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Developmental Biology - Limbs
How Limbs Form
Reflex Contractions Trigger Limbs to Form...
It normally takes about 21 days for chicken embryos to develop into chicks. By observing chicken hindlimbs forming, Université de Paris (the French National Center for Scientific Research - CNRS) research team has just identified their mechanism of origin. It consists of a sequence of reflexive contractions which researchers were able to recreate and accelerate by a factor 20times faster.
Their findings published in the European Physical Journal on August 15, 2019.
In its first days of life, a chicken embryo appears somewhat like a flat disc, organised into concentric rings from the center out. During development, the embryo is stretched, rolled up and twisted by its confinement within a small area up against its ever increasing number of growing cells. This spacial confinement seggregates the previous disc of concentric rings into a number of pinched and folded protuberances — each eventually giving rise to a separate anatomical structure.
Chick embryo spine
Researchers observed that during formation of the chick's future tail, one of these rings stretches and deforms into the hind region of the embryo. This particular shape deformation is set off by a series of reflexive contractions, a domino effect owing to the surrounding rings. These contracting rings then fold into the primitive contours of each opposing embryonic hind limb.
In order to prove the physical nature of this phenomenon, researchers administered brief (1-3 seconds), low-intensity (1 volt), shocks to the hind portion of the embryo. These electric pulses mimick the effect of natural deformation produced during tail formation, triggering faster embryonic development in a cascading pattern — even accelerating that pattern by 20-fold.
The scientists want to continue investigating the technical limits of their discovery, as this technique may be useful against certain effects of disease.
Abstract
Current knowledge on limbs development lacks a physical description of the forces leading to formation of the limbs precursors or “buds”. Earlier stages of development are driven by large scale morphogenetic movements, such as dipolar vortical flows and mechanical buckling, pulled by rings of cells. It is a natural hypothesis that similar phenomena occur during limb formation. However it is difficult to experiment on the developmental forces, in such a complex dynamic system. Here, we report a physical study of hindlimb bud formation in the chicken embryo. We use electrical stimulation to enhance the physical forces present in the tissue, prior to limb bud formation. By triggering the physical forces in a rapid and amplified pattern, we reveal the mechanism of formation of the hindlimbs: the early presumptive embryonic territory is composed of a set of rings encased like Russian dolls. Each ring constricts in an excitable pattern of force, and the limb buds are generated by folding at a pre-existing boundary between two rings, forming the dorsal and ventral ectoderms. The amniotic sac buckles at another boundary. Physiologically, the actuator of the excitable force is the tail bud pushing posteriorly along the median axis. The developmental dynamics suggests how animals may evolve by modification of the magnitude of these forces, within a common broken symmetry. On a practical level, localized electrical stimulation of morphogenetic forces opens the way to in vivo electrical engineering of tissues.
Authors
Vincent Fleury and Ameya Vaishnavi Murukutla.
Acknowledgements
The authors report no conflict of interest.
This study was supported by grant funds from AMAG Pharmaceuticals (to R.M.).
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Aug 30 2019 Fetal Timeline Maternal Timeline News
 During development, embryos are continually stretched, rolled and twisted due to their continuing cell expansion within a tightly confined space. Each movement seggregates the previous concentric tubes into unique tissues with every twist and turn. Each tissue giving rise to a unique anatomical function. CREDIT the authors.
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