Regional differences in actomyosin contraction shape the primary vesicles in the embryonic chicken brain

Abstract

In the early embryo, the brain initially forms as a relatively straight, cylindrical epithelial tube composed of neural stem cells. The brain tube then divides into three primary vesicles (forebrain, midbrain, hindbrain), as well as a series of bulges (rhombomeres) in the hindbrain. The boundaries between these subdivisions have been well studied as regions of differential gene expression, but the morphogenetic mechanisms that generate these constrictions are not well understood. Here, we show that regional variations in actomyosin-based contractility play a major role in vesicle formation in the embryonic chicken brain. In particular, boundaries did not form in brains exposed to the nonmuscle myosin II inhibitor blebbistatin, whereas increasing contractile force using calyculin or ATP deepened boundaries considerably. Tissue staining showed that contraction likely occurs at the inner part of the wall, as F-actin and phosphorylated myosin are concentrated at the apical side. However, relatively little actin and myosin was found in rhombomere boundaries. To determine the specific physical mechanisms that drive vesicle formation, we developed a finite-element model for the brain tube. Regional apical contraction was simulated in the model, with contractile anisotropy and strength estimated from contractile protein distributions and measurements of cell shapes. The model shows that a combination of circumferential contraction in the boundary regions and relatively isotropic contraction between boundaries can generate realistic morphologies for the primary vesicles. In contrast, rhombomere formation likely involves longitudinal contraction between boundaries. Further simulations suggest that these different mechanisms are dictated by regional differences in initial morphology and the need to withstand cerebrospinal fluid pressure. This study provides a new understanding of early brain morphogenesis.

Morphomechanics: transforming tubes into organs

“…The primary mechanism in the chick is localized circumferential contraction at the apical (inner) side of the wall, which decreases the BT circumference within the boundary regions (Fig. 2A) [47*]. In zebrafish, on the other hand, radial cellular shortening first generates a local circumferential groove that establishes the midbrain-hindbrain boundary, which is then sharpened by local laminin-dependent basal constriction [48*] (Fig.…”

Supracellular actomyosin assemblies during development

“…26 Whether vertebrate compartment boundaries between rhombomeres also employ actomyosin cables to restrict cell movement is still to be determined, but, in support if this idea, regional differences in F-actin and phosphorylated myosin II can be observed between central rhombomere cells and the boundary cells in the chick. 27 In Drosophila, both in the embryo at the parasegment boundary a well as at the DV boundary in the wing disc, the boundary cable is formed by the accumulation of actomyosin in each row of cells on either side of the boundary. Although the upstream signals in the embryo are still unclear, Notch signaling has been shown to be the upstream activator of cable formation at the DV boundary in the larval wing disc ( Table 1).…”

Mechanical effects of the surface ectoderm on optic vesicle morphogenesis in the chick embryo

“…Cytoskeletal contraction is involved in numerous morphogenetic processes (Ettensohn, 1985; Filas et al, 2012; Martin, 2010). To explore the possible role of contraction in OV morphogenesis, we cultured HH9 embryos in bleb for 12 h and used PCA to compare OV shapes between bleb-treated and control embryos.…”

“…As in previous studies, the material properties of the early brain tube are assumed to be approximately isotropic and nearly incompressible (Filas et al, 2012; Xu et al, 2010a). Because stress is associated only with elastic deformation, the constitutive relation for a compressible pseudoelastic material can be written in the form (Taber, 2004)…”