Endochondral ossification

The resting zone is composed of typical hyaline cartilage that serves as a reserve cell source for the epiphyseal plate.

Chondrocytes, in the zone of proliferation between black lines, undergo interstitial growth to maintain the thickness of the plate. Instead of forming spherical isogenous groups, these dividing cells become aligned in a linear array (arrows) like tennis balls in a can. Such an orientation guarantees that the calcified cartilage spicules remaining after chondrocyte death will be vertically arranged.

Chondrocytes begin regressive changes that produce a sequence of identifiable zones: cells mature, hypertrophy and begin secreting alkaline phosphatase, which allows the matrix to calcify. These zones are grouped here into a single zone, the zone of maturation, hypertrophy and calcification (Zones of M-H-C).

The zone of degeneration (between the black lines) is formed as the calcified cartilage matrix prohibits nutrients and oxygen from diffusing into the chondrocytes, and these cells degenerate. The intervening calcified cartilage remains between columns of empty chondrocyte lacunae (empty tennis ball cans), forming cylinders on which bone will be deposited. In section the cylinders appear as linear spicules.

The zone of ossification (blue arrow) is formed as osteoblasts deposit bone (black arrows) on the calcified cartilage cylinders formed in the epiphyseal plate. The addition of this new bone adds to the length of the shaft, essentially pushing the epiphyses farther apart. Thus, growth in length of the bone occurs as new bone is added to the diaphysis.

The zone of resorption is formed by the resorption of much of the newly formed bone/calcified cartilage cylinders in the area of the developing marrow cavity. The presence of this zone is indicated by osteoclasts in the endosteum lining the dual-colored cylinders. This zone cannot be identified here because the magnification is too low to identify osteoclasts.

The periosteal band (arrows) forms the diaphysis of the long bone. This band was laid down intramembranously on the surface of the hyaline cartilage template by the periosteum. The periosteum lies external to the periosteal band.

The periosteal band (arrows) forms the diaphysis of the long bone. This band was laid down intramembranously on the surface of the hyaline cartilage template by the periosteum. The periosteum lies external to the periosteal band.

The next three images were taken from areas similar to those outlined by the rectangles.