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The asymmetric Nodal signaling cascade as a prerequisite for asymmetric body plan specification is conserved among deuterostomes. In this review we argue that symmetry breakage by cilia-driven leftward flow presents an ancestral character of vertebrates, likely the chordate phylum and maybe all deuterostomes. In vertebrates, leftward flow occurs in a transient structure, a monociliated epithelium, which is derived from superficial mesoderm and localizes to the archenteron roof during gastrulation. The chick as an example for the highly derived birds lacks superficial mesoderm and flow. This loss should be secondary, as flow is present from fish and amphibians to mammals.
Fig. 1.
Sites of leftward flow in the vertebrates. (A and B) Mammals. PNC in a 3-somite rabbit (A) and an E8.0 mouse embryo (B). (CâE) Amphibians. GRP in neurula embryos of Xenopus laevis (C; stage 18), Xenopus tropicalis (D; stage 15/16) and axolotl (E; stage 17/18). (F and G) Fish. KV in medaka (F) and GRP in the white sturgeon (G, stage 21; Gâ², stage 25). SEM pictures of embryos shown in ventral view and oriented anterior up. Dashed lines highlight ciliated epithelia. (Aâ²), (Bâ²), (Fâ²) and insets in (CâE) and (G) show higher power magnifications of ciliated cells. Red arrowheads in inset of (G) mark two monocilia. Scale bars represent 50 μm (AâD, F, G, and Gâ²), 200 μm (E), and 10 μm (Aâ², Bâ², Fâ², insets in CâE, G). (Panels F + Fâ² reprinted from [15] with permission from Elsevier. Panel G reprinted from [42] with permission from Wiley.)
Fig. 2.
Localization and size of KV, GRP and PNC. SEM pictures of medaka larva (A), posterior half of X. laevis neurula (B), rabbit blastodisc (C) and mouse egg cylinder (D), shown in ventral view and oriented anterior up in (A, C and D), or in anterior view dorsal side up in (B). KV, GRP and PNC are highlighted in green. (E) Variation of size and shape of ciliated field of cells in various embryos. Cilia numbers are indicated where known (medaka and rabbit numbers from [15], others from own analyses). n, notochord; s, somite. Red arrowheads mark organizer tissue. Scale bars represent 50 μm (A + D), 200 μm (B + C) and 100 μm (E). (Panel A reprinted from [15] with permission from Elsevier.)
Fig. 3.
The laterality coordinator. Schematic representation of an idealized vertebrate laterality coordinator. A field of cells with posteriorly polarized cilia (green) is bordered by Nodal expressing cells (blue) on either side, and situated in the roof of the archenteron. Ventral view from posterior (front) to anterior (back). Posterior polarization, the ensuing tilt and the clockwise rotation of cilia result in a net leftward flow of extracellular fluid (arrow). Inset highlights tilt and rotation of a single ciliated cell.
Fig. 4.
Amphioxus and sea urchin. (A) Amphioxus. Schematic drawing of a neurula stage amphioxus embryo cut transversely (A) and mid-sagittally (Aâ²). The bilateral tell-tale Nodal expression indicated in blue (A) predicts an in-between GRP in the archenteron (green in A, Aâ²). (B and C) Sea urchin. 3D schematic drawings of early (B and Bâ²) and late (C and Câ²) gastrula sea urchin embryos in whole-mount (B and C) and bisected along the dorso-ventral (Bâ²) and left-right (Câ²) axis, respectively. Proposed right half shown in (Bâ²), and oral half shown in (Câ²), Early (dorsal/aboral) symmetric Nodal organizer domain in (B) and (Bâ²) and late asymmetric expression domains in ectoderm and archenteron in (C) and (Câ²) are indicated in blue. The supposed position of a field of polarized ciliated cells is indicated in green. The conventional echinoderm body plan is inverted to account for (a) conserved left asymmetry of Nodal expression and (b) organizer gene expression on the dorsal side. Expression domains are indicated according to [9] (A), [72] (B and Bâ²) and [10] (C and Câ²). Schemes were redrawn from [9] (A) and [73] (Aâ²). a, anterior; ar, archenteron; bc, blastocoel; bp, blastoporus; d, dorsal; ec, ectoderm; en, endoderm; l, left; n, notochord; nc, neuroenteric canal; ne, neuroectoderm; pm, paraxial mesoderm; r, right; v, ventral.
