How do embryos know their left side from right ? Role of Nodal signalling in Left-right asymmetry in Snails – PART I

By | May 7, 2021
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Introduction Symmetry:

A major and long-standing goal of developmental biology is to understand how embryos initially know their left and right hand sides, a process known as “symmetry breaking”. If you look for symmetry in wikipedia

” Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically-pleasing proportionality and balance[citation needed]; such that it reflects beauty or perfection. The second meaning is a precise and well-defined concept of balance or “patterned self-similarity” that can be demonstrated or proved according to the rules of a formal system: by geometry, through physics or otherwise”-Wikipedia

When it comes to biology, symmetry can be put in following words: regularity in form and arrangement; orderly and similar distribution of parts, such that an animal may be divided into parts which are structurally symmetrical.

Animals can be divided in Bilateral symmetry as seen in majority of animals eg: vertebrates, etc, is that in which the body can be divided into symmetrical halves by a vertical plane passing through the middle.Some animals exhibit ,what called as radial symmetry, as in echinoderms (exhibit bilateral symmetry in their larvae, and are hence classed as bilaterian), is that in which the individual parts are arranged symmetrically around a central axis and some animals are also devoid of any symmetry like sponges.

At first glance, the left and right sides of our bodies are identical to one-another. However, internally, as we dig deeper, some of our visceral organs are displaced to one side. Our hearts are larger on the left than the right, brains also show significant differences and when one take a look at long coil of guts with appendix near one end, it generally folds one side or to the another , but most interesting for many people in the field is that how things so consistently folds over in the same direction in individual after individual.Lot of work has gone and going on in this particular field of left right asymmetry and so far a decent understanding is available for the entire mechanism especially in vertebrates.

Nearly all visceral organs of the thorax and abdomen are Left-Right (LR) asymmetrical in their anatomy, placement and, in some cases,physiology.Bilateral symmetry appears mainly in structures such as the brain, nervous system, skin, hair, and nails, and in parts of the eye and ear — all of which arise from the outer germ layer of the embryo called as ectoderm and some structures, including the skeleton and skeletal muscles, tendons, glands, and reproductive organs, which develop from the mesoderm (middle germ layer). The heart, originating in the mesoderm, and the liver, stomach, pancreas, and intestines, which arise from endoderm (the inner germ layer) appear singly to one side of the midline.

Asymmetry plays a vital role in development and any abnormalities in LR asymmetry of internal organs can lead health defects in the individuals.Hence a complete understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes.

With this brief introduction about LR asymmetry we shall look ( in the next post) into an article from Cristina Grande & Nipam H. Patel in nature describing the role of Nodal in generating Left right asymmetry in snail and these results also indicate that the involvement of the Nodal pathway in left-right asymmetry might have been an ancestral feature of the Bilateria.

Further reading on LR asymmetry :

1) Left-right asymmetry in embryonic development: a comprehensive review.
Levin M.
Mech Dev. 2005 Jan;122(1):3-25. Review. Erratum in: Mech Dev. 2005 Apr;122(4):621.
PMID: 15582774 [PubMed – indexed for MEDLINE]

2) The embryonic origins of left-right asymmetry.
Levin M.
Crit Rev Oral Biol Med. 2004 Jul 1;15(4):197-206. Review.
PMID: 15284185 [PubMed – indexed for MEDLINE]

3) What’s left in asymmetry?
Aw S, Levin M.
Dev Dyn. 2008 Dec;237(12):3453-63.
PMID: 18488999 [PubMed – in process]

4) Type ID unconventional myosin controls left-right asymmetry in Drosophila.
SpÃder P, Adam G, Noselli S.
Nature. 2006 Apr 6;440(7085):803-7.

Mark Mercola and Michael Levin
Annual Review of Cell and Developmental Biology
Vol. 17: 779-805 (Volume publication date November 2001)