Abstract
Anterior sclerotome is presumed to be the only somitic tissue that guides neural crest cells as they migrate ventrally. In contrast, we report here that crest cells prefer the myotome's basal lamina over the sclerotome as a substratum. This conclusion stems from four observations. First, crest cells migrating between the neural tube and somite invade lumbar and thoracic somites only after the myotome has formed a basal lamina, as though they use this basal lamina to penetrate the somite. Second, crest cells alter their trajectories dramatically when they contact this basal lamina. They abruptly turn laterally and align closely with the myotome's basal surface. Third, crest cells invade sclerotome only when they fail to contact this basal lamina. For instance, the lateral half of each myotome is initially devoid of basal lamina. When the first crest cells reach the lateral myotome, they depart from the myotome's basal surface and penetrate lateral sclerotome. Only later, when a higher population density prevents some cells from contacting the basal lamina, do crest cells penetrate medial sclerotome. Conversely, crest cells that migrate between somites do not have access to myotome and fail to turn laterally. Fourth, when we prevent myotome development by surgically removing its precursor (the dermamyotome), crest cells fail to turn laterally within the somite. Instead, they move directly ventrally and colonize medial sclerotome. The preference for myotomal basal lamina implies that anterior sclerotome is a suboptimal environment for neural crest migration. The myotome's basal lamina may facilitate rapid migration through the somite before impediments to ventral migration develop.
