Day 14: The Skeleton

What is Bone?

Bone is a connective tissue derived from mesoderm. Bone consists of minerals - mostly calcium hydroxyapatite which is composed of calcium and phosphate - laid into a framework of connective tissue, like bricks laid between the wooden framework of a house. There are three types of bone cells: osteoblasts, osteocytes and osteoclasts. Osteoblasts are responsible for first making the framework and then laying in the mineral. Once the bone is constructed, osteoblasts mature into osteocytes which are then responsible for nourishing the bone surrounding them. Osteoclasts are bone sculptors; they remove bone to help remodel and shape the bones as they grow.

New bone is first laid down in a rather random fashion; consequently, it is referred to as woven bone. Woven bone is then remodeled into lamellar bone. As the name implies, lamellar bone is laid in concentric layers, like the rings of a tree. Lamellar bone can be either compact or spongy. A cross-section of a long bone, such as the femur, shows a good example of both compact bone forming the perimeter of the femur and spongy bone in the form of trabeculae and spicules filling its inside cavity. The lamellae of compact bone are fairly uniform in width and highly organized. Canals carrying blood vessels and nerves run both lengthwise and horizontally through the compact bone. The lamellae of spongy bone, on the other hand, is laid down in varying shapes and thicknesses. Compact bone on the outside of the long bone provides strength, support, and protection. The trabeculae and spicules within the cavity of the bone allow the bone to be lightweight and provide a space in which bone marrow can grow.

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