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Chapter 6- Skeleton System

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I. Structure of bones

A. Functions 

1. The skeleton has many functions, but the most obvious is supporting the weight of the body.

 

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2. The skeletal system includes the bones of the skeleton and the cartilages, ligaments, and other connective tissues that stabilize or connect them. This system has five primary functions:

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a.      Support- Provides structural support for the entire body. Individual bones or groups of bones provide a framework for attachment of soft tissue organs.

 b.      Storage-The calcium salts of bones are

very important to bones. Also, bones store lipids in areas of yellow marrow as energy reserve.

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c. Blood cell production- RBC, WBC and other blood elements are produced within the RED marrow.

 d. Protection- Delicate tissues and organs are surrounded by skeletal elements for protection.

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e. Leverage- Bones of the skeleton function as levers that change the magnitude and direction of the forces generated by skeletal muscles.

 

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B. Organization of the Skeleton System 1.      Osteology is the study of bones.  a.      Each bone is an organ that plays a part in

the total functioning of the skeleton system. b.      An adult human has approximately 206

bones. 

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c. Number of bones differs from person to person depending on age and genetic variations.

 d. At birth, a baby has about 270 bones. As the bones develop (ossification – hardening of the bones) the number decreases.

 e. During adolescence, the number decreases, due to gradual fusion of separate bones.

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f. Some adults have extra bones within the sutures (joints) of the skull called suture.

 g. Additional bones may develop in tendons in response to stress as the tendons repeatedly move across a joint.

 

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C. Macroscopic features of bone 1.      The bones of the human skeleton have four

general shapes, long, short, flat and irregular. (See page 128)

 2.      Long bones are longer than they are wide

whereas short bones are of roughly equal dimensions.

 3. Long bones examples: bones in limbs. (Femur or humerus)

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4. Short bones include the bones of the wrist (carpal) and ankles (tarsal)

 5. Flat bones are thin and relatively broad, such as the parietal bones in the skull, the ribs and the scapulae.

 6. Irregular bones have complex shapes that do not fit easily into any other category. Example- vertebrae of the spinal column.

 

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7. A long bone has a central shaft, or diaphysis and an expanded portion at each end or epiphysis.

 8. The diaphysis surrounds a central marrow cavity.

 9. The ends or epiphyses are covered by cartilage.

  

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10. The two types of bone tissue are compact/dense bone or spongy bone/cancellous.

 11. The outer surface of a bone is covered by a periosteum, which consists of a fibrous outer layer and a cellular inner layer.

 

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12. Inside the bone, a cellular endosteum lines the marrow cavity and other surfaces.

 13. See figure 6.2 on page 129.

 

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D. The Axial skeleton consists of the bones that form the axis of the body and that support and protect the organs of the head, neck and torso.

 

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1. Skull- has two main sets of the bones: the cranial bones that form the cranium and the facial bones that support the eyes, nose, and jaws. There are 22 bones total in the skull.

 

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2. Auditory ossicles are present in the middle ear chamber of each ear and serve to transmit sound. There are 6 bones total.

 

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3. The hyoid bone is located above the larynx and below the lower jaw, supports the tongue and assists in swallowing. One bone total.

 

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4. Vertebral column (backbone) consists of 26 individual vertebrae separated by cartilaginous intervertebral discs. In the pelvic region several vertebrae are fused to form the sacrum.

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5. The rib cage or thoracic cage includes the 12 pairs of ribs, the flattened sternum and the costal cartilage that connects the ribs to the sternum.

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E.   The appendicular skeleton is composed of bones of the upper and lower extremities and the bony girdles, which anchor the appendages to the axial skeleton.

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1. Pectoral girdle is made up of the paired scapulae and the clavicles. The primary function of the pectoral girdle is to provide attachment for the muscles that move the brachium and forearm. Pectoral girdle has 4 bones.

 

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2. Upper extremity- Each upper extremity consists of a proximal humerus, an ulna and radius, carpal bones of the wrist and the metacarpal and phalangeal bones of the hand. Upper extremity has 60 bones.

 

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3. Pelvic girdles are formed by two ossa coxae (hipbones) untied by the symphysis pubis. The pelvic girdle has 2 bones.

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4. The lower extremity consists of a femur, tibia, fibula, tarsals bones of the ankle, and the metatarsal and phalangeal bones of the foot. The patella is also included. The lower extremity has 60 bones.

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F. Cells in bone 1.      Osteocytes, mature bone cells.

Osteocytes maintain normal bone structure by recycling the calcium salts in bony matrix around themselves.

