Chondrocyte

Chondroblasts differentiate from mesenchymal cells (under the transcription factor SOX9) and from the chondrogenic layer of the perichondrium. They secrete type II collagen and aggrecan and other matrix molecules until they become surrounded and enclosed in small cavities, the lacunae, at which point they are called chondrocytes. Chondrocytes can still divide, forming isogenous groups of two to eight cells within the matrix (interstitial growth). They have abundant RER and Golgi, lipid droplets and glycogen, and in routine sections often shrink away from the lacunar wall.

Because cartilage is avascular, chondrocytes rely on diffusion of nutrients and O₂ through the matrix, and largely produce ATP by glycolysis. Mechanical loading (compression-driven fluid flow) helps nutrient exchange and stimulates matrix synthesis. In the epiphyseal growth plate, chondrocytes progress through zones of reserve, proliferation (stacked columns), hypertrophy and calcification before dying and being replaced by bone — the engine of longitudinal bone growth, controlled by growth hormone/IGF-1, thyroid hormone, sex steroids, Indian hedgehog/PTHrP and FGFR3 signalling.