The Metaphysis

Between the cartilaginous head and the shaft of a developing long bone, there is a transitional zone called the metaphysis (meh-TAF-ih-sis). It exhibits five histological zones of transformation from cartilage to bone (fig. 7.9):

  1. Zone of reserve cartilage. In this zone, farthest from the marrow space, the resting cartilage as yet shows no sign of transforming into bone.
  2. Zone of cell proliferation. A little closer to the marrow space, chondrocytes multiply and become arranged into longitudinal columns of flattened lacunae.
  3. Zone of cell hypertrophy. Next, the chondrocytes cease to divide and begin to hypertrophy, just as they did in the primary ossification center. The cartilage walls between lacunae become very thin. Cell multiplication in zone 2 and hypertrophy in zone 3 continually push the zone of reserve cartilage toward the ends of the bone and make the bone grow longer.
  4. Zone of calcification. Minerals are deposited in the matrix between columns of lacunae and calcify the cartilage for temporary support.
  5. Zone of bone deposition. Within each column, the walls between lacunae break down and the

Multiplying chondrocytes

Enlarged chondrocytes

Calcifying cartilage

Breakdown of cartilage lacunae

Trabecula of spongy bone

Osteoblasts depositing bone matrix

Osteocytes

Bone marrow

Multiplying chondrocytes

Enlarged chondrocytes

Calcifying cartilage

Breakdown of cartilage lacunae

Trabecula of spongy bone

Osteoblasts depositing bone matrix

Osteocytes

Bone marrow

Figure 7.9 The Metaphysis. This micrograph shows the transition from cartilage to bone in the growth zone of a long bone.

- Zone of cell proliferation

Zone of cell hypertrophy

Zone of calcification

Zone of bone deposition

Figure 7.9 The Metaphysis. This micrograph shows the transition from cartilage to bone in the growth zone of a long bone.

Saladin: Anatomy & 7. Bone Tissue Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

228 Part Two Support and Movement chondrocytes die. This converts each column into a longitudinal channel, which is quickly invaded by marrow and blood vessels from the primary marrow space. Osteoclasts dissolve the calcified cartilage while osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix. The channel therefore grows smaller and smaller as one layer after another is laid down, until only a narrow channel remains in the middle—now a central canal. Osteoblasts trapped in their own matrix become osteocytes and stop producing matrix.

dissolve, and the chondrocytes die. Vascular buds arise from the perichondrium and grow into the cartilage, bringing osteogenic cells and osteoclasts with them. The cartilage is eroded from the center of the epiphysis outward in all directions. Thin trabeculae of cartilage matrix are calcified to form spongy bone. Hyaline cartilage persists in two places—on the epiphyseal surfaces as the articular cartilages and at the junction of the diaphysis and epiph-ysis, where it forms the epiphyseal plate (fig. 7.11). Each side of the epiphyseal plate has a metaphysis, where the transformation of cartilage to bone occurs.

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