Cellular Characterization of Microvascular Fragments and Stromal Vascular Fraction for the Treatment of Composite Bone-Muscle Defects

Abstract

The current treatment for critical-sized boned defects, bone morphogenetic protein 2 (BMP-2) loaded collagen sponge grafting, is often inadequate for composite bone-muscle defects, which heal more slowly with higher rates of non- or malunion. Due the additional muscle defect, there is a vacancy of vasculature around the bone defect, causing insufficient nutrient and cytokine delivery and thus hindering bone regeneration. Hypothesizing that increasing angiogenesis will improve regeneration, we have previously proposed to co-deliver microvascular fragments (MVF) or stromal vascular fraction (SVF), which have been shown to mature into microvascular networks in vitro and in vivo, with BMP-2 in a collagen sponge. However, in preliminary studies, collagen sponge co-loaded with BMP-2 and MVF showed augmented release of BMP-2 compared to those co-loaded with SVF or with BMP-2 only. Although they are both derived from the same adipose tissue source, we hypothesize that their cellular composition may differ and thus affect BMP-2 kinetics. Therefore, this project aims to characterize the cellular components of MVF and SVF using flow cytometry to elucidate the potential mechanism of differential BMP-2 release. It was found that MVF has a relatively lower percentage of mesenchymal stem cells (MSCs) and relatively higher percentage of mature endothelial cells (ECs) than SVF, suggesting a role for ECs in the augmented BMP-2 release from MVF-loaded collagen sponges.