The quail-chick chimera system (see Box 1) has been used over the years to establish a fate map of the NC along the neural axis. These studies have shown that melanocytes arise from the entire length of the NC in higher vertebrates, whereas mesectodermal derivatives originate only from the cephalic NC region. NC-derived cells that contribute to the PNS and ENS arise only from some areas of the neural axis (Fig. 1).Using the quail-chick system, well-defined areas of the NC have been exchanged to assess NCC plasticity (see Fig. 1). For example, in one study, the vagal region of the NC (which is located between somites 1 and 7, and gives rise to the enteric ganglia) was exchanged with NC from between somites 18 and 24 (which gives rise to the adrenal medulla and sympathetic ganglia) (Le Douarin and Teillet, 1974). This swap resulted Review 4638 in the normal colonisation of the suprarenal gland and sympathetic ganglia by NCCs fated to colonise the gut. However, although the adrenomedullary trunk NCCs invaded the pre-umbilical gut wall and differentiated into normal enteric plexuses, they failed to reach the post-umbilical bowel .This experimental system has since been used together with various molecular markers, such as the Schwann cell myelin protein (SMP), which is present on Schwann cells but not on other PNS and ENS glial cells, to allow a more refined analysis of NCC plasticity. These studies have shown that NCC differentiation into a specific type of glia depends upon the environment in which they develop (Dulac et al., 1988;Dulac and Le Douarin, 1991; Cameron-Curry et al., 1993). Similarly, the differentiation of the various types of autonomic neurons varies according to the milieu in which they differentiate (for reviews, see Le Douarin, 1982;Le Douarin and Kalcheim, 1999).The conclusion of these heterotopic grafting experiments was that the fate of the NCCs that form the PNS and ENS is not fully determined before these cells migrate, but instead remains plastic until they receive differentiation signals at the end of, and possibly during, their migration. This finding raised the issue of whether all the precursors of PNS ganglion cells became fully differentiated and/or committed soon after reaching their sites of arrest, or whether some remained as quiescent undifferentiated cells. This was explored in the experiments discussed in the following section. Undifferentiated precursors in PNS gangliaTo investigate the developmental potentials of PNS ganglion cells, fragments of sensory and autonomic ganglia from quail embryos, taken from embryonic day (E) 4 up to the end of the incubation period, were implanted into NCC migration pathways of E2 chick hosts when their own NCCs were migrating. The grafted neurons themselves died (probably because the necessary survival factors are not present in the younger host). However, the non-neuronal cells of implanted ganglia migrated and homed to host sensory and autonomic ganglia, where they differentiated into the types of neurons and glia corresponding to their novel...

Generating a precise cellular and molecular cartography of the human embryo is essential to our understanding of the mechanisms of organogenesis in normal and pathological conditions. Here, we have combined whole-mount immunostaining, 3DISCO clearing, and light-sheet imaging to start building a 3D cellular map of the human development during the first trimester of gestation. We provide high-resolution 3D images of the developing peripheral nervous, muscular, vascular, cardiopulmonary, and urogenital systems. We found that the adult-like pattern of skin innervation is established before the end of the first trimester, showing important intra- and inter-individual variations in nerve branches. We also present evidence for a differential vascularization of the male and female genital tracts concomitant with sex determination. This work paves the way for a cellular and molecular reference atlas of human cells, which will be of paramount importance to understanding human development in health and disease. PAPERCLIP.