Growth, proliferation, and cell death in the ontogeny of transient DRG (Froriep's ganglia) of chick embryos.

A striking example of axial patterning in nervous system development is the unusual fate of dorsal root ganglia (DRG) that develop in the most rostral somites, the Froriep's ganglia. In amniotes, the DRG that develop adjacent to the occipital (cranial) and the first cervical segments of the CNS "disappear" early in embryonic development. In contrast, all other DRG are present throughout the animal's life. We here reexamine in greater detail the ontogeny of the longest surviving Froriep's ganglion of the chick embryo, DRG C-2. By 50 h of development (stage, st. 15), an anlagen of a DRG had formed in C-2 that was indistinguishable from those of adjacent "permanent" ganglia. At st. 18 [embryonic day (E) 2.5+], the C-2 DRG had the same shape and volume as permanent ganglia C-5 and C-6. C-2's development first diverged from that of normal DRG at st. 19 (E3-), when C-2 was observed to be half the size and shaped differently from its neighbors, and its peripheral nerve root began to degenerate. Two cellular mechanisms appear to contribute to the reduced size of C-2 compared to normal DRG at st. 20 at this early stage: lower proliferation and higher apoptosis rates. One-third fewer C-2 cells were found to be in the S phase when compared to neighboring ganglia, and apoptotic cells were more than three times more abundant in C-2 than in conventional DRG at this stage. The C-2 DRG continued to grow, but at a slower pace than neighboring ganglia through st. 32 (E7). At the height of the normal programmed DRG cell death in normal cervical DRG at st. 28 (E6), even more massive apoptosis occurred in C-2, which resulted in the absence of this ganglion in 80% of st. 36 (E10) embryos. A recent study demonstrated that the overexpression of a single Hox gene can "rescue" the C-2 DRG in transgenic mice. We speculate that Hox genes may produce the difference in fate between C-2 and normal DRG by modulating proliferation and apoptosis via modified neurotrophic factor and/or receptor expression.