Figure 1 – Sculpting tissues via spatiotemporal variations of key physical quantities.

A. The functional, morphological phenotype is ultimately achieved by controlling in space and time three physical quantities (physical level): volumetric growth, tissue material properties and active forces. Such control is, at least partially, due to gradients in signaling molecules in the tissue (signaling level). Cells within the tissue both sense and respond to local values of biochemical and physical cues (cellular level). B. Examples of morphogenetic processes for which spatiotemporal control of specific physical quantities have been identified. A localized cell proliferation zone (controlled region of volumetric growth) is essential to shaping bird beaks. Changes in the shape of the proliferation zone change the shape of the resulting beak, as illustrated for chicken and duck (dorsal views of developing beak) [5,6] (left). A fluid-to-solid transition between tissue physical states along the anteroposterior axis guides posterior body elongation in zebrafish[10]. In this case, the spatiotemporal control of fluid-like and solid-like tissue regions shapes the tissue (middle). In Drosophila, high levels of actomyosin contractility are restricted to a ventral region to enable invagination of the ventral furrow[9] (right). Dotted line indicates location of transverse cross sections shown on right.