<P> For a short time, the neural tube is open both cranially and caudally . These openings, called neuropores, close during the fourth week in humans . Improper closure of the neuropores can result in neural tube defects such as anencephaly or spina bifida . </P> <P> The dorsal part of the neural tube contains the alar plate, which is associated primarily with sensation . The ventral part of the neural tube contains the basal plate, which is primarily associated with motor (i.e., muscle) control . </P> <P> The neural tube patterns along the dorsal - ventral axis to establish defined compartments of neural progenitor cells that lead to distinct classes of neurons . According to the French flag model of morphogenesis, this patterning occurs early in development and results from the activity of several secreted signaling molecules . Sonic hedgehog (Shh) is a key player in patterning the ventral axis, while bone morphogenic proteins (BMPs) and Wnt family members play an important role in patterning the dorsal axis . Other factors shown to provide positional information to the neural progenitor cells include fibroblast growth factors (FGFs) and retinoic acid . Retinoic acid is required ventrally along with Shh to induce Pax6 and Olig2 during differentiation of motor neurons . </P> <P> Three main ventral cell types are established during early neural tube development: the floor plate cells, which form at the ventral midline during the neural fold stage; as well as the more dorsally located motor neurons and interneurons . These cell types are specified by the secretion of the Shh from the notochord (located ventrally to the neural tube), and later from the floor plate cells . Shh acts as a morphogen, meaning that it acts in a concentration - dependent manner to specify cell types as it moves further from its source . </P>

What is the function of the proteins secreted by cells of the ventral midline of the spinal cord