Bone Growth and Bone Remodeling
Overview of Bone Growth
Bone growth and bone remodeling initiate during embryologic development and continue throughout skeletal maturation under continuous hormonal regulation. Long bones expand through two distinct physiological growth mechanisms:
Interstitial Growth: Growth in bone length, which relies on the division and hypertrophy of cartilage within the epiphyseal plate.
Appositional Growth: Growth in bone diameter or thickness, which occurs within the periosteum.
Interstitial Growth and Epiphyseal Plate Zonation
Interstitial growth is dependent upon the growth of hyaline cartilage within the epiphyseal plate. Located between the diaphysis (shaft) and the epiphysis (end) of a long bone, the epiphyseal plate contains five continuous microscopic zones, arranged from the epiphysis toward the medullary cavity of the diaphysis:
Zone 1: Zone of Resting Cartilage
Location: Farthest from the medullary cavity of the diaphysis and nearest to the epiphysis.
Histology: Composed of small chondrocytes distributed throughout the extracellular cartilage matrix, closely resembling mature, healthy hyaline cartilage.
Function: Anchors and secures the epiphysis to the epiphyseal plate.
Zone 2: Zone of Proliferating Cartilage
Dynamic: Chondrocytes in this zone undergo rapid mitotic cell division.
Morphology: Newly formed chondrocytes enlarge slightly and align within their lacunae into longitudinal columns parallel to the diaphysis, resembling stacks of coins.
Zone 3: Zone of Hypertrophic Cartilage
Dynamic: Chondrocytes cease dividing and undergo substantial hypertrophy (enlargement in size).
Matrix Changes: As the chondrocytes hypertrophy, they resorb surrounding cartilage matrix, causing the walls of the lacunae to become markedly thin.
Zone 4: Zone of Calcified Cartilage
Composition: Consists of two to three layers of chondrocytes.
Calcification Process: Mineral salts are deposited into the cartilage matrix between the longitudinal columns of lacunae.
Cellular Outcome: Matrix calcification restricts the diffusion of nutrients, resulting in the death and loss of chondrocytes. The matrix becomes opaque in appearance.
Zone 5: Zone of Ossification
Structural Remodeling: The thin lacunar walls between columns break down, forming longitudinal open channels.
Invasion: These longitudinal spaces are invaded by capillaries and osteoprogenitor cells migrating from the medullary cavity.
Bone Matrix Deposition: Osteoprogenitor cells differentiate into osteoblasts, which deposit new bone matrix on the remaining fragments of calcified cartilage matrix.
Dynamics of Longitudinal Growth and Epiphyseal Closure
Mechanism of Elongation:
Actual growth in bone length occurs specifically within Zone 2 (as chondrocytes undergo mitotic cell division to increase in total cell number) and Zone 3 (as chondrocytes hypertrophy to increase in cell size).
The expansion of tissue within zones 2 and 3 pushes Zone 1 (the zone of resting cartilage) and the epiphysis outward, away from the central diaphysis.
This physical extension is made possible by the flexible matrix of hyaline cartilage, rather than the rigid, calcified matrix of bone.
As cartilage expands in zones 2 and 3, osteoblasts in Zone 5 produce new bone connective tissue at an equivalent rate, replacing the calcified cartilage. This process mirrors endochondral ossification during embryonic skeletal development.
Epiphyseal Plate Closure:
Throughout childhood, the epiphyseal plate maintains a steady thickness as it is pushed outward from the center of the bone shaft.
Upon reaching maturity, the rate of hyaline cartilage production slows within zones 2 and 3, while osteoblast activity accelerates in zone 5.
Consequently, the epiphyseal plate continuously narrows until the cartilage is completely replaced by bone, causing interstitial growth to stop.
The remnant of the epiphyseal plate is an internal thin line of compact bone known as the epiphyseal line.
The complete disappearance of hyaline cartilage and the appearance of the epiphyseal line mark the permanent cessation of longitudinal bone growth and the attainment of full height.
Appositional Bone Growth
Appositional growth increases the overall diameter and thickness of a bone and takes place within the periosteum.
Peripheral Deposition by Osteoblasts:
Osteoblasts situated in the inner cellular layer of the periosteum synthesize and deposit new bone matrix along the outer surface.
This matrix is laid down in layers parallel to the surface, called external circumferential lamellae.
Analogous to tree rings, as external circumferential lamellae increase in number, the bone expands in diameter at its periphery.
Internal Resorption by Osteoclasts:
Concurrently, osteoclasts residing along the endosteal lining of the medullary cavity resorb inner bone matrix.
This resorption causes the medullary cavity to continuously expand outward as the exterior surface grows.
Developmental Progression:
The coordinated combination of peripheral bone deposition by osteoblasts and internal matrix resorption by osteoclasts transforms an infant bone sequentially through childhood and young adulthood into a larger adult bone.
This process allows the bone to increase its structural diameter and strength while enlarging the medullary cavity to prevent the bone from becoming excessively heavy.
Clinical Applications and Review Questions
Radiographic Assessment of Height Maturation:
Question: How can a physician determine whether a person has reached full height by examining x-rays of the patient's bones?
Answer: On a pediatric radiograph, the hyaline cartilage of the epiphyseal plate appears as a distinct dark (radiolucent) band between the epiphysis and diaphysis. In an adult who has finished growing, the cartilage has been entirely replaced by dense bone tissue, visible on an x-ray as a thin line called the epiphyseal line. The presence of an open dark band indicates remaining growth potential, whereas the appearance of the epiphyseal line confirms that interstitial growth has permanently ceased and full height has been achieved.
Mechanism of Growth in Bone Width:
Question: How does a bone grow in width?
Answer: A bone increases in width through appositional growth. Osteoblasts in the inner layer of the periosteum add new external circumferential lamellae to the outer surface of the bone, enlarging its external diameter. At the same time, osteoclasts resorb bone along the borders of the medullary cavity, widening the cavity in proportion to the outer growth.