Bone Development and StructureBecause bone is made up of minerals and is hard, many people think that it is not living material. But a bone in a living animal consists of both living tissue and non-living substances. Within the "alive bone" are blood vessels, nerves, collagen, and living cells including:
Besides the metabolically active cellular portion of bone tissue, bone is also made up of a matrix (a bonding of multiple fibers and chemicals) of different materials, including primarily collagen fibers and crystalline salts. The crystalline salts deposited in the matrix of bone are composed principally of calcium and phosphate, which are combined to form hydroxyapatite crystals. As you can see, the chemical formula for hydroxyapatite crystals includes molecules of calcium (Ca), phosphate (PO4), and hydroxide (OH):
In particular, it is the collagen fibers and the calcium salts that help to strengthen bone. In fact, the collagen fibers of bone have great tensile strength (the strength to endure stretching forces), while the calcium salts, which are similar in physical properties to marble, have great compressional strength (the strength to endure squeezing forces). These combined properties, plus the degree of bondage between the collagen fibers and the crystals, provide a bony structure that has both extreme tensile and compressional strength (Figure 3).
Thus, bones are constructed in exactly the same way that reinforced concrete is constructed. The steel of reinforced concrete provides the tensile strength, while the cement, sand, and rock provide the compressional strength. However, the compressional strength of bone is greater than that of even the best reinforced concrete, and the tensile strength approaches that of reinforced concrete. But, even with their great compressional and tensile strengths, neither bone nor concrete has a very high level of torsional strength (the strength to endure twisting). In fact, bone fractures often occur as a result of torsional forces that are exerted on an arm or a leg. First, let's examine the major structural components of our bones and then let's briefly discuss how bone develops.
Bones can be classified according to their shapes as: long (including the arm and leg bones), short (including the bones of the wrists and ankles), flat (including the ribs and the bones of the skull), and irregular (including the vertebrae along our spine). In describing the general structure of bone, a long bone will be used as an example (Figure 4). At each end of such a bone there is an expanded portion called an epiphysis, which forms a joint with another bone. The shaft of the bone, which is located between the epiphyses, is called the diaphysis. Except for the articular cartilage that covers the very ends of each epiphysis, the bone is completely enclosed by a tough covering called the periosteum. Within the periosteum lies a bony layer called compact bone, which is solid, strong, and resistant to bending. The epiphyses are composed largely of spongy (cancellous) bone, which provides the greatest amount of elastic strength since the epiphyses are subjected to the greatest forces of compression.
In the fetus, most of the skeleton is made up of cartilage, a tough, flexible connective tissue that has no minerals or salts. As the fetus grows, osteoblasts and osteoclasts slowly replace cartilage cells and ossification begins.
Ossification is the formation of bone by the activity of osteoblasts and osteoclasts and the addition of minerals and salts. Calcium compounds must be present for ossification to take place. Osteoblasts do not make these minerals, but must take them from the blood and deposit them in the bone. By the time we are born, many of the bones have been at least partly ossified.
In long bones, the growth and elongation (lengthening) continue from birth through adolescence. Elongation is achieved by the activity of two cartilage plates, called epiphyseal plates, located between the shaft (the diaphysis) and the heads (epiphyses) of the bones (Figure 5). These plates expand, forming new cells, and increasing the length of the shaft. In this manner, the length of the shaft increases at both ends, and each head of the bone moves progressively apart. As growth proceeds, the thickness of the epiphyseal plates gradually decreases and this bone lengthening process ends. In humans, different bones stop lengthening at different ages, but ossification is fully complete by about age 25. During this lengthening period, the stresses of physical activity result in the strengthening of bone tissue.
In contrast to the lengthening of bone, the thickness and strength of bone must continually be maintained by the body. That is, old bone must be replaced by new bone all the time. This is accomplished as bone is continually deposited by osteoblasts, while at the same time, it is continually being reabsorbed (broken down and digested by the body) by osteoclasts (Figure 5). Osteoblasts are found on the outer surfaces of the bones and in the bone cavities. A small amount of osteoblastic activity occurs continually in all living bones (on about 4% of all surfaces at any given time) so that at least some new bone is being formed constantly. Normally, in fact, except in growing bones, the rates of bone deposition and absorption are equal to each other so that the total mass of bone remains constant.
Usually, osteoclasts exist in small but concentrated masses, and once a mass of osteoclasts begins to develop, it usually eats away at the bone for about three weeks, eating out a tunnel that may be as large as 1 millimeter in diameter and several millimeters in length. At the end of this time the osteoclasts disappear and the tunnel is invaded by osteoblasts instead; then new bone begins to develop, Bone deposition then continues for several months, the new bone being laid down in successive layers of concentric circles on the inner surfaces of the cavity until the tunnel is filled. Deposition of new bone ceases when the bone reaches the surface of the blood vessels supplying the area. The canal through which these blood vessels run, called the haversion canal (Figure 5), therefore, is all that remains of the original cavity. This process continues until about age 40, when the activity of osteoblasts slows and bones become more brittle. Let's look at some important factors that are necessary to produce healthy bone.
Bone development is influenced by a number of factors, including nutrition, exposure to sunlight, hormonal secretions, and physical exercise. For example, exposure of skin to the ultraviolet portion of sunlight is favorable to bone development, because the skin can produce vitamin D when it is exposed to such radiation. Vitamin D is necessary for the proper absorption of calcium in the small intestine. In the absence of this vitamin, calcium is poorly absorbed, the bone matrix is deficient in calcium, and the bones are likely to be deformed or very weak. Vitamins A and C also are needed for normal bone growth and development.
Hormones that affect bone growth and development include those secreted by the pituitary gland, thyroid gland, parathyroid glands, and the ovaries and testes (Figure 6). The pituitary gland, for instance, secretes growth hormone (GH), also called somatotropin, which stimulates activity in the epiphyseal plates. This hormone is the main regulator of height. Somatotropin plays many roles in the body: it stimulates bone and muscle growth, maintains the normal rate of protein synthesis in all body cells, and speeds the release of fats as an energy source for growth. Other hormones play a part in maintaining the strength and health of the bone matrix by functioning to control the level of blood calcium. In fact, calcium is needed for a number of metabolic processes other than for bone formation, including blood clot formation, nerve impulse conduction, and muscle cell contraction. When a low blood calcium condition exists, the parathyroid glands respond by releasing parathyroid hormone (PTH). This hormone stimulates osteoclasts to break down bone tissue, and as a result, calcium salts are released into the blood. On the other hand, if the blood calcium level is excessively high, the thyroid gland responds by releasing a hormone called calcitonin. Its effect is opposite that of parathyroid hormone; it inhibits osteoclast activity allowing osteoblasts to form bone tissue. As a result, the excessive calcium is stored in bone matrix. The actions of these hormones are both excellent examples of some important negative feedback loops present in our bodies (Figure 7). Without adequate supplies of these important chemicals, the bones will not develop or grow normally.
Bone is deposited in proportion to the compressional load that the bone must carry. For instance, the bones of athletes become considerably heavier than those of nonathletes. Also, if a person has one leg in a cast but continues to walk on the opposite leg, the bone of the leg in the cast becomes thin and as much as 30% decalcified within a few weeks, while the opposite bone remains thick and normally calcified. Therefore, continual physical stress stimulates calcification and osteoblastic deposition of bone, producing stronger bones. Now that we are familiar with what bone is and how it is formed, let's examine how bones serve to support, protect, and move our bodies.