Here the marginal zone will be referred to as the involuting marginal zone (IMZ). One of the regional, ‘formative tendencies’ observed by Walter Vogt and Hans Spemann was the ‘constriction’ and ‘stretching’ of the marginal zone, the collar of involuting tissue that surrounds the blastopore and turns inside during amphibian gastrulation ( Vogt 1929 Spemann 1938). Summary of the function of convergent extension and convergent thickening in Xenopus gastrulation Finally, the question of how these very precise spatial and temporal progressions are regulated, and how their regulation is linked to the much better understood regulation of patterning tissue types is an important one and will require analysis of signalling events with high spatio-temporal resolution in living tissues.Ģ. Fifth, the specific combination of cellular mechanisms used to generate force varies in different species, but the pattern of forces is conserved within eggs of the particular architecture examined here. Fourth, different cellular mechanisms of force generation are used in the same presumptive tissues, and in similar spatial patterns, even in fairly closely related species. Third, these cell behaviours have boundaries of expression and axial properties that impart function in the context of the geometry and biomechanics of a particular gastrulation strategy. Second, the progressive aspect of their expression is important for the morphogenic function of these cell behaviours. First, the cell behaviours driving blastopore closure are expressed in a progressive pattern of high spatio-temporal resolution. Some of these formative movements were already described at the tissue level in Spemann's day, but what we have learned about the cell behaviours underlying these regional movements since then may offer new insights into how genes encode the forces that shape the embryo. Here we will discuss the aspects of patterning the morphogenic cell behaviours that close the blastopore during gastrulation of amphibians. Spemann (1938) distinguished between patterning of morphogenesis, which he referred to as ‘dynamic determination’ of regionally autonomous ‘formative tendencies’, and patterning of tissue types, which he referred to as ‘material determination’, and he discussed at length how the two might be related. Patterning is often thought of as the process of specifying a region of the embryo to differentiate into a particular cell or tissue type, but in the context of morphogenesis, patterning is the specification of a dynamic, spatio-temporal pattern of cell behaviours and tissue properties that generate and transmit the forces driving morphogenic (shape generating) processes. Understanding the patterning of these dynamic features of cell behaviour is important and will require analysis of signalling at much greater spatial and temporal resolution than that has been typical in the analysis of patterning tissue differentiation. It is not the nature of the cell behaviour alone, but the context, the biomechanical connectivity and spatial and temporal pattern of its expression that determine specificity of morphogenic output during gastrulation and blastopore closure. They are expressed progressively along presumptive lateral–medial and anterior–posterior axes of the body plan in highly ordered geometries of functional significance in the context of the biomechanics of blastopore closure, thereby accounting for the production of similar patterns of circumblastoporal forces. Although these cell behaviours are quite different and involve different germ layers and tissue organization, they are expressed in similar patterns. Mediolateral cell intercalation behaviour and epithelial–mesenchymal transition are used in different combinations in several species of amphibian to generate a conserved pattern of circumblastoporal hoop stresses. We review the dynamic patterns of cell behaviours in the marginal zone of amphibians with a focus on how the progressive nature and the geometry of these behaviours drive blastopore closure.
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