OSSEOUS TISSUE
SKELETAL STRUCTURE
Skeletal System
• 206 bones, cartilage, ligaments, and connective tissues
• Functions:
• support
– provides a rigid framework
• storage
– calcium & phosphorus
– lipids
• production of blood cells
– formed in red marrow
• protection
– brain is encased in skull
– heart and lungs are surrounded by boney sternum and rib cage
• leverage
– allows for movement due to interaction of muscular & skeletal
systems
• acid-base balance
– absorbs or releases alkaline salts
Divisions of the Skeletal System
• Axial skeleton
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consists of bones forming axis of the body
Skull
Hyoid
Sternum
Ribs
Vertebrae
sacrum & cocyx
auditory ossicles (not a part of either; but put here by
convention)
• Appendicular skeleton
• consists of bones that anchor appendages to axial skeleton
• upper & lower extremities, shoulder and pelvic girdles
Types of Bones
• Long
• Flat
• Short
• Irregular
• Sesamoid
• Sutural
Long Bones
• longer than wide
• function as levers
• act on skeletal
muscles to
produce
movements
• found in
appendages
• fingers & toes
Short Bones
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boxy & small
nearly cube-shape
found in wrist-carpals
ankle-tarsals
limited movements
Flat Bones
• thin
• roughly parallel surfaces
• found in the roof of the
skull
• sternum, ribs & scapula
• enclose & protect soft
organs
• provide broad surfaces for
muscle attachment
Irregular Bones
• bones that do not fall into any other
category
• varied, complex shapes, sizes & surface
features
• vertebrae, sacrum, coccyx, temporal,
sphenoid, ethmoid, zygomatic, maxilla,
mandible, palatine, inferior nasal concha,
& hyoid
Sesamoid Bones
• shaped like sesame
seeds develop in
areas where there is
a great deal of
friction
• most only a few
mms number in each
person differs
• patella present in
everyone
Sutural Bones
• also called
Wormian bones
• small
• located in
sutures
• classified by
location-not by
shape
Bone Composition
• Osseous Tissue
• supporting connective tissue
• Composed of an extra cellular matrix and specialized cells
– give flexibility
• two types
• compact or dense bone
– dense, hard, & relatively solid
– forms protective exterior of all bones
• spongy or cancellous bone
– found inside most compact bone
– very porous
• full of tiny holes forming open networks of struts & plates
– lighter than compact bone
• reduces skeletal weight
• makes it easier for muscles to move bones
Extracellular Matrix
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composed of collagen fibers &
ground substance
hardened by inorganic calcium
phosphate deposits
– called mineralization or
calcification
solid calcium phosphate salts
deposited around protein fibers
Calcium phosphate makes up 2/3rd
of bone weight
Calcium phosphate + calcium
hydroxide  hydroxyapatiteCa10(PO4)6(OH)2
Calcium phosphate is hard, brittle &
inflexible
– can withstand compression
Collagen fibers are stronger than
steel, flexible- can be twisted &
bent
– not good at being compressed
collagen makes a frame around
which calcium minerals deposit
combination makes bone flexible,
strong & resistant to shattering
Bone Cell Types
• Osteogenic cells
– stem cells  produce other
bone cells
– found in cellular layer of
periosteum, endosteum &
central canals
– continually divide
– only bone cell that can
divide
• Osteoblasts
– bone-forming cells
– make organic matter of
bone matrix
• Osteocytes
– mature bone cells
– most of bone cell population
– former osteoblasts that
have become trapped in
matrix they have deposited
• Osteoclasts
– bone destroying cells
Osteocytes
• cannot divide
• function to maintain &
monitor protein &
mineral content of
matrix
• participate in bone
repair by converting
back into osteoblasts
or osteogeneic cells at
the site of injury
• sense strain &
regulate bone
remodeling
Osteoclasts
• bone dissolving cells
• function to remove bone by
osteolysis
• secrete acids & proteolytic enzymes
which degrade minerals & fibers and
dissolve boney matrix
• releases matrix components into the
blood restoring calcium and
phosphorus concentrations in body
fluids
Types of Bone Tissue
• Compact Bone
– dense
– covers exterior of all
bones
• Spongy Bone
– cancellous
– trabecular
– inside compact bone
– lighter
Compact Bone
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basic functional unit -osteon or
Haversian system.
