Developmental expression of the amphioxus <Emphasis Type="Italic">Tbx1</Emphasis>/<Emphasis Type="Italic">10</Emphasis> gene illuminates the evolution of vertebrate branchial arches and sclerotome

We have isolated an amphioxus T-box gene that is orthologous to the two vertebrate genes, Tbx1 and Tbx10, and examined its expression pattern during embryonic and early larval development. AmphiTbx1/10 is first expressed in branchial arch endoderm and mesoderm of developing neurulae, and in a bilateral, segmented pattern in the ventral half of newly formed somites. Branchial expression is restricted to the first three branchial arches, and disappears completely by 4 days post fertilization. Ventral somitic expression is restricted to the first 10–12 somites, and is not observed in early larvae except in the most ventral mesoderm of the first three branchial arches. No expression can be detected by 4 days post fertilization. Integrating functional, phylogenetic and expression data from amphioxus and a variety of vertebrate model organisms, we have reconstructed the early evolutionary history of the Tbx1/10 subfamily of genes within the chordate lineage. We conclude that Tbx1/10-mediated branchial arch endoderm and mesoderm patterning functions predated the origin of neural crest, and that ventral somite specification functions predated the origin of vertebrate sclerotome, but that Tbx1 was later co-opted during the evolution of developmental programs regulating branchial neural crest and sclerotome migration.Edited by M. Akam     

Contribution of somitic cells to the avian ribs

The traditional view that all parts of the ribs originate from the sclerotome of the thoracic somites has recently been challenged by an alternative view suggesting that only the proximal rib derives from the sclerotome, while the distal rib arises from regions of the dermomyotome. In view of this continuing controversy and to learn more about the cell interactions during rib morphogenesis, this study aimed to reveal the precise contributions made by somitic cells to the ribs and associated tissues of the thoracic cage. A replication-deficient lacZ-encoding retrovirus was utilized to label cell populations within distinct regions of somites 19-26 in stage 13-18 chick embryos. Analysis of the subsequent contributions made by these cells revealed that the thoracic somites are the sole source of cells for the ribs. More precisely, it is the sclerotome compartment of the somites that contributes cells to both the proximal and distal elements of the ribs, confirming the traditional view of the origin of the ribs. Results also indicate that the precursor cells of the ribs and intercostal muscles are intimately associated within the somite, a relationship that may be essential for proper rib morphogenesis. Finally, the data from this study also show that the distal ribs are largely subject to resegmentation, although cell mixing may occur at the most sternal extremities.     

Mediolateral patterning of somites: multiple axial signals, including Sonic hedgehog, regulate Nkx-3.1 expression

The axial structures, the notochord and the neural tube, play an essential role in the dorsoventral patterning of somites and in the differentiation of their many cell lineages. Here, we investigated the role of the axial structures in the mediolateral patterning of the somite by using a newly identified murine homeobox gene, Nkx-3.1, as a medial somitic marker in explant in vitro assays. Nkx-3.1 is dynamically expressed during somitogenesis only in the youngest, most newly-formed somites at the caudal end of the embryo. We found that the expression of Nkx-3.1 in pre-somitic tissue explants is induced by the notochord and maintained in newly-differentiated somites by the notochord and both ventral and dorsal parts of the neural tube. We showed that Sonic hedgehog (Shh) is one of the signaling molecules that can reproduce the effect of the axial structures by exposing explants to either COS cells transfected with a Shh expression construct or to recombinant SHH. Shh could induce and maintain Nkx-3.1 expression in pre-somitic mesoderm and young somites but not in more mature, differentiated ones. The effects of Shh on Nkx-3.1 expression were antagonized by a forskolin-induced increase in the activity of cyclic AMP-dependent protein kinase A. Additionally, we confirmed that the expression of the earliest expressed murine myogenic marker, myf 5, is also regulated by the axial strucutres but that Shh by itself is not capable of inducing or maintaining it. We suggest that the establishment of somitic medial and lateral compartments and the early events in myogenesis are governed by a combination of positive and inhibitory signals derived from the neighboring structures, as has previously been proposed for the dorsoventral patterning of somites.     

Morphogenesis of sclerotome and neural crest in avian embryos

Summary The distribution of sclerotome and neural crest cells of avian embryos was studied by light and electron microscopy. Sclerotome cells radiated from the somites towards the notochord, to occupy the perichordal space. Neural crest cells, at least initially, also entered cell-free spaces. At the cranial somitic levels they moved chiefly dorsal to the somites, favouring the rostral part of each somite. These cells did not approach the perichordal space. More caudally (i.e. trunk levels), neural crest cells initially moved ventrally between the somites and neural tube. Adjacent to the caudal half of each somite, these cells penetrated no further than the myosclerotomal border, but opposite the rostral somite half, they were found next to the sclerotome almost as far ventrally as the notochord. However, they did not appear to enter the perichordal space, in contrast to sclerotome cells.When tested in vitro, sclerotome cells migrated towards notochords co-cultured on fibronectin-rich extracellular material, and on collagen gels. In contrast, neural crest cells avoided co-cultured notochords. This avoidance was abolished by inclusion of testicular hyaluronidase and chondroitinase ABC in the culture medium, but not by hyaluronidase from Streptomyces hyalurolyticus. The results suggest that sclerotome and neural crest mesenchyme cells have a different distribution with respect to the notochord, and that differential responses to notochordal extracellular material, possibly chondroitin sulphate proteoglycan, may be responsible for this.     

Somite polarity and segmental patterning of the peripheral nervous system

The analysis of the outgrowth pattern of spinal axons in the chick embryo has shown that somites are polarized into anterior and posterior halves. This polarity dictates the segmental development of the peripheral nervous system: migrating neural crest cells and outgrowing spinal axons traverse exclusively the anterior halves of the somite-derived sclerotomes, ensuring a proper register between spinal axons, their ganglia and the segmented vertebral column. Much progress has been made recently in understanding the molecular basis for somite polarization, and its linkage with Notch/Delta, Wnt and Fgf signalling. Contact-repulsive molecules expressed by posterior half-sclerotome cells provide critical guidance cues for axons and neural crest cells along the anterior-posterior axis. Diffusible repellents from surrounding tissues, particularly the dermomyotome and notochord, orient outgrowing spinal axons in the dorso-ventral axis ('surround repulsion'). Repulsive forces therefore guide axons in three dimensions. Although several molecular systems have been identified that may guide neural crest cells and axons in the sclerotome, it remains unclear whether these operate together with considerable overall redundancy, or whether any one system predominates in vivo.