Practice question

Synovial joint formation requires segmentation of continuous chondrogenic condensation into separate skeletal elements. GDF5, also called Cartilage-Derived Morphogenetic Protein-1 belonging to BMP superfamily, expressed in interzone region marks future joint stripe. It maintains joint progenitors expressing Wnt14, Wnt9a, activates beta-catenin, inhibits cartilage maturation locally, induces expression of joint markers like suppression of Collagen type II. Mutations cause brachydactyly type C, symphalangism, joint fusions in humans. Exogenous GDF5 induces ectopic joints in developing limbs, while Noggin removal perturbs pattern, establishing GDF5 as essential factor initiating joint cavitation and boundaries.