The metaphysis is a transitional zone in long bones responsible for the mechanism by which bones increase their length throughout childhood and adolescence. As a site of intense cell division and tissue turnover, the metaphysis exhibits a unique structure and high metabolic rate.
Anatomical Location and Definition
The metaphysis is the flared portion of a long bone situated between the central shaft, known as the diaphysis, and the terminal end, called the epiphysis. In a growing skeleton, the metaphysis directly abuts the growth plate, or physis, which is a layer of cartilage separating it from the epiphysis.
Structurally, the metaphysis is primarily composed of spongy, or cancellous, bone tissue. This contrasts with the dense, compact bone that makes up the bulk of the diaphysis. Its internal matrix forms a lattice-like network of porous bony spicules called trabeculae.
This cancellous bone is richly supplied by metaphyseal arteries, resulting in high vascularity. The extensive blood flow and porous architecture support a diverse population of cells, including mesenchymal stem cells and bone-forming osteoblasts.
Primary Role in Bone Growth
The metaphysis is the engine of longitudinal skeletal growth, a process driven by endochondral ossification. This mechanism involves replacing a cartilage template with new bone tissue, centered at the growth plate. The growth plate is a structured layer of hyaline cartilage, organized into distinct zones that facilitate elongation.
Starting from the epiphyseal side, the zone of reserve cartilage contains resting chondrocytes that anchor the growth plate to the epiphysis. Moving toward the diaphysis is the zone of proliferation, where chondrocytes rapidly divide, stacking themselves into distinct longitudinal columns. This multiplication physically pushes the epiphysis away, thereby lengthening the bone.
Following this division, the cells enter the zone of hypertrophy, where they significantly enlarge and mature. These swollen chondrocytes prepare the surrounding matrix for mineralization by secreting specific factors. The subsequent zone of calcification sees the cartilage matrix harden with calcium deposits, which causes the hypertrophied chondrocytes to die.
Finally, in the zone of ossification, blood vessels from the metaphysis invade the calcified cartilage framework. Bone-forming osteoblasts arrive, depositing new bone matrix onto the remnants of the calcified cartilage spicules. Simultaneously, bone-resorbing osteoclasts remove the oldest parts, converting the cartilage scaffold into true bone tissue on the metaphyseal side.
Clinical Relevance and Conditions
The intense biological activity and unique structure of the metaphysis make it susceptible to specific medical conditions. Its location adjacent to the growth plate means that injuries to the region can severely impact future growth. Metaphyseal fractures, particularly in children, often involve the growth plate and are classified using systems like Salter-Harris. Type II fractures are the most common, involving a break through the physis and a fragment of the metaphysis.
The high rate of cell turnover also contributes to its vulnerability to certain bone tumors. Osteosarcoma, a common bone cancer, frequently originates in the metaphysis, particularly around the knee, a region with high growth rates. The rapid proliferation of cells provides a fertile environment for malignant transformation.
Metabolic disorders that affect bone mineralization directly manifest in this region. Rickets, caused by insufficient calcium or vitamin D, is characterized by a failure of the cartilage matrix to properly calcify in the metaphysis. This lack of mineralization leads to a visible widening and fraying of the growth plate on X-rays, resulting in the characteristic bowing deformities of the long bones.