The Skeletal System
Three Types of Skeletons
• Three Types of Skeletons
• 1. Hydrostatic (water)
2. Exoskeleton (arthropods)
3. Endoskeleton (vertebrates)
• The Skeleton of Humans is composed of a
special connective tissue called BONE
• There are 206 bones in the human body
THE HUMAN SKELETON HAS
TWO DIVISIONS
• THE AXIAL SKELETON - CONSIST OF THE SKULL,
VERTEBRATE COLUMN, AND THE RIB CAGE
– The Skull consists of 8 CRANIAL BONES and 13 FACIAL
BONES
– The Ears consists 6 BONES
– FLOATING IN THE THROAT 1 BONE THE HYOID
– Ribcage - 12 pairs of ribs
– Sternum (breastbone)
– The Vertebral Column (Spinal Column or Backbone)
7 CERVICAL (NECK) VERTEBRAE,
12 THORACIC
5 LUMBAR,
5 FUSED VERTEBRAE INTO 1 SACRUM,
4 TO 5 SMALL FUSED VERTEBRAE INTO 1
COCCYX (YOUR TAIL BONE)
• THE APPENDICULAR SKELETON - CONSIST
OF THE BONES OF THE ARMS AND LEGS,
SHOULDER, AND THE PELVIC GIRDLE.
– THE PECTORAL GIRDLE CONSISTS OF 4 BONES,
AND UPPER LIMBS CONSISTS OF 60 BONES (the
hands and wrist contain 54 separate bones).
– THE PELVIC GIRDLE CONSISTS OF 2 BONES AND
THE LOWER LIMB CONSISTS OF 60 BONES (the
ankles and feet contain 54 separate bones)
Types of Skeletal Cartilages
Skeletal Cartilage:
–
–
–
–
Made of variety of cartilage tissue, consists primarily of water
Has resilience: the ability to spring back to original shape
Contains no nerves or blood vessels, surrounded by
perichondrium that acts as a girdle
Three Basic types of Cartilage
•
•
•
Hyaline Cartilage
Elastic Cartilage
Fibrocartilages
Hyaline Cartilage
• The cells, chondrocytes, appear spherical
and are made of collagen fibers
• Provide support and flexibility (resilience)
• Most abundant type
–
–
–
–
–
Articular cartilage (cover end of bone)
Costal cartilage (connect rib to sternum)
Laryngeal cartilage (voicebox)
Tracheal and Bronchial cartilage (reinforce respiratory system)
Nasal (support the external nose)
Elastic Cartilage
• Looks very much like hyaline cartilage
• Contains more elastic fibers, stands up to
repeated bending
• Only found in 2 places
– The external ear
– The epiglottis (flap that covers the larynx
when swallowing)
Fibrocartilage
• Chondrocytes allign in parallel rows with
thick collagen fibers
• High tensile strength (bend ability without
breaking)
• In sites of heavy pressure and stretch
– Knee
– Vertebral disks
Growth of Cartilage
• Appositional Growth
– “growth from outside”
– The perichondrium secrete new matrix
against the surface, expanding the tissue
• Interstitial Growth
– “growth from within”
– Chondrocytes inside cartilage divide and
secrete matrix, expanding tissue
Functions of Bones
•
•
•
•
•
Support and Structure
Protection of Vital Organs
Movement—attachment site for muscles
Mineral Storage: inorganic salts
Blood Cell Formation (hematopoiesis):
contains red marrow and produces red
blood cells (erythrocytes) and some White
Blood Cells (Leukocytes)
Classification of Bones
• Long Bones
– Longer than wide
– Consists of shaft and two ends
– Constructed of primarily of compact bone, but may contain
spongy (cancellous) bone
– Examples: humerus, femur
• Short Bones
– Roughly cubelike
– Made mostly of spongy (cancellous) bone
– Examples: carpals and tarsals, sesamoid (patella)
Classification of Bones
• Flat Bones
– Thin, flattened, and usually curved
– Two layers of compact bone with a spongy (cancellous) layer in
between
– Examples: sternum, ribs, skull bones
• Irregular Bones
– Bones that do not fit the preceding classifications are placed in
this category
– Examples: vertebrae, and pelvic bones
