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Veiled chameleon embryos form a pre-gastrulation cavity long credited only to mammals
Veiled chameleon embryos form the pre-gastrulation cavity once thought unique to mammals, a Nature Communications study reports. The team proposes the lizard as a model for the earliest human stages, a period scientists rarely get to observe directly.
The Scientist · Science desk
What happened
- Human embryo cells arrange themselves around a central space, whereas a chameleon embryo begins as a flat sheet that folds inward and closes like a drawstring bag.
- Chameleon embryos are still very early when the eggs are laid, while many other reptiles are much further along by then, putting those stages out of easy reach.
- Oxford's Antonia Weberling, reviewing images Natasha Shylo took at the Stowers Institute, asked whether they were really chameleon embryos because they looked exactly like a human one.
- Shylo, now at Rowan University, co-authored the study with researchers from Stowers, Oxford, Cambridge and the University of Kansas Medical Center.
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Why it matters
- capability Labs can sample the cavity-forming stage from laid eggs, which gives them a reptile series to line up against human material that is scarce at the same stage.
- constraint Findings about how the cavity is assembled will need separate checking in mammalian or human embryos, because the two species use different cellular steps to build it.
- precedent With the mammal-only account of this cavity overturned, comparative work on how early development evolved across species has a new question to answer.
Gastrulation is the stage at which an embryo starts laying out its body plan and the tissues that later become organs [15]. In mammals, a cavity forms just before it and was thought to help organize the embryo for that step. Scientists had long treated that cavity as a mammalian feature [1]. The paper's title states the revision directly: "Epiblast lumenogenesis is not a mammalian-specific trait" [2].
Chameleons and humans are separated by 300 million years of evolution [5]. The two embryos arrive at similar structures by different steps. The phys.org account takes that to suggest evolution can reach similar outcomes through different biological processes [14]. I think this is the firmest part of the work. It depends only on chameleons forming the cavity, and the study reports that they do [1].
The human-health case comes from Shylo. "By understanding how embryos develop normally, we can also better understand what happens when development goes wrong," she said [8]. She also said: "Those early developmental processes are connected to questions about infertility, early pregnancy loss and how complex organisms form in the first place." [9]
The account does not say how many embryos were imaged or which molecular markers were compared. Nor does it say whether the chameleon cavity was shown to do the organizing work it does in mammals. As Shylo tells it, Weberling's first reaction was to how the embryos looked [10]. For the lizard to stand in for human embryos in studies of infertility or early pregnancy loss, its cavity would need to play the same part. So far the reported evidence is that the cavity forms [1].
What to watch
- Whether the full paper or follow-up work shows the chameleon cavity organizing the embryo before gastrulation the way the mammalian one does.
- Whether other egg-laying species, sampled at equally early stages, also form the cavity; a wider spread would bear on whether it evolved more than once.
- What Shylo's Rowan lab publishes as she continues work on the earliest stages of human development.