 2.      Osteoclasts, acids and enzymes secreted

by osteoclasts dissolve the bony matrix and release the stored minerals through osteolysis or resorption.

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3. Osteoblasts- the cell responsible for the production of new bone, a process called ostogenesis

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II. Bone development

A. Skeletal growth begins about 6 weeks after fertilization, when the embryo is about 0.5 in long.

 1. At this time, all skeletal elements are made of cartilage

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2. Bone growth continues through adolescence, and portions of the skeleton usually do not stop growing until around age 25.

 3. Osteogenesis is bone formation and growth.

 4. During development, cartilage or connective tissue is replaced by bone.

 

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5. Replacing tissue with bone is called ossification.

 6. There are two major forms of ossification.

 a.      Intramembraneous ossification bone

develops within sheets.  b.      Endochondral ossification bone is

replaced with cartilage.

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B. Intramembranous ossification 

1. Intramembranous ossification begins when osteoblasts start to differentiate into stem cells where they become calcified.

 

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2. The place where ossification first occurs is called an ossification center.

 3. An ossification proceeds and new bone branches.

 

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C. Endochondral ossification 1.    Most of the bones of the skeleton are

formed through the endochondral ossification of existing hyaline cartilage.

 2.    The cartilages develop first; they are like

miniature cartilage models of the future bone.

 

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3. By the time an embryo is 6 weeks old, the cartilage models of the future limb bones begin to be replaced by true bone.

 a. Step 1- Endochondral ossification starts

as chondrocytes within the cartilage model enlarge and the surrounding matrix begins to calcify.

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b. Step 2- Bone formation first occurs at the shaft surface. Blood vessels invade the area and osteoblasts begin producing bone matrix.

c. Step 3- Blood vessels invade the inner region of the cartilage and newly osteoblasts from spongy bone within the center of the shaft at the primary center of ossification

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d. Step 4- As the bone enlarges, osteoclasts break down some of the spongy bone and create a marrow cavity.

 e. Step 5- The centers of the epiphyses begin to calcify.

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4. When sex hormone production increases at puberty, bone growth accelerates dramatically, and osteoblasts begin to produce bone faster than epiphyseal cartilage expands. = growing pains.

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D. Bone growth and body portions 1.    The timing of epiphyseal closure varies from

bone to bone and individual to individual. 2.    Toes may complete ossification by age 11. 3.    Some of the pelvis or the wrist may

continue to enlarge up to 25 years old! 

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4. The epiphyseal plates in the arms and legs usually close by age 18 in women and 20 in men.

 5. Differences in sex hormones account for variations in body size and proportions between men and women.

 

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E. Requirements for normal bone growth 1.    Normal bone growth and maintenance

cannot occur without a reliable source of mineral, especially calcium.

 2.    During prenatal development these

minerals are absorbed from the mother’s bloodstream.

 

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3. The demands are so great that the maternal skeleton often loses bone mass during pregnancy.

 4. From infancy to adulthood, the diet must provide adequate amounts of calcium and phosphate to be able to absorb and transport these minerals to sites of bone formation.

 

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5. Vitamin D plays an important role in normal calcium metabolism.

 6. The vitamin is converted in the liver into calcitriol, a hormone that stimulates the absorption of calcium and phosphate ions in the digestive track.

 

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7. Rickets is a condition marked by a softening and bending of bones that occurs in growing children as a result of vitamin D deficiency.

 

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8. Vitamin A and Vitamin C are also important.

 9. A deficiency of vitamin C will cause scurvy. This condition causes a reduction in osteoblast activity that leads to weak and brittle bones.

 

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10. Hormones can also play an important role in normal skeletal growth and development – such as growth hormones, thyroid hormones, and sex hormones.

 

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III. Remodeling and Homeostatic Mechanisms

A.   Support and storage depends on the bones.

 1.    The turn over rate for bone is

quite high.  

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2. In adults, roughly 18% of the portion and mineral components are removed and replaced each year through the process of remodeling.

 3. Each part of every bone may not be affected, but new bone is always being formed.

 

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B. Remodeling and support 1.    Regular mineral turnover gives each bone the

ability to adapt to new stress. 

2. Heavily stressed bones become thicker, stronger, and develop more pronounced surface ridges; bones not subject to ordinary stresses become thin and brittle

3. Regular exercise is very important in maintaining bone strength.

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4. Degenerative changes in the skeleton occur after even brief periods of inactivity. Such as using a crutch.

