Skeletal Development Health Article

Definition

Skeletal development refers to the development of the human skeletal system from the early days of pregnancy until the bones have reached full development in late puberty.

Description

The early development of the skeletal system begins in the third week after conception with the formation of the notochord (a rod-like structure along the back of the embryo that later becomes the spine, spinal cord, and brain), 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 becomes the coccyx (lowermost tip of the backbone). By the seventh week the embryo is about 2 cm (1 in) long and facial features are visible. At this stage, the 206 bones of the human body 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 are "bony." Ossification (the process whereby tissue becomes bone) of most bony nuclei of the long bones and round bones does not complete until after birth.

The major types of human bones are:

• long (e.g. the arm and leg bones)
• short (e.g. the small bones in the wrists and ankles)
• flat (e.g. the bones of the skull or the ribs)
• irregular (e.g. vertebrae)

Long, short, and irregular bones develop by endochondral ossification, where cartilage is replaced by bone. Flat bones develop by intramembranous ossification, where bone develops within sheets of connective tissue. Compact cortical bone, representing about 80 percent 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.

Bone growth is more complicated than simple elongation or simple enlargement. Most long bones add width on the outside by a process referred 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 bones gain in length by adding to the epiphyseal plate (the surface at the end of the bone). As they elongate, bones of this type go through a process called remodeling during which they change in outer shape as well. Conversely, 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 occurs by a different process. At birth, long bones have more than one ossification center (regions from which bone growth starts). These grow during childhood until the ends of the bone (epiphyseal plates) become fused with the shaft of the bone (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. Both round and flat bones of the skeleton are capable of continued growth throughout life.