SKELETAL
SYSTEM
Function
of the Skeletal System
Humans are vertebrates, animals having a vertabral
column or backbone. They rely on a sturdy internal frame that is centered on a
prominent spine. The human skeletal system consists of bones, cartilage,
ligaments and tendons and accounts for about 20 percent of the body weight.
The living bones in our bodies 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 frame work, known as the
skeleton, that support and protect the soft organs of 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 make it less vulnerable to
injury. Vertebrae surround and protect the spinal cord and bones of the rib
cage help protect the heart and lungs of the thorax.
Bones work together with muscles as simple
mechanical lever systems to produce body movement.
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.
Structure
of Bone Tissue
There are two types of bone tissue: compact and
spongy. The names imply that the two types of differ in density, or how tightly
the tissue is packed together. There are three types of cells that contribute
to bone homeostasis. Osteoblasts are bone-forming cell, osteoclasts resorb or
break down bone, and osteocytes are mature bone cells. An equilibrium between
osteoblasts and osteoclasts maintains bone tissue.
Compact Bone
Compact bone consists of closely packed osteons or
haversian systems. The osteon consists of a central canal called the osteonic
(haversian) canal, which is surrounded by concentric rings (lamellae) of
matrix. Between the rings of matrix, the bone cells (osteocytes) are located in
spaces called lacunae. Small channels (canaliculi) radiate from the lacunae to
the osteonic (haversian) canal to provide passageways through the hard matrix.
In compact bone, the haversian systems are packed tightly together to form what
appears to be a solid mass. The osteonic canals contain blood vessels that are
parallel to the long axis of the bone. These blood vessels interconnect, by way
of perforating canals, with vessels on the surface of the bone.
Spongy (Cancellous) Bone
Spongy (cancellous) bone is lighter and less dense
than compact bone. Spongy bone consists of plates (trabeculae) and bars of bone
adjacent to small, irregular cavities that contain red bone marrow. The
canaliculi connect to the adjacent cavities, instead of a central haversian
canal, to receive their blood supply. It may appear that the trabeculae are
arranged in a haphazard manner, but they are organized to provide maximum
strength similar to braces that are used to support a building. The trabeculae
of spongy bone follow the lines of stress and can realign if the direction of
stress changes.
Bone
Development and Growth
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 stature 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.
There are two types of ossification:
intramembranous and endochondral.
Intramembranous
Intramembranous ossification involves the replacement
of sheet-like connective tissue membranes with bony tissue. Bones formed in
this manner are called intramembranous bones. They include certain flat bones
of the skull and some of the irregular bones. The future bones are first formed
as connective tissue membranes. Osteoblasts migrate to the membranes and
deposit bony matrix around themselves. When the osteoblasts are surrounded by
matrix they are called osteocytes.
Endochondral Ossification
Endochondral ossification involves the replacement
of hyaline cartilage with bony tissue. Most of the bones of the skeleton are
formed in this manner. These bones are called endochondral bones. In this
process, the future bones are first formed as hyaline cartilage models. During
the third month after conception, the perichondrium
that surrounds the hyaline cartilage "models" becomes infiltrated
with blood vessels and osteoblasts and changes into a periosteum. The
osteoblasts form a collar of compact bone around the diaphysis. At the same
time, the cartilage in the center of the diaphysis begins to disintegrate.
Osteoblasts penetrate the disintegrating cartilage and replace it with spongy
bone. This forms a primary ossification center. Ossification continues from
this center toward the ends of the bones. After spongy bone is formed in the
diaphysis, osteoclasts break down the newly formed bone to open up the
medullary cavity.
The cartilage in the epiphyses continues to grow so
the developing bone increases in length. Later, usually after birth, secondary
ossification centers form in the epiphyses. Ossification in the epiphyses is
similar to that in the diaphysis except that the spongy bone is retained
instead of being broken down to form a medullary cavity. When secondary
ossification is complete, the hyaline cartilage is totally replaced by bone
except in two areas. A region of hyaline cartilage remains over the surface of
the epiphysis as the articular cartilage and another area of cartilage remains
between the epiphysis and diaphysis. This is the epiphyseal plate or growth
region.
