Ed

Lore & Background

Bones serve a variety of mechanical functions. Together the bones in the body form the skeleton. They provide a frame to keep the body supported, and an attachment point for skeletal muscles, tendons, ligaments and joints, which function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space. Bones protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs. Because of the way that bone is formed, bone has a high compressive strength of about 170 MPa (1,700 kgf/cm2), poor tensile strength of 104–121 MPa, and a very low shear stress strength (51.6 MPa). Mechanically, bones also have a special role in hearing. The ossicles are three small bones in the middle ear which are involved in sound transduction.

In Their Own Story

The cancellous part of bones contain bone marrow. Bone marrow produces blood cells in a process called hematopoiesis. Blood cells that are created in bone marrow include red blood cells, platelets and white blood cells. Progenitor cells such as the hematopoietic stem cell divide in a process called mitosis to produce precursor cells. These include precursors which eventually give rise to white blood cells, and erythroblasts which give rise to red blood cells. Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called megakaryocytes. This process of progressive differentiation occurs within the bone marrow. After the cells are matured, they enter the circulation. Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes are produced in this way. As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed.

Reader's Guide

Bones act as reserves of minerals important for the body, most notably calcium and phosphorus. Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone. Mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others. Marrow adipose tissue (MAT) acts as a storage reserve of fatty acids. Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts. Bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion. Bone controls phosphate metabolism by releasing fibroblast growth factor 23 (FGF-23), which acts on kidneys to reduce phosphate reabsorption. Bone cells also release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat. The process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important process in regulating calcium balance.

Did You Know?

• The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.
• In the human body at birth, approximately 300 bones are present, many of which fuse together during development, leaving a total of 206 separate bones in the adult.
• Bone has a high compressive strength of about 170 MPa (1,700 kgf/cm2), poor tensile strength of 104–121 MPa, and a very low shear stress strength (51.6 MPa).
• Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes are produced in bone marrow.
• The Ancient Greek word for bone is ὀστέον ('osteon'), and in anatomical terminology the word for a bone is os.