What are the four zones of the epiphyseal plate, in order from the epiphysis to the diaphysis, and what happens in each zone?

From the epiphysis to the diaphysis, the four zones of the epiphyseal plate are the proliferation zone, hypertrophic zone, calcification zone, and ossification zone. In the proliferation zone, cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis and lengthening the bone. The older chondrocytes of the hypertrophic zone enlarge, and in the calcification zone the surrounding matrix calcifies and those cells die and deteriorate. In the ossification zone, osteoclasts partially erode the calcified cartilage spicules, osteoblasts cover them with new bone, and spongy bone replaces them.

The epiphyseal plate has a relatively inactive resting zone on the side facing the epiphysis, and the cells next to it are arranged in four zones running toward the diaphysis. The proliferation zone is closest to the epiphysis; its cartilage cells divide rapidly, which pushes the epiphysis away from the diaphysis and lengthens the bone. In the hypertrophic zone, older chondrocytes closer to the diaphysis enlarge, and their lacunae erode and enlarge, leaving large interconnecting spaces. In the calcification zone, the surrounding cartilage matrix calcifies, the chondrocytes die and the matrix begins to deteriorate, and blood vessels invade the cavities. Finally, in the ossification zone at the epiphysis-diaphysis junction, the calcified cartilage spicules are invaded by marrow elements, osteoclasts partly erode the spicules, and osteoblasts quickly cover them with new bone, so spongy bone replaces the cartilage and the medullary cavity lengthens as osteoclasts digest the spicule tips.