Bone Growth
Bones grow in length at the epiphyseal plate by a
process that is similar to endochondral ossification. The cartilage in the
region of the epiphyseal plate next to the epiphysis continues to grow by
mitosis. The chondrocytes, in the region next to the diaphysis, age and
degenerate. Osteoblasts move in and ossify the matrix to form bone. This
process continues throughout childhood and the adolescent years until the
cartilage growth slows and finally stops. When cartilage growth ceases, usually
in the early twenties, the epiphyseal plate completely ossifies so that only a
thin epiphyseal line remains and the bones can no longer grow in length. Bone
growth is under the influence of growth hormone from the anterior pituitary
gland and sex hormones from the ovaries and testes.
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.
Classification
of Bones
Long Bones
|
The bones of the body come in a variety of sizes
and shapes. The four principal types of bones are long, short, flat and
irregular. Bones that are longer than they are wide are called long bones.
They consist of a long shaft with two bulky ends or extremities. They are
primarily compact bone but may have a large amount of spongy bone at the ends
or extremities. Long bones include bones of the thigh, leg, arm, and forearm.
Short Bones
Short bones are roughly cube shaped with vertical
and horizontal dimensions approximately equal. They consist primarily of
spongy bone, which is covered by a thin layer of compact bone. Short bones
include the bones of the wrist and ankle.
|
|
Flat Bones
Flat bones are thin, flattened, and usually curved.
Most of the bones of the cranium are flat bones.
Irregular Bones
Bones that are not in any of the above three
categories are classified as irregular bones. They are primarily spongy bone
that is covered with a thin layer of compact bone. The vertebrae and some of
the bones in the skull are irregular bones.
All bones have surface markings and characteristics
that make a specific bone unique. There are holes, depressions, smooth facets,
lines, projections and other markings. These usually represent passageways for
vessels and nerves, points of articulation with other bones or points of
attachment for tendons and ligaments.
The adult human skeleton usually consists of 206 named bones. These bones can be grouped in two divisions: axial skeleton and appendicular 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 breastbone or sternum. 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)
Skull
(28)
|
Cranial Bones
|
|
Facial
Bones
|
|
Auditory
Ossicles
|
Hyoid (1)
Vetebral Column
|
Thoracic Cage
|
Appendicular Skeleton
(126 bones)
Pectoral girdles
|
Upper Extremity
|
Pelvic
Girdle
|
Lower Extremity
|
Articulations
An articulation, or joint, is where two bones come
together. In terms of the amount of movement they allow, there are three types
of joints: immovable, slightly movable and freely movable.
Synarthroses
Synarthroses are immovable joints. The singular
form is synarthrosis. In these joints, the bones come in very close contact and
are separated only by a thin layer of fibrous connective tissue. The sutures in
the skull are examples of immovable joints
Amphiarthroses
Slightly movable joints are called amphiarthroses.
The singular form is amphiarthrosis. In this type of joint, the bones are
connected by hyaline cartilage or fibrocartilage. The ribs connected to the
sternum by costal cartilages are slightly movable joints connected by hyaline
cartilage. The symphysis pubis is a slightly movable joint in which there is a
fibrocartilage pad between the two bones. The joints between the vertebrae and
the intervertebral disks are also of this type.
|
Diarthroses
Most joints in the adult body are diarthroses, or
freely movable joints. The singular form is diarthrosis. In this type of
joint, the ends of the opposing bones are covered with hyaline cartilage, the
articular cartilage, and they are separated by a space called the joint
cavity. The components of the joints are enclosed in a dense fibrous joint
capsule. The outer layer of the capsule consists of the ligaments that hold
the bones together. The inner layer is the synovial membrane that secretes
synovial fluid into the joint cavity for lubrication.
|
|
Because all of these joints have a synovial
membrane, they are sometimes called synovial joints.
