might be controlled during normal development is discussed.

Lumbar lateral motor columns and hindlimbs of two Xenopus laevis chromosome mosaics

“…Correspondences between numbers of muscle fibers at metamorphosis (Sperry, 1987) or primary myotube clusters at an earlier stage of development (McLennan, 1988) and numbers of motoneurons in metamorphic diploid Xenopus may be evidence that adult numbers of motoneurons are controlled epigenetically through a match to some feature of or substance from the peripheral target that allows motoneurons to survive the period of naturally occurring cell death. Afferents are also necessary for normal motoncuron development in anurans (Hughes and Tschumi, 1958;Davis et al, 1983); but whether they help determine how many motoneurons survive cell death is unclear, since the numbers of lumbar ganglion cells and L-LMC motoneurons in normally developing diploid Xenopus are not highly correlated (Sperry, 1990). Certainly the asymmetrical total number ofmotoneurons in the stage 66 mosaic, or even the reduced but symmetrical number of motoneurons in triploids (Sperry, 1988a), could be the result of a match between some feature of the periphery or afferents and the L-LMC motoneurons that is the same as the match believed to occur in diploids.…”