How to attach the limb to the body axis

Abstract

While the question “How to make a limb” is extensively addressed in terms of pattern formation, morphogenesis, and genetic background, the ontophyletic scenario of attaching the limb to the axial skeleton via girdles has been paid much less attention. In a comparative study we have investigated pelvis morphogenesis and axio‐appendicular linkage in species from different vertebrate classes. Series of developmental stages from frog tadpoles1, chick embryos and mouse embryos2 were serially sectioned, and 3D‐reconstructed3. The 3D‐models were aligned to 4D‐visualizations of developmental dynamics and further analyzed.Our data reveal, that in all three species ‐ at the morphological level ‐ the pelvic element originates from a single anlage in close relation to the femur. Shape and orientation, however, differ already at initial stages and are followed by divergent subsequent morphogenesis. The timing and patterns of ossification are significantly different. In frog tadpoles a connective tissue strand serves a guiding function for attachment to the sacral vertebra1.These results demonstrate substantial differences in the establishment of the axio‐appendicular linkage among vertebrates and prompt for clarification of their genetic and epigenetic background.