How is bilateral symmetry broken?

Gastrulation results in the anterior–posterior and dorsal–ventral patterning of the embryo, but all of the mechanisms discussed produce bilateral symmetry. At the opening of this chapter we introduced an early mechanism that breaks that symmetry to produce the well-known left–right asymmetries of the body. The heart is on the left; the major lobe of the liver is on the right; the pancreas and spleen are on the left; the appendix is on the right; the major blood vessel leaving the heart, the aorta, bends to the left; the right lung has three lobes and the left lung has two; and so on. At the very beginning of gastrulation, the location where the involution of cells begins in a mammal is marked by a node of cells. The ventral surface of the node has ciliated cells. Some of these cilia are motile and others are not. The motile cilia create a leftward flow of extracellular fluid over the node. The experiments in Investigating Life: Does the Direction of Nodal Flow Influence the Development of Left–Right Asymmetry in the Mouse Embryo? investigate whether that directional flow is both necessary and sufficient to initiate signaling processes that break bilateral symmetry of the embryo.

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investigating life

Does the Direction of Nodal Flow Influence the Development of Left–Right Asymmetry in the Mouse Embryo?

experiment

Original Paper: Nonaka, S., H. Shiratori and H. Hamata. 2002. Determination of left–right patterning of the mouse embryo by artificial nodal flow. Nature 418: 96–99.

image

work with the data

The experiment illustrated in the graph was done on presomite embryos—before any somites had formed. Developmental signals usually have a time window of effectiveness given that developmental events are sequential and different events must be coordinated. Is there a time window within which nodal flow determines left–right symmetry in the embryo? To investigate the timing of sensitivity to nodal flow, another experiment was conducted using only fast right flow on early embryos at different stages of development.

Genotype, stage Results
Wild type, presomite 2N, 21R, 24T
Wild type, 1-somite 9N, 0R, 12T
Wild type, 2-somite 22N, 0R, 22T
Wild type, 3-somite 14N, 0R, 14T

QUESTIONS

Question 1

Plot the data as the percent of responses on the y axis and developmental stage on the x axis.


Question 2

Why did this experiment not include left flow as a stimulus?

The experiment did not include left flow because we know left flow is the normal stimulus and that it is only disrupted by a fast right flow. This experiment was aimed at determining the sensitive period for the ability of that disruptive stimulus to alter the normal developmental pattern.

Question 3

Do these data provide evidence for a sensitive period for nodal flow determining left–right asymmetry?

The data from embryos of different developmental stages support the conclusion that there is a sensitive phase of development where the left–right symmetry of the embryo is determined, and that it is between the presomite and 1-somite stages.

Question 4

What do you conclude from the results on the 1-, 2-, and 3-somite wild-type embryos?

The results on 1-, 2-, and 3-somite wild-type embryos exposed to fast right flow show that the left–right asymmetry was determined at an earlier stage and that the embryo is no longer sensitive to direction of flow over the node.

A similar work with the data exercise may be assigned in LaunchPad.