5. After a few weeks, the unstressed leg will lose up to about a third of its bone mass.

 6. The bones rebuild just as quickly when they again carry their normal weight.

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C. Homeostasis and mineral storage 1.    Bones are important mineral

reservoirs, and calcium is the most abundant mineral in the human body.

 

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2. By providing a calcium reserve, the skeleton helps maintain calcium homeostasis in body fluids.

 3. This function can directly affect the shape and strength of the bones in the skeleton.

 

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D. Injury and repair 1.    Despite its mineral strength, bone

cracks or even breaks if subjected to extreme loads, sudden impact, or stresses from unusual directions.

 2.    All such cracks and breaks in bones

constitute a fracture.

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3. Fractures are classified according to their external appearance, the site of the fracture and the nature of the break.

 4. See page 135.

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Comminuted fracture

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Transverse fractures

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Spiral Fracture

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5. Bones will usually heal even after they have been severely damaged, as long as the circulatory supply and the cellular components of the endosteum and periosteum survive.

 6. To repair a break can take from 4 months to a year following a fracture.

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7. Steps to heal a break:

a. Step 1- Many blood vessels are broken and extensive bleeding occurs. A hematoma forms to clot the blood.

 

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b.   Step 2- An internal callus forms as a network of spongy bone unities the inner surfaces, and an external callus of cartilage and bone stabilizes the outer edges.

 c. Step 3- The cartilage of the external callus has been replaced by bone. Fragments of dead bone and areas of bone closest to the break have been removed and replaced.

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d. Step 4- A swelling initially marks the location of the fracture. Over time this region will be remodeled with little evidence of a fracture.

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E. Aging and the Skeletal System 1.    The bones of the skeleton become

thinner and relatively weaker as normal part of the aging process.

 2.    Inadequate ossification is called

osteopenia. 

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3. The reduction in bone mass occurs between the ages of 30-40.

 4. Osteoblast activity decline, while osteoclast activity continues at normal levels.

 5. Once the reduction begins, women lose about 8% of their skeletal mass every decade.

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6. Men’s skeletons deteriorate about 3% per decade.

 7. Osteoporosis is a condition that produces a reduction in bone mass great enough to compromise normal function.

 8. About 29% of women between 45-79 can be considered to have this disease.

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9. The increase in incidence after menopause has been linked to decreases in the production of estrogens.

 10. The incidence in men for the same age is 18%.

 11. Because bones are more fragile, vertebrae may collapse.

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 IV. An overview of the skeleton system

A.   Skeletal terminology 1.    Each bone in the human skeleton

not only has a distinctive shape, but also has characteristics external features.

 

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2. Depressions and openings indicate sites where blood vessels and nerves lie alongside or penetrate the bone.

 3. These landmarks are called bone

markings, or surface features.

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B. Skeletal divisions 1.    The skeletal system consists of

206 bones that are divided into axial and the appendicular.

 2.    See page 139!!!!

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C. The axial division 1.    The axial division creates a framework that

supports and protects organ systems in the dorsal and ventral body cavities.

 2. The axial has three functions: adjust the positions of the head, neck and trunk, performs respiratory movements, stabilize or position elements of the appendicular skeleton.

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3. Remember that the axis skeleton consists of the head neck and torso.

 4. Look on pages 140-141 for the bones in the skull

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V. Bones of the face

The maxillary bones 1.    The maxillary bones articulate with all

other facial bones except the mandible.  2.    The maxillary bones contain large

maxillary sinuses, which lighted the portion of the maxillary bones above teeth.

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 3. Know the sinuses on page 144.

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IV. Skulls of infants and children

A. Many centers of ossification are involved in the formation of the skull.

 1.    As the fetus develops, the

individual centers begin to fuse.

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2. At birth, the fusion has yet to be completed, and there are two frontal bones, four occipital bones, and several sphenoid and temporal elements.

 3. The skull organizes around the developing brain, and as the time of birth approaches, the brain enlarges rapidly.

 

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4. The bones in the skull are also growing, but they fail to keep pace and at birth the cranial bones are connected to fontanels.

 5. The fontanels, or “soft spots,: are quite flexible and permit distortion of the skull without damage.

 6. Such distortions normally occur during delivery to allow the baby out of the birth canal.

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C. Spinal curvature 1.    The vertebral column is not straight and

rigid. 2.    The spinal column has four spinal curves. 3.    The thoracic and sacral curves are called

primary curves because they appear late in fetal development, as the thoracic and abdominal organs enlarge.

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4. The cervical and lumbar curves known as secondary curves do not appear until months after birth.

