Dependence of Bone Microarchitecture Upon Collagen α1(XI) Expression During Embryonic Development

Skeletogenesis occurs through both intramembranous ossification, and endochondral ossification, the latter of which features a cartilage intermediate. The cartilage intermediate is comprised of heterotypic molecules formed from collagens type II, IX, and XI. Recently, a number of cell culture and transgenic model studies have posited a function for the alpha 1 chain of collagen XI beyond contributing to cartilage formation. Further, collagen α1(XI) mutations in humans generate a series of spondyloepiphyseal dysplasias, including Stickler syndrome and Marshall syndrome. These disorders present clinical skeletal symptoms, including abnormal epiphyseal development, irregularity of the margins of the vertebral bodies, thick calvaria, short stature, and intracranial calcifications, supporting a more extensive role in skeletal formation for collagen α1(XI). These skeletal symptoms are similar to those seen in the homozygous chondrodysplasia (cho) mouse model, a functional knockout for collagen α1(XI). This study investigates the role of the alpha 1 chain of collagen XI at the organismal level by quantifying the skeletal abnormalities in the bones of the homozygous cho mouse using microcomputed x-ray tomography (micro-CT) analysis. Geometric analysis was conducted to quantify the decreases in length and increases in width of long bones compared with littermate controls. Rib length and ribcage shape, as well as spinal column and vertebral dimensions were also quantified for the homozygous cho mouse, heterozygous, and wildtype mice. The absence of a deltoid tuberosity and the abnormal formation of vertebral bodies were also described for the first time in the homozygous cho mouse. Microarchitectural analysis was conducted on endochondrally formed bones of the skeleton and both endochondrally and intramembranously formed bones of the skull. Regarding the microarchitecture of the endochondrally formed bones, the homozygous cho mouse differs from wildtype mice with respect to trabecular thickness, trabecular number, trabecular separation, and trabecular percent bone volume. Relative to wildtype, homozygous cho mouse trabeculae are generally increased in thickness and number, while decreased in separation. Early homozygous cho mouse metaphyseal growth is also characterized by a disorganization in structure, and also variation in trabecular percent bone volume with a trend toward increased mineralization of the primary spongiosa. Craniofacial analysis was completed on both intramembranously and endochondrally formed bones. Most intramembranously formed bones of the homozygous cho mouse exhibited decreases in all microarchitectural indices except for the mesoderm-derived intramembranous parietal bone, which exhibited decreased trabecular separation but increased trabecular thickness, number, and trabecular percent bone volume. Endochondral craniofacial bones exhibited an increase in trabecular thickness and an increase in trabecular separation. These findings are different than the results seen in endochondrally formed long bones, ribs, and verebrae of the homozygous cho mouse and warrant further investigation. In this study, micro-CT proved to be a robust quantitative method to study bone and an ideal means to evaluate early skeletal abnormalities during development. Furthermore, the application of micro-CT technology to existing and newly developed collagen α1(XI) mouse models will allow for better understanding of collagen XI’s function during skeletogenesis at the full skeletal, individual bone, and microarchitectural levels. This study supports a structural and signaling role for Collagen α1(XI) in bone formation and provides the foundation for future research