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Morphogenesis and form  ·  Article

Left-right asymmetry

Left-right asymmetry is the consistent difference between the two sides of a body that looks symmetrical from outside. In vertebrates the heart and viscera take fixed positions, and errors in this patterning form a class of human birth defects. Authors in the held texts treat it as a problem of pattern. It is tied to the other body axes and open to bioelectric and chemical control.5

Earliest held
1852, Du Bois-Reymond, E. (ed. H.…
Most discussed in
The Principles of Psychology, Vols. 1-2, 1890
In the library
3,007 passages in 152 works
Rewritten
2026-10-05
18501875190019251950197520002025Bateson, W., 18941Loeb, J., 19182Child, C. M., 19243Thompson, D. A. W., 19924Carneiro K, Donnet C, Rejtar T, Karger…, 20115Carneiro K, Donnet C, Rejtar T, Karger…, 20116Vandenberg LN, Pennarola BW, Levin M, 20117Pai VP, Vandenberg LN, Blackiston D…, 20128McMillen P, Levin M, 20249
01

Early observations

Bateson (1894) noted that in many substantially symmetrical animals an organ present on both sides can differ between them. He set aside the unpaired viscera, such as the heart and liver, as a separate matter.1 Loeb (1918) reported Jennings's account of certain infusorians. Jennings read their behaviour as a trap action that depends on the asymmetric arrangement of the cilia, not as a tropism. Loeb thought Jennings was probably correct in this.2 Child (1924) wrote that in partial duplications in vertebrates the visceral asymmetry is often reversed in one component. He had no experimental analysis and offered little more than surmise.3 Thompson quoted Japp's claim that only asymmetry can beget asymmetry.4

02

Types of error

Carneiro and colleagues (2011) list the failures of laterality. They are isomerism, which is loss of asymmetry, situs inversus, which is complete inversion, and heterotaxia, in which organs are placed at random and lose concordance.5 Vandenberg, Pennarola and Levin (2011) vibrated Xenopus embryos at different stages. Vibration beginning at the one-cell stage gave large amounts of situs inversus. Situs inversus keeps asymmetry but orients the left-right axis randomly against the other two axes. The authors suggest that early vibration uncouples a mechanism that orients the axis.7 Later vibration disrupted concordance instead, as reagents acting on ion flux did.7

03

Signals and the wider body

Carneiro and colleagues (2011) state that in Xenopus the movement of maternal serotonin through gap-junctional paths at cleavage stages dictates asymmetry upstream of Nr1. They add that the link between earlier biophysical asymmetries and transcriptional control was not known.6 Pai, Vandenberg, Blackiston and Levin (2012) point to a disconnect between organs and brain. Human situs inversus patients show normal levels of right handedness and language lateralization.8 They argue that left-right identity may be more widespread in the body than is appreciated, and they searched for asymmetries in functional physiology.8

04

Concordance within an individual

McMillen and Levin (2024) describe the vertebrate left-right axis as an example of collective behaviour. Many treatments randomize the outcome, yet the randomization acts above the level of the individual. A given embryo has a consistent identity on its left and right sides, and all of its cells agree.9 They know of only one study that breaks this concordance. It disrupted planar polarity by down-regulating VANGL signaling.9 They place this among their examples of cells acting in agreement.9

