Bone tissue is a type of connective tissue that makes up the skeletal system.
Bones are made of specialized connective tissue into which are deposited calcium and phosphate. This process results in a connective tissue that is very hard, and the bones are therefore said to be mineralized or ossified. Bones are living tissue and as such must be supplied with nerves and blood vessels. Because they are living tissue, they are able to repair themselves after being fractured. Bones continue to change throughout an individual's lifetime, even after growth stops. Most often they change in response to the hormones that control calcium balance in the body but also in response to stresses placed upon them.
Formation of bones
Bones are a mixture of collagen, a tough, fibrous connective tissue, and the mineral calcium phosphate. The cells that form bones are called osteoblasts. Osteoblasts form bones in two ways. In some bones, such as the bones in the skull and certain irregular bones, sheets of connective tissue are formed first. Osteoblasts then migrate to this connective tissue and secrete calcium phosphate around themselves until the cartilage is hardened. This process is called intermembranous ossification. Once the bone-forming osteoblasts are completely surrounded by a hard matrix of calcium phosphate, they are called osteocytes.
Most bones are formed by a process called endochondral ossification. During fetal development, the body lays down a cartilage template of the bone. Blood vessels invade the cartilage bringing with them osteoblasts. The osteoblasts form a layer of hard, dense bone around the outside of the cartilage. This hard, dense bone is called compact bone. Meanwhile, the cartilage in the center of the bone begins to disintegrate. Osteoblasts move into this area and replace it with less dense bone known as spongy bone. In some bones, a cavity forms in the center of the long bones and a few other bones. This cavity will become filled with red bone marrow. Red bone marrow contains stem cells that form red blood cells, white blood cells, and platelets throughout an individual's life.
The process of mineralization of bone during growth or maturation moves from the middle of the bone toward either end. A band of non-ossified cartilage called a growth plate found at each end of the bone. Until puberty, children's bones grow wider, longer, and thicker. The hormones released at sexual maturity cause the bone to become more dense. The cartilage in the growth plate does not become mineralized until adulthood. Bones are at their strongest between the ages of about 18 to 35, after which they begin to become less dense. Density of bone is regulated by two hormones, calcitonin, a hormone produced by the thyroid gland and parathyroid hormone (PTH), a hormone produced by the parathyroid gland. Calcitonin stimulates osteoblast cells to remove calcium from the blood and deposit it in bones. PTH stimulates another type of bone cell, called an osteoclast, to remove calcium from the bone and return it to the blood. The process of laying down new bone and reabsorbing old bone is known as bone remodeling. It occurs throughout an individuals life, but after about age 35, more calcium is removed from the bone by the osteoclasts than is deposited by the osteoblasts, so that bones become progressively weaker.
Types of bones
Four types of bones are found in humans. These are:
- long bones. These include the bones of the upper leg (femur), lower leg (tibia and fibula), upper arm (humerus), forearm (radius and ulna), hands and feet (tarsals and metatarsals). They are mainly made of hard, compact bone.
- short bones. These roughly cube-shaped bones are found mainly in the wrist and ankle. They are made of mainly less dense spongy bone.
- flat bones. These form the bones of the skull, ribs and breastbone (sternum).
- irregular bones. Any bones that do not fall into the other categories, such as the bones of the spinal column (vertebrae).
Bones provide support, protection of soft tissue organs, and attachments for muscles. The combination of bones and muscles allows movement of body parts. They also act as storage sites for calcium and phosphate and play an active role in maintaining calcium homeostasis within the body.
Long bones of the upper arm and leg contain red bone marrow. Red bone marrow produces red blood cells, white blood cells, and platelets. Short bones allow a wide range of joint rotation. Flat bones, such as the bones of the skull, mainly protect soft tissue. Irregular bones provide large surface areas for the attachment of muscles.
Role in human health
Calcium ions play a critical role in muscle contraction. Bone tissue acts as a reservoir for calcium, and is therefore involved in the critical function of maintaining calcium homeostasis in the body. Strong healthy bones are also essential for movement and support. Healthy bone marrow is needed to replace lost blood and to produce immune system cells that fight infection.
Common diseases and disorders
Bone diseases can be congenital or develop with age. Osteoporosis is a common bone disease in the elderly and more often found in women. With osteoporosis, bone loses minerals and the bone structure is disrupted. This weakens the bone and makes it vulnerable to fractures. Paget's disease also affects older individuals. It results in large but weak bones. The cause of Paget's disease is unknown. Osteogenesis imperfecta (OI) is a disease of unknown origin that causes bones to break easily. People with OI may have dozens of fractures during their lifetime. Rickets is a bone disease that results in weak, soft bones. It is caused by an inadequate supply of vitamin D. Rickets is rare in the United States where vitamin D is added to milk, cereal, and other processed foods.
Other bone diseases include:
- myeloma bone disease—a tumor made of bone cells that causes bone to be broken down and reabsorbed osteopetrosis—a congenital disease in which bones are too dense
- fibrous dysplasia—an abnormal growth of connective tissue within the bone, causing bones to become deformed and brittle
- metabolic disorders such as hypophosphatasia or hyperparathyroidism that disrupt the phosphate or calcium balance of the body.
- leukemia and lymphoma—not strictly a bone disease, but blood cell cancers that arise from defects in the stem cells found in bone marrow.
Homeostasis— The chemical, fluid, and metabolic balance the body must maintain to remain healthy.
Osteoblast— A bone-forming cell.
Osteoclast— A cell that removes minerals from bone and returns them to the bloodstream.
Osteocyte— A mature bone cell.
Platelets— cellular material that arises from stem cells in red bone marrow and is involved in causing blood to clot.
Red blood cells— Oxygen-carrying cells that arise from stem cells in red bone marrow.
White blood cells— The main cells involved in immune system response to disease. There are several types of white blood cells, all of which arise from stem cells in red bone marrow.
Stem cells— Cells that remain undifferentiated and able to produce several different types of cells when new cells are needed.
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