J Cell Sci
August 15, 2003;
The striking left
asymmetry of visceral organs is known to depend on left
- and right
-side-specific cascades of gene expression during early embryogenesis. Now, developmental biologists are characterizing the earliest steps in asymmetry determination that dictate the sidedness of asymmetric gene expression. The proteins and structures involved control fascinating physiological processes, such as extracellular fluid flow and membrane voltage potential and yet little is known about how their activities are coordinated to control laterality. By analogy with intercellular signalling in certain epithelial and endothelial cells, however, it is reasonable to speculate that at least three of these players, monocilia, gap junction communication and the Ca2+ channel polycystin-2
, participate in a signalling pathway that propagates left
cues through multicellular fields.
J Cell Sci
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The mouse node and monocilia. Scanning electron micrographs (provided by Daisuke Watanabe and Hiroshi Hamada, Osaka University) showing the node at the distal tip of an E8.0 mouse embryo (A). Higher magnification views show the ventral node cells (B) and individual monocilia (C). Anteroposterior body axes (A, P) and direction of flow (arrow in B) to left side (L) are indicated. Magnifications: ×200 (A); ×700 (B); ×7000 (C).
Conservation of node monocilia. mRNA encoding endogenous left-right dynein (LRD), a protein involved in node monocilia in the mouse, is seen in Hensen's node in the chick, dorsal blastopore cells of the early neurula stage Xenopus, and the zebrafish shield (A,D,G,J; arrows). Monocilia are revealed on the apical surfaces of mouse ventral node cells and in cells of the related structures in chick, Xenopus and zebrafish embryos by immunostaining with anti-acetylated tubulin (B,E,H,K; arrows); a schematic representation is also shown (C,F,I,L). Reproduced, with permission, from the Nature Publishing Group (http://www.nature.com) (Essner et al., 2002).
dnah3 (dynein, axonemal, heavy chain 3) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 14, lateral view, anterior left, dorsal up.