Endochondral Ossification

Endochondral Ossification

Most bones are formed by the transformation of cartilage "bone models", a process called endochondral ossification. A periosteal bud invades the cartilage model and allows osteoprogenitor cells to enter the cartilage. At these sites, the cartilage is in a state of hypertrophy (very large lacunae and chondrocytes) and partial calcification, which eventually leads to the death of the chondrocytes.
Invading osteoprogenitor cells mature into osteoblasts, which use the framework of calcified cartilage to deposit new bone. The bone deposited onto the cartilage scaffold is lamellar bone. The initial site of bone deposition is called a primary ossification centre. Secondary ossification centres occur in the future epiphyses of the bone.
A thin sheet of bone, the periosteal collar, is deposited around the shaft of the cartilage model. The periosteal collar consists of woven bone.

Close to the zone of ossification, the cartilage can usually be divided into a number of distinct zones :

1. Reserve cartilage, furthest away from the zone of ossification, looks like immature hyaline cartilage.
2. A zone of chondrocyte proliferation contains longitudinal columns of mitotically active chondrocytes, which grow in size in
3. the zone of cartilage maturation and hypertrophy.
4. A zone of cartilage calcification forms the border between cartilage and the zone of bone deposition.

Primary and secondary ossification centres do not merge before adulthood. Between the diaphysis and the epiphyses a thin sheet of cartilage, the epiphyseal plate, is maintained until adulthood. By continuing cartilage production, the epiphyseal plate provides the basis for rapid growth in the length of the bone. Cartilage production gradually ceases in the epiphyseal plate as maturity is approached. The epiphyseal plate is finally removed by the continued production of bone from the diaphyseal side.

Bone formation and bone resorption go hand in hand during the growth of bone. This first deposited trabecular bone is removed as the zone of ossification moves in the direction of the future epiphyses. This process creates the marrow cavity of the bones. Simultaneously, bone is removed from the endosteal surface and deposited on the periosteal surface of the compact bone which forms the diaphysis. This results in a growth of the diameter of the bone.

Suitable Slides

sections of bones during the early stages of their development - H&E, van Gieson

Foetal vertebral column, human - H&E
Hold the section against a light background. The cartilage models will stain very light, and the outlines of the skeletal structures they will form can often be identified. If an ossification centre is present, it will appear as a darker area within the cartilage model. The zonation of the cartilage should be visible in all ossification centres. How much bone is present depends on how far ossification has proceeded. The newly formed bone trabeculae will often consist of lamellar bone with a more or less extensive core of darkly staining calcified cartilage. The lamellar organisation of the bone may not be visible.
How the ossification centre exactly will look also depends on the plane of the section in relation to the ossification centre.

Suitable Slides

sections of the epiphyseal disc of growing bones - H&E

Growing bone, rabbit - H&E The cartilage model has almost entirely been transformed into bone. The only remaining cartilage is found in the epiphyseal disk. Zones of cartilage proliferation, hypertrophy and calcification are visible at high magnification, but only on one side of the epiphyseal disk - towards the diaphysis, which increases in length as the cartilage generated by the epiphyseal disc is transformed into bone. Osteoclasts may be found on the newly formed trabeculae or associated with parts of the cartilage scaffold. Draw the region of the epiphyseal disks. Identify in your drawing the epiphyseal disk and the bone of the epiphysis and diaphysis. Indicate the direction of cartilage proliferation in the epiphyseal disk and the direction of bone growth.