Bone - cells, structure, bone types and bone health and disease

What is Bone and Definition

The rigid tissue that gives the body structure and mobility. Bone consists of living cells contained within a mineralized structure called the bone matrix. Collagen fibers form intricate networks to which crystals of calcium phosphate, calcium carbonate, and other mineral compounds adhere, forming the dense and rigid structure familiar as bone. Despite its density and its impression as a static structure, bone is in a perpetual state of change, called remodeling. Throughout life certain processes destroy old bone and other processes construct new bone.

Bone Cells

Three types of cells make up bone tissue:

• Osteoblasts form new bone. In response to stimulation from hormones such as CALCITONIN, ESTROGENS, and TESTOSTERONE, osteoblasts draw calcium and other minerals into the bone to strengthen and solidify the bone matrix. Osteoblasts produce a collagen-based substance called osteoid. Calcium, phosphorus, magnesium, and other minerals bind with the osteoid to form mineralized bone (the bone matrix).
• Osteocytes make up the structure of existing bone. Contained within the bone matrix, osteocytes have a lifespan of 20 years or more. Osteocytes begin their lives as osteoblasts, then become enclosed in the bone’s mineralized structure. The spaces they occupy within the mineralized framework are lacunae. Each lacuna has a rich BLOOD supply to nourish and support the osteocytes.
• Osteoclasts remove old bone. Osteoclasts derive from monocytes and are phagocytic; they encircle and consume cellular debris. As they consume old bone tissue, osteoclasts release its calcium into the blood circulation. PARATHYROID HORMONE plays a key role in regulating this release. The process of bone resorption leaves vacant lacunae on the surface of the bone that subsequently fill with new bone structure.

In health, osteoblasts and osteoclasts function in relative balance so the rate of new bone formation matches the rate of old bone destruction. This process of bone remodeling is one of maintenance, not growth. Bone growth, in which the bones increase in size, occurs through ossification (conversion of CARTILAGE cells to bone cells). Imbalance may result from disease processes that alter HORMONE levels in the body or when calcium levels in the blood circulation are too low. Calcium is vital to numerous cellular activities and crucial for MUSCLE contraction and the conduction of NERVE signals; when its levels in the blood circulation are inadequate, the body accelerates bone resorption so it can withdraw calcium from the bones.

Bone Structure

Were they solid, bones would weigh more than the body could support or the muscles could move. So instead they are a combination of densities that provide a balance between STRENGTH and mass. Though all bones contain the same elements of structure, the particular combination of those elements varies according to the bone’s role.

The outermost layer of bone, called compact bone or cortical bone, is made of multiple thin layers, called lamellae, that contain tightly packed osteocytes. Each lamella contains a somewhat different structure of cells, altering the density and orientation of the bone structure for maximum strength. Compact bone is heavily mineralized and very dense; tooth enamel is the only other substance in the body that is harder than compact bone. An intricate network of canals, called the Haversian systems, bring blood vessels and nerves through the lamellae to nourish and support communication among the osteocytes. Compact bone protects the inner bone structures and provides the stiffness necessary to leverage the muscles for movement.

The middle layer of bone is cancellous, or spongy, bone, also called trabecular bone, where mineralized filaments form intricate networks of walls and spaces. The spaces contain osteocytes, fluids, and other cells. The structure of cancellous bone is more elastic than that of compact bone, allowing the bones to absorb compression such as occurs with walking, running, and jumping. Cancellous bone has less than half the density of compact bone but many times more the surface area.

Some bones contain a center channel, the medullary canal, that houses BONE MARROW. In children every bone contains red bone marrow, the type of bone marrow that produces new blood cells. By adulthood only the long bones, sternum, and hip bones contain appreciable amounts of red bone marrow. Yellow bone marrow, a mix of collagen and fatty tissues, occupies the innermost layer of most other bones. Some bones do not contain any marrow.

A thin but tough membrane called the periosteum covers the surface of the bones except at the joints. It forms the attachment surface for tendons and ligaments. The periosteum contains a rich network of blood vessels and nerves that help nourish the compact bone. Osteoblasts in the periosteum are “first responders” when there is injury to the bone, rapidly forming new bone for repair. The nerves in the periosteum are largely responsible for PAIN signals when there is injury to the bone.

Types of Bones

The SKELETON contains four basic types of bones: long, short, flat, and irregular. Long bones, such as those in the arms and legs, must support the body’s weight and mass. Their length and structure also allows them to function as levers to make movement possible. A thin layer of compact bone provides the rigidity the long bones require; a substantial middle layer of cancellous bone provides added bone mass for strength and stability. The intricate trabecular structure of cancellous bone makes it much stronger for supporting weight, though more vulnerable to impact. At each end of a long bone is the epiphysis, or growth plate, where ossification takes place during growth in childhood. The shaft of a long bone is its diaphysis. Lengthwise through the center of a long bone is a medullary canal that contains bone marrow.

Short bones, such as those that form phalanges (metacarpals in the fingers and metatarsals in the toes), are structurally long bones on a much smaller scale. However, a short bone does not have a medullary canal or bone marrow. Flat bones, such as the scapulae (shoulder blades), sternum (breastbone), and pelvis (hip bones), serve as attachment surfaces for the large muscles of movement. They contain a substantial thickness of compact bone with a thin layer of cancellous bone in the center. The sternum and the pelvis also contain bone marrow. Irregular bones, such as the vertebrae (bones of the spine), carpals (bones of the wrist), and tarsals (bones of the ankle), are primarily structures of compact bone with cancellous bone centers.

Bone Health and Disease

Bones require a steady intake of dietary calcium and other minerals as well as an adequate amount of vitamin D, VITAMIN K, and various hormones to maintain themselves. Deficiencies (and less commonly, excesses) of these substances alter bone structure in ways that can affect bone function. Though a certain degree of demineralization occurs naturally as a component of the aging process, excessive calcium loss results in thin and weak bones that are particularly vulnerable to FRACTURE. Fracture is the most common health condition that affects the bones. Other health conditions involving the bones include OSTEOPOROSIS, INFECTION (OSTEOMYELITIS), and congenital musculoskeletal anomalies (BIRTH DEFECTS that affect muscle and bone structure and function).

For further discussion of bone structure and function, please see the overview section “The Musculoskeletal System.”

See also AGING, MUSCULOSKELETAL CHANGES THAT OCCUR WITH; CALCIUM AND BONE HEALTH; CLEFT PALATE/CLEFT PALATE AND LIP; JOINT; LIGAMENT; MONOCYTE; MUSCLE; PHAGOCYTE; SKELETON; TENDON.

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