4D-analysis of early pelvic girdle development in the mouse (Mus musculus).

The formation of limb girdles is a key-novelty in vertebrate evolution. Although the knowledge of pattern formation, genetic, and molecular analysis of limb development has prodigiously grown over the past four decades, the morphogenesis of the pelvic element, joining the appendicular with the axial skeleton has poorly been investigated. Because of their heterochrony in development and evolution, axial and appendicular skeletal elements have seldom been seen as a cojoined morphological complex. The present study examines the pelvis morphogenesis in the mouse (Mus musculus), with special focuses on the axio-appendicular linkage, the formation and number of elements, and the joint formation. Serial histological sections of specimens from Theiler stages (TH) 18-25 (Theiler, 1972) were examined using bright field microscopy. 3D-models of the growing pelvis were reconstructed from these serial sections. The generated 3D-models were subsequently integrated into a computer-animated 4D-visualization illustrating the complex developmental dynamics of the mammalian pelvis morphogenesis. The findings demonstrate that the pelvic element forms from a single mesenchymal condensation in close vicinity to the appendicular skeleton. From the early start of development the pelvic element is limb-associated, and quite lately connects to the axial skeleton. Additionally, the 4D-visualization of the entire developmental process reveals a yet unnoticed reorientation of the mouse pelvic element from an initial posteriorly oblique developmental position to a ventrally oblique definitive position.