In the zone of ossification, woven bone is deposited on calcified cartilage cylinders. With time, the endosteum lining these cylinders produces lamellar bone, forming osteons. Addition of osteons converts the spongy-bone area to compact bone, still containing calcified cartilage remnants. The woven bone and calcified cartilage will be resorbed eventually, replaced by lamellar bone. 400x
Calcified cartilage >
The gray, calcified cartilage remnants form almost a circular rim around the osteon, thus indicating the hollow cylinder these remnants once formed.
Woven bone >
Woven bone surrounding the calcified cartilage represents the bone deposited in the zone of ossification during endochondral ossification of this region.
An osteon has filled in the space originally existing within the calcified cartilage-bone cylinders. Because of the vertical orientation of the cylinders, forming osteon columns will be organized parallel to the long axis of the bone, providing additional strength to the long bone. See the three-dimensional drawing for orientation.