At the DMDD one of our projects is to measure the amount of variation in wild-type embryos. But this led us to ask the question – what is normal? We asked some leading scientists for their opinions and found that it can be quite hard to define.

What does normal mean in your research field? Comment on this post or join the debate on Twitter #dmddnormal

'Define normal' cartoon


 Tim Mohun, DMDD Programme

‘Normal’ is what we expect – so the real answer turns on what drives our expectation – Experience? Prior knowledge? Inference? Prejudice?

For embryo structure and development, we rely on experience and prior knowledge (our own, or more commonly that of acknowledged experts like Kaufman). But even reference atlases are incomplete in lots of ways. They show only individual embryos, using a limited number of stages and based on a particular 2D imaging technique.

Embryos, like people, will lie in a bell-shaped distribution – we really need to study and compare many embryos and many stages to appreciate the extent of the distribution we call ‘normal’. The atlases also cannot encompass all the many variations we know can be observed between different mouse strains and genetic backgrounds – so there are many subtly different ‘normals’.

What you see depends on how you look, which affects what you understand as ‘normal’. 3D analysis shows things that are difficult or impossible to see or interpret by 2D. So ‘normal’ is often a misleading shorthand that can hide enormous range and variation. Perhaps its more useful to ask what is ‘abnormal’ – how do we set the limits of what we understand as ‘normal’?

Robin Lovell-Badge, the Francis Crick Institute

Of course, I think I am (fairly) normal – but others might disagree. I also suspect a C57BL/6J mouse might think a 129S8 mouse is abnormal and vice versa. They certainly behave differently. And from studies on sex determination, we know that, in the early gonad, the balance of gene expression relevant to ovary versus testis differentiation is skewed in favour of the former in C57BL6/J compared to that seen in 129S8.

This explains why C57BL/6J mice are far more sensitive than many other strains to mutations or allelic variants of genes involved in testis differentiation and are far more likely to show XY female sex reversal. This means that ‘wild-type’ is very context dependent and in the context of an experiment the term relates to the particular strain used. And of course, no inbred mouse strain is directly comparable to any mouse found in the wild.

John Skehel, the Francis Crick Insitute

We work with RNA viruses, which have high rates of mutation, about 10-4 to 10-5, without repair mechanisms. So the product of a replication cycle is viewed as a quasi-species and normal, genetically and phenotypically, as a consensus.

Jacqui White, Wellcome Trust Sanger Institute

I confess we have migrated away from the using the term ‘normal’, as what is normal for one strain of mouse is entirely different for another.

The inbred strain 129 does not have a corpus callosum (the white matter tracks linking the left and right hemispheres of the brain), but agenesis of that structure in a C57BL/6 sub-strain is an interesting find. Even within sub-strains ‘normal’ is different. For example, C57BL/6J mice are virtually resistant to seizures and retain a normal retina into adulthood, whilst C57BL/6N mice are more susceptible to seizures and will develop retinal degeneration as adults.

We have moved away for using the term normal and now use ‘as expected’ in its place. ‘As expected’ describes the observation on the basis of a well-established baseline of control data, and deviation from that baseline can then be described as abnormal.

Dorota Szumska, University of Oxford

In analysing mouse embryos, ‘normal’ means ‘developing as expected’ or ‘developing in a way so to form a healthy animal’ and is based on thousands of observations and the anatomical knowledge of the studied object. It means that all embryonic structures are formed as expected to provide a shape and a function characteristic for the given organism (in this case, a mouse). This would depend on the developmental stage, for example unfused palatal shelves are absolutely ‘normal’ at the early stages while it would be considered a malformation later on in development.

‘Normal’ can also mean ‘typical’ for the genetic background or the mouse line, as these may differ in some features like coat colour or eye pigmentation. But variability of a given feature can also be ‘normal’ within a mouse strain, for example the number of splits in some nerves or the number of canals in some bones.

Janet Rossant, Hospital for Sick Kids

Normal embryogenesis means that phenotypic variation falls within a fairly narrow range and is compatible with full development to term.

Antonella Galli, Wellcome Trust Sanger Institute

Normal is independent of any phenotype observed, as it is relative to your point of comparison.  Sometimes we may say that it’s normal for a particular mouse mutation to show a certain phenotype – they might have no kidneys or no limbs. Then the embryos not displaying that phenotype would be abnormal.


Whether you agree or disagree, tell us what you think! Comment on this post or join the debate on Twitter #dmddnormal