Albumin-mediated vascular osmotic microenvironment as a bioactive regulator of endochondral ossification.

Endochondral ossification is a tightly regulated developmental process in which vascular invasion and chondrocyte hypertrophy precede mineral deposition. While the cellular and molecular regulators of this process have been extensively studied, the mechanisms underlying blood flow-mediated changes in the extracellular microenvironment that regulate bone formation at the mesoscale remain poorly understood. Here, we investigated the impact of impaired blood supply on the secondary ossification center in mice. Unilateral ligation of the femoral artery during early postnatal development markedly suppressed chondrocyte hypertrophy and reduced bone formation in the epiphysis. Based on the known sensitivity of chondrocytes to osmotic conditions, we hypothesized that reduced oncotic pressure associated with blood-borne proteins represents an additional mechanism contributing to these effects, beyond those associated with cellular hypoxia and nutrient deprivation. Focusing on serum albumin as a representative circulating factor, we demonstrate that albumin promotes chondrocyte hypertrophy both in vitro and in vivo. Consistently, local administration of collagen gel containing serum albumin into the secondary ossification center enhanced chondrocyte hypertrophy and bone formation. Together, our findings reveal that the extracellular microenvironment associated with vascular invasion, particularly that established by serum proteins, plays a critical role in regulating endochondral ossification. Modulating this developmental microenvironment may therefore represent a biomimetic strategy for bone regeneration.