osteocytes are arranged in concentric
circles or layers-lamellae
around a central or Haversian canal
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perforating central canal are
Volkmann’s canals
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run perpendicular to surface
canaliculi run through layers
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runs parallel to surface
contains blood vessels
connect osteocytes to each other
interstitial lamellae fill spaces
between
Spongy Bone
• matrix compositionsame
• osteocytes, canalicui &
lamellae-different
arrangements
• has no osteons
• matrix forms plates or
struts called trabeculae
(little beams)
• form a thin, branching
open network filled with
red bone marrow
• makes bone lighter
Bone Type & Bone Tissue
Type Location
• the relationship between
compact & spongy bone and
the relative proportions of each
varies with bone shape & with
the function of the bone
Long Bone Structure
• Diaphysis or shaft-long & cylindrical
• Outside made of dense bone
– medullary canal or marrow
cavity is filled with marrow
– Yellow bone marrow is dominated
by fat cells & red marrow is
responsible for forming blood cells
• Epiphysis-expanded extremities at
either end of the bone
– articulates with other bonesforming joints
– have broad surfaces for muscle
attachment.
– filled with cancellous tissue
surrounded by thin layer of
compact bone
• Metaphysis
– connects diaphysis to epiphysis
Flat Bone Composition
• function
– provide protection for underlying structures
– broad surfaces for muscle attachment
• function can be seen by structure
• resembles a spongy bone sandwich
• composed of 2 thin layers of compact bone
covering a layer of spongy bone
• bone marrow is present
• there is no marrow cavity
Periosteum & Endosteum
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Periosteum
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covers all portions of compact bone
except at joint cavities
has fibrous outer layer & an inner
cellular layer
isolates bones from surrounding
tissues
provides route for blood vessels &
nerves
participates in bone growth & repair
continuous with other connective
tissues that mesh with-tendons &
ligaments
perforating or Sharpey’s fibers bond
tendons & ligaments into the general
structure of bone
endosteum
consists of an incomplete cellular layer
lines marrow cavities
covers trabeculae of spongy bones
lines inner surfaces of central canals
active during bone growth, repair, and
remodeling
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Blood & Nerve Supply
• bone tissue is highly
vascular
• Vessels pass into the
bone through the
periosteum
• Periosteal arteries
enter via perforating
canals
• nutrient artery & vein
• enter through a nutrient
foramen located in
middle of the bone
Bone Growth
• new bone matrix is made through
osteogenesis or ossification
• process makes & releases
proteins & other organic
components of matrix
• substance is osteoid
–bone matrix before calcium salts
have been added
• calcium salts are laid down in a
process called calcification
Bone Development & Growth
• skeleton begins to
form at 6 weeks post
fertilization
• does not stop until
around age 25
• develops by two
methods
• intramembranous
ossification
• endochondral
ossification
Intramembranous Ossification
 bone forms from
mesenchyme or
fibrous connective
tissue
 produces flat bones
of skull, most of the
facial bones,
mandible & medial
part of the clavicle
 bone develop within
a fibrous sheet
similar to dermis of
the skin
 bones are called
dermal bones
Intramembranous Ossification
Steps
• Step1: Development of
Ossification Center
• Step 2: Calcification
• Step 3: Formation of Trabeculae
• Step 4: Development of
Periosteum
Step1: Development of Ossification
Center
• at site where the bone is to
form, chemical messages
cause mesenchymal cells
(embryonic connective tissue)
to cluster together into a layer
of soft tissue
• cells enlarge & differentiate
into osteogenic cells and then
into osteoblasts.
• site is the ossification center
• osteoblasts begin to secrete
organic matrix
• eventually become trapped &
become osteocytes
Step 2: Calcification
• Calcium & other salts
deposit on organic
extracellular matrix
made by osteoblasts
• As trabeculae
continue to grow
calcium phosphate is
deposited
• causes matrix to
harden or calcify
Step 3: Formation of Trabeculae
• osteoblasts
continue to deposit
matrix
• continue to be
calcified producing
struts of trabeculae
• connective tissue
present
differentiates into
red bone marrow
Step 4: Development of the
Periosteum
• Mesenchyme condenses
at periphery of the
boneperiosteum.
