Day 14: The Skeleton

Development of Cartilage Bones

Most bones begin as cartilage that is gradually replaced by bone. The chrondroblasts that build the cartilage skeleton of the embryo are derived from the somites (vertebrae and ribs) and from somatopleural mesoderm ( sternum, pectoral girdle, wings, pelvic girdle, and legs). As the embryonic bones grow in length and diameter, chondroblasts continually divide and form new cartilage at the active growth sites. After chondroblasts have laid down their cartilage, they mature into chondrocytes and take on the responsibility of nourishing and maintaining the cartilage surrounding them until it becomes ossified.

The cartilage begins to ossify, or turn to bone, when its chondrocytes mature and die. Chondrocytes on the advancing edge of ossification enlarge die, and leave large holes in the cartilage. Blood vessels find their way to the area, bringing osteoblasts and osteoclasts with them to form and remodel bone on the thin columns of remaining cartilage. The original bone is woven and later remodeled into lamellar bone. The outer layers are remodeled into compact bone, while the bone within the central cavity becomes spongy bone. Primitive mesenchymal cells trapped within the spongy bone specialize into bone marrow. Bone marrow is responsible for producing most of our blood cells. In the newborn, all bones contain bone marrow. By adulthood, most of the marrow in long bones is replaced by fat.

Long bones are made up of a long central shaft with an articulating knob on either end. A long bone begins to ossify at the center of its shaft. This is called its primary ossification center. Later, secondary ossification centers begin to form within each articulating knob. Eventually, all that remains of the actively dividing cartilage are two zones, located between the articulating knobs and the shaft at either end of the bone. These two zones are called growth plates and are responsible for lengthening the bone. Hormonal changes cause growth plates to ossify, and thereby fuse the articulating knobs and shaft sometime during late puberty. A thin layer of hyaline cartilage remains on the surface of the articulating knobs to cushion the bones at the joints.

This photo of a 40-day old cat fetus shows its cartilaginous skeleton stained with alcian blue. The specimen was also stained with alizarin red, which stains calcium. Primary ossification centers can be seen (red or violet) in the middle of the shafts of long bones (blue). Secondary centers of ossification in the articulating knobs of the long bones are not yet clearly visible.

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