Neoangiogenesis drives the replacement of mineralised cartilage by trabecular bone during bone growth regulated by molecules like e.g. VEGF, OPG and RANKL. The Heparan sulfate proteoglycan Syndecan-1 (Sdc1) plays a role in the interaction of osteoclasts and osteoblasts and the development of blood vessels. We expected Sdc1 to have an influence on bone structure and vessel development. Therefore, bone structure and angiogenesis at the growth plate in mice was compared and the influence of Syndecan-1 deficiency was characterised.

Animals: Femura of male and female C57BL/6 WT (5♀, 6♂) and Sdc1-/- (9♀, 5♂) mice were used for native bone analysis at 4 month age. Histology: Bone structure was analysed using microCT scans with a resolution of 9µm. Vascularisation was visualised using an anti-Endomucin antibody in 80µm thick cryosections. In vitro angiogenesis: Bone marrow isolates were used to generate endothelial progenitor cells by sequential cultivation on fibronectin. Microvessel development was analysed 4h after plating on matrigel.

Bone structure in male Sdc1 deficient mice was significantly reduced compare to male WT, whereas female mice of both genotypes did not differ. Sdc1 deficient mice at the age of 4 month showed a high decrease in the number of vessel bulbs at the chondro-osseous border (growth plate) compared to WT mice. However, no sex related differences were shown. Quantification of microvessel outgrowth of endothelial cells revealed a decreased amount of sprouting, but increased length of microvessels of Sdc1-/- cells compared to WT.

Syndecan-1 has a significant impact on neoangiogenesis at the chondro-osseous border of the native bone, but the impact of Syndecan-1 deficiency on the loss of bone structure was significantly higher in male mice. This emphasises the importance to further characterise the function of Syndecan-1 regulated processes during enchondral ossification in a sex dependent manner.