Formation of synovial joints and articular cartilage

Chondrocytes differentiate from mesenchymal progenitors and produce templates(anlagen) for the developing bones. Chondrocyte differentiation is controlled bySox transcription factors. Templates for the neighbour bones are subsequentlyseparated by conversion of differentiated chondrocytes into non-chondrogeniccells and emergence of interzone in which joints cavitation occurs. A central rolein initiating synovial joint formation plays Wnt-14/beta-catenin signalling pathway.Moreover, bone morphogenetic proteins and growth and differentiation factorsare expressed at the site of joint formation. Joint cavitation is associated withincreased hyaluronic acid synthesis. Hyaluronic acid facilitates tissue separationand creation of a functional joint cavity. According to the traditional view articularcartilage represents part of cartilage anlage that is not replaced by bone throughendochondral ossification. Recent studies indicate, however, that peri-joint mesenchymalcells take part in interzone formation and that these interzone cellssubsequently differentiate into articular chondrocytes and synovial cells. Thus,anlage chondrocytes have a transient character and disappear after cessation ofgrowth plate function while articular chondrocytes have stable and permanentphenotype and function throughout life.