Chapter 7 - The Skeletal System

• Individual bones are the organs of the skeletal system. A bone contains very active tissues.

Bone Structure

Bone classification

• Classified according to their shapes:

• Long - longitudinal axis and expanded ends

Ex: forearm and thigh bones

• Short - cube like with lengths and widths about equal Ex: wrist and ankle

• Flat - platelike with broad surfaces

Ex: ribs, scapula, some skull bones • Irregular - variety of shapes, usually

connected to several other bones.

Ex: vertebrae and facial bones • Sesamoid - round, usually small and

nodular embedded within tendons next to joints where tendons are compressed Ex: kneecap

Parts of a long bone

• Epiphysis at each end that articulates (forms a joint) with another bone

• Outer surface of epiphysis covered with a layer of hyaline cartilage called articular cartilage

• The shaft of a bone is called the diaphysis

• Except for the articular cartilage, a bone is covered by fibrous tissue called the periosteum.

• Wall of diaphysis made of compact bone that has a continuous matrix with no gaps.

• Epiphysis made of spongy bone that has irregular interconnecting spaces between bony plates (trabeculae).

• Both compact and spongy bone are strong and resist bending.

• The diaphysis contains a medullary cavity filled with marrow.

Microscopic structure

• Osteocytes (bone cells) located in lacunae which form concentric circles around osteonic canals

Compact bone

• Osteocytes and layers of intercellular material concentrically clustered around an osteonic canal to form osteons cemented together.

• Osteonic canals contain blood vessels that nourish the cells of osteons.

• Perforating canals connect osteonic canals transversely and communicate with the bone's surface and the medullary cavity.

Spongy bone

• Composed of osteocytes and intercellular material but the cells don not aggregate around osteonic canals

• Cells lie within trabeculae

• Diffusion from the surface of thin bony plates nourishes cells of spongy bones.

Bone Development and Growth

• Bones form by replacing existing connective tissue in an embryo and continue to grow and develop into adulthood.

Intramembraneous Ossification

• Flat, broad bones in skull

• Membranelike layers of unspecialized connective tissue appear at future bone sites

• Unspecialized connective tissue cells arrange blood vessels in layers

• Connective tissue cells differentiate into osteoblasts, which deposit spongy bone

• Osteoblasts become osteocytes when a bony matrix completely surrounds them.

• Connective tissue on the surface of each developing structure forms a periosteum.

• Osteoblasts on the inside of periosteum deposit compact bone over the spongy bone.

• Process of bone formation is called ossification. Ossification timeline - see table 7.2 page 209.

Endochondral Ossification

• Masses of hyaline cartilage form models of future bone

• Cartilage tissue breaks down, periosteum develops

• Blood vessels and osteoblasts invade tissue• Osteoblasts form spongy bone• Osteoblasts deposit a thin layer of compact

bone

• Osteoblasts become osteocytes when bony matrix surrounds them.

• Osteocytes are mature bone cells

Growth of an endochondral bone • Primary ossification center appears in the

diaphysis and bone develops towards the ends

• Secondary ossification centers appear in the epiphysis and spongy bone forms around them

• An epiphyseal disk (band of cartilage) remains between the primary and secondary ossification centers.

An epiphyseal disk consists of 4 layers of cells

• resting cells - anchor disk to bony tissue of epiphysis

• young reproducing cells - undergo mitosis

• older enlarging cells - enlarge and thicken the disk causing bone to lengthen

• dying cells - thin layer

• Long bones continue to lengthen until the epiphyseal disks are ossified.

• Growth in thickness occurs as compact bone is deposited beneath the periosteum.

• Osteoclasts erode bone tissue inside which later fills with marrow.

• Throughout life osteoclasts resorb bone tissue (resorption) and osteoblasts replace bone (deposition).

• 3% -5% of calcium exchanged each year. • The total mass of bone remains nearly constant.

Factors affecting bone development, growth, and repair.

Nutrition

• Vitamin D necessary for proper calcium absorption

• Bones deformed without - rickets in children and osteomalacia in adults

• Vitamin A needed for bone resorption

Lack of retards bone development • Vitamin C needed for collagen synthesis

Lack of results in bones abnormally slender and fragile

Sunlight

• Converts dehydrocholesterol produced by cells in the digestive tract into vitamin D.

