Chapter 5 Individual Objectives
1.) Describe fetal bone formation and summarize the events of bone remodeling throughout life.
Fetal bones start as hyaline cartilage. Bones go through two major phases in order to completely form in the stage of ossification. During ossification the hyaline cartilage model is first completely covered with bone matrix by bone-forming cells called osteoblasts. For a short period of time the fetus has cartilage "bones" enclosed by "bony" bones. Eventually the enclosed hyaline cartilage is digested away opening up a medullary cavity within the newly forming bone. By birth, or shortly after, most hyaline cartilage models have been converted to bone, except for the articular cartilages and epiphyseal plates. New cartilage is formed continuously on the external face of the articular cartilage and on the epiphyseal plate surface that faces the bone end. At the same time, the old cartilage and medullary cavity are broken down and replaced by bony matrix. Bone remodeling starts from birth till about the time an individual has become an adolescent and past puberty. During this long stage bones both lengthen and widen. Bonds widen when osteoblasts in the periosteum add bone tissue to the external/face of the diaphysis as the osteoclasts in the endosteum remove bone from the inner surface of the diaphysis wall. Because these two processes occur at the same rate, the circumference of the long bone expands and the bone widens. This process is called appositional growth. The process of long bone growth is controlled by hormones, the most important of which are the growth hormones and, during puberty, the sex hormones. It ends during adolescence when the epiphyseal plates are completely converted to bone. Bones are remodeling continually in response to calcium levels in the blood and the pull of gravity and muscles in the skeleton. Bone remodeling is essential if bones are to retain normal proportions and strength during long bone growth as the body increases in size and weight. It also accounts for the fact that bones become thicker and form large projections to increase their strength in areas where bulky muscles are attached.
Fetal bones start as hyaline cartilage. Bones go through two major phases in order to completely form in the stage of ossification. During ossification the hyaline cartilage model is first completely covered with bone matrix by bone-forming cells called osteoblasts. For a short period of time the fetus has cartilage "bones" enclosed by "bony" bones. Eventually the enclosed hyaline cartilage is digested away opening up a medullary cavity within the newly forming bone. By birth, or shortly after, most hyaline cartilage models have been converted to bone, except for the articular cartilages and epiphyseal plates. New cartilage is formed continuously on the external face of the articular cartilage and on the epiphyseal plate surface that faces the bone end. At the same time, the old cartilage and medullary cavity are broken down and replaced by bony matrix. Bone remodeling starts from birth till about the time an individual has become an adolescent and past puberty. During this long stage bones both lengthen and widen. Bonds widen when osteoblasts in the periosteum add bone tissue to the external/face of the diaphysis as the osteoclasts in the endosteum remove bone from the inner surface of the diaphysis wall. Because these two processes occur at the same rate, the circumference of the long bone expands and the bone widens. This process is called appositional growth. The process of long bone growth is controlled by hormones, the most important of which are the growth hormones and, during puberty, the sex hormones. It ends during adolescence when the epiphyseal plates are completely converted to bone. Bones are remodeling continually in response to calcium levels in the blood and the pull of gravity and muscles in the skeleton. Bone remodeling is essential if bones are to retain normal proportions and strength during long bone growth as the body increases in size and weight. It also accounts for the fact that bones become thicker and form large projections to increase their strength in areas where bulky muscles are attached.