Chapter 6
The Skeletal System:Bone Tissue
Dynamic and ever-changing throughout life
Skeleton composed of many different tissues
cartilage, bone tissue, epithelium, nerve, blood forming
tissue, adipose, and dense connective tissue
Functions of Bone
Supporting & protecting soft tissues
Attachment site for muscles making movement possible
Storage of the minerals, calcium & phosphate -- mineral
homeostasis
Blood cell production occurs in red bone marrow (hemopoiesis)
Energy storage in yellow bone marrow
Anatomy of a Long Bone
Diaphysis = shaft
Epiphysis = one end of a long bone
Metaphysis = growth plate region
Articular cartilage over joint
surfaces acts as friction & shock
absorber
Medullary cavity = marrow cavity
Endosteum = lining of marrow
cavity
Periosteum = tough membrane
covering bone but not the cartilage
fibrous layer = dense irregular CT
osteogenic layer = bone cells
& blood vessels that nourish or help with repairs
Histology of Bone
A type of connective tissue as seen by widely spaced cells
separated by matrix
Matrix of 25% water, 25% collagen fibers & 50% crystalized mineral salts
4 types of cells in bone tissue
Cell Types of Bone
Osteoprogenitor cells ----
undifferentiated cells
can divide to replace themselves & can become osteoblasts
found in inner layer of periosteum
and endosteum
Osteoblasts--form matrix &
collagen fibers but can’t divide
Osteocytes ---mature cells that no
longer secrete matrix
Osteoclasts---- huge cells from
fused monocytes (WBC)
function in bone resorption at
surfaces such as endosteum
Matrix of Bone
Inorganic mineral salts provide bone’s hardness
Organic collagen fibers provide bone’s flexibility
their tensile strength resists being stretched or torn
Mineralization (calcification) is hardening of tissue when
mineral crystals deposit around collagen fibers
Bone is not completely solid since it has small spaces for
vessels and red bone marrow
spongy bone has many such spaces
compact bone has very few
Compact or Dense Bone
Looks like solid hard layer of bone
Makes up the shaft of long bones and the external layer of
all bones
Resists stresses produced by weight and movement
Histology of Compact Bone
Osteon is concentric rings
(lamellae) of calcified matrix surrounding a vertically oriented blood vessel
Osteocytes found in spaces called
lacunae
Osteocytes communicate through canaliculi filled with extracellular
fluid that connect one cell to the next cell
The Trabeculae of Spongy Bone
Latticework of thin plates of bone called trabeculae oriented along lines of stress
Spaces in between these struts are filled with red marrow
where blood cells develop
Found in ends of long bones and inside flat bones such as
the hipbones, sternum, sides of skull, and ribs.
Bone Scan
Radioactive tracer is given intravenously
Amount of uptake is related to amount of blood flow to the
bone
“Hot spots” are areas of increased metabolic activity that
may indicate cancer, abnormal healing or growth
“Cold spots” indicate decreased metabolism of decalcified
bone, fracture or bone infection
Blood and Nerve Supply of Bone
Periosteal arteries
supply periosteum
Nutrient arteries
enter through nutrient foramen
supplies compact bone of diaphysis
& red marrow
Metaphyseal & epiphyseal aa.
supply red marrow & bone tissue of epiphyses
Bone Formation or Ossification
All embryonic connective tissue begins as mesenchyme.
Intramembranous bone formation =
formation of bone directly from mesenchymal cells.
Endochondral ossification =
formation of bone within hyaline cartilage.
Intramembranous
Bone Formation
Mesenchymal cells become osteoprogenitor cells then osteoblasts.
Osteoblasts surround themselves
with matrix to become osteocytes.
Matrix calcifies into trabeculae
with spaces holding red bone marrow.
Mesenchyme condenses as periosteum at the bone surface.
Superficial layers of spongy bone are replaced with compact
bone.
Endochondral Bone Formation (1)
Development of Cartilage model
Mesenchymal cells form a cartilage
model of the bone during development
Growth of Cartilage model
in length by chondrocyte cell division and matrix formation (
interstitial growth)
in width by formation of new matrix on the periphery by new chondroblasts from the perichondrium
(appositional growth)
Endochondral Bone Formation (2)
Development of
nutrient artery penetrates center of cartilage model
osteoblasts deposit bone matrix
over calcified cartilage forming spongy bone trabeculae
osteoclasts form medullary cavity
Endochondral Bone Formation (3)
Development of
blood vessels enter the epiphyses around time of birth
spongy bone is formed but no medullary
cavity
Formation of Articular Cartilage
cartilage on ends of bone remains as articular
cartilage.
