Establishment of left-right asymmetry in vertebrate development: the node in mouse embryos

Abstract

Establishment of vertebrate left-right asymmetry is a critical process for normal embryonic development. After the discovery of genes expressed asymmetrically along the left-right axis in chick embryos in the mid 1990s, the molecular mechanisms responsible for left-right patterning in vertebrate embryos have been studied extensively. In this review article, we discuss the mechanisms by which the initial symmetry along the left-right axis is broken in the mouse embryo. We focus on the role of primary cilia and molecular mechanisms of ciliogenesis at the node when symmetry is broken and left-right asymmetry is established. The node is considered a signaling center for early mouse embryonic development, and the results we review here have led to a better understanding of how the node functions and establishes left-right asymmetry.

Fig. 1

Establishment of left–right asymmetry during early mouse embryogenesis. Whole-mount view of wild-type mouse embryos from embryonic day (E) 7.75–E10.5. Around E7.75–E8.0, left–right symmetry is broken at the node (left panel, red marked on the ventral side of embryo). Next, Nodal-Lefty2-Pitx2 gene cascades are initiated to develop the patterning of lateral plate mesoderm (middle panel, showing Pitx2 gene expression). Finally, situs-specific organogenesis is seen in heart looping at E10.5 (right panel). A anterior, P posterior, L left, R right

Fig. 2

Generation of leftward fluid flow at the node in mouse embryos. a Scanning electron microscopic analysis of the node at E8.0. Wild-type mouse embryos show a cup-shaped node with evenly distributed ciliated cells when viewed from the ventral side. b High magnification images from (a). Around 3–4 μm length of primary cilia at E8.0 are developed in each node cell. c Model illustrating the posterior tilting of nodal cilium in pit cell. Nodal cilia are posteriorly tilted to generate a leftward laminar flow (a, b) were reproduced from [34]

Fig. 3

Two hypothetical models to sense the nodal flow. a Nodal cilia expressing PKD2/PKD1L1 complex function as a mechanosensors unit that relays the information of nodal flow direction by inducing left-side-specific Ca²⁺ signaling. b Nodal flow transports chemicals that trigger the initiation of left-side-specific gene regulation. Nodal vesicular parcels (NVPs) that carry SHH and retinoic acid are transported by nodal flow

Fig. 4

Importance of cell-cycle arrest in ventral surface of node cells. Cell-cycle progression was examined by Ki-67 immunohistochemistry in E8.0 mouse embryos. Note that Ki-67 is not detectable in cells of the ventral surface of node where nodal cilia are developed

Fig. 5

Cell quiescence is required for nodal cilia formation and is controlled by BMP signaling. Ventral node was stained by Ki-67 (red) (a–f), p27^Kip1 (green) (a, c, e) and p27^Kip1 phospho-Ser¹⁰ (green) (b, d, f) in wild-type (WT) and Acvr1 cKO embryos. A anterior, P posterior (c–f) were reproduced from [34]