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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
6-1

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
6-2

Anatomy of a Long Bone
•
•
•
•

Diaphysis = shaft
Epiphysis = one end of a long bone
Metaphysis = growth plate region
Articular cartilage over joint surfaces
decreases friction
• 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
6-3

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
6-4

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
6-5

Matrix of Bone
• Inorganic mineral salts provide bone’s hardness
– Hydroxyapatite: calcium phosphate & calcium hydroxide

• Organic collagen fibers provide bone’s flexibility
– their tensile strength resists being stretched or torn
– remove minerals with acid - rubbery structure results

• Mineralization (ossification) 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

6-6

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

6-7

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
• Interstitial lamellae represent older osteons that have been
partially removed during tissue remodeling

6-8

6-9

6-10

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
...


6-11

Blood and Nerve Supply of Bone
• Periosteal arteries
– supply periosteum

• Nutrient arteries
– enter through nutrient foramen
– supply compact bone of
diaphysis & bone marrow of the
central canal

• Metaphyseal & epiphyseal aa
...

• Intramembranous bone formation = formation
of bone directly from mesenchymal cells
...


6-13

Intramembranous Bone Formation

•
•
•
•
•

Mesenchymal cells become osteoprogenitor cells then osteoblasts
...

Matrix calcifies into trabeculae with spaces holding red bone marrow
...

Superficial layers of spongy bone are replaced with compact bone
...

– nutrient artery penetrates center of
cartilage model stimulating
perichondreal cells to convert to
osteogenic; thus producing a
periosteal collar
– Nutrient artery grows into calcified
center creating 10 ossification center
– osteoblasts deposit bone matrix over
calcified cartilage forming spongy
bone trabeculae up & down the shaft
– osteoclasts form medullary cavity

Initially medullary cavity
has spongy bone, prior to
osteoclast activity
6-17

Endochondral Bone Formation (3)
remains

20 ossification
centers proceed
outward
• Development of Secondary Ossification Center
– 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
...

– cartilage cells stop dividing and bone
replaces the cartilage (epiphyseal line)

• Growth in length stops at age 25
6-19

Bone Growth in Width

• Only by appositional growth at the bone’s surface
• Periosteal cells differentiate into osteoblasts and form bony ridges
and then form a tunnel around periosteal blood vessel
...


6-20

Factors Affecting Bone Growth
• Nutrition
– adequate levels of minerals and vitamins
• calcium and phosphorus for bone growth
• vitamin C for collagen formation (Scurvy)
• 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 6-21

Wolff’s Law
A theory developed by the German anatomist
and surgeon Julius Wolff (1836–1902)
• Bone in a healthy person or animal will adapt to the loads under
which it is placed
...

• The internal architecture of the trabeculae undergoes adaptive
changes, followed by secondary changes to the external cortical
portion of the bone, perhaps becoming thicker as a result
...

6-22

Bone Remodeling
• Ongoing since osteoclasts carve out small
tunnels and osteoblasts rebuild osteons
...


Comminuted
fractures, such as
this fracture of the
femur, shatter the
affected area into a
multitude of bony
fragments
...


Displaced fractures
produce new and
abnormal bone arrangements;
nondisplaced
fractures retain the
normal alignment of
the bones or
fragments
...
A clean
transverse fracture along this line generally
heals well
...


Compression fractures
occur in vertebrae
subjected to extreme
stresses, such as those
produced by the forces that
arise when you land on your
seat in a fall
...


In a greenstick
fracture, such as this
fracture of the radius,
only one side of the
shaft is broken, and
the other is bent
...


A Colles fracture, a
break in the distal
portion of the radius,
is typically the result
of reaching out to
cushion a fall
...

• 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
6-33

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)
6-34

Comparison of Normal and
Osteoporotic Spongy Bone

A, B: From Dempster DW et al: A simple method for correlative light and scanning electron microscopy of human iliac crest bone bipsies, J Bone Miner Res
1:15, 1986

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