GROWTH AND DEVELOPMENT OF BONE

Bone grows in length (Longitudinal growth) and in diameter (Appositional growth)

• Longitudinal Growth of bone: The epiphyseal plate in the long bone is region for the elongation or increase in the length of the bone. The epiphyseal plate has two sides (i) the diaphyseal side (near to diaphysis) and (ii) the epiphyseal side (near to the epiphysis). The hyaline cartilage cells in the epiphyseal side of the epiphyseal plate actively divide and produce the hyaline cartilage matrix. The hyaline cartilage in the diaphyseal side of the epiphyseal plate calcifies and replaced by the bone. As the cartilage grows the entire structure elongates and develops in to a bone. The epiphyseal plate is composed of five zones- reserve zone, proliferative Zone, maturation & hypertrophy zone, calcification zone and Ossification Zone
  • Reserve Zone: It is the region very close to the epiphyseal end of the plate and contains small chondrocytes within the matrix that do not participate in bone growth but protects the epiphyseal plate
  • Proliferative Zone: This is the next layer of the epiphyseal plate close to the diaphysis that contains stacks of slightly larger chondrocytes. This Zone develops new chondrocytes through mitosis to replace the cells that die at the diaphyseal end plate.
  • Maturation & Hypertrophy Zone: This is the next layer of chondrocytes; with more older and larger than the chondrocytes. The more mature chondrocytes are located at the diaphyseal end of the epiphyseal plate. Cellular division occurs in the proliferative zone results in longitudinal growth of the bone and maturation of cells occur in the zone of Maturation and hypertrophy. This growth of cells within the tissue is called Interstitial growth
  • Calcified Zone: This Zone is closest to the diaphysis and connects the epiphyseal plate to the diaphysis. Most of the chondrocytes in this zone are dead because the matrix around them is calcified preventing diffusion of nutrients. Capillaries and osteoblasts from the diaphysis penetrate this zone and the osteoblasts develop on the remaining calcified cartilage. The bone grows longitudinally when the osseus tissue develops on the diaphysis
  • Ossification Zone: As the bone continues to grow in length and when all the cartilages are replaced by the bone at the epiphyseal plate, the longitudinal growth of the bone stops, and the chondrocytes no longer proliferate. The epiphyseal plate then completely ossifies, so that only a thin epiphyseal line remains and the bones can no longer grow in length.

Appositional growth of the bone: when the bones grow longitudinally, they also grow in diameter and continue to increase in thickness even after the longitudinal growth ends, this growth is called as Appositional growth. This growth occurs at the endosteum or periosteum where osteoclasts resorb old bone that lines the medullary cavity while osteoblasts develops new bone tissue. The erosion of old bone and deposition of new bone beneath the periosteum increases the diameter of the bone and thus contributes to the Appositional growth.