• Trabeculae at surface
continue to calcify until
spaces between them are
filled in converting spongy
bone to compact bone
• process gives rise to
sandwich like arrangement
of flat bones
Intramembranous Ossification
Endochondral Ossification
• bone forms by
replacing pre-existing
hyaline cartilage
model with bone
• most bones are made
this way
• begins around sixth
week of fetal
development
• continues into the 20’s
Endochondral Ossification Steps
• Step 1: Development of Hyaline Cartilage
Model
• Step 2: Growth of Cartilage Model
• Step 3: Development of Primary
Ossification Center
• Step 4: Development of Medullary Cavity
• Step 5: Development of Secondary
Ossification Centers
• Step 6: Formation of Articular Cartilage &
Epiphseal Growth
Step 1: Development of Hyaline Cartilage
Model
• at site when bone will form
chemical messengers
cause mesenchymal cells
to crowed together in
general shape of future
bone
• cells develop into
chondroblasts.
• begin to secrete cartilage
extracellular matrix which
develops into a hyaline
cartilage bone covered with
a perichondrium
Step 2: Growth of Cartilage Model
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once chondroblasts become
embedded in extracellular matrix 
become chrondrocytes.
cartilage model continues to grow
longer from either end via interstitial
or endogenous growth.
grows in diameter or thickness via
appositional or exogenous growth
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new cartilage is laid on the outside of model
by chondroblasts
as model continues to grow
chondrocytes in area get larger in the
mid-region area & the cartilage matrix
begins to calcify
enlarged chondrocytes are deprived of
nutrients due to their size and
calcification & diffusion cannot occur
die and disintegrate
dying leaves spaces which merge into
small cavities called lacunae
Step 3: Development of Primary
Ossification Center
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ossification continues inward from
surface of bone to inside in the
middle of model- primary
ossification center
a nutrient artery penetrates
perichondrium
stimulates osteogenic cells there to
become osteoblasts
once this occurs perichondrium is
termed periosteum
in the primary ossification center most
of cartilage will be replaced with bone
osteoblasts begin to deposit a thin
collar of boney matrix around middle of
cartilage model forming trabeculae of
spongy bone
primary ossification spreads from
central area toward both ends of the
cartilage model
Step 4: Development of Medullary
Cavity
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as primary ossification center
grows osteoclast cells break
down some newly formed
spongy bone trabeculae
leaves a cavity
capillaries & fibroblasts migrate
to the inside of the cartilage and
take over the spaces left by the
dying chondrocytes
as center is hollowed out & filled
with blood and stem cells, it
becomes primary marrow
cavity.
region of transition from
cartilage to bone at the end of
the primary marrow cavity is
called the metaphysis
Step 5: Development of
SecondaryOssification Centers
• when branches of the
epiphyseal artery enter the
epiphyses the secondary
ossification centers form
• bone formation is similar to
as described in the center
of the bone
• here however spongy bone
remains in the epiphyses
• secondary ossification
proceeds outward from
center of each epiphysis
toward outer surface of the
bone
Step 6: Formation of Articular
Cartilage & Epiphseal Growth
• hyaline cartilage covering
epiphyses develop into articular
cartilages
• during infancy & childhood
epiphyses fill with spongy bone
• cartilage is limited to articular
cartilages
• prior to adulthood there is some
hyaline cartilage that remains
between the diaphysis and the
epiphysis
• called epiphyseal or growth
plate
• area where bone will continue to
grow in length until it becomes
adult sized
Endochondral Ossification
Endochondral Ossification
Bone Growth
• bone increases in length & width
• increases in length at epiphyseal
plate
• interstitital growth
• diameter of bone increases
through appositional growth
• new tissues is deposited at
surface of the bone
Interstitital Growth
• occurs at epiphyseal
plate
• consists of hyaline
cartilage in middle with a
transitional zone on either
side
• in transitional zone
cartilage is turning into
bone
• epiphysis makes cartilage
& ostoblasts try to
overtake it by making
bone
• osteoblasts cannot catch
up bone gets longer
Interstitital Growth
• epiphyseal plate
consists of four zones
• zone of resting
cartilage
• zone of proliferating
cartilage
• zone of hypertrophic
cartilage
• zone of calcified
cartilage
Interstitital Growth
• In zone of resting cartilage small
chondrocytes present
• do not participate in bone growth
• cells anchor plate to the epiphysis
• in zone of proliferating cartilage contains
slightly larger chondrocytes
– undergo interstitial growth
• cells divide replacing those that die on
diaphysis side of plate
• in zone of hypertrophy there are large,
maturing chondrocytes arranged in
columns
• zone of calcified cartilage contains few
cells
– cells are mostly dead due to extracellular matrix
around them having been calcified and no blood
or nutrients can reach them
Interstitital Growth
• at puberty rising
levels of sex & thyroid
hormones cause
osteoblasts to
outpace manufacture
of cartilage at
epiphyseal end
• growth plate
eventually fuses shut,
leaving an
epiphyseal line
• completes length of
bone
Appositional Growth
• way diameter of bone increases
• new tissues is deposited at
surface of bone
• at surface periosteal cells
differentiate into osteoblasts
• begin to secrete organic parts of
matrix.
