The skeletal shift from dermal to endochondral bones in the pectoral shoulder girdles. (A–C) The pectoral girdles of Eusthenopteron(A), Ichthyostega(B), Eryops(C). Brown shaded bones (extrascapular, post-temporal, supracleithrum, and anocleithrum) in (A) have been lost during the fish-to-tetrapod transition. Shaded bones depict endochondral bones. Note that the endochondral bones have enlarged with the concomitant decrease of the dermal bones. In (B,C), the spaces between the skull and pectoral girdle demonstrate the origin of the neck. (D) The developmental mechanisms of the scapula in mouse and chicken embryos. The epithelial-mesenchymal transition produces prospective scapular cells from the dermomyotome. The understanding of developmental programs underlying the dermal bone development is still poor (see text). acl, anocleithrum; acr, acromion; cl, cleithrum; cla, clavicle; g, glenoid fossa; ic, interclavicle; sc, scapula; sca, scapulocoracoid; scl, supracleithrum; sp, spine; Illustrations in (A–C) are redrawn with permissions form Andrews and Westoll (1970), Jarvik (1980), and Gregory (1951). The illustration of the mouse scapula is adapted with permission from Kuijper et al. (2005).
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