Stress fracture healing: Fatigue loading of the rat ulna induces upregulation in expression of osteogenic and angiogenic genes that mimic the intramembranous portion of fracture repair

Woven bone is formed in response to fatigue-induced stress fractures and is associated with increased local angiogenesis. The molecular mechanisms that regulate this woven bone formation are unknown. Our objective was to measure the temporal and spatial expression of osteo- and angiogenic genes in woven bone formation in response to increasing levels of fatigue-induced damage. We used the rat forelimb compression model to produce four discrete levels of fatigue damage in the right ulna of 115 male Fischer rats. Rats were killed at 0 (1 h), 1, 3 and 7 days after loading. Using qRT-PCR, we quantified gene expression associated with osteogenesis (BMP2, Msx2, Runx2, Osx, BSP, Osc), cell proliferation (Hist4), and angiogenesis (VEGF, PECAM-1) from the central half of the ulna. The spatial distribution of BMP2, BSP and PCNA was assessed by immunohistochemistry or in situ hybridization in transverse histological sections 1, 4, and 7 mm distal to the ulnar mid-diaphysis. One hour after loading, BMP2 was significantly upregulated in neurovascular structures in the medial ulnar periosteum. Expression of angiogenic markers (VEGF, PECAM-1) increased significantly between Day 0 and 1 and, as with BMP2 expression, remained upregulated through Day 7. While Osx and BSP were upregulated on Day 1, the other osteogenic genes (Msx2, Runx2, Osx, BSP and Osc) were induced on Day 3 in association with the initiation of periosteal woven bone formation and continued through Day 7. The magnitude of osteogenic gene expression, particularly matrix genes (BSP, Osc) was significantly proportional the level of fatigue damage. The woven bone response to fatigue injury is remarkably similar to the "intramembranous" portion of fracture repair - rapid formation of periosteal woven bone characterized by early BMP2 expression, cell proliferation, and upregulation of osteogenic genes. We speculate that woven bone repair of fatigue damage may be an abbreviated fracture response without the requirement for endochondral repair. We conclude that bone fatigue repair is a process similar to intramembranous fracture repair characterized by increases in the expression of genes associated with angiogenesis, cell proliferation and osteoblastogenesis, and that the response from the local vasculature precedes the osteogenic response to fatigue loading.