Skeletal Development Health Article

Skeletal Development

Development of the bones of the human body.

Scientists who study the development of the human being from conception to birth begin calling the embryo a fetus around eight weeks after conception, when the first bone cells appear. The beginnings of the skeletal system begins prior to this, however. In the third week after conception, the notochord—a rod-like structure along the back of the embryo that will later become the spine, spinal cord, and brain—develops, followed in the fourth week by the first signs of arms and legs. Between the fifth and eighth weeks, the limbs (first the arms, hands, and fingers, followed by the legs, feet, and toes) begin to extend and take on a definite shape. By the end of the fifth week, the embryo has doubled in size and has grown a tail-like structure that will become the coccyx, or lowermost tip of the backbone. By the seventh week it is about 2 cm (1 in) long and facial features are visible. At this stage, the 206 bones of the human body—tubular, round, and flat—are all set down, in surprisingly adult form. However, the process of osteogenesis—development of bone—has not progressed to the point where the bones will become "bony." Indeed, ossification—the process whereby tissue hardens completely and becomes bone—of most bony nuclei of the long bones and round bones does not take place until after birth. It is therefore commonly but incorrectly stated in many elementary texts that the infant and child have more bones than adults do; this is a conceptual error resulting from confusing ossification centers (more than one bony spot that will spread and merge with other similar bony spots to form one bone mass) with anatomical bones.

Bone growth is more complicated than simple elongation or simple enlargement. Most long bones or tubular bones add in width on the outside by a process scientists refer to as subperiosteal apposition (layers added to those already existing), while losing bone on the inside by endosteal resorption (breaking down and reabsorbing material at the center of a mass). At the same time, long or tubular bones gain in length by additions to the epiphyseal plate (the surface at the end of the bone). As they elongate, bones of this type go through a process, remodeling, where they change in outer shape as well.

The individual bones of the skull grow by circumferential apposition (adding layers at the circumference), while gaining in thickness by adding layers (apposition) at the surface with simultaneous resorption at the inner surface. By this process, the skull expands and becomes thicker while allowing for more brain space within.

Linear growth of the long bones continues until the bone is completely hardened. At birth, long bones have more than one ossification center. These grow during childhood until the epiphyseal plates (at the ends of the bone) becomes fused with the shaft of the long bone, known as the diaphysis. This process is stimulated by the hormones produced by the testes and ovaries, which provide the developmental signal that the linear growth of the long bones should reach completion, or full development. Compact cortical bone, representing about 80% of the mature skeleton, supports the body, and features extra thickness at the midpoint in long bones to prevent the bones from bending. Cancellous bone, whose porous structure with small cavities resembles sponge, predominates in the pelvis and the 33 vertebrae from the neck to the tailbone.

Both round and flat bones of the skeleton are capable of continued growth throughout life. Deficiencies in bone growth may be caused by insufficient thyroid hormone or growth hormone (hypopituitarism). Conversely, hyperpituitarism (too much growth hormone) before puberty results in bone overgrowth, and, if left untreated, giantism, characterized by statures of eight feet (244 cm) or greater.

During childhood, bones are growing rapidly. Bone growth is fueled by a positive energy balance, created by a well-balanced diet and healthy living environment. Even in circumstances of severe malnutrition, there may be some formation of new bone; however, it will occur while bone formed earlier is deteriorating. During protein malnutrition, bone growth is largely halted and existing bone is cannibalized by the body as a source of protein. Both conditions exist in low-income families worldwide. Bone growth may also be limited by vitamin D deficiency, resulting in rickets in children (osteomalacia in adults) and other conditions characterized by insufficient dietary calcium.

With growing concern about adult osteoporosis, it is important to realize that the mass of skeleton built during childhood and into early adulthood constitutes bone banked against inevitable later withdrawals. For this reason there is much interest in the proposition that a calcium intake over 1500 mg per day may build a greater skeletal mass. However, this proposition, currently favored in the United States, is met with great skepticism in the United Kingdom. Moreover, calcium intakes during childhood are far greater in the United States than in most countries, but there is no particular evidence that the adult North American skeleton has a greater bone density (bone mass divided by bone volume).