SourcesEach quotation was checked word for word against the passage it opens.
  1. Nevertheless in many substantially symmetrical forms there is asymmetry in the condition of some one or more organs present on both sides.Bateson, W., 1894 · Materials for the Study of Variation Treated with Especial Regard to… · open at passage 1397
  2. He interprets both phenomena as a trap action due to the asymmetry of certain infusorians; a sudden change in the concentration of a solution causes a reverse of the stroke of their cilia by which the organism is driven back.Loeb, J., 1918 · Forced Movements, Tropisms, and Animal Conduct · open at passage 258
  3. In partial duplications in vertebrates it is often found that the usual visceral asymmetry is reversed in one of the components.Child, C. M., 1924 · Physiological Foundations of Behavior · open at passage 435
  4. Only the living organism, or the living intelligence with its conception of asymmetry, can produce this result. Only asymmetry can beget asymmetry.Thompson, D. A. W., 1992 · On Growth and Form · open at passage 929
  5. Errors in LR patterning include loss of asymmetry (isomerism), complete inversions (situs inversus), and random placement of individual organs (loss of concordance known as heterotaxia).Carneiro K, Donnet C, Rejtar T, Karger BL, Barisone GA…, 2011 · Histone deacetylase activity is necessary for left-right patterning during… · open at passage 3
  6. In Xenopus, movement of maternal serotonin (5HT) through gap-junctional paths at cleavage stages dictates asymmetry upstream of Nr1.Carneiro K, Donnet C, Rejtar T, Karger BL, Barisone GA…, 2011 · Histone deacetylase activity is necessary for left-right patterning during… · open at passage 0
  7. Because individuals with situs inversus maintain LR asymmetry but orient their LR axes randomly compared to the other two axesVandenberg LN, Pennarola BW, Levin M, 2011 · Low frequency vibrations disrupt left-right patterning in the Xenopus embryo · open at passage 17
  8. Human situs inversus patients (who exhibit complete reversal of the left-right body axis) show normal levels of right handedness and language lateralizationPai VP, Vandenberg LN, Blackiston D, Levin M, 2012 · Neurally Derived Tissues in Xenopus laevis Embryos Exhibit a Consistent… · open at passage 24
  9. the randomization is once again above the level of the individual: any given embryo has a consistent identity on the L and R side, and all of the cells agree.McMillen P, Levin M, 2024 · Collective intelligence: A unifying concept for integrating biology across… · open at passage 18
Linked ideas
Gap junctionspart of / contains
Carneiro et al. describe serotonin moving through gap-junctional paths at cleavage stages as setting asymmetry upstream of Nodal-related 1 in Xenopus.
Pai et al. (2017) cite ion-flux regulation of membrane voltage as a determinant of left-right laterality, feeding the Nodal-Lefty-Pitx2 node.
Pai et al. (2017) report that the HCN4 channel is required for early events regulating left-right asymmetry.
Polarityrelated to
Lobo et al. link left-right to dorso-ventral axes in vertebrates and describe medial-lateral polarity in planarians; Carneiro et al. note the left-right axis is oriented relative to other axes.
Pai et al. (2012) searched for left-right asymmetries in functional physiology, suggesting sidedness information may be present in anatomically symmetrical tissues.
Carneiro et al. cite serotonin's asymmetric accumulation in frog blastomeres, and its requirement in chick and frog, as a pre-neural step in left-right patterning.
Child treats a persistent physiological gradient as the primary regional differential; the Levin-lab passages likewise explain left-right patterning by serotonin and voltage gradients, but never cite Child or metabolic rate.
Levin shows membrane voltage experimentally controls left-right patterning, an instance of identifiable conditions fixing a response, as Bernard's determinism holds; Bateson's irregular tapeworm sidedness illustrates variability. No passage links them explicit
Both depend on bioelectric/gap-junctional signaling for body-wide patterning; planaria also show cryptic left/right asymmetry and lateralization. The passages don't link asymmetry to memory directly.
Randomized left-right outcomes stay consistent within an embryo, so cells agree on identity above the level of the individual cell.
Regenerationrelated to
Durant et al. (2016) say gap junctions are required for regeneration (retina, zebrafish fins) and for patterning the left-right axis, so both depend on shared gap-junctional, bioelectric signalling.
Tropismrelated to
Passages tie tropism to differences between two sides: Jennings/Davenport's lateral illumination of a bilateral worm, Bose's torsion from differential excitability of two halves. None describes body left-right asymmetry itself.
Only loosely linked: the planarian passage lists a cryptic left/right asymmetry and lateralized behavior alongside EEG-recorded brain waves, while Beck's passage covers only animal cortical potentials and says nothing on asymmetry.
Davison proposes inherent cytoskeletal asymmetry as an early, cilia-independent source of left-right pattern. Morgan treats polarity and bilaterality as inherent properties of living substance. Neither passage mentions mesoscopic tissue physics.
Where it is discussedPassages matching left-right, laterality, situs, asymmetry
1890The Principles of Psychology, Vols. 1-2 · James, W.230
1880The Power of Movement in Plants · Darwin, C.213
1915Principles of General Physiology · Bayliss, W. M.213
1906Plant Response as a Means of Physiological Investigation · Bose, J. C.174
1901Regeneration · Morgan, T. H.151
1924Physiological Foundations of Behavior · Child, C. M.111
1899General Physiology: An Outline of the Science of Life · Verworn, M.101
1992On Growth and Form · Thompson, D. A. W.96
1907Comparative Electro-Physiology: A Physico-Physiological Study · Bose, J. C.91
1930The History of Biological Theories · Rádl, E.73