Chemical Composition of
Bone
• Organic Components
– Cells: osteoblasts, osteoclasts, and osteocytes
• Osteoblasts: bone forming cells
• Osteoclasts: large cells that resorb or break down bone matrix
• Osteocytes: mature bone cell
– Osteoid:
• Makes up 1/3 of the matrix, includes
proteoglycans, glycoproteins, and collagen fibers
• Contribute to a bone’s structure and are
responsible for flexibility and tensile strength
Chemical Composition of Bone
• Inorganic Components
– Make up 65% by mass
– Hydroxyapatites (mineral salts)
• Largely calcium phosphates
• Allow for exceptional hardness
½ as strong as steel in resisting compression, and strong as steel in
resisting tension--stretching
• Allow bones to persist after death
Osteogenesis
• Intramembranous Ossification
–
–
–
–
Defn.: when bone forms from fibrous membrane
Most of the bones in skull and clavicles are examples
Usually flat bones
Mesenchymal cells form the initial structure and ossification
begins at about the 8th week of development
– The four steps in formation
•
•
•
•
Ossification center of fibrous membrane forms
Formation of bone matrix w/in fibrous membrane
Formation of woven bone and periosteum
Formation of compact bone plates and red marrow
Osteogenesis
• Endochondral ossification
–
–
–
–
Defn.: bone formed by replacing hyaline cartilage structures
Process begins late in second month of development
More complex than intramembranous ossification
Steps of ossification:
• A bone collar forms around the diaphysis (shaft) of the hyaline
cartilage model
• Cartilage in the center of diaphysis calcifies
• The periosteal bud invades the internal cavities and spongy bone
forms
• The medullary (marrow) cavity forms
• The epiphysis (bone ends) ossify
• See pg 174
Postnatal Bone Growth
• Grows appositionally, usually stops in late
adolescence or early adulthood
• Some facial bones continue to grow
throughout life—Nose and Lower jaw
Growth of Long Bones
• Mimics endochondral ossification
– Cartilage on epiphyseal plate form tall column, ones on top
divide quickly, pushing epiphysis away from diaphysis—entire
bone lengthens
– Older chondrocytes closer to shaft enlarge, and the surrounding
cartilage matrix calcifies. The chondrocytes die, leaving spicules
of calcified cartilage at the epiphysis-diaphysis junction.
Cartilage spicules are covered with bone matrix by osteoblasts,
forms spongy bone. Spongy bone is digested by osteoclasts,
thus medullary cavity grows longer as well.
Growth in Width
• Done by process of appositional growth
– Osteoblasts beneath periosteum secrete bone matrix on externl
bone surface, osteoclasts on te endosteal surface of the
diaphysis remove bone.
– There is slightly less breakdown than building, thus produces
thicker and stronger bone
Hormones and Bone
Growth
• During infancy and childhood controlled by the growth
hormone produced by pituitary gland
• Thyroid produces T3 and T4 to ensure proper
proportions
• At puberty, testosterone and estrogen cause growth
spurt, masculinization or feminization of specific parts of
skeleton
• Testosterone and estrogen causes the closure of the
epiphyseal plate and end the longitudinal growth---stop
increasing in height
Bone Homeostasis
• Bone Remodeling
– Consists of bone deposit and bone resorption (removal)
– In healthy adults total bone mass remains constant
– BONE DEPOSIT
• Occurs when bone is injured or additional bone strength is needed
• Need diet rich in proteins, vitamin C (collagen synthesis), Vitamin A,
B12, and minerals ( Ca, P, Mg, Mn, etc.)
