4D formation of human embryonic forelimb musculature

Abstract

The size, shape and insertion sites of muscles enable them to carry out their precise functions in moving and supporting the skeleton. Although forelimb anatomy is well described, much less is known about the embryonic events that ensure individual muscles reach their mature form. A description of human forelimb muscle development is needed to understand the events that control normal muscle formation and to identify what events are disrupted in congenital abnormalities in which muscles fail to form normally. We provide a novel, 4D anatomical characterisation of the developing human upper limb muscles between Carnegie Stage 18-22 using Optical Projection Tomography. We show muscles develop in a progressive wave, proximal to distal and superficial to deep. We show some muscle bundles undergo splitting events to form individual muscles, while others translocate to reach their correct position within the forelimb. Finally, we show palmaris longus fails to form from early in development. Our study reveals the timings of, and suggests mechanisms for, critical events that enable nascent muscle bundles to reach their mature form and position within the human forelimb.