BONE Flashcards
introduction to the bone
defination-
-Bone is a living tissue.
-it makes up the body skeleton.
-it is one of the hardest structures of the animal body.
-Provides the site for attachment of muscles and tendons
-it protects vital organs of the body
-it serves as storage site for minerals like calcium and phosphorous
-it is imporatant for muscle contraction and nerve activity.
-it is essential for locomotion.
has a connective tissue (red bone marrow ) that produce RBC , WBC , platlets.
and (yellow bone marrow ) that tore lipids cells and is a importent energy reserve.
- periosteum
-it is present on the outer surface of the bone
- exception- articular surface and muscle attachments
- has two parts outer dense fibrous connective tissue
and nner loose connective tissue - endosteum
- it is the inner layer
- has a loose connective tissue which contain oestogenic cells that separates the bone marrow from within.
At the periosteal and endosteal surfaces, the lamellar is arranged in parallel layers surrounding the bony surface and is called circumferential lamella
There are also small concentric layers around a central vascular canal- haversian canal
Haversian canals+ circumferential lamella= haversian system
There is a line that marks the end of bone resorption = cementline/reversal line
It is highly irregular and thus difference from a resting line
Haversian canals are interconnected by volkmanns canal which contains BVs
There are canaliculi in osteocyts that connect them together and provide nutrition and waste relayed one to another.
Interstitial lamella- remnants of osteons, lest behind during bone remodelling.
Classification on the
basis of their shape
- FLAT
-long and slender
humerus
,radius
,ulna
,femