Axial level‐dependent differences in size of avian dorsal root ganglia are present from gangliogenesis

Dorsal root ganglia (DRG) form from neural crest cells that colonize the rostral sclerotomes of the somites of the trunk of higher vertebrates. Differences in sensory field size and innervation density are reflected by the size of the ganglia at different axial levels. Some of the position‐dependent differences in ganglionic size derives from asymmetry in embryonic programmed cell death, when more DRG neurons die at non‐limb‐innervating levels than at the levels that innervate the limbs. I have now examined early chick embryos determine whether there is asymmetry in DRG size at the time of their condensation at stage 20 [embryonic day (E) 3], before the onset of cell death. Ganglia in brachial segments 14 and 15 are more than 80% larger on average than those in cervical segments 5 and 6 at this stage of development. This difference in volume is due to increased numbers of cells in the brachial sensory ganglia. Several other morphometric parameters of the DRG and sclerotomes were then determined. The rostro‐caudal length was found to be significantly greater for brachial ganglia. The greater length of the brachial ganglia was found to be correlated with (1) a greater length of brachial than cervical sclerotomes and (2) the occupation by brachial ganglia of a larger proportion of the rostro‐caudal extent of the sclerotome. These results demonstrate that the mature pattern of axial differences in ganglionic size are foreshadowed by a pattern set during the period of gangliogenesis, which is then further sculpted by apoptosis. This initial axial asymmetry in size preceding cell death parallels that recently shown for the development of the ventral motor column. Thus the sensory ganglia, like the somites, have distinct axial‐position‐dependent characteristics from their outset, characteristics that are not dependent on cell death resulting from competition for target‐derived survival factors as was previously thought. DRG positional identities, like their segmentation, may be imposed by the paraxial mesoderm or may alternatively be intrinsically determined as is the case for the rhombomeres of the hindbrain. © 1993 John Wiley & Sons, Inc.