Morphogenesis of the node and notochord: The cellular basis for the establishment and maintenance of left–right asymmetry in the mouse

Abstract

Establishment of left-right asymmetry in the mouse embryo depends on leftward laminar fluid flow in the node, which initiates a signaling cascade that is confined to the left side of the embryo. Leftward fluid flow depends on two cellular processes: motility of the cilia that generate the flow and morphogenesis of the node, the structure where the cilia reside. Here, we provide an overview of the current understanding and unresolved questions about the regulation of ciliary motility and node structure. Analysis of mouse mutants has shown that the motile cilia must have a specific structure and length, and that they must point posteriorly to generate the necessary leftward fluid flow. However, the precise structure of the motile cilia is not clear and the mechanisms that position cilia on node cells have not been defined. The mouse node is a teardrop-shaped pit at the distal tip of the early embryo, but the morphogenetic events that create the mature node from cells derived from the primitive streak are only beginning to be characterized. Recent live imaging experiments support earlier scanning electron microscopy (SEM) studies and show that node assembly is a multi-step process in which clusters of node precursors appear on the embryo surface as overlying endoderm cells are removed. We present additional SEM and confocal microscopy studies that help define the transition stages during node morphogenesis. After the initiation of left-sided signaling, the notochordal plate, which is contiguous with the node, generates a barrier at the embryonic midline that restricts the cascade of gene expression to the left side of the embryo. The field is now poised to dissect the genetic and cellular mechanisms that create and organize the specialized cells of the node and midline that are essential for left-right asymmetry. Developmental Dynamics 237:3464 -3476, 2008.

Cytoskeletal Symmetry Breaking and Chirality: From Reconstituted Systems to Animal Development

“…Evidence that cytoskeletal proteins play a role in establishing these asymmetries emerged with the report of dysfunctional cilia causing situs inversus totalis [230]. In the 1990s it became clear that directional flow in the ciliated node (functionally orthologous structures are also called Hensen's node, Kupffer's vesicle, or dorsal blastopore lip), a transient embryonic structure that forms at the anterior end of the primitive streak in a gastrulating embryo mediates L/R symmetry breaking through establishment of L/R asymmetric gene expression (reviewed in [231][232][233][234]). The rotating monocilia of the late gastrula node are microtubule-based structures that differ from regular motile cilia in that they are only composed of nine peripheral microtubule doublets but lack the central pair of singlets.…”

Human Heterotaxy Syndrome

“…Monocilia consist of 2 different, highly sensitive subtypes: motile and immotile monocilia ( Figure 1A). [23][24][25][26][27] Motile cilia are predominantly located in the central region of the node and contain left-right dynein (lrd) in mice, 28 and the gene responsible for Kartagener's syndrome, DNAH5, in human 29 as a motor protein on microtubules ( Figure 1B). In contrast, the immotile mechanosensory cilia expressing the calcium ion channel polycystin-2 (Pkd2) are located in the peripheral region of the node.…”

Extracellular matrix motion and early morphogenesis

“…Nodal flow-based mechanisms for left-right symmetry breaking were also demonstrated in fish and frog embryos (Essner et al, 2005;Schweickert et al, 2007). Nevertheless, a role for tissue displacement during determination of left-right asymmetry (see below) has not been ruled out in the case of embryos that manifestly exhibit nodal flow (Lee and Anderson, 2008). Data from bird and pig embryos show that symmetry breaking proceeds readily in the absence of nodal cilia (Gros et al, 2009).…”

“…In mice and rabbitsbut not all vertebrate systems -the earliest steps of this process are initiated by the monocilia found on the cells of the posterior notochordal plate (Levin, 2005;Raya and Izpisua Belmonte, 2006). The primary cilia at the node, termed the nodal cilia, rotate to generate a leftward fluid flow, called the nodal flow, within a pit-like teardrop-shaped space that is devoid of subjacent endoderm (Lee and Anderson, 2008;Nonaka et al, 1998). The nodal flow stimulates signaling cascades that ultimately result in the induction of asymmetric gene expression (Komatsu and Mishina, 2013).…”