ENST: Growing a Bone Requires Foresight

Keeping bones aligned with
attachment points as they grow
is no small engineering feat

This article by CEH editor David Coppedge appeared anonymously at Evolution News ten years ago this month. It discusses an amazing feature of animals with skeletons: how bones keep parts aligned as ratios of attachment points change during growth.

Growing a Bone Requires Foresight
Evolution News and Science Today, August 31, 2015

Picture a linear bone growing. Say it’s a leg bone, with attachment points for muscles and tendons along its length. Picture one such protrusion located a third of the way from one end. If the bone grows only at one end, the protuberance will migrate from its 1/3 position, causing problems for the tissues that need to attach there. If the bone grows at both ends, the same problem can occur.

How does the bone “know” to keep its structures at proper ratios along its length as it grows? That problem was investigated by a team of Israeli scientists publishing in PLOS Biology.

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Although bidirectional elongation is a universal mechanism for bone growth, it nevertheless introduces a major challenge to bone morphogenesis. A fundamental characteristic of the unique morphology of each long bone is a set of protrusions of varying shapes and sizes, which are scattered along the exterior of the bone and thus break its morphological symmetry. These superstructures, known as bone ridges, tuberosities, condyles, etc., are necessary for the attachment of tendons and ligament as well as for articulation. To perform these functions they are located at specific positions along the bone. Bone superstructures emerge during early skeletogenesis. During growth, bones elongate extensively by advancement of the two growth plates away from the superstructures. It is therefore expected that during elongation, superstructures would remain at their original position near the center of the bone. Nevertheless, the end result is proper spreading of superstructures along the mature bone, which clearly implies the existence of a morphogenetic mechanism that corrects their locations. [Emphasis added.]

Bones end up with the right ratios, in other words, but how do they get that way? The team wanted to know if bone growth is isometric (“same-measure”) or allometric (“other-measure”). If isometric, the bone’s ratios should be maintained during growth. If allometric, the ratios should converge on the proper position at the end of growth. They were surprised at the result and the implications….

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