Crête neurale et évolution des vertébrés

Abstract

The neural crest (NC) is a remarkable structure of the Vertebrate embryo, which forms from the lateral borders of the neural plate (designated as neural folds) during neural tube closure. As soon as the NC is formed, its constitutive cells detach and migrate away from the neural primordium along definite pathways and at precise periods of time according to a rostro-caudal progression. The NC cells aggregate in definite places in the developing embryo, where they differentiate into a large variety of cell types including the neurons and glial cells of the peripheral nervous system, the pigment cells dispersed throughout the body and endocrine cells such as the adrenal medulla and the calcitonin producing cells. At the cephalic level only, in higher Vertebrates (but along the whole neural axis in Fishes and Amphibians), the NC is also at the origin of mesenchymal cells differentiating into connective tissue chondrogenic and osteogenic cells. Vertebrates belong to the larger group of Cordates which includes also the Protocordates (Cephalocordates and the Urocordates). All Cordates are characterized by the same body plan with a dorsal neural tube and a notochord which, in Vertebrates, exists only at embryonic stages. The main difference between Protocordates and Vertebrates is the very rudimentary development of cephalic structures in the former. As a result, the process of cephalization is one of the most obvious characteristics of Vertebrates. It was accompanied by the apparition of the NC which can therefore be considered as an innovation of Vertebrates during evolution. The application of a cell marking technique which consists in constructing chimeric embryos between two species of birds, the quail and the chicken, has led to show that the vertebrate head is mainly formed by cells originating from the NC, meaning that this structure was an important asset in Vertebrate evolution. Recent studies, described in this article, have strengthened this view by showing that the NC does not only provide the cells that build up the facial skeleton and most of the skull but plays a major role in early brain neurogenesis. It was shown that the cephalic NC cells produce signaling molecules able to regulate the activity of the two secondary organizing centers previously identified in the developing brain: the anterior neural ridge and the midbrain-hindbrain junction, which secrete Fgf8, a potent stimulator of early brain neurogenesis.