Further Development 17.2: From Cell Surface Interactions to Cytoskeletal Rearrangements

Paraxial Mesoderm: The Somites and Their Derivatives

A mesenchymal-epithelial transition requires significant cytoskeletal rearrangements, which are typically governed by modulating the Rho family of small GTPases (Rho GTPases), such as Cdc42. Activated ephrin B2 signaling leads to the suppression of Cdc42 activity, which establishes a lower level of Cdc42 function in the anterior half of somitomere zero (S0) compared with the posterior half of S0. A forced reduction in Cdc42 activity leads to hyperepithelialized somites, whereas induced Cdc42 activity inhibits epithelialization (Nakaya et al. 2004; Watanabe et al. 2009). Therefore, the progressive formation of boundaries by MET in the PSM is fulfilled by reducing Cdc42 levels only in the peripheral cells of the somitomere, which creates a box around the remaining mesenchymal cells, which exhibit a higher level of Cdc42 until the next fissure event. Although reductions in Cdc42 are required for epithelialization, differential regulation of other Rho GTPases, such as Rac1, are also involved in epithelialization of somites (Burgess et al. 1995; Barnes et al. 1997; Nakaya et al. 2004).

In contrast to the repressive outcome of Eph A4-ephrin B2 signaling on Cdc42, this signaling enhances the activity of Integrin α5, which then serves to promote fibronectin assemblies in the extracellular matrix surrounding the immature somite (see Figure 17.11C; Lash and Yamada 1986; Hatta et al. 1987; Saga et al. 1997; Durbin et al. 1998; Linask et al. 1998; Barrios et al. 2003; Koshida et al. 2005; Jülich et al. 2009; Watanabe et al. 2009). This assembly of fibronectin reinforces epithelial cell separation and completes somite boundary formation (Jülich et al. 2015; reviewed by McMillen and Holley 2015).

Literature Cited

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