• Vitamin D allows calcium to be absorbed

Hormones

• Insufficient secretion of pituitary growth hormone may result in dwarfism.

• excessive secretion of GH may result in giantism (>8ft.) or acromegaly( hands, feet, and jaw enlarge).

• Deficiency of thyroid hormone delays bone growth.

• Male and female sex hormones promote bone formation and stimulate ossification of the epiphyseal disks.

Physical stress

• Skeletal muscle contractions pulls on bone attachments which stimulates bone tissue to thicken and strengthen.

Bone Function

• Support and Protection

• Body movevement

• Blood Cell Formation (hematopoiesis)

Begins in the yolk sac, outside the embryo

Occurs later in the liver, spleen and bone marrow

• Red marrow houses developing red blood cells, white blood cells, and blood platelets.

• Red in color due to hemoglobin.

• In adults found in spongy bone of the skull, ribs, sternum, clavicles, vertebrae, and pelvis.

• Yellow marrow stores fat and replaces red marrow as we age.

Inorganic salt storage • Bone matrix is made of collagen and

inorganic mineral salts. • Salts are about 70% of matrix by weight. • Most salt is calcium phosphate in the form

of hydroxyapatite. • Calcium is needed for blood clot formation,

nerve impulse conduction, and muscle cell contraction

• When blood is low in calcium osteoclasts resorb bone, releasing calcium salts.

• When blood is high in calcium osteoblasts are stimulated to form bone tissue and store calcium salts.

• Bone also stores small amounts of sodium, magnesium, potassium, and carbonate ions.

Skeletal Organization

Number of bones

• Usually a human has 206 bones, but the number may vary.

• Extra bones in sutures (flat bones in skull grow together) are called sutural bones.

Divisions of the skeleton

• Axial- skull, hyoid bone (supports tongue), vertebral column, and thorax

• Appendicular - pectoral girdle, upper limbs, pelvic girdle, and lower limbs.

Skull

• Consists of twenty-two bones, which include eight cranial bones, thirteen facial bones, and one mandible.

Cranium

• Encloses and protects the brain, and provides attachments for muscles.

• Some bones contain air-filled sinuses that help reduce the weight of the skull.

• Cranial bones include the frontal bone, parietal bones, occipital bone, temporal bones, sphenoid bone, and ethmoid bone.

Facial skeleton

• Form the basic shape of the face and provide attachments for muscles.

• Consist of 13 immovable bones and a movable lower jawbone (mandible).

• Facial bones include the maxillary bones(2), palatine bones(2), zygomatic bones(2), lacrimal bones(2), nasal bones(2), vomer bone, inferior nasal conchae(2) and mandible.

Infantile skull

• Incompletely developed bones, connected by fontanels (soft spots) enable the infantile skull to change shape slightly during childbirth.

• Small face with a prominent forehead and large orbits.

• Bones are thin and somewhat flexible and less easily fractured than adults.

Vertebral Column • Extends from the skull to the pelvis and

protects the spinal cord. • Composed of vertebrae separated by

intervertebral disks. • Vertebrae connected to one another by

ligaments. • An infant has thirty-three vertebral bones - 5

eventually fuse to become the sacrum and 4 fuse to become the coccyx.

• An adult has twenty-six vertebrae. • The vertebral column has four curvatures -

cervical, thoracic, lumbar, and pelvic.

Cervical Vertebrae (7)

• The bones of the neck. • Smallest and most dense vertebrae • Transverse processes have transverse

foramina (passageways for arteries leading to the brain).

• 2 - 5 are forked to provided attachments for muscles

• 7th vertebrae is longer and called vertebra prominens

• The atlas (first vertebra) supports the head.

• The axis (second vertebra) provides a pivot when the head is turned from side to side.

Thoracic vertebrae(12)

• Larger than cervical vertebrae.

• Their long spinous processes slope downward, and facets on the sides of bodies articulate with the ribs.

• Increase in size starting with 3 rd one.

Lumbar vertebrae(5)

• Large and strong to support more weight.

• Their transverse processes project posteriorly at sharp angles, and their spinous processes are directed horizontally.

Sacrum

• Formed of 5 vertebrae that gradually fuse between ages 18-30.

• Triangular structure that has rows of dorsal sacral foramina ( nerves and blood vessels pass through).

