Bone Evolution & Growth
The terms osteogenesis and ossification are often used synonymously to signal the procedure of bone formation. Parts of the skeleton form during the outset few weeks later on conception. By the end of the eighth week after formulation, the skeletal pattern is formed in cartilage and connective tissue membranes and ossification begins.
Os development continues throughout adulthood. Even after developed stature is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles. Osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodeling of bones. Osteoblasts are bone-forming cells, osteocytes are mature bone cells and osteoclasts break downward and reabsorb bone.
There are two types of ossification: intramembranous and endochondral.
Intramembranous ossification involves the replacement of canvass-like connective tissue membranes with bony tissue. Basic formed in this way are called intramembranous bones. They include sure flat bones of the skull and some of the irregular bones. The futurity basic are kickoff formed every bit connective tissue membranes. Osteoblasts migrate to the membranes and deposit bony matrix around themselves. When the osteoblasts are surrounded by matrix they are called osteocytes.
Endochondral ossification involves the replacement of hyaline cartilage with bony tissue. Well-nigh of the basic of the skeleton are formed in this manner. These bones are called endochondral bones. In this procedure, the future bones are get-go formed as hyaline cartilage models. During the third month after conception, the perichondrium that surrounds the hyaline cartilage “models” becomes infiltrated with blood vessels and osteoblasts and changes into a periosteum. The osteoblasts class a collar of compact bone around the diaphysis. At the same time, the cartilage in the center of the diaphysis begins to disintegrate. Osteoblasts penetrate the disintegrating cartilage and supervene upon it with spongy bone. This forms a primary ossification center. Ossification continues from this center toward the ends of the bones. After spongy os is formed in the diaphysis, osteoclasts break down the newly formed bone to open up the medullary cavity.
The cartilage in the epiphyses continues to abound so the developing bone increases in length. After, ordinarily subsequently birth, secondary ossification centers form in the epiphyses. Ossification in the epiphyses is like to that in the diaphysis except that the spongy bone is retained instead of being broken downwardly to course a medullary cavity. When secondary ossification is complete, the hyaline cartilage is totally replaced past bone except in two areas. A region of hyaline cartilage remains over the surface of the epiphysis as the articular cartilage and another area of cartilage remains between the epiphysis and diaphysis. This is the epiphyseal plate or growth region.
Bones grow in length at the epiphyseal plate past a process that is similar to endochondral ossification. The cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes, in the region next to the diaphysis, age and degenerate. Osteoblasts motility in and congeal the matrix to form bone. This process continues throughout childhood and the adolescent years until the cartilage growth slows and finally stops. When cartilage growth ceases, commonly in the early twenties, the epiphyseal plate completely ossifies and then that only a thin epiphyseal line remains and the basic can no longer grow in length. Bone growth is under the influence of growth hormone from the inductive pituitary gland and sex hormones from the ovaries and testes.
Fifty-fifty though basic stop growing in length in early on adulthood, they tin continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight. The increase in diameter is chosen appositional growth. Osteoblasts in the periosteum grade compact bone effectually the external bone surface. At the aforementioned time, osteoclasts in the endosteum break downwards bone on the internal bone surface, effectually the medullary crenel. These 2 processes together increase the diameter of the os and, at the same time, go on the bone from becoming excessively heavy and bulky.
Bones Do Not Have a Role in __________