Evidence for an extensive collagen type III proximal domain in the rat femur. II. Expansion with exercise.

Exercise in youth may affect bone "quality" as well as quantity. Using the rat model, 1.5-month-old females were divided into four weight-matched groups, exercised short-term (6 weeks, E(s), n = 20) and long-term (14 weeks, E(L), n = 10) by access to monitored running wheels, and corresponding "sedentary" controls (S(S) short-term, n = 20; S(L) long-term, n = 10). Femora were either plastic-embedded or fresh-frozen. Transverse histological slices, 100 microm thick, were cut midshaft, while similar cryosections, 8 microm thick, were prepared from the same site and also coronal to the femoral neck region. An image analyser measured femoral neck and midshaft microarchitecture, while immunostaining localized collagen type III-rich fibres (CIII, an index of Sharpey fibre insertions) and osteopontin-rich osteons (OPN, an index of remodelling). Exercise increased cortical bone (proximal width +18%, midshaft area +7%). It also raised cancellous bone volume (+25%) by trabecular thickening (+30%) with more intraosseous vascularity and new trabecular interconnections (node-terminus ratio, +57%; trabecular pattern factor, -147%; marrow star volume. -48%). In the cortex a prominent discrete subperiosteal domain became wider (+50% midshaft) with exercise and contained more numerous (+15%) CIII-stained fibres. In contrast the encircled inner bone developed more numerous (+14%) OPN-rich osteons. It is concluded that short-term voluntary exercise augments both cortical and cancellous microarchitecture. It also alters protein composition, such that expanding arrays of Sharpey's fibres within a circumferential proximal domain (Part I) interconnect more powerfully with the musculature and interface more robustly with the core bone that in response becomes more vascular and biodynamic, providing further insight into how muscle mass may be skeletally translated.