• United with the coxal bones at the sacroiliac joints.

• The sacral promontory (1 st sacral vertebrae) provides a guide for determining the size of the pelvis.

Coccyx

• Composed of four vertebrae that fuse by age 25.

• Acts as a shock absorber when a person sits.

Thoracic Cage

• includes the ribs, thoracic vertebrae, sternum, and costal cartilages .

• Supports the pectoral girdle and upper limbs, protects viscera, and functions in breathing.

Ribs

• Twelve pairs are attached to the twelve thoracic vertebrae.

• Costal cartilages of the true ribs (first 7) join the sternum directly.

• False ribs (last 5) join indirectly or not at all to sternum. - last 2 pairs called floating ribs

• A typical rib has a shaft, head, and tubercles that articulate with the vertebrae.

Sternum

• The sternum consists of a manubrium

(upper), middle body, and xiphoid process (projects downward).

• Articulates with costal cartilages and clavicles.

Pectoral Girdle

• Composed of two clavicles and two scapulae.

• Forms an incomplete ring that supports the upper limbs and provides attachments for muscles that move the upper limbs.

Clavicles • Rodlike bones that run horizontally between the

sternum and shoulders. • Hold the shoulders in place and provide

attachments for muscles.

Scapulae • Broad, triangular bones located on either side of

upper back. • Articulate with the clavicle and humerus and

provide attachments for muscles.

Upper Limb

• Provide the frameworks and attachments of muscles, and function in levers that move the limb and its parts.

Humerus

• Extends from the scapula to the elbow.

• Smooth rounded head with narrow depression called the anatomical neck

Radius

• Located on the thumb side of the forearm between the elbow and wrist.

• Disklike head, to allow for rotation.

Ulna • Longer than the radius and overlaps the

humerus posteriorly. • Articulates with the radius laterally.

Hand • Composed of a wrist, palm, and five fingers. • It includes 8 carpal bones that form a

carpus, five metacarpals and fourteen phalanges.

Pelvic Girdle

• Consists of two coxal bones that articulate

with each other anteriorly and with the sacrum posteriorly.

• The sacrum, coccyx, and pelvic girdle form the pelvis.

• The girdle provides support for body weight and attachments for muscles, and protects visceral organs.

Coxal bones

• Each coxal bone consists of an ilium, ischium and pubis which are fused in the region of the acetabulum

• • Ilium isthe largest portion of the coxal bone, joins the sacrum at the sacroiliac joint.

• The ischium is the lowest portion of the coxal bone.

• The pubis is the anterior part of coxal bone and is fused anteriorly at symphysis pubis.

Greater and lesser pelves

• The lesser pelvis is below the pelvic brim; the greater pelvis is above it.

• The lesser pelvis functions as a birth canal; the greater pelvis helps support abdominal organs.

Differences between male and

female pelves • Differences related to the function of the

female pelvis as a birth canal. • Usually the female iliac bones are more

flared (broader hips), pubic arch angle is greater, more distance between the ischial spines and the ischial tuberosities, and the sacral curvature is shorter.

• Bones of female pelvis usually lighter and show less evidence of muscle attachments.

Lower Limb

• Bones of the lower limb provide the frameworks of the thigh, leg, and foot.

Femur

• Extends from the hip to the knee.

Patella

• Flat sesamoid bone in the tendon that passes anteriorly over the knee.

• Controls the angle of this tendon and functions in lever actions associated with lower limb movements.

Tibia

• Located on the medial side of the leg.

• Larger of the 2 lower leg bones

• Articulates with the talus of the ankle.

Fibula

• Located on the lateral side of the tibia.

• articulates with the ankle but does not bear body weight.

Foot

• Consists of an ankle, an instep, and five toes.

• It includes seven tarsals that form the tarsus, five metatarsals and fourteen phalanges.

Types of Fractures

Healing of Fractures

• Compound (open) fracture is where the bone sticks through the skin

• Simple (closed) fracture is when the bone does not go through the skin

• Complete fracture is when the break is all the way through the bone

• The neck of the femur is the site of most hip fractures

Osteoporosis

• Bones become thinner and brittle

Risk factors

• Postmenopausal women

• Lack of exercise

• Increased alcohol consumption

• Light skin

• Low calcium intake