Development And Growth Of A Long Bone
Most bones take many years to grow and mature. The humerus (arm bone), for example, begins to ossify at the end of the embryonic period (8 weeks); however, ossification is not complete until age 20. All bones derive from mesenchyme (embryonic connective tissue) by two different processes: intramembranous ossification (directly from mesenchyme) and endochondral ossification (from cartilage derived from mesenchyme). The histology (microscopic structure) of a bone is the same by either process (Pawlina, 2016). The two processes of bone development proceed as follows:
- In intramembranous ossification (membranous bone formation), mesenchymal models of bones form during the embryonic period, and direct ossification of the mesenchyme begins in the fetal period.
- In endochondral ossification (cartilaginous bone formation), cartilage models of the bones form from mesenchyme during the fetal period, and bone subsequently replaces most of the cartilage.
A brief description of endochondral ossification helps explain how long bones grow. The mesenchymal cells condense and differentiate into chondroblasts , dividing cells in growing cartilage tissue, thereby forming a cartilaginous bone model . In the midregion of the model, the cartilage calcifies (becomes impregnated with calcium salts), and periosteal capillaries (capillaries from the fibrous sheath surrounding the model) grow into the calcified cartilage of the bone model and supply its interior. These blood vessels, together with associated osteogenic (bone-forming) cells , form a periosteal bud. The capillaries initiate the primary ossification center , so named because the bone tissue it forms replaces most of the cartilage in the main body of the bone model. The shaft of a bone ossified from the primary ossification center is the diaphysis , which grows as the bone develops.
A. The formation of primary and secondary ossification centers is shown.
B. Growth in length occurs on both sides of the cartilaginous epiphysial plates ( double-headed arrows ). The bone formed from the primary center in the diaphysis does not fuse with that formed from the secondary centers in the epiphyses until the bone reaches its adult size. When growth ceases, the depleted epiphysial plate is replaced by a synostosis (bone-to-bone fusion), observed as an epiphysial line in radiographs and sectioned bone.
Most secondary ossification centers appear in other parts of the developing bone after birth; the parts of a bone ossified from these centers are epiphyses . The chondrocytes in the middle of the epiphysis hypertrophy, and the bone matrix (extracellular substance) between them calcifies. Epiphysial arteries grow into the developing cavities with associated osteogenic cells. The flared part of the diaphysis nearest the epiphysis is the metaphysis . For growth to continue, the bone formed from the primary center in the diaphysis does not fuse with that formed from the secondary centers in the epiphyses until the bone reaches its adult size. Thus, during growth of a long bone, cartilaginous epiphysial plates intervene between the diaphysis and epiphyses . These growth plates are eventually replaced by bone at each of its two sides, diaphysial and epiphysial. When this occurs, bone growth ceases and the diaphysis fuses with the epiphyses. The seam formed during this fusion process (synostosis) is particularly dense and is recognizable in sectioned bone or radiographs as an epiphysial line. The epiphysial fusion of bones occurs progressively from puberty to maturity. Ossification of short bones is similar to that of the primary ossification center of long bones, and only one short bone, the calcaneus (heel bone), develops a secondary ossification center.
Source: Clinically Oriented Anatomy
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