Skeletal System Health Article

Definition

The skeletal system is a living, dynamic, bony framework of the body, with networks of infiltrating blood vessels.

Description

Inside every person is a skeleton, a sturdy framework of about 206 bones that protects the body's organs, supports the body, provides attachment points for muscles to enable body movement, functions as a storage site for minerals such as calcium and phosphorus, and produces blood cells. Living mature bone is about 60% calcium compounds and about 40% collagen. Hence, bone is strong, hard, and slightly elastic. Humans are born with over 300 bones but some bones, such as those in the skull and lower spine, fuse during growth, thereby reducing the number. Although mature bones consist largely of calcium—70% calcium salts and about 30% organic matrix, mostly collagen fibers—most bones in the skeleton of vertebrates, including humans, began as cartilage. Cartilage is a type of connective tissue, and contains collagen and elastin fibers. The hard outer part of bones are comprised mostly of a proteins such as collagen, in addition to a substance called hydroxyapatite. This substance is composed primarily of calcium and other minerals, and stores much of the body's calcium; it is primarily responsible for the strength of bones. At the center of each bone is the marrow, which is softer and less dense than the rest of the bone. The marrow contains specialized cells that produce blood cells that run through a bone, with nerves surrounding it.

Individual bones meet at areas called joints and are held in place by connective tissue. Most joints, such as the elbow, are called synovial joints, for the synovial membrane which envelopes the joint and secretes a lubricating fluid. Cartilage lines the surface of many joints and helps reduce friction between bones. The connective tissues linking the skeleton together at the joints are tendons and ligaments. Ligaments and tendons are both made up of collagen, but serve different functions. Ligaments link bones together and help prevent dislocated joints. Tendons link bone to muscle.

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton. Some animals, such as the crab, have an external skeleton called an exoskeleton.

Types of bone

Bones may be classified according to their various traits, such as shape, origin, and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones, and irregular bones. Long bones have a long central shaft, called the diaphysis, and two knobby ends, called the epiphysis. In growing long bones, the diaphysis and epiphysis are separated by a thin sheet of cartilage. Examples of long bones include bones of the arms and legs, the metacarpals of the hand, metatarsals of the foot, and the clavicle. Short bones are about as long as wide. The patella, carpels of the wrist, and tarsals of the ankle are short bones. Flat bones take several shapes, but are characterized by being relatively thin and flat. Examples include the sternum, ribs, hip bones, scapula, and cranial bones. Irregular bones are the odd-shaped bones of the skull, such as the sphenoid, the sacrum, and the vertebrae. The common characteristic of irregular bones is not that they are similar to each other in appearance, but that they cannot be placed in any of the other bone categories.

Bones may also be classified based on their origin. All bone (as well as muscles and connective tissue) originates from an embryonic connective tissue called mesenchyme, which makes mesoderm, also an embryonic tissue. Some mesoderm forms the cartilaginous skeleton of the fetus, the precursor for the bony skeleton. However, some bones, such as the clavicle and some of the facial and cranial bones of the skull, develop directly from mesenchyme, thereby bypassing the cartilaginous stage. These types of bone are called membrane bone (or dermal bone). Bone which originates from cartilage is called endochondral bone.

Finally, bones are classified based on texture. Smooth, hard bone called compact bone forms the outer layer of bones. Inside the outer compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, the two types of bone produce a light, but strong, skeleton.