Differentiation of dorsal root ganglion cells with processes in their synaptic target zone of embryonic mouse spinal cord: a retrograde tracer study.

The differentiation of dorsal root ganglion (DRG) cells with central processes in their synaptic target zones was studied in the developing spinal cord of embryonic mice (C57BL/6J). On embryonic days 12-15 (E12-E15), horseradish peroxidase (HRP) was pressure injected into the intermediate region of developing spinal grey matter and the embryos were cultured in an oxygenated medium to allow retrograde HRP transport to the dorsal root ganglia. Labelled DRG cells were measured and classified into four categories representing successive developmental stages: primitive bipolar cells, early transitional bipolar neurons, late transitional bipolar cells, and pseudounipolar neurons. In the period between E12 and E15, retrogradely labelled DRG cells became larger and less elongated as a population. Furthermore, a quantitative analysis of the cell types labelled on successive embryonic days of injection indicated an increase in the relative morphological maturity of labelled cells. On E12, the labelled cell population consisted solely of primitive bipolar and early transitional bipolar cells whereas the more mature late transitional bipolar and pseudounipolar neurons were predominant at E15, with the change between these two relationships occurring between E14 and E15. The principal finding of this study was that even the less differentiated forms of DRG neurons had axons within their central synaptic target fields during embryonic stages of spinal cord development.