Skeletal System: Bones and Bone Tissue - Anatomy Freaks

spyfleaUrban and Civil

Nov 25, 2013 (3 years and 10 months ago)

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Chapter 6

Skeletal System:

Bones and Bone Tissue

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Functions of the Skeletal System


Support
. Bone is hard and rigid; cartilage is
flexible yet strong. Cartilage in nose, external ear,
thoracic cage and trachea. Ligaments
-

bone to
bone


Protection
. Skull around brain; ribs, sternum,
vertebrae protect organs of thoracic cavity


Movement
. Produced by muscles on bones, via
tendons. Ligaments allow some movement
between bones but prevent excessive movement
(joint: formed where two or more bones come together
------
allows movement between bones)


Storage
. Ca and P. Stored then released as
needed. Fat stored in marrow cavities


Blood cell production
. Bone marrow that gives
rise to blood cells and platelets

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Components of Skeletal System


Bone


Cartilage: three types


Hyaline


Fibrocartilage


Elastic


Tendons and ligaments

(Tendons: muscle to bone







Ligament: bone to bone)

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Hyaline Cartilage


Consists of specialized cells that produce matrix


Chondroblasts
: form matrix


Chondrocytes
: surrounded by matrix; are
lacunae


Matrix
. Collagen fibers for strength, proteoglycans for resiliency


Perichondrium
. Double
-
layered C.T. sheath. Covers cartilage except at
articulations


Inner. More delicate, has fewer fibers, contains chondroblasts


Outer. Blood vessels and nerves penetrate. No blood vessels in
cartilage itself


Articular cartilage
. Covers bones at joints; has no perichondrium


Growth


Appositional
. New chondrocytes and new matrix at the periphery





(chondroblasts in perichondrium lay down new matrix & new chondrocytes)


Interstitial
. Chondrocytes within the tissue divide and add more
matrix between the cells.

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Bone Histology


Bone matrix
. Like reinforced concrete. Steel bars
is collagen fibers, cement is
hydroxyapatite


Organic: collagen and proteoglycans
(35%)


Inorganic: hydroxyapatite. CaPO
4

crystals
(65%)


If mineral removed, bone is too bendable


If collagen removed, bone is too brittle


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Bone Histology


Bone cells


Osteoblasts


Osteocytes


Osteoclasts


Stem cells or
osteochondral

progenitor cells


Woven bone
: collagen fibers randomly oriented


Lamellar bone
: mature bone in sheets


Cancellous

bone (spongy)
: contain
trabeculae






-
less bone matrix & more space


Compact bone
: dense





-
more bone matrix & less space

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Bone Cells


Osteoblasts


Formation of bone through
ossification or osteogenesis.
Collagen produced by E.R. and
golgi. Released by exocytosis.
Precursors of hydroxyapetite stored
in vesicles, then released by
exocytosis.


Ossification
: formation of bone by
osteoblasts.
It occurs by appositional
growth on surface of previously existing
bone.
Osteoblasts communicate
through gap junctions. Bone matrix
produced by osteoblasts covers the
older bone & surrounds osteoblasts


results in new layer of bone.

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Bone Cells


Osteocytes
. Mature bone cells
once surrounded by matrix. They
are relatively inactive, but can
make small amounts of matrix to
maintain it.


Lacunae
: spaces occupied by
osteocyte cell body


Canaliculi
: canals occupied by
osteocyte cell processes


Nutrients diffuse through tiny
amount of liquid surrounding cell
and filling lacunae and canaliculi.
Then can transfer nutrients from one
cell to the next through gap
junctions.

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Bone Cells


Osteoclasts
.
Responsible for resorption of bone






(breakdown of bone)



Ruffled border
(projections)
: where cell membrane
borders bone and resorption is taking place.


H ions pumped across membrane, acid forms, eats
away bone.


Release enzymes that digest the bone.


Derived from monocytes (which are formed from stem
cells in red bone marrow)


Multinucleated


Stem Cells
.
Mesenchyme

(
Osteochondral
Progenitor Cells
) become chondroblasts or
osteoblasts.
(Remember: osteocytes derived from osteoblasts)

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Woven and Lamellar Bone


Woven bone
. Collagen fibers randomly oriented.


-

first formed during fetal development or repair of fracture


-

after formation, osteoclasts break down woven bone & osteoblasts

build new matrix
---------------
this process called remodeling








Remodeling


Removing old bone and adding new


Woven bone is remodeled into lamellar bone



Lamellar bone


Mature bone in sheets called
lamellae
. Fibers are
oriented in one direction in each layer, but in different
directions in different layers for strength.

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Cancellous (Spongy) Bone


Trabeculae
: interconnecting rods or plates of bone. Like
scaffolding.


Spaces filled with marrow & blood vessels.


Covered with endosteum.


