(C44-48) Slide 194: Child’s fingertip, H&E

This slide presents an almost diagrammatic view of the zones in endochondral ossification.

After identifying the tendons (C44 t), joint cavity (c) and distal phalanx, turn to higher power (C45) to study the epiphyseal region of the distal phalanx. Although the zones grade into one another, it is possible to distinguish the typical appearances of the zones of resting (C45 r), proliferating (p) hypertrophying (h) and calcifying (c) cartilage. Notice the striking differences in staining of the calcified cartilage (C46 c) and bone (b).

You can find many multinucleate cells (C46 arrow), which because they are destroying cartilage, are called chondroclasts. Histologically, they are indistinguishable from osteoclasts. Those found on cartilage are chondroclasts and those on bone are osteoclasts (C47).

Be sure to notice the differences between osteocytes and chondrocytes. Here are some distinguishing characteristics.

Cells: Chondrocytes are dividing cells found in isogenous groups.

Osteocytes are hermits. They are found alone because they don’t divide.

Remember that cartilage grows interstitially so you will see isogenous groups. Bone can’t grow interstitially because the matrix doesn’t allow expansion, therefore osteocytes can’t divide like chondrocytes.

Lacunae: Chondrocytes have lacunae which are "rounded".

Osteocytes have lacunae which have points.

The points projecting from the osteocyte’s lacunae are actually the canaliculi; the home of the filopodia, and communication network for the osteocytes. Cartilage does not have an elaborate nutrient exchange network thus it takes so long for cartilage to heal. In cartilage, exchange occurs strictly by diffusion, whereas supplies are distributed throughout the bone matrix via filopodia.

Look for a thick periosteum and a layer of osteoblasts along the bone’s diaphysis distal to the epiphysis. Deep to them is a layer of osteoid (C48). Remember that intramembranous ossification occurs around the circumference of long bones and endochondral ossification occurs at the epiphyseal plate. Therefore, bones can still grow in width even after the epiphyseal plate is fused.