- Compact bone is the hard material that makes up the shaft of long bones and the outside surfaces of other bones. Compact bone consists of cylindrical units called osteons. Each osteon contains concentric lamellae (layers) of hard, calcified matrix with osteocytes (bone cells) lodged in lacunae (spaces) between the lamellae. Smaller canals, or canaliculi, radiate outward from a central canal, which contains blood vessels and nerve fibers. Osteocytes within an osteon are connected to each other and to the central canal by fine cellular extensions. Through these cellular extensions, nutrients and waste are exchanged between the osteocytes and the blood vessels. Perforating canals provide channels that allow the blood vessels that run through the central canals to connect to the blood vessels in the periosteum that surrounds the bone.
- Spongy bone consists of thin, irregularly shaped plates called trabeculae, arranged in a latticework network. Trabeculae are similar to osteons in that both have osteocytes in lacunae that lie between calcified lamellae. As in osteons, canaliculi present in trabeculae provide connections between osteocytes. However, since each trabecula is only a few cell layers thick, each osteocyte is able to exchange nutrients with nearby blood vessels. Thus, no central canal is necessary.
Figure 1.Main features of a long bone.
Here are the main features of a long bone (refer to Figure 1):
- The diaphysis, or shaft, is the long tubular portion of long bones. It is composed of compact bone tissue.
- The epiphysis (plural, epiphyses) is the expanded end of a long bone. It is in the epiphyses where red blood cells are formed.
- The metaphysis is the area where the diaphysis meets the epiphysis. It includes the epiphyseal line, a remnant of cartilage from growing bones.
- The medullary cavity, or marrow cavity, is the open area within the diaphysis. The adipose tissue inside the cavity stores lipids and forms the yellow marrow.
- Articular cartilage covers the epiphysis where joints occur.
- The periosteum is the membrane covering the outside of the diaphysis (and epiphyses where articular cartilage is absent). It contains osteoblasts (bone‐forming cells), osteoclasts (bone‐destroying cells), nerve fibers, and blood and lymphatic vessels. Ligaments and tendons attach to the periosteum.
- The endosteum is the membrane that lines the marrow cavity.
Here are the main features of short, flat, and irregular bones:
- In short and irregular bones, spongy bone tissue is encircled by a thin layer of compact bone tissue.
- In flat bones, the spongy bone tissue is sandwiched between two layers of compact bone tissue. The spongy bone tissue is called the diploë.
- The periosteum covers the outside layer of compact bone tissue.
- The endosteum covers the trabeculae that fill the inside of the bone.
- In certain bones (ribs, vertebrae, hip bones, sternum), the spaces between the trabeculae contain red marrow, which is active in hematopoiesis.