Oriented along stress lines


Trabeculae consist of several lamellae
(thin sheets)

w/ osteocytes
located in lacunae between them


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Compact Bone


Central

or
Haversian canals
: parallel
to long axis



Lamellae
: thin sheets/layers


concentric
(circles)
, circumferential
(perimeter, periphery)
, interstitial
(between
osteon)



Osteon
or

Haversian system
: central
canal
(also called haversian canals
------
contains
blood vessels that run parallel to long axis of
bone
)
, associated concentric lamellae
and osteocytes



Perforating

or
Volkmann’s canal
:
perpendicular to long axis.
Both
perforating and central canals contain
blood vessels. Central canals receive
blood vessels from perforating canals &
nutrients in blood enter central canals,
pass into canaliculi, move through
osteocytes to osteocytes
(by gap junctions)
.
Waste removed in reverse direction.


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Bone Shapes


Long


Ex. Upper and lower
limbs


Short


Ex. Carpals and tarsals


Flat


Ex. Ribs, sternum,
skull, scapulae


Irregular


Ex. Vertebrae, facial

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Structure of a Long Bone


Diaphysis


Shaft


Compact bone


Epiphysis


End of the bone


Cancellous bone


Epiphyseal plate:
growth plate
(separates
epiphysis from diaphysis)


Hyaline cartilage; present until
growth stops


Epiphyseal line:
bone stops growing in
length
(epiphyseal plate becomes epiphyseal line)


Medullary cavity
(space)
:

In children
medullary cavity is red marrow,
gradually changes to yellow in limb
bones and skull (except for epiphyses of
long bones). Rest of skeleton is red.

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Structure of a Long Bone, cont.


Periosteum


Outer is fibrous
(contains blood vessels & nerves)


Inner is single layer of bone cells
including osteoblasts, osteoclasts and
osteochondral progenitor cells


Tendons & ligaments attach to bone &
become continuous with fibers of
periosteum.


Sharpey’s fibers
: some collagen fibers
of tendons or ligaments penetrate the
periosteum and into the bone.
Strengthen attachment of tendon or
ligaments to bone.


Endosteum
.
Similar to periosteum,
but more cellular. Lines all internal
surfaces of all cavities including
spaces in cancellous bone.

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Structure of Flat, Short,

and Irregular Bones


Flat Bones


No diaphyses, epiphyses


Sandwich of cancellous
between compact bone


Short and Irregular Bone


Compact bone that surrounds
cancellous bone center; similar
to structure of epiphyses of
long bones
(bec. have processes
that possess epiphyseal growth plates


No diaphyses and not
elongated


Some flat and irregular
bones of skull have sinuses
(air filled spaces)

lined by mucous
membranes.


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Bone Development


(during fetal development)


Intramembranous ossification


Takes place in connective tissue membrane


Endochondral ossification


Takes place in cartilage


Both methods of ossification


Produce woven bone that is then remodeled


After remodeling, formation cannot be
distinguished as one or other

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Intramembranous Ossification

(8
th

week


2 years of development)


Takes place in connective tissue membrane
formed from embryonic mesenchyme


Forms many skull bones, part of mandible,
diaphyses of clavicles


When remodeled, indistinguishable from
endochondral bone.


Centers of ossification
: locations in membrane
where ossification begins
(centers of ossification expand
outwards to form a bone by gradually ossifying the membrane)


Fontanels
: large membrane
-
covered spaces
between developing skull bones; unossified
(bones
eventually grow together & all fontanels have closed by 2 years of age)



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Intramembranous Ossification

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Endochondral Ossification


Bones of the base of the skull, part of the
mandible, epiphyses of the clavicles, and
most of remaining bones of skeletal system


Cartilage formation begins at end of fourth
week of development


Some ossification beginning at about week
eight; some does not begin until 18
-
20 years
of age


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Endochondral Ossification

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Endochondral Ossification

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Endochondral Ossification

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Endochondral Ossification

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Growth in Bone Length


Growth in length occurs at the epiphyseal plate


Involves the formation of new cartilage by


Interstitial cartilage growth


Appositional growth on the surface of the cartilage


Closure of epiphyseal plate
: epiphyseal plate is
ossified becoming the epiphyseal line. Between 12
and 25 years of age


Articular cartilage:

does not ossify, and persists
through life


Appositional growth

only


Interstitial growth cannot occur because matrix is solid


Occurs on old bone and/or on cartilage surface


ex: trabeculae grow in size by the deposition of new bone matrix by




osteoblasts onto the surface of the trabeculae.

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Zones of the Epiphyseal Plate

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Growth in Bone Length

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Growth at Articular Cartilage


Increases size of bones with no epiphyses: e.g.,
short bones


Chondrocytes near the surface of the articular
cartilage similar to those in zone of resting cartilage


The process of growth in articular cartilage is
similar to that occurring in the epiphyseal plate,
except that the chondrocyte columns are not as
obvious.


When epiphyses reach their full size, the growth of
cartilage & its replacement by bone
cease
.


