Developmental dynamics of pelvis formation in the chick

While the knowledge about body axis and limb development especially in terms of pattern formation and genetic background has enormously grown, the establishment of the axio‐appendicular junction ‐ the development of the girdles ‐ is vaguely comprehended. The present study examines pelvis morphogenesis in the chick, with respect to axio‐appendicular linkage, origin and number of elements, and the attachment process. Specimens from developmental stage HH1 27 to 34 were serially sectioned, 3D‐reconstructed and analyzed2. In contrast to amphibians3 and mammals4 the chick pelvis ab initio locates close to the body axis, although the primary skeletal element in contact ‐ the femur ‐ suggests an appendicular origin. The pelvis anlage does not reorient from the developmental to the final position as observed in other vertebrates3,4. Ossification commences later and follows a heterochronic pattern. The formation of the synsacrum, the synostotic junction with the ilium, and the omission of ventral closure significantly distinguishes pelvis formation in the chick. These result demonstrate substantial differences in the ontophyletic organization of axio‐appendicular linkage in vertebrates.