This image shows a section from a developing fetal finger bone. In the upper half of the image is hyaline cartilage that forms the cartilage model of the bone that first appears in the embryo. In the lower half of the image, the cartilage is being replaced by bone in the process called both cartilage replacement and endochondral bone formation. Four of the five cartilage zones characteristic of cartilage replacement can be seen. They are, from top to bottom: zone of chondrocyte proliferation (ZCP); zone of chondrocyte hypertrophy (ZCH); zone of calcified cartilage (ZCC); and zone of cartilage resorption (ZCR). The zone of resting cartilage, which is not seen, is located above the zone of proliferation. In the lower half of the image look for the periosteal collar (PC) of bone that forms the outer wall of the bone shaft and the numerous trabeculae (Tr) of forming bone within the marrow cavity. The spaces between the trabeculae are occupied by developing bone marrow cells, blood vessels and blood sinuses (Si) filled with red blood cells. The connective tissue covering on the outside of the bone shaft is a perichondrium (Pch) where it covers the cartilage and a periosteum (Pos) where it covers the bone.
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Copyright by: Paul B. Bell, Jr. & Barbara Safiejko-Mroczka
The University of Oklahoma