Solute transport in growth plate cartilage: in vitro and in vivo.

Bone elongation originates from cartilaginous discs (growth plates) at both ends of a growing bone. Here chondrocytes proliferate and subsequently enlarge (hypertrophy), laying down a matrix that serves as the scaffolding for subsequent bone matrix deposition. Because cartilage is generally avascular, all nutrients, oxygen, signaling molecules, and waste must be transported relatively long distances through the tissue for it to survive and function. Here we examine the transport properties of growth plate cartilage. Ex vivo, fluorescence photobleaching recovery methods are used in tissue explants. In vivo, multiphoton microscopy is used to image through an intact perichondrium and into the cartilage of anesthetized mice. Systemically introduced fluorescent tracers are monitored directly as they move from the vasculature into the cartilage. We demonstrate the existence of a relatively permissive region at the midplane of the growth plate, where chondrocytes transition from late proliferative to early hypertrophic stages and where paracrine communication is known to occur between chondrocytes and cells in the surrounding perichondrium. Transport in the living mouse is also significantly affected by fluid flow from the two chondro-osseus junctions, presumably resulting from a pressure difference between the bone vasculature and the cartilage.