Skeletal System

Humans are vertebrates, animals that have a vertebral column or backbone. They rely on a sturdy internal frame that is centered on a prominent spine. The human skeletal system consists of bones and cartilages, and accounts for about 20% of body weight. Most adults have 206 bones.

The female skeleton. Source: Wikimedia Commons. Author: Bernhard Ungerer. License: Attribution 3.0 Unported (http://creativecommons.org/licenses/by/3.0/deed.en)

Other Names

Synonyms

• Bones
• Skeleton

Related terms

• Musculoskeletal System (also includes Muscular System and Articular System)

Types

Bone with compact and spongy tissue. Source: SEER training site.

There are two types of bone tissue. These two types differ in density, or how tightly the tissue is packed together.

• Compact. Compact tissue contains closely packed systems that include blood vessels. This part of the bone appears solid.
• Spongy. Spongy tissue, also called cancellous tissue, is lighter and less dense than compact bone. This tissue surrounds pockets in the bone that contain bone marrow. This tissue may appear to be arranged in a haphazard manner, but it is organized to provide maximum strength similar to braces that are used to support a building. Spongy bone is organized to follow the lines of stress and can realign if the direction of stress changes.

Description

The adult human skeleton usually consists of 206 named bones, which vary in size and shape. These bones can be grouped in two divisions:

• Axial skeleton. The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs, and sternum ("breastbone").
• Appendicular skeleton. The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower extremities, or limbs, and their attachments, which are called girdles. The named bones of the body are listed below by category.

Axial Skeleton (80 bones)

Cranial bones (Cranium, hyoid and mandible: 29 bones)

• Bones of neurocranium
  • Parietal bone (2)
  • Temporal bone (2)
  • Frontal bone (1)
  • Occipital bone (1)
  • Ethmoid (1)
  • Sphenoid (1)
  • Nasal bone (2)
  • Inferior nasal concha (2)
  • Lacrimal bone (2)
  • Vomer (1)
• Bones of viscerocranium
  • Maxilla (2)
  • Palatine bone (2)
  • Zygomatic bone (2)
  • Auditory ossicles
    • Malleus (2)
    • Incus (2)
    • Stapes (2)
  • Mandible (1)
  • Hyoid bone (1)

Vertebral column (26 bones)

• Cervical vertebrae (7)
• Thoracic vertebrae (12)
• Lumbar vertebrae (5)
• Sacrum (1)
• Coccyx (1)

Thoracic skeleton (25 bones)

• Sternum (1)
• Ribs (24)
• Costal cartilage

Appendicular Skeleton (126 bones)

Bones of upper limb (two sets of 32 bones)

• Bones of pectoral or shoulder girdle
  • Clavicle (2)
  • Scapula (2)
• Bones of free part of upper limb
  • Humerus (2)
  • Radius (2)
  • Ulna (2)
  • Carpals (16)
  • Metacarpals (10)
  • Phalanges (28)

Bones of lower limb (two sets of 31 bones)

• Bones of pelvic girdle
  • Hip (or coxal or innominate) bone (2)
• Bones of free part of lower limb
  • Femur (2)
  • Tibia (2)
  • Fibula (2)
  • Patella (2)
  • Tarsals (14)
  • Metatarsals (10)
  • Phalanges (28)

Illustrations

Cranial bones. Source: SEER training site.	Facial bones. Source: SEER training site.	Auditory bones. Source: SEER training site.
Vertebral column. Source: SEER training site.	Thoracic cage. Source: SEER training site.	Pectoral girdles. Source: SEER training site.
Upper extremity. Source: SEER training site.	Pelvic girdle. Source: SEER training site.	Lower extremity. Source: SEER training site.

Role of the Skeletal System in the Body

The skeletal system has many roles in the body:

• The skeleton supports the body against the pull of gravity. The large bones of the lower limbs support the trunk when standing.
• The skeleton also protects the soft body parts. The fused bones of the cranium surround the brain to protect it from injury. Vertebrae surround and protect the spinal cord, and bones of the rib cage help protect the heart and lungs.
• Bones work together with muscles as simple mechanical lever systems to produce body movement.
• Bones are also important for providing electrolytes such as calcium to the body and producing some of the cells that make up blood.
• The bones of the ear canal are important for sound transduction.

How It Works

The living bones use oxygen and give off waste products in metabolism. They contain active tissues that consume nutrients, require a blood supply, and change shape or remodel in response to variations in mechanical stress. Bones provide a rigid framework, known as the skeleton, that support and protect the soft organs of the body.

Bones contain more calcium than any other organ. The intercellular matrix of bone contains large amounts of calcium salts, the most important being calcium phosphate. When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs. When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously.

Hematopoiesis, the formation of blood cells, mostly takes place in the red marrow of the bones. In infants, red marrow is found in the bone cavities. With age, it is largely replaced by yellow marrow for fat storage. In adults, red marrow is limited to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae, and pelvis. Red marrow functions in the formation of red blood cells, white blood cells and blood platelets.

Bone development

The terms osteogenesis and ossification are often used synonymously to indicate the process of bone formation. Parts of the skeleton form during the first few weeks after conception. By the end of the eighth week after conception, the skeletal pattern is formed in cartilage and connective tissue membranes and ossification begins.

Bone development continues throughout adulthood. Even after adult height is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles. Osteoblasts, osteocytes, and osteoclasts are the three cell types involved in the development, growth, and remodeling of bones. Osteoblasts are bone-forming cells, osteocytes are mature bone cells, and osteoclasts break down and reabsorb bone. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.

Bone growth

Bones grow in length at the epiphyseal plate. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight. The increase in diameter is called appositional growth. Osteoblasts in the periosteum form compact bone around the external bone surface. At the same time, osteoclasts in the endosteum break down bone on the internal bone surface, around the medullary cavity. These two processes together increase the diameter of the bone and, at the same time, keep the bone from becoming excessively heavy and bulky.

Diseases of the Skeletal System

Diseases involving the skeletal system include the following:

• Achondroplasia
• Fracture
• Osteogenesis imperfecta
• Osteoporosis

Related Professions

• A chiropractor is a health care professional trained to provide medical care for disorders of the musculoskeletal system and the nervous system.

• An orthopedic surgeon is a medical doctor who is trained in the diagnosis and management of disorders of the musculoskeletal system.

History

Jean-Andre Venel is credited as the father of orthopedics. He established the first hospital for the treatment of children's skeletal deformities in 1780.^[1]

Research

Contrary to long-standing belief, vitamin D is now thought to be metabolized by the skeleton, rather than just by the endocrine system. Research has shown that bone cells are able to convert vitamin D into its various forms.^[2]

References

1. ↑ WorldOrtho Web site. The Foundations of Modern Orthopaedics.
2. ↑ Anderson PH, Atkins GJ. The skeleton as an intracrine organ for vitamin D metabolism. Mol Aspects Med. 2008 Jul 3. [Epub ahead of print]. Abstract

External Links

American Academy of Orthopaedic Surgeons

American Orthopaedic Association