Bone Growth in Length
Epiphyseal plate or cartilage
growth plate
cartilage cells are produced by mitosis on epiphyseal side of plate
cartilage cells are destroyed and replaced by bone on diaphyseal side of plate
Between ages 18 to 25, epiphyseal
plates close.
cartilage cells stop dividing and bone replaces the
cartilage (epiphyseal line)
Growth in length stops at age 25
Zones of Growth in Epiphyseal
Plate
Zone of resting cartilage
anchors growth plate to bone
Zone of proliferating cartilage
rapid cell division (stacked coins)
Zone of hypertrophic cartilage
cells enlarged & remain in columns
Zone of calcified cartilage
thin zone, cells mostly dead since matrix calcified
osteoclasts removing matrix
osteoblasts & capillaries move
in to create bone over calcified cartilage
Factors Affecting Bone Growth
Nutrition
adequate levels of minerals and vitamins
calcium and phosphorus for bone growth
vitamin C for collagen formation
vitamins K and B12 for protein synthesis
Sufficient levels of specific hormones
during childhood need insulinlike
growth factor
promotes cell division at epiphyseal
plate
need hGH (growth), thyroid (T3
&T4) and insulin
sex steroids at puberty
growth spurt and closure of the epiphyseal
growth plate
estrogens promote female changes -- wider pelvis
Hormonal Abnormalities
Oversecretion of hGH during childhood produces giantism
Undersecretion of hGH or thyroid hormone during childhood produces short
stature
Both men or women that lack estrogen receptors on cells grow
taller than normal
estrogen responsible for closure of growth plate
Bone Remodeling
Ongoing since osteoclasts carve
out small tunnels and osteoblasts rebuild osteons.
release calcium and phosphorus into interstitial fluid
Continual redistribution of bone matrix along lines of
mechanical stress
distal femur is fully remodeled every 4 months
Fracture & Repair of Bone
Fracture is break in a bone
Healing is faster in bone than in cartilage due to lack of
blood vessels in cartilage
Healing of bone is still slow process due to vessel damage
Clinical treatment
closed reduction = restore pieces to normal position by
manipulation
open reduction = surgery
Fractures
Named for shape or position of
fracture line
Common types of fracture
closed -- no break in skin
open fracture --skin broken
comminuted -- broken ends of
bones are fragmented
greenstick -- partial fracture
impacted -- one side of fracture
driven into the interior of other side
Pott’s -- distal fibular fracture
Colles’s -- distal radial fracture
stress fracture -- microscopic fissures
from repeated strenuous activities
Repair of a Fracture (1)
Formation of fracture hematoma
damaged blood vessels produce clot in 6-8 hours, bone cells
die
inflammation brings in phagocytic
cells for clean-up duty
new capillaries grow into damaged area
Formation of fibrocartilagenous
callus formation
fibroblasts invade the procallus
& lay down collagen fibers
chondroblasts produce fibrocartilage to span the broken ends of the bone
Repair of a Fracture (2)
Formation of bony callus
osteoblasts secrete spongy bone
that joins 2 broken ends of bone
lasts 3-4 months
Bone remodeling
compact bone replaces the spongy in the bony callus
surface is remodeled back to normal shape
Calcium Homeostasis & Bone Tissue
Skeleton is reservoir of Calcium & Phosphate
Calcium ions involved with many body systems
nerve & muscle cell function
blood clotting
enzyme function in many biochemical reactions
Small changes in blood levels of Ca+2 can be deadly (plasma
level maintained 9-11mg/100mL)
cardiac arrest if too high
respiratory arrest if too low
Exercise & Bone Tissue
Pull on bone by skeletal muscle and gravity is mechanical
stress .
Stress increases deposition of mineral salts &
production of collagen (calcitonin prevents bone
loss)
Lack of mechanical stress results in bone loss
reduced activity while in a cast
astronauts in weightlessness
bedridden person
Weight-bearing exercises build bone mass (walking or
weight-lifting)
Aging & Bone Tissue
Bone is being built through adolescence, holds its own in
young adults, but is gradually lost in aged.
Demineralization = loss of minerals
very rapid in women 40-45 as estrogens levels decrease
in males, begins after age 60
Decrease in protein synthesis
decrease in growth hormone
decrease in collagen production which gives bone its tensile
strength
bone becomes brittle & susceptible to fracture
Osteoporosis
Decreased bone mass resulting in porous bones
Those at risk
white, thin menopausal, smoking, drinking female with family
history
athletes who are not menstruating due to decreased body fat
& decreased estrogen levels
people allergic to milk or with eating disorders whose
intake of calcium is too low
Prevention or decrease in severity
adequate diet, weight-bearing exercise, & estrogen
replacement therapy (for menopausal women)
behavior when young may be most important factor
Disorders of Bone Ossification
Rickets
calcium salts are not deposited properly
bones of growing children are soft
bowed legs, skull, rib cage, and pelvic deformities result
Osteomalacia
new adult bone produced during remodeling fails to ossify
hip fractures are common