Bone
 It is a calcified osteoid tissue.
 It is rich in blood supply.
 It has solid matrix.
It consists of cells, ground substance (matrix) and fibers.
 In contrast to cartilage, bone has high degree of structural
organization especially the packed collagen fibers. This
enable the bone to heal properly and recoveIn contrast to
cartilage, they has high degree of structural organization
especially the packed collagen fibers. This enable the bone
to heal properly & recover to perform its function with
minimal damage.
 Therefore it is an ideal tissue for support and
protective skeletal function.
Matrix
o Bone matrix is a mineralized organic matrix, consist of:.
(1) Inorganic substance = mineral salts.
o These constitute 60% of bone volume.
o Give hardness , rigidity and act as a
reservoir for Ca++.
o Primarily constitution are: calcium
phosphate, carbonate, nitrate, citrate,
Na, magnesium, fluride inform of
hydroxy apitate , crystals.
o Small amount of radioactive material
(minerals) may incorporated into bone.
(2) Organic materials = hydrated organic component:
 These constitute 40% of matrix bone volume.
 Collagen fibers composes 95% of this organic
materials, glycoprotein include , osteoclacin,
osteopontin.
The amorphous ground substance accounts for only 5% of the
matrix.
Most of proteoglycan of bone lack acidic sulfate groups (but
rich in cartilage). So decalcified bone is acidophilic because its
abundant in acidophilic collagen but fibers little acidophilic
ground substance.
This can different between the two (bone & cartilage) if other
morphological clues are absent.
 Water constitute little of its volume
Bone marrow
red in young
at the bone end
Yellow increase by age replace by fat in
adult shaft.
Cellular constitution
There are four types of cells in the bone tissue:
Osteogenic, osteoblasts, osteocytes, osteoclast.
1. Osteogenic cells
 There are not highly differentiated, primitive cells developed
from undifferentiated cells called pericytes.
 They can proliferate and differentiate into chondroblasts or
osteoblasts depend on O2 tension.
In well vascularized tissue( O2 tension)
differentiate into osteoblast.
In poorly vascularized area ( O2 tension)
differentiate into chondroblasts.
they
they
Shape:
They are flat with pale staining elongated nuclei,
cytoplasm is acidophilic.
Site :In Mature bone:
 Osteogenic cells form the cellular layer of periosteum.
 Lines all blood vessels – within the (Volkmann's canal
& haversian canals).
 Lines the endosteum (single layer) of the bone
marrow.
Function:
 They are responsible for growth and development of bone,
and repair of bone fracture.
Play a role in blood cell formation (myelopoiesis).
Osteoblasts
Osteoblasts
- Bone forming cells
 They are responsible for bone formation and growth .
 They are responsible for secretion (deposition) of new
unmeniralized matrix called osteoid
subsequently
mineralization will occur.
Shape
o They are oval (pulmp) cells slightly elongated.
o They have intensity stained basophilic cytoplasm
because it is rich in rER, mitochondria, RNA, Golgi
apparatus (with negative Golgi image).
o Their axes are perpendicular to the surface which they
are aligned and polarized.
Matrix granules:
o They are rich in granules constitute of alkaline
phosphatase activity & PAA – positive.
o Osteoblast may transform into osteocytes when they
trapped in bone tissue.
o They are usually present at the surface, periosteum
and endosteum.
o When the osteoblast release their content throught
matrix vesicles by exocytosis, the phospholipid of the
vesicle membrane attract Ca++ and facilitate
calcification of osteiod tissue. This result in formation
of crystals resembling hydroxyapatite
o In rickets and osteomalasia; this process is stoped, so
result in formation of excessire osteoid tissue which is
weak and lack weight bearing properties
bending
of bone.
Osteocyte
 They are the principle cells of the bone.
Site:
Shape :
Function:
 They are surrounded by bone matrix & located
in potential space called lacunae.
 They are connected by their end by nearby
perivascular region via cytoplasmic process
extended through aperatures in the lacunar
wall (canaliculi).
 They has elongated nucles with condensed
chromatin.
 Cytoplasm is slightly basophilic.
 RER & golgi apparatus are less prominent.
 They are connect with each other in the
canaliculi by gap junction (macula
communicates)
 They are involved in the maintenance of the
bone matrix. They are able to concentrate Ca
phosphate in their cytoplasm and in the
surrounding matrix.
Osteoclasts
They are derived from blood monocyte, not from
osteogenic cell.
Site:
Near the bone surfaces. In shallow depression of bone
(resorption bags, Howship’s lacunae) which they have
excavated.
Shape
 Multinucleated (fusion of multiple monocytes) giant cell
contain 30 or more nuclei.
 Acidophilic cytoplasm.
 Resemble megakaryocytes (megakaryotes has multilobed
nucleus present in bone marrow.
 They have irregular striated cell membrane = ruffled
border toward the surface of bone where they apposed.
EM
 Ruffled border: appears as an array of irregular villus like
projections of the cytoplasm a butting on the bony
surface.
