Figure 3. Early development of the spinal cord.

(a) Schematic cross sections through the developing mouse spinal cord showing the patterning and specification of early spinal cord progenitors and their neuronal progeny. At E9, a gradient of Sonic hedgehog (red) ventrally and BMP/GDF7 (yellow) dorsally provide instructive positional signals to dividing progenitors in the ventricular zone. This leads to the restricted activation of patterning factors in discrete dorsventral domains, which are represented by Nkx6.1 (ventral), Pax6 (intermediate) and Pax3 and Pax7 (dorsal). At E11, eleven early classes of postmitotic neuron are present in the embryonic spinal cord. dI1-dI5 neurons that are derived from dorsal progenitors (grey) primarily contribute to sensory spinal pathways, while dI6, MN and V0-V3 neurons from ventral progenitors (yellow) are elements of the locomotor circuitry. Some of the postmitotic transcription factors that mark each of the eleven early generic populations are indicated. (b) Six classes of embryonic neurons are proposed to give rise to the core elements of the spinal locomotor circuitry. The neurotransmitter phenotypes and axonal projections of these embryonic neurons are indicated along with some of the known adult cells types that are derived from each population. (Right) Schematic of the adult spinal cord showing the position and projections of somatic motor neurons (MN, yellow), V1-derived Renshaw cells (RC, green) and Ia inhibitory interneurons (Ia, green) and V0 commissural neurons (blue). The laminae of the spinal cord are indicated by Roman numerals.