The massive cow femur I keep on a shelf right in front of me in my office clearly demonstrates that the shaft of a long bone is anything but a straight, smooth, symmetric tube. It is unevenly flattened and covered with ridges and grooves, bulges and depressions. This extremely intricate topography matches perfectly with adjacent organs and tissues, tendons and joints, thereby enabling the musculoskeletal system to perform its vital functions. Oddly enough, the mechanisms that sculpt the unique and elaborate surface of each bone have been largely overlooked by the study of bone development.
My lab has studied musculoskeletal development for quite a few years, focusing on aspects such as the contribution of muscle contraction to joint formation (Kahn et al., 2009) and the involvement of tendons and muscles in the development of bone protrusions (Blitz et al., 2009). Fascinated by the morphogenetic riddle of the circumferential shape of bones, I have assembled a multidisciplinary team of scientists and students to tackle it. I first approached Prof. Ron Shahar from the Hebrew University, a scientist, veterinarian and engineer, who is an expert in bone orthopedics and mechanobiology. Together, we brought in the then Ph.D. student Amnon Sharir, also a vet, who took upon himself to integrate the developmental and biomechanical aspects of the work. I also recruited specifically for this project Tomer Stern, then an M.Sc. student with a background in mathematics and informatics.
We began by developing scanning protocols and data processing algorithms for the analysis of bone development using micro-CT images. Soon enough, we discovered that the minute dimensions of embryonic mouse bones at the onset of ossification, the low and varying mineral levels and the complex and diverse morphology all presented major obstacles to our efforts. When we finally obtained the first lucid images of how a long bone is formed, our frustration turned into excitement. We could clearly see how a ring of mineral is formed around the developing cortex, followed by construction of perpendicular struts on which the next layer of mineral is laid. The bone shaft gradually became wider and the cortex thicker, until the latter was eroded from within to reach its final thickness.
A prominent and fascinating observation was that this process turned the circumference of the bone shaft from an almost perfect circle in the cartilage anlage, to the typical uneven outline of each fully ossified bone. Using a technique we had designed to visualize and quantify three-dimensional bone features by two-dimensional color maps, we realized that this shaping process involved nonuniform distribution of mineral deposition. We therefore termed this developmental program preferential periosteal growth.
Our next challenge was to uncover the regulatory mechanisms that underlie this morphogenetic process. Using muscular dysgenesis (mdg) mice, which lack muscle contractility, we showed that muscle-induced force is required for the mineralization patterns observed in wild type embryos and for the emergence of the resulting circumferential shape. Mechanical testing revealed that the properly shaped wild type bones had a larger load-bearing capacity. Finally, analysis of the distribution of osteoblasts showed that in bones that experience muscle loads, differential distribution of these bone-forming cells is responsible for preferential bone growth.
Our study expands the prevailing model of bone development by incorporating the contribution of periosteal bone formation, under regulation of muscle forces, to the shaping of the specific three-dimensional design of each long bone. Further study is required to uncover the entire molecular and cellular regulatory pathway that transduces mechanical signals into specific patterns of mineral deposition and bone formation.
Amnon Sharir, Tomer Stern, Chagai Rot, Ron Shahar, & Elazar Zelzer (2011). Muscle force regulates bone shaping for optimal load-bearing capacity during embryogenesis Development, 138 (15), 3247-3259 : 10.1242/dev.063768
Kahn, J., Shwartz, Y., Blitz, E., Krief, S., Sharir, A., Breitel, D., Rattenbach, R., Relaix, F., Maire, P., & Rountree, R. (2009). Muscle Contraction Is Necessary to Maintain Joint Progenitor Cell Fate Developmental Cell, 16 (5), 734-743 DOI: 10.1016/j.devcel.2009.04.013
Blitz E, Viukov S, Sharir A, Shwartz Y, Galloway JL, Pryce BA, Johnson RL, Tabin CJ, Schweitzer R, & Zelzer E (2009). Bone ridge patterning during musculoskeletal assembly is mediated through SCX regulation of Bmp4 at the tendon-skeleton junction. Developmental cell, 17 (6), 861-73 PMID: 20059955