 Also there is clear zone which lack villus like projection.
 The cytoplasm subjacent to the ruffled border in LM having
foamy appearance , in EM it consists of numerous vacuoles.
Function
They are responsible for degrading bone matrix at the
sites
where bone will be remolded i.e. remolding takes place:
 During normal development and growth of bone.
 During maintenance of mature bone matrix.
 During repair of bone fractures
Osteocytes
Oseoclasts
General comments:
Osteoclastic resorption (catabolic action) accompanied by
osteoblastic (anabolic action) in normal bone growth.
Osteoclastic resorption of bone matrix have two stage:
Initial dissolution of minerals(acid phosphatase activity)
Enzymatic degradation of collagen(collagenase activity)
 Osteoclast sent acid phostase
increase solubility of
bone mineral salt acidification
demineralization.
 Then acid hydrolases (secret in between = the intervillous
space) to degredate collagen.
 Mainly two stages occur extracellular, but also osteoclast
phagocytse bit, of bone matrix and degraded by lysosomes.
Osteoclastcell also play a role in calcium mobilization from the
bone and makes it available to the blood stream.
-Two hormones affect this process a.PTH increase activity of
ruffled border and accelerates osteoclustic activity .
b. calcitonin
diminish ruffled border and decrease size
reduce osteoclastic activity.
Note = Another process to increase Ca++ osteocytic osteolysis ;
this to aim bone resorption in adult to maintain proper plasma
calcium levels Occur in deep, oldest and most mineralize portion
of bone tissue.
Types of bone tissue
Morphologically, bone tissue is divided into:
1- Compact bone
when the bone tissue forms a compact solid mass known as
compact tissue:
-It forms the outer supportive shell of bone organs
Site: i.e. shaft of long bone.
2- Spongy
When the bone tissue forms a three dimensional network of
intercommunicated osseoys projection called trabeculae with
many intervening spaces. It is known as spongy bone =
trabecular, or cancellous bone.
Site : Present in epiphysis of long bone, ribs, vertebra,
flat bone.
Long bone consists of tow epiphysis (tow ends) and one
diaphysis.
 Epiphysis
spongy bone.
 Outer layer of diaphysis is (cortical)
compact bone
but hollow diaphysis is medullary or indirect
communication with can cell out spaces at two ends.
There are 2 types of bone marrow:
1- Red = active,
Site: ribs, vertebrae, pelvis, sternum.
2-Yellow marrow = fat infiltrated bone marrow in the adult
diaphysis.
The bone cavity filled primarily with yellow marrow
Compact bone is formed of:
1- Haversian lamellae = Haversian system or osteons.
 The bone lamellae arranged concentrically around a
central haversian canal (H.C)
 Haversian canal contains blood, nerves, and some
connective tissue. The canal are lined by osteogenic
cell.
 The osteocytes are present within the lamellae in
lacunar spaces.
 Their canaliculi radiating toward the H.C.
 In some regions adjacent H.C. are connected by
volkman’s canal. It runs approximately perpendicular to
their planes.
 They contain branches of blood vessels originate from
those in H.C.
Osteons are longitudinally arranged in the cortex of long bone.
Compact bone
copyright 1998 Gwen V. Childs, Ph.D.
URL Address: http://cellbio.utmb.edu/microanatomy/
Gwen V. Childs, Ph.D., WebMistress
www.orthoteers.org/(S(gtrq0d45wzrxc5ecsrjgfhz
2. Interstitial lamellae
They are fragments of laminated bone tissue which are
present in between the cylindrical osteons.
The boundaries between osteons and interstitial lamellae
are called cement lines.
3. Circumfential lamella
They are circular lamella that form external and internal
lamination of cortical bone.
Then are arrange perpendicular to that of Haversian lamellae.
Outer Circumnfential lamella = present underneath periosteum.
Inner Circumnfential lamella = adjacent to endosteum.
Sharpey’s fibers (collagen fibers) from the tendon or ligament
(that attach to periosteum) an anchored in to the outer
circumterential lamellae.
Spongy bone
 It consists of an interconnected network of bony plates or
bars with many intervening spaces. The bony bars
composed of lamellae.
 The orientation of collagen fibers
There is parallel orientation in one lamellae and next
lamellae 90 degree angle with respect to the adjacent lamellae
to provide bone strength.
Located within lamellae are lacunae contains osteocyte.
 There is canaliculi between the lacunae and surface of
lamellae to provide nutrition and O2 from the bone
marrow spaces intervening the bars oxygen.
Two types of preparation of bone tissue:
1- Decalcified bone preparations:
Bone speciment treated with acid such as nitric acid & formic
acid.
It is in section of bone that contain only organic materials
such as cells, matrix and blood vessels
2- Ground bone preparations
Piece of bone is orinded to the appropriate thinness by stones
only the bone minerals is demonstrated.
Haversian canal, volkmans, canaliculum.
Development of bone organs & bone tissues
The distinction between bone as tissue and bones as organ by
bone tissue is the mineralized supportive connective tissue
that forms the framework of bone organs.