• New deposit has osteoid seam surrounding a calcification front
• Requires the enzyme alkaline phosphatase for mineralization
Bone Homeostasis
– BONE RESORPTION
• Must eliminate old bone matrix and release minerals as
needed
• Accomplished by osteoclasts
– Osteoclasts secrete lysosomal enzymes that digest
organic matrix and metabolic acids that convert calcium
salts into soluble forms
– The dissolved mineral salts are sent into the interstitial
fluid and then the blood
Hormonal Mechanism that Controls
Remodeling of Bone
• Occurs due to interaction of parathyroid hormone (PTH—produced
by parathyroid glands) and calcitonin (produced by the thyroid
gland)
– When ionic Ca level drops in blood, PTH released to stimulate
osteoclasts to resorb bone
– Calcitonin is released when Ca levels in blood rise, stops resorption of
bone and increases deposition of calcium salt in bone
– HOMEOSTASIS
##Ca needed for nerve impulse transmission, muscle contractions, blood
coagulation (clotting), gland and nerve cell secretions, and cell division
**Hypercalcemia (disease of high blood Ca) leads to deposits of calcium
salt deposit in blood vessels, kidneys and soft organs, hampering
functionability
Mechanical Stress and
Remodeling of Bone
• Caused by Muscle Pull and Gravity
– Wolff’s Law: Bone grows or remodels in response to the forces or
demands placed on it
• Supported by weightlifters—have enormous thickenings in bone at muscle
attachment sites
• Supported by ballet dancers—have thickened bones in feet
• Supported by anthropologic studies of human remains and class system
• Explains atrophy in bedridden individuals and fetuses
Differences in Males and Females
• Females are shorter
– During puberty, estrogen seals the epiphyseal plates sooner
than testosterone in males
• Males have larger arm and leg bones
• Females have wider pelvic inlet and outlet
Fractures of Bone
• Simple (closed fracture): bone breaks cleanly, doesn’t penetrate
skin
• Compound: broken bone protrude through soft tissues and skin—
often lead to osteomyelitis
• Comminuted: bone fragments into many pieces (most common in
elderly)
• Compression: bone is crushed—common in osteoporotic bones
• Depressed: broken bone portion is pressed inward—typical of skull
• Impacted: broken bone ends are forced into each other—hip
fractures
• Spiral: ragged break caused by twisting forces applied to bone—
sport fractures
• Greenstick: bone break incompletely—most common in children
Healing a Fracture
• Hematoma forms
– A mass of clotted blood forms at the fracture site
– Bone cells are deprived of nutrition and die, area becomes swollen,
painful, and obviously inflammed
• Fibrocartilagionous callus forms
– Capillaries grow in hematoma and phagocytic cells clean up debris
– Fibroblasts and osteoblasts migrate to fracture site and begin
reconstructing bone
– Fibroblasts release collagen fibers that connect bone ends and will also
secrete cartilage matrix
– Osteoblasts begin formation of spongy bone that will eventually harden
– “Splints the bone”
Healing A Fracture Continued
• Bony Callus Forms
– Osteoblasts and osteoclasts migrate inward and multiply rapidly in the
fibrocartilaginous callus. This will be converted to the bony callus of
spongy bone
– Occurs 3-4 weeks after injury and continues up to 2-3 months later
• Remodeling
– During bony callus formation and continuing for several months, bony
callus remodeled
– Excess building material on shaft of bone and within the meduliary
cavity removed and compact bone reconstructed
Disease of Bone
• Osteoporosis
– A group of diseases in which bone resorption outpaces deposit
– Composition of bone remains same, mass reduced—bones become
porous and lighter
– Occurs most frequently in postmenopausal women, but cessation of sex
hormone synthesis can cause disorder on both sexes
– Other causes: insufficient exercise, calcium and protein poor diet,
abnormal vitamin D receptors, smoking, and hormone related conditions
– Treatments—Is preventable
•
•
•
•
Calcium and vitamin D increase in diet
Weight bearing exercise
Hormone replacement therapy
Drinking fluorinated water
Bone disease continued
• Osteomalacia
– “Soft Bones”
– Includes a number of disorders that cause bones to inadequately
mineralize
– Weight bearing bones fracture, bend, and deform
– Symptom: pain when weight is put on affected bone
– Cause: insufficient calcium in diet and Vit D deficiency
• Rickets
– Analogous disease in children
– Causes bowed legs, deformities of pelvis, skull, and ribcage
– Epiphyseal plates can’t be calcified so they continue to widen and ends
of long bones are enlarged
– Cause: insufficient calcium in diet or Vit D deficiency
Bone Disease Continued
• Paget’s Disease
–
–
–
–
–
Excessive bone breakdown and abnormal bone formation
Can occur anywhere, most common in spine, pelvis, femur, and skull
Rarely seen before age 40
Cause unknown—maybe virus?
Treatment: drug therapy
• Osteomyelitis
– Inflammation of bone caused by puss forming bacteria
– Commonly affects long bone
– Acute pain, fever, joint stiffness, bone destruction, shortening of limb
common symptoms
– Treatment: antibiotic therapy, drainage of abscesses, removal of bone
fragments
Bone Diseases Continued
• Osteosarcoma
–
–
–
–
–
–
–
Form of Bone cancer
Typically in long bones of limbs and most often in 10-25 age group
Grows aggressively
Painful eroding of bone
Metastasizes (moves to) lungs
Treatment: amputation of affected bone or limb, chemotherapy
Survival rate of 50% (if caught early)
Paget’s Disease
Rickets
osteomyelitis
osteosarcoma
Osteomalacia