Human bones are incredible living structures which continually regenerate. Bones support and protect the body, store minerals and are the centers of blood production. Muscle attaches to skeletal bones which are used as levers to move our body's limbs. Some cells in bone secrete osteoclacin, a substance which has recently been linked to insulin sensitivity and blood sugar metabolism.
Bone Composition
Is bone a tissue? The answer is both yes and no. Overall bone anatomy is composed of several different types of tissues including bone tissue, nervous tissue, blood tissue, cartilage and epithelial tissue. So bone tissue exists within overall bone anatomy, but bones as a whole are considered organs. Organs by definition are structures made of more than one type of tissue.
Human bones are composed of a harder outer surface called compact bone, and an inner, lower density filling called spongy bone. Spongy bone is made of trabeculae, which are small needle-like bone protrusions. The space within the trabeculae is filled with yellow or red bone marrow, which produces and stores blood cells.
Surrounding most of the compact bone is a connective tissue membrane called the periosteum. The periosteum is composed of two layers. The outer layer is made of dense irregular connective tissue which functions to resist structural forces. The inner layer of the periosteum is osteogenic, which means it is involved with both bone formation and bone decomposition. Perforating fibers in the periosteum, called Sharpey's fibers penetrate into the bone matrix, securely attaching the periosteum outer covering to the bone tissue itself. Periosteum is vascularized and innervated, which means that it is supplied with blood vessels and nervous tissues. A thinner, osteogenic membrane called the endosteum overlays the trabeculae in spongy bone.
Bone Tissue
Bone tissue is classified as connective tissue and consists of cells separated by extracellular matrix. Bone matrix is made of 35% organic material and 65% inorganic materials. The inorganic materials, which are mainly mineral salts, give bone the strength to support the frame without bending. Organic bone materials, specifically collagen, give bones tensile strength and flexibility. Organic bone matrix is referred to as osteoid.
Bone Cells
- osteogenic cells
- osteoblasts (secrete osteoclacin)
- osteocytes
- osteoclasts
Osteogenic cells are stem cells which differentiate into osteoblasts. Osteoblasts secrete organic matrix called osteoid which becomes calcified with inorganic salts. Once the osteoid surrounding an osteoblast is completely calcified the cell is called an osteocyte, and no longer produces matrix. Osteocytes signal increases or decreases in bone formation in response to wear and tear on bone from use or environmental situations. Bone cells which break down other bone cells using hydrochloric acid and lysosomal enzymes are called osteoclasts.
Bone Types
- long bones
- short bones
- flat bones
- irregular bones
Long bones are found in the arms and legs of the body. A cavity in the center of long bones, called the medullary cavity, contains yellow bone marrow. Long bones have an elongated shaft and distinct bulbous ends. Short bones are somewhat square shaped and found in the wrists and ankles. The knee cap is classified as a short bone. Flat bones, such as the cranial bones, are thin, flat and frequently curved. Spongy bone inside of flat bones is called diploe. Irregular bones do not fall easily into any of the above categories. The hip bones are examples of irregular bones.
Resources
www.umm.edu. "Bone Marrow Transplantation" (Accessed February 12, 2011).
www.btec.cmu.edu. "Cells." (Accessed February 12, 2011).
Marieb, Elaine, Patricia Brady and Jon Mallatt. Human Anatomy, sixth ed. Pearson Education. 2011.
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