Bone tissue development occurs as a result of two processes:
(a) Formation of osteoid tissue (unmineralized matrix) =
ground substance and collagen.
(b) Bone formation and mineralization resorption and
Remodeling include of matrix
Development start with the formation of bone tissue
2 Stages:
(1) Formation of bone tissue without a pre existing surface this
stage is a transitory event in early embryonic life.
Mesenchymal tissue differentiate in certain areas into bone
tissue. Certain mesenchymal cells
osteoblasts
elaborate fibers and ground substance
osteocytes when trapped in mineralized bone tissue.
The result is a non lamellated, mineralized matrix forming primitive
bone tissue. This called
woven bone or primary bone .
Collagen fibers are arranged randomly and no lamellae.
(This an initiation process)
(2) Subsequently bone tissue exist
formation of bone tissue
against an Existing surface
Osteoprogenitor cells arrange themselves against the newly
primitive bone
osteoblasts
elaborate ground
substance, collagen fibers and matrix.
They oriented with respect to existing surface
orientation of matrix & collagen give either
compact
spongy bone
SPONGY BONE
copyright 1998 Gwen V. Childs, Ph.D.
URL Address: http://cellbio.utmb.edu/microanatom
Gray's
Dorlands/Elsevier
Development of bone organ after forming osteoid tissue.
Intermembranous ossification
It occurs in flat bone such as temporal, parietal & part of
temporal bones, part of mandible, facial bones, and clavicles.
The bone formed by this way called membrane or mesenchymal
bones (losly vascular)
It is followed the development of bone tissue without prexisty
surface then the process continue by appositional growth against
existing surface leading to formation of spongy bone (collagen
randomly arrange).
The mesenchyme surround the bone differentiates to periosteum.
The bone organ continues by remodeling & reabsorbtion.
The free surface
compact bone.
Intracartilagenous ossification
 The bone formed by intracartilage ossification called
cartilage bone or endochondral bones.
Site:
They include the majority of the bone organs of the axial
appendicular portion of the skeleton.
They develop from mesenchymally derived hyaline cartilage
models.
 It starts from epipteseal centers (2ry ossification centers
after birds).
Stages
1. Resting stage = small flat chondrocytes
The cartilage cell of the cartilage models acts as reserve
areas for next stages.
2. Proliferative stage = cartilage cells increase in number
flat young chondrocytes – new formed chondrocytes
arrange in rows and show signs of mitosis.
3. Machination & hypertrophy stage
The chondrocyte will increase in size in their lacunae, become
nature cells and start to accumulate matrix.
4. Calcification stage of cartilage
The surrounding matrix of cartilage cells starts to calcified.
Subsequently chondrocyte degenerate edic.
5. The surrounding perichndrium of the developing bone
become osteogenic and called periosteum, its cellular portion
give rise to steoblasts, osteoblast start to lay down a thin
sheet of bone tissue around the circumference of the model
in its mid portion, which is termed the bone collar (this my
refered as intramembranous ossification) when the
membrane is periosteum.
6. Stage of invasion of the previous empty spaces by
mesenchymal vascular tissue (mesenchymal bud)
They erode the calcified cartilage.
Cells of mesenchymal bud: - Blood
- osteoblast forming cells
Osteoblast lay down matrix
lamella or bone trabecullae.
7. Stage of remodeling & reconstruct
Osteoclast
reabsorbtion + destruction of central
irregulatices
Osteoblast
lay organized lamella
compact bone.
Healing of bone fracture
(1) Formation of new soft tissue around the perimeter of broken
bone fragments
external callus + between them
internal callus
(2) Osteogenic cell differentiate from periosteum & endosteum
to chondroblasts which deposit hyaline cartilage in the
callus & osteoblast (deep in the callus).
X-ray and H&E histology of fracture callus healing in a stabilized rat tibia fracture model
at: www.lerner.ccf.org/bme/midura/lab/
at: www.technion.ac.il/%7Emdcourse/274203/lect5.html
(3) Deep in the callus bone tissue is starting to form
interstitial growth.
(4) Endochondral ossification then start to begin at cartilage site in
the callus (periphery).
(5) External callus cancellous bone provide temporary support
which then will remodeling by the action of osteoclast &
osteoblast into compact bone.
Clinical notes
Scurvy =
Vitamin C which result in reduce
hydroxylation of Protein + lysine
collagen fiber deposition
although ostoid tissue is radiated but calcification of remaining
ostoid is completed.
Rickets =
Vit. D
inadequate Ca & phosphate
osteoid is inadequately calcified
weak bone inability to
bear wt.
Bowing leg.
Osteomalacia = Vit. D deficiency in adult (common in female)
- It leads to bone ache due to decreased calcification during the
processes of maintenance of the matrix.
Osteoprosis
It is a condition of weak of weak bones, which
commonly occurs in postmenopausal as result of
estrogen reduction.
Estrogen is important for bone matrix formation by
osteoblasts.