Chapter 5: the Skeletal System s1
Chapter 5: The Skeletal System
Bones: An Overview
· There are 206 bones in the human body, which makes up about 20% of our body mass.
· The parts of the skeletal system include:
1. Bones- internal framework that supports and anchors all organs.
2. Joints- allow for flexibility and movement
3. Cartilages- provides additional support for structures in the body
4. Ligaments- connect bone to bone
· The skeletal system is subdivided into two groups: Axial and Appendicular
1. Axial Skeleton- includes the skull, vertebral column and rib cage
2. Appendicular Skeleton- includes bones of the upper and lower limbs
· Functions of the Bones
1. Support- bones act as pillars to support the body
2. Protection- for soft body organs
3. Storage- for fat in the internal cavities (yellow marrow), also bones store minerals such as calcium and phosphorous
4. Blood Cell Formation (Hematopoiesis) - blood cells are formed in the red marrow of certain bones
Two types of bone tissue
· Compact bone- the external layer that is dense and looks smooth and homogeneous
1. Contains the functioning units of bone called osteons.
2. Spongy bone- the internal layer that is composed of a honeycomb of needle like or flat pieces called trabeculae
· The open spaces between trabeculae are filled with red or yellow bone marrow.
Chemical Composition of bone- inorganic and organic components
· Inorganic- 65% of bone mass consists of hydroxyapatities, or mineral salts (largely calcium phosphates)
1. Calcium salts account for the exceptional hardness of bone and allows it to resist compression.
2. Organic- cells, osteoblasts (bone making cells), and osteiod
o Osteiod makes up 33% of bone matrix and includes proteoglycans, glycoproteins and collagen fibers.
o Organic components allow for bone flexibility and great tensile strength to allow bone to resist stretch and twisting.
Classification of bones according to shape
· Long Bones
1. Typically longer than wide
2. Have a shaft with heads at both ends
3. Contain mostly compact bone
4. Examples: femur and humerus
· Short Bones
1. Generally cube shaped
2. Contain mostly spongy bone
3. Examples: carpals and tarsals
4. Sesamoid bones are special type of short bones that are embedded in a tendon. Ex. Patella
· Flat Bones
1. Thin and flattened
2. Usually curved
3. Thin layers of compact bone around spongy bone
4. Example: skull, ribs, sternum
· Irregular Bones
1. Irregular shape that does not fit other bone classifications
2. Example: Vertebrae and Hips
Bone structure
· Gross anatomy of a typical long bone
1. Diaphysis- the bone shaft that makes up most of the bones length and is composed of compact bone
· The diaphysis is covered with a fibrous connective tissue membrane called the periosteum. Sharpey’s Fibers (connective tissues) secure the periosteum to long bone.
2. The diaphysis surrounds a central medullary cavity or marrow cavity (yellow marrow) primarily used for storing adipose (fat) tissue. In infants this area forms blood cells, and red marrow is found here. In adults, the red marrow is confined to the cavities of spongy bone of flat bones and the epiphyses of some long bones.
· Epiphysis- the ends of long bones
1. Consists of a thin layer of compact bone enclosing an area filled with spongy bone
2. In adult bones a thin line of bony tissue spans the epiphysis and separates it from the rest of the bone in that area, this is called the epiphyseal line. The epiphyseal line is a remnant of the epiphyseal plate (flat plate of hyaline cartilage) you would seen in a young growing bone.
· Bone Markings- the bumps, holes, and ridges seen on bones and reveal where muscles, ligaments and tendons attach.There are two categories of bone markings:
1. Projections or Processes- grow out from the bone surface, (see chart on page 119) Ex. spinous process of vertebrae
2. Depressions or cavities- indentions in the bone, all depressions begin with the letter F; ex. foramen magnum at the base of the skull
Microscopic Anatomy of Compact Bone
· Osteocytes -Mature bone cells
· Osteons- Units of bone (Haversian System)
· Haversian system
1. opening in the center of an osteon
2. Run lengthwise through the bone carrying blood vessels and nerves to all areas of the bone.
· Perforating (Volkmann’s) canals
1. perpendicular to central canals
2. carries blood vessels and nerves
· Lacunae - tiny cavities
1. contain bone cells (Osteocytes)
2. arranged in concentric rings
· Lamellae- rings around the central canal
1. sites of lacunae
· Canaliculi – tiny canals that radiate from central canals to connect all bone cells to the nutrient supply.