 5. The lumbar curve forms as an in infant starts to crawl. Actually crawling for a long time promotes good curves in the back.

 

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6. The cervical curve forms as infants learn to balance their head and lift their head up.

 7. Several abnormal distortions of spinal curvature may appear during childhood and adolescence.

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C. The cervical vertebrae 1.    The seven cervical vertebrae extend from the

head to the thorax. 2.    Distinctive features of a typical cervical vertebra

include; 1- an oval, concave body, 2- a relatively large vertebral foremen, 3- a stumpy spinous process, usually with a notched tip and 4- round transverse foramina within the transverse processes.

 

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3.   The first two vertebrae have unique characteristics that allow for specialized movements.

 4. The atlas holds up the head, articulating with the occipital. It is named after Atlas according to Greek myth that held the world on his shoulders.

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D. The thoracic vertebrae 1.    There are 12 thoracic vertebrae. 2. Features of the thoracic vertebra include: 

a. Characteristic heart shaped bodyb. Large, slender c. One or more pairs of ribs

See pages 148-149

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E. The lumbar vertebrae 1.    The distinctive features of lumbar vertebrae include: 

a. Vertebral body that is thicker and more oval than that of a thoracic vertebra.

 b. A relatively massive, stumpy spinous process.

 c. Bladelike transverse process.

 

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VII. The appendicular division

A.   The pectoral girdle 1.    Movement of the clavicle and

scapula position the shoulder joint and provide a base for arm movement.

 2.    See pages 150-156.

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VIII. Articulations A.   Joints, or articulations, exist wherever

two bones meet.  1.    The characteristic structure of a joint

determines the type of movement that may occur.

 2.    Each joint reflects a workable

compromise between the need for strength and the need for movement.

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B. Classification of a joint 1.    Joints are classified according to

their structure or function. 2.    The structure classification is

based on the anatomy of the joint. 

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3. Joints are classified as fibrous, cartilaginous or synovial.

 4. The first two reflect the type of connective tissue binding them together.

 5.   Synovial joints prevent bone-to-bone

contact. 

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6. An immovable joint is a synarthrosis. 

7. A slightly movable joint is amphiarthrosis.

 8. A freely moveable joint is diarthrosis.

 

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C. Immovable joints/synarthroses 1.    A synarthrosis can be fibrous or cartilaginous. 2.    Two examples can be found in the skull.  3.    In a suture the bones of the skull are interlocked

and bound together by dense connective tissue. 4.    In a gomphosis a ligament binds each tooth in

the mouth within a bony socket.

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D. Slightly movable joints (amphiarthroses)

 1.    Permits very

limited movement- example in the leg the fibula and the tibula.

 

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E. Diarthrosis (free movement)

 1.    Most of the joints

give free range of movement such as the shoulder or wrist.

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F. Types of joints 1.    There are five types of joints 1- gliding

(wrist and ankle), hinge (knee and elbow) 3- ball and socket (shoulder and hip) 4- immovable – (skull) 5- pivot (neck)

 G. Types of movement

 1.    See pages 160-161

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Abduction Abduction is movement away from the midline, or to abduct.

Adduction Adduction is movement toward the midline, or to add.

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Flexion Flexion is to bend at a joint, or to reduce the angle.

Extension Extension is to straighten at a joint, or to increase the angle, for example, from 90 degrees to 180 degrees.

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Medial Rotation Medial rotation is to turn inward.

Lateral Rotation Lateral rotation is to turn outward.

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Supination Supination is to rotate the forearm so that the palm faces forward.

Pronation Pronation is to rotate the forearm so that the palm faces backward.

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IX. Clinical considerationA.   Development disorders 1.    Minor defects of the extremities are relatively

common malformation’s 2.    Extra digits – polydactyly is the most common

limb deformity. 3.    Usually an extra digit is incompletely formed

and does not function. 

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4. Syndactyly, or webbed digits is another common limb deformity.

 

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5. Talipes- clubfoot- the sole of the foot is twisted medially. It isn’t sure if it is caused by restricted movement in utero causing this condition.

 

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6. Cleft palate is a treatable birth defect in which the roof of the mouth (palate) does not develop normally during pregnancy, leaving an opening (cleft) that may go through to the nasal cavity

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B. Trauma and injury 1.    The most common type of bone injury is a

fracture- a cracking or breaking of a bone. 2.    Spontaneous, or pathologic fractures result

from diseases that weaken the bones.  3.    Most fractures are called traumatic

fractures- because they are caused by injury.

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THE END