However, articular cartilage persists throughout life
& does
not
become ossified as does epiphyseal
plate.



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Fracture of the Epiphyseal Plate

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Growth in Bone Width

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Factors Affecting Bone Growth


Size and shape of a bone determined genetically but can be
modified and influenced by nutrition and hormones


Nutrition


Lack of calcium, protein and other nutrients during growth and
development can cause bones to be small


Vitamin D


Necessary for absorption of calcium from intestines


Can be eaten or manufactured in the body


Rickets
: lack of vitamin D during childhood
(bowed bones)


Osteomalacia
: lack of vitamin D during adulthood leading to
softening of bones


Vitamin C


Necessary for collagen synthesis by osteoblasts


Scurvy: deficiency of vitamin C
(causes ulceration & hemorrhage in





body)


Lack of vitamin C also causes wounds not to heal
(because requires
collagen synthesis)
, teeth to fall out
(because ligaments that hold them in
place break down)


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Factors Affecting Bone Growth
(cont.)


Hormones


Growth hormone from anterior pituitary. Stimulates
interstitial cartilage growth and appositional bone
growth


Thyroid hormone required for growth of all tissues


Sex hormones such as estrogen and testosterone


Cause growth at puberty, but also cause closure of the
epiphyseal plates and the cessation of growth

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Bone Remodeling


Converts woven bone into lamellar bone


Caused by migration of
Basic
Multicellular Units (BMU)


Groups of osteoclasts and osteoblasts that
remodel bones


Involved in bone growth, changes in bone
shape, adjustments in bone due to stress,
bone repair, and Ca ion regulation


Relative thickness of bone changes as
bone grows. Bone constantly removed by
osteoclasts and new bone formed by
osteoblasts.


Formation of new osteons in compact
bone


Osteoclasts enter the osteon from blood in
the central canal and internally remove
lamellae. Osteoblasts replace bone


Osteoclasts remove bone from the
exterior and the bone is rebuilt

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Bone Repair

1.
Hematoma formation. Localized mass of blood released from blood vessels
(damaged from fracture)
but confined within an organ or space. Clot formation
(consists of fibrous proteins that stop the bleeding).

2.
Callus formation
.
Callus
: mass of tissue that forms at a fracture site and
connects the broken ends of the bone.


-

Internal
-

blood vessels grow into clot in hematoma
(several days after fracture)
.


Macrophages clean up debris, osteoclasts break down dead tissue,
fibroblasts produce collagen and granulation tissue.


Chondroblasts from osteochondral progenitor cells of periosteum and
endosteum produce cartilage within the collagen.


Osteoblasts invade. New bone is



formed.

-

External
-

collar around opposing
ends. Periosteal osteochondral
progenitor cells


osteoblasts
and chondroblasts. Bone/cartilage
collar stabilizes two pieces.

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Bone Repair, cont.

3.
Callus

ossification
. Callus replaced by woven,
cancellous bone through endochondral ossification.

4.
Bone remodeling
. Replacement of cancellous bone and
damaged material by compact bone. Sculpting of site by
osteoclasts



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Calcium Homeostasis


Bone is major storage site for calcium


The level of calcium in the blood depends
upon movement of calcium into or out of
bone.


Calcium enters bone when osteoblasts create
new bone; calcium leaves bone when
osteoclasts break down bone


Two hormones control blood calcium levels
-

parathyroid hormone
(produced by parathyroid gland
----
blood calcium decreases then secretion of PTH increases which
increases # of osteoclasts)

and calcitonin
( produced by
thyroid gland
----
decreases osteoclast activity

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Calcium Homeostasis

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Effects of Aging on Skeletal
System


Bone matrix decreases. More brittle due to lack of collagen;
but also less hydroxyapatite.


Bone mass decreases.
Highest around 30. Men denser due to
testosterone and greater weight. African Americans and
Hispanics have higher bone masses than Caucasians and
Asians. Rate of bone loss increases 10 fold after menopause.
Cancellous bone lost first, then compact.



Increased bone fractures


Bone loss causes deformity, loss of height, pain, stiffness


Stooped posture


Loss of teeth

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Bone Fractures


Open (compound)
-

bone break with
open wound. Bone may be sticking
out of wound.


Closed (simple)
-

Skin not perforated.


Incomplete
-

doesn’t extend across the
bone. Complete
-

does


Greenstick: incomplete fracture that
occurs on the convex side of the curve
of a bone


Hairline: incomplete where two
sections of bone do not separate.
Common in skull fractures


Comminuted fractures: complete with
break into more than two pieces


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Bone Fractures, cont.


Impacted fractures: one
fragment is driven into the
cancellous portion of the
other fragment.


Classified on basis of
direction of fracture


Linear


Transverse


Spiral


Oblique


Dentate: rough, toothed,
broken ends


Stellate radiating out from a
central point.