Bone Formation and Growth
· Bone formation is also known as osteogenesis (bone beginning) or ossification.
1. Formation of the skeleton
o In embryos the skeleton is composed mainly hyaline cartilage (fibrous membranes at skull and clavicles)
2. Intramembranous ossification- process in which the bones form to replace fibrous membranes of the skull and clavicles.
3. Endochondral ossification- process in which hyaline cartilage is replaced by bone (includes all bones except skull and clavicles)
o The hyaline cartilage is covered in osteoblasts (bone forming cells).
o The fetus then has cartilage “bones” enclosed by bony bones.
o The enclosed hyaline cartilage is digested away, opening up a medullary cavity within the newly formed bone.
· Cartilage remains in the bridge of the nose, parts of the ribs, joints (articular cartilage), and epiphyseal plates.
· Bone Growth- occurs at epiphyseal plates
1. Provides for longitudinal bone growth during childhood.
2. Controlled by growth hormones and sex hormones
3. New cartilage is continuously formed
4. Old cartilage becomes ossified
o Cartilage is broken down and replaced by bone
5. Growing bones must also widen as they lengthen (called appositional growth)
o Osteoblasts begin adding bone to the outside of the diaphysis
o Osteoclasts (bone destroying cells) in the endosteum remove bone from the inside diaphysis wall
6. The work of osteoblasts and osteoclasts occur at almost the same rate allowing the bone to expand and widen
Bone Remodeling- the two processes of bone deposits and removal
· In an adult skeleton bone deposits and bone removal are occurring all the time (yes, bone is a dynamic, active living tissue)
· In a healthy adult the rate of bone deposit and bone removal should remain constant and equal.
1. Bone deposits occur where bone is injured or additional bone strength is needed.
o Deposits are accomplished through osteoblasts
o Optimal bone deposits require a diet rich in proteins, vitamin C, A, and B12, calcium, phosphorous, and magnesium
2. Bone Removal or resorption occurs when the blood levels of calcium become too low
o Resorption is accomplished by osteoclasts
3. If bone removal (resorption) occurs faster than bone deposition takes place homeostatic balance is lost. This results in porous, lightweight bones that easily break. This condition is called osteoporosis. This occurs most often in aged, postmenopausal women, but can occur in both sexes.
· Bone remodeling is controlled by hormones, diet and mechanical stress
1. Hormonal mechanism- involves the parathyroid hormone (PTH) and calcitonin in a negative feedback loop to maintain homeostasis of blood calcium.
o PTH is released when blood calcium levels are too low. It stimulates osteoclasts to resorb (remove) bone releasing the calcium into the blood.
o When blood calcium levels rise calcitonin is secreted to encourage calcium deposits into the bone.
2. Diet
o Calcium is necessary for transmission of nerve impulses, muscle contraction, blood coagulation, secretion by gland and nerve cells, and cell division
ü Your daily recommended levels of calcium is 1200- 1400 mg.
ü Calcium is absorbed by the intestines under the control of vitamin D metabolites
3. Mechanical Stress
o This set of controls serves the needs of the skeleton itself- keeping bones strong where stressors are present.
o Wolff’s Law- holds that bones grow or remodel in response to the forces or demands placed on it. Bone is laid down where bone is needed- for example large bony projections occur where heavy, active muscles attach)
Bone Fractures
· A break in the bone. There are two types:
1. Open (compound) fracture- the broken bone penetrates through the skin
2. Closed (simple) fracture- the broken bone does not penetrate through the skin
· Bone fractures are treated by reduction and immobilization
1. Realignment of the bone
· Common Types- comminuted, compression, depression, compacted, spiral and green stick- see book for pictures
· Repair of Bone Fractures
1. After the fracture occurs, blood vessels rupture forming a blood-filled swelling called a hematoma.
2. A fibrocartilage callus forms (made of various connective tissues) and acts to splint the broken bone.
3. A bony callus forms as osteoblasts replace the fibrocartilage with bone.
4. The bony callus continues to be remodeled in response to mechanical stress placed upon the bone.
Skeletal Joints- also called articulations (place where two or more bones meet)
· Functions of Joints:
o Give skeleton mobility (allow you to move!)
o Hold skeleton together (protective function)
· Classifying Joints
o Structural classification focuses on materials used to bind bones together and/or the presence of a joint cavity. There are three classifications.
