Cartilage and Bone

Endochondrial ossification, Knee joint, Monkey

First orientate your section with the naked eye by locating the following structures: patella(cartiginous) and the ligamentum patella(dense regular connective tissue) which is inserted to the tibia on the right side of your section. At low power under the microscope, locate now the epiphysis and the epiphyseal plate of both tibia and the femur. In the epiphyseal plate differentiate the various zones of chondrocytes beginning from the epiphyseal end:

• zone of cell rest
• zone of cell proliferation
• zone of cell hypertrophy
• zone of cell death(why do cells degenerate here?).

Note that many free(mixed) spicules project into the marrow cavity of the diaphysis. Can you recognize the supporting(mixed) spicules anchoring to the walls at the sides? Examine a mixed(free) spicule at higher magnification and identify:

• the osteoblasts aligning on the surface of the spicule where bone formation is actively in progress,
• the chondrocytes present in the centre of some spicules especially near the epiphyseal cartilage,
• the osteocytes in the bone matrix of the spicules,
• the osteoclasts.

Describe the features of an osteoclast. Where are they located? Between the spicules massive darkly stained bone marrow cells are present. Move your field now to the surface of the tibia/femur. Identify their covering periosteum. Just as in the perichondrium, it is composed of an inner cellular(osteoblasts) and an outer fibrous layer. The shaft(diaphysis) of the tibia and femur(osteoblasts) is composeed of woven bone at this stage. What are the histological features of the woven bone?