The pea-shaped human pisiform results from the evolutionary loss of a growth plate

The human pisiform is a small nodular bone that serves as the distal attachment for the powerful forearm muscle, the flexor carpi ulnaris. In contrast, most other mammals, including primates and the ancestral hominid Australopithecus afarensis, possess a pisiform that is substantially elongated. An underappreciated fact is that in these species the pisiform develops from two separate ossification centers. A secondary center implies the presence of a growth plate (similar to the calcaneus) whose loss could serve as an evolutionary mechanism underlying human pisiform reduction. Here we perform a histological analysis of pisiform ossification in juvenile mice. We confirm the existence of a pisiform growth plate with distinct organized columnar and hyptertrophic chondrocyte zones. We also conducted a comparative analysis of hominoid pisiform ossification and confirm that epiphyses are present in Pan, Gorilla, and Pongo but not in humans. In addition, the initial ossification occurs substantially earlier in apes relative to humans, thus raising questions concerning the homology of the human pisiform and the two distinct ossification centers of other mammals. The research is funded by Penn State University.