The eventful somite: patterning, fate determination and cell division in the somite

Abstract

The segmental somites not only determine the vertebrate body plan, but also represent turntables of cell fates. The somite is initially naive in terms of its fate restriction as shown by grafting and rotation experiments whereby ectopically grafted or rotated tissue of newly formed somites yielded the same pattern of normal derivatives. Somitic derivatives are determined by local signalling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. The correct spatio-temporal specification of the deriving tissues, skeletal muscle, cartilage, endothelia and connective tissue is achieved by a sequence of morphogenetic changes of the paraxial mesoderm, eventually leading to the three transitory somitic compartments: dermomyotome, myotome and sclerotome. These structures are specified along a double gradient from dorsal to ventral and from medial to lateral. The establishment and controlled disruption of the epithelial state of the somitic compartments are crucial for development. In this article, we give a synopsis of some of the most important signalling events involved in somite patterning and cell fate decisions. Particular emphasis has been laid on the issue of epithelio-mesenchymal transition and different types of cell division in the somite.

Modular development of the teleost trunk along the dorsoventral axis and zic1/zic4 as selector genes in the dorsal module

“…Like dorsomedial-ventrolateral patterning of amniote somites, teleost somites are first dorsoventrally patterned at segmentation stages by the signals derived from surrounding tissues. This pattern thus reflects the initial DV pattern determined by the gradient of the Wnt and BMP activities (Hirsinger et al, 1997;Marcelle et al, 1997;Pourquié et al, 1996;Yusuf and Brand-Saberi, 2006). However, once established, their dorsal expression stops being dependent on the external signals by the hatching stage and is maintained for life.…”

In vivo analyzes of dystroglycan function during somitogenesis in Xenopus laevis

“…Indeed the patterning and the fate determination of somitic cells occur in response to extrinsic signals, produced and secreted from the adjacent tissues in particular the neural tube, the notochord, the surface ectoderm, and the somitic compartment themselves. Candidate molecules for these signaling activities include Sonic Hedgehog, the Wnt proteins, Noggin, and BMP-4 (Yusuf and Brand-Saberi, 2006). It is possible that in MO-treated explants, one or more of these proteins are absent or misexpressed leading to an incomplete muscle cell differentiation in synergy with the depletion of Dg and explaining the lack of 12/101 staining that is specific of differentiated muscle cells.…”

Dynamic somite cell rearrangements lead to distinct waves of myotome growth

“…In addition to their shared position, gene expression and fate, external cells and early dermomyotome cells are proliferative, and Hedgehog promotes their differentiation into muscle (Buckingham, 2006;Feng et al, 2006;Hammond et al, 2007;Johnson et al, 1994). We do not yet know if later zebrafish external cells also share features with later amniote dermomyotome cells, which give rise to dermis, endothelial cells, cells that migrate into limbs, and cells that enter the myotome as mitotically active, Pax7-positive myogenic precursors (Yusuf and Brand-Saberi, 2006). The zebrafish dermis is acellular until 4 weeks of life (Le Guellec…”