§ Fibrous- the bones are united by fibrous tissues. The best examples are sutures of the skull. No movement allowed, no joint cavity
§ Cartilagenous- bone ends are connected by cartilage that acts as a shock absorber. Forms a strong joint. Examples include the pubic symphyses, intervertebral discs, hyaline cartilage, costal cartilage.
§ Synovial- articulating bones are separated by a joint cavity containing synovial fluid. These joints are moveable, and have four distinguishing features.
Ø Articular cartilage- covers the ends of the bones forming the joint
Ø Fibrous Articular Capsule- the sleeve or capsule of fibrous connective tissue
Ø Joint Cavity- Cavity enclosed by the capsule that contains the synovial fluid
Ø Reinforcing Ligaments- the capsule is supported with ligaments (connect bone to bone)
o Functional Classification (how much movement is allowed at a joint?)
§ Synarthroses: immovable joints, Ex. Cranial Sutures
§ Amphiarthroses: slightly moveable. Ex. Pubic Symphysis
§ Diarthroses: freely moveable. Ex. Ball and Socket
· Types of Synovial Joints
o Plane Joint-allows nonaxial gliding movements, articular surfaces are flat.
Ex. intercarpal joints
o Hinge Joint- uniaxial movements, the cylindrical end of one bone fits into a trough-shaped surface of another bone. Ex. Elbow joint, phalanges, knee
o Pivot Joint- uniaxial movements, the rounded end of one bone fits into a sleeve or ring of another, this allows rotation of one bone.
Ex. Atlas and dens of axis, proximal radioulnar joint
o Condyloid Joint- biaxial movements, both articulating surfaces are oval and allow all angular motions. Ex. Radio carpal (wrist joint), occipital bone with atlas
o Saddle Joint- biaxial movements; each articular surface has both concave and convex areas that fit together. Ex. Carpo-metacarpal joint of the thumb.
o Ball and Socket Joint- multiaxial movements, spherical head of one bone articulates with the cuplike socket of another, most freely moving joint.
Ex. Hip and shoulder joints
Joint Problems
· Causes
o Inflammation of synovial cavity
o Degeneration of the joint or articular cartilage
· Most common trauma-induced joint injuries
o Sprain: the ligaments reinforcing a joint are stretched or torn.
o Dislocation: bones are forced out of normal positions at the joint.
o Bursitis: inflammation of bursa through excessive friction, or direct injury
o Tendonitis: inflammation of the tendon sheath, usually an overuse injury
o Arthritis: cumulative effects of compression abrasions at joint surfaces
§ Osteoarthritis: most common form, “wear and tear arthritis” that commonly affects older people. The articular cartilage softens, frays and begins to break down.
§ Rheumatoid Arthritis: A Chronic inflammatory disorder. An autoimmune disorder (the body’s immune system attempts to destroy its own tissues). The disorder begins with an inflammation of synovial membranes that thicken and accumulate fluid. The scar tissue can eventually ossify and bones look deformed.
§ Gouty Arthritis or Gout: a disease which uric acid accumulates in the blood and may be deposited as needle-shaped crystals in the soft tissues or joints. Gout tends to run in families.
Developmental Aspects of the Skeleton
· At birth, some fontanels are still present in the skull. Fontanels allow for brain growth and ease birth passage. The growth of the cranium is related to brain growth and the increase in size of the facial skeleton follows tooth development and the respiratory passages.
· The vertebral column is C-shaped at birth due to curvatures present in the thoracic and lumbar vertebrae (similar to a four-legged animal). When the baby begins to lift its head and walk the spine changes into its S-shaped form.
· During youth the skeleton changes not only in size but in body proportions as well. At birth the head and trunk are one and half times as long as the lower limbs. The lower limbs then begin to grow much more rapidly than the trunk. During puberty the female pelvis widens to prepare for childbearing. Once the adult height is reached the skeleton changes very little until late middle age.
· As people age fractures and osteoporosis are the most common bone problems for elderly. The bones must be physically stressed as we age in order to remain healthy. In order to maintain healthy bones through your adult years, a diet rich in calcium and vitamin D are important as well as weight bearing exercise to keep bones strong.