Middle figure
1 – Zone of calcified cartilage
The large chondrocytes (highlighted in green) are placed at the end of the stacks of hypertrophic chondrocytes.
Calcium salts are progressively deposited in the extracellular matrix that surrounds the chondrocytes (highlighted in blue). This induces the death of the chondrocytes, now seen as fragmented or already dead cells. Their lacunae show remainings of cells or appear to be empty.
2 – Zone of ossification
This Zone is highlighted in pink. Compare it with the previous one and notice an important difference: the presence of small blood vessels containing red blood cells (highlighted in red). Cartilage is not a vascularized tissue and only its calcified extracellular matrix remains in the Zone of ossification.
The blood vessels are branches of periostal vessels that have grown inside the bone together with mesenchymal cells. The blood vessels and mesenchymal cells occupy the empty spaces formerly inhabited by chondrocytes, spaces now delimited by the walls of calcified cartilage extracellular matrix.
The mesenchymal cells that invaded the spaces include osteogenic cells. They establish on the surface of the calcified cartilage matrix and originate osteoblasts. They begin secreting bone extracellular matrix, become surrounded by this matrix and are then recognized as osteocytes. More details on the next pages.
Epiphyseal disc. Staining: HE. Medium magnification.
3 – Lower figure – image of a section prepared to demonstrate calcium salts. They are seen as black areas in the matrix of the calcified cartilage and, in the lower part of the tissue, in the matrix of recently formed bone.
Epiphyseal disc: Zones of serial cartilage, hypertrophic carilage and ossification. Staining: Safranin + demonstration of calcium in black. Medium magnification.