PARTS OF A LONG BONE
The activity of osteoclasts and osteoblasts is particularly rapid at the ends of long bones that extend in length. The region (see Figure 3.3) at the ends of bones is theepiphysis (plural, epiphyses). New carti-lage is constantly being formed here to increase the length. Adjacent to this new cartilage, is a thin region known as theepiphyseal plate, where the os-teoblasts constantly turn cartilage into bone. As more cartilage is formed, the epiphyseal plate advances, leaving bone behind it. Thus, bone is remodeled by cellular activity. Diaphysis is the region of bone be-tween the epiphysis. The diaphysis forms the middle, cylindrical part of the bone. The metaphysis is the region of bone that lies between the diaphysis and the epiphysis, and it includes the epiphyseal plate.
The ends of long bones, adjacent to the joint, are covered with hyaline cartilage - articular cartilage. The articular cartilage absorbs shock and reduces fric-tion in joints. The inner region of long bones houses the medullary or marrow cavity. In children this cavity is filled with red bone marrow (where blood cells are formed). In older individuals, the red bone marrow is replaced by yellow marrow that is largely made up of adipose tissue. The medullary cavity is lined by the endosteum membrane. It contains containing bone-forming cells (osteogenic cells and osteoblasts).
Bone continues to lengthen rapidly during puberty and stops in adulthood. However, bone deposition and resorption continues throughout life and is mod-ified by diet and endocrine, mechanical, chemical, and psychological factors.
For proper bone formation, there must be adequate protein, calcium, and phosphorus, among others, es-pecially at rapid growth phases such as childhood, adolescence, pregnancy, and lactation. Because bone is also a calcium reservoir, if demands are increased, calcium is removed from the bones to meet those needs and the bones can get weaker. Small quantities of fluoride, magnesium, manganese, and iron are also needed. Vitamin C is needed for proper collagen fiber development. Other vitamins, such as vitamin A, B12, and K, are needed for protein synthesis and os-teoblastic activity.
Three hormones are important in maintaining blood levels of calcium. This implies that they affect the mineralization of bone in the process. Parathormone, from the parathyroid gland, and vitamin D increase the blood levels of calcium while the hormone calcitonin, from the thyroid gland, de-creases the levels. In bone, parathormone and vita-min D increase osteoclastic activity and resorption of bone and decrease excretion of calcium by the kid-neys and increase absorption in the gut. Calcitonin does the opposite; if dietary calcium is inadequate, bone resorption occurs.
In addition to these hormones, growth hormone, thyroid hormone, insulin, and sex hormones are all required for proper bone formation.
The plasticity of bone can be illustrated in many ways. Bones of athletes are stronger and denser as com-pared with sedentary individuals. Similar to other tis-sue that atrophy with disuse, bone becomes weaker and less dense when not stressed. Conversely, exces-sive stress placed on one or more bones makes those bones alone stronger and denser. Hence, posture, muscle tone, and weight can all affect the remodeling process.