• oteoid tissue is calcified
• as osteoblasts become trapped
osteocytes
• lay down matrix in layers parallel
to surface
• produce circumferential lamellae
of bone
Bone Dynamics
• bones constantly adapt to demands placed
on them and are continually remodeled
throughout life
• part of normal growth & maintenance
• 10% of skeleton tissue is replaced each
year
• organic and mineral components are
continuously recycled & removed through
remodeling
• gives bone the ability to adapt to new
stresses
Bone Dynamics
• activities of both cells types are
continuous
• activities must be balanced
• when osteoclasts remove calcium
faster than osteoblasts can deposit
itbone weakens
• when osteoblast activity
predominates bones get stronger
and more massive
Wolff’s law
• bone’s structure is determined by mechanical stresses placed on it
• one such stress is exercise
• when bone is stressedmineral crystals generate small electrical
fields which attract osteoblasts
• bony landmarks or bumps and ridges on surface of bone where
tendons attach may become more pronounced as muscles work to
withstand increased forces
• regular exercise is needed to maintain normal bone structure
• bone degeneration results from inactivity
• changes in mineral content does not necessarily change shape of
bones because boney matrix contains protein fibers
• bones can okay but may be soft due to no mineral deposition
– this is called osteomalacia
• one form of this is rickets
– typically due to a vitamin D3 deficiency
• not properly mineralized bones are flexible
– legs will bend under the weight of the body
Nutritional Needs
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bone growth and maintenance requires
– calcium
– phosphorous
– magnesium
– fluoride
– manganese
Calcitriol
– from kidneys
– absorption of calcium & phosphate from GI tract
– synthesis of calcitriol depends on Vitamin D3
• therefore Vitamin D3 is needed for proper bone growth
Vitamin C
– needed for enzymatic reactions
– needed for collagen synthesis
– needed to stimulate osteoblast differentiation
– without vitamin C there is a loss of bone strength and mass-scurvy
Vitamin A
– stimulates osteoblast activity
– especially important for bone growth in children
Vitamins K, and B12
– needed for protein synthesis
Hormonal Needs
• Growth hormone
• Thyroxine
• Sex hormones
–androgens in males
–estrogens in females
–help to close epiphyseal plates
–stimulate osteoblasts to produce
bone at rate faster than
epiphyseal cartilage can expand
Calcium Balance
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most abundant mineral in the body
90% is in bones
crucial to membrane functions
needed for activities of neurons & muscle
cells
for homeostatic balance three hormones
are needed
Calcitriol
Calcitonin
Parathyroid hormone
Calcitriol
• active form of
vitamin D
• principle
functionraise
blood calcium
• increases
absorption of
calcium by
small intestine
Calcitonin & Parathyroid
Hormones
• opposite effects
• Targets
–bones where calcium is stored
–digestive tract where calcium
is absorbed
–kidneys where calcium is
excreted
Calcitonin
• made in thyroid gland
• blood calcium levels rise
parafollicular or C cellsrelease
calcitoninlowers blood calcium
• inhibits osteoclast activity 
slowing rate of calcium release
from bone
• stimulates osteoblasts
• encourages calcium to be
deposited into bones
– more important during
childhood
– also important in reducing loss
of bone mass during prolonged
starvation & during late stages
of pregnancy
– role in healthy adults is
unknown
Parathyroid Hormone
• made by parathryroid gland
• calcium levels fall parathyroid
glandssecrete parathyroid
hormone
• raises blood calcium levels
– increases osteoclast
acitivty  increases release
of calcium from bones
– promotes calcium
reabsorption by kidneys
– promotes final step of
calcitriol synthesis in kidneys
enhancing calcium uptake
by intestine
– inhibits collagen synthesis
by osteoblastscalcium
deposition into bone
decreases
Calcium Balance