Summary
Cilia-driven flow in the embryonic node breaks symmetry, leading to asymmetric organ placement.
In depth
Motile cilia in the node create a leftward flow of fluid, leading to Nodal signalling on the left side. Defects cause situs inversus (mirror image) or heterotaxy with heart defects. Kartagener syndrome features ciliary defects.
Nodal flow is generated by rotating cilia in the node. A Nodal-Lefty-Pitx2 cascade on the left side positions organs. Disrupting cilia leads to randomised laterality.
Key idea: asymmetry arises from a physical flow signal.
Vertebrates look bilaterally symmetric outside but have asymmetrical internal organs: the heart loops to the left, the liver sits on the right and the stomach on the left. Symmetry is broken in a transient structure, the node (or Kupffer's vesicle in zebrafish).
Motile cilia in the node rotate and produce a leftward fluid flow, detected by immotile cilia, leading to calcium signalling and expression of Nodal and Pitx2 on the left lateral plate. Mutations affecting ciliary motion cause situs inversus in about half of cases, as in primary ciliary dyskinesia (Kartagener syndrome).
Example: Kartagener patients have sinus infections and may have mirror-image organs.
Remember: a small fluid flow sets a body-wide pattern.
Frequently asked questions
What is Left-Right Asymmetry?
Cilia-driven flow in the embryonic node breaks symmetry, leading to asymmetric organ placement.
Which area of biology covers Left-Right Asymmetry, and how advanced is it?
Left-Right Asymmetry is one of the 54 topics in Developmental Biology. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Hedgehog Signalling, BMP and TGF-Beta Signalling, Organogenesis.
Can you give an example of Left-Right Asymmetry?
The heart loops rightward and the liver sits on the right in most people.
What should I learn before Left-Right Asymmetry?
Work through the Beginner topics in Developmental Biology first, such as Fertilisation, Zygote, Embryo, then return to Left-Right Asymmetry. Advanced topics assume the Intermediate vocabulary.
What topics are related to Left-Right Asymmetry?
The closest topics to Left-Right Asymmetry are Hedgehog Signalling, BMP and TGF-Beta Signalling, Organogenesis, Neural Crest Cells. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Left-Right Asymmetry?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Left-Right Asymmetry. To practise the whole of Developmental Biology, use the Developmental Biology quiz.
How is Left-Right Asymmetry different from Hedgehog Signalling?
Left-Right Asymmetry: Cilia-driven flow in the embryonic node breaks symmetry, leading to asymmetric organ placement. Hedgehog Signalling: A morphogen pathway patterning the neural tube and limbs through Gli transcription factors. Read both side by side and note what each one explains.
How should I study Left-Right Asymmetry step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Hedgehog Signalling and explain the difference aloud. Finally, take the Developmental Biology quiz and come back to this page for any question you miss.