Page 64 - Headlines Histology2024_Neat

Growth of bone

I) Growth in width
• Bone grows in width by appositional growth. The cell in the inner

osteogenic layer of the periosteum is differentiated into osteoblasts that
produce new bony matrix to be added to the pre-exiting bone. Unlike
cartilage, interstitial growth does not occur in bone.
II) Growth in length

• The long bone grows in length through continuous interstitial growth of the
cartilage cells in the epiphyseal disk located between the epiphyses and
diaphysis.

• The epiphyseal disks have 4 regions:
Zone of resting cartilage
• It consists of typical hyaline cartilage with the chondrocytes arranged in

small clusters surrounded by a large amount of moderately stained matrix.
2) Zone of proliferation
• The cartilage cells undergo successive mitotic divisions to form columns of

chondrocytes separated by strongly stained glycoprotein rich matrix.
3) Zone of maturation
• The cell division stopped and the chondrocytes increase in size.
4) Zone of hypertrophy and calcification

• The chondrocytes become greatly enlarged and vacuolated and the matrix
becomes calcified leading to their death.

5) Zone of ossification
The osteogenic cells and capillaries from the marrow cavities of the diaphysis invade
the calcified matrix. The osteogenic cells differentiate into osteoblasts that cluster on
the surface of spicules of calcified cartilage matrix and start bone formation.

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