Embryonic nodal flow and the dynamics of nodal vesicular parcels

Abstract

We address with fluid-dynamical simulations using direct numerical techniques three important and fundamental questions with respect to fluid flow within the mouse node and left-right development. First, we consider the differences between what is experimentally observed when assessing cilium-induced fluid flow in the mouse node in vitro and what is to be expected in vivo. The distinction is that in vivo, the leftward fluid flow across the mouse node takes place within a closed system and is consequently confined, while this is no longer the case on removing the covering membrane and immersing the embryo in a fluid-filled volume to perform in vitro experiments. Although there is a central leftward flow in both instances, we elucidate some important distinctions about the closed in vivo situation. Second, we model the movement of the newly discovered nodal vesicular parcels (NVPs) across the node and demonstrate that the flow should indeed cause them to accumulate on the left side of the node, as required for symmetry breaking. Third, we discuss the rupture of NVPs. Based on the biophysical properties of these vesicles, we argue that the morphogens they contain are likely not delivered to the surrounding cells by their mechanical rupture either by the cilia or the flow, and rupture must instead be induced by an as yet undiscovered biochemical mechanism.

Figure 1

Sketch of a vertical slice across the node viewed from the ventral side showing the monocilia producing the leftward flow that transports NVPs. The mouse node is some 50 μm across by 10 μm depth. Note that, following the convention in this field, in this and all subsequent vertical slices of the node shown here, the node is seen from the ventral side, and thus the left side of the embryo is on the viewer's right.

Figure 2

General three-dimensional views of particle trajectories in simulated nodal flow: (a) the upper recirculation within the in vivo node and (b) the equivalent situation in vitro.

Figure 3

Horizontal slices of the velocity field in the node: on the left in vivo and on the right in vitro. (a) The upper slices are at 8 μm, near the top of the node; (b) the middle slices are at 5 μm, in the centre of the node; and (c) the lower slices are at 0.2 μm, next to the floor of the node. The cilia appear in the upper slices just as a guide for the eye.

Figure 4

Vertical views corresponding to y-direction averages of the velocity field in the node: (a) in vivo and (b) in vitro. (c) General view of the in vitro node immersed within a container of much larger volume; so (b) is seen at bottom centre.

Figure 5

Histograms showing the relative frequency with which a NVP breaks as a function of its position from right to left along the floor of the node. (a) The in vivo case and (b) the in vitro flow.