Long Distance Wanderers and the Various Fates of the Neural Crest Cells
In the development of animals extensive cell migrations take place. This applies especially to vertebrate embryos. They are like cities full of tourists. In a translucent fish embryo migratory cells can be seen crawling and swarming. In avian and mammalian blastodiscs the cells of the meso‘ derm’ do not form a ‘skin’ (Greek derma = skin) or a coherent germ ‘layer’; instead the mesodermal cells creep around like amoebae to colonize the spaces between the ectoderm/epiblast and the endoderm/hypoblast towards their target positions and form a loose primary mesenchyme (see Fig. 5.22 fish, Fig. 5.25 bird). Only later they aggregate forming compact structures such as the somites. When the somites split up, the cells of the sclerotome and dermatome emigrate to form compact structures or soft tissues elsewhere (Fig. 15.1). Primordial germ cells, blood cells and neural crest cells travel particularly long distances. Even the germ cells and the cells of the peripheral nervous system as well as the musculature and the skeletal elements of the extremities (fingers, toes) derive from migratory precursors.