Development of human limb muscles based on whole-mount immunostaining and the links between ontogeny and evolution.

We provide the first detailed ontogenetic analysis of human limb muscles using whole-mount immunostaining. We compare our observations with the few earlier studies that have focused on the development of these muscles, and with data available on limb evolution, variations and pathologies. Our study confirms the transient presence of several atavistic muscles - present in our ancestors but normally absent from the adult human - during normal embryonic human development, and reveals the existence of others not previously described in human embryos. These atavistic muscles are found both as rare variations in the adult population and as anomalies in human congenital malformations, reinforcing the idea that such variations/anomalies can be related to delayed or arrested development. We further show that there is a striking difference in the developmental order of muscle appearance in the upper versus lower limbs, reinforcing the idea that the similarity between various distal upper versus lower limb muscles of tetrapod adults may be derived.