(A) Signals that direct the dorsoventral pattern: Sonic hedgehog protein from the notochord and the floor plate (the ventral midline of the neural tube) and BMP proteins from the roof plate (the dorsal midline) act as morphogens to control gene expression. (B) The resulting patterns of gene expression in the ventral part of the developing spinal cord. Different groups of proliferating neural progenitor cells (in the ventricular zone, close to the lumen of the neural tube) and of differentiating neurons (in the mantle zone, further out) express different combinations of gene regulatory proteins. Those indicated in this diagram are almost all members of the homeodomain superfamily; various other genes of the same superfamily (including Islet/Lim proteins) are expressed in the differentiating neurons. Neurons expressing different gene regulatory proteins will form connections with different partners and may make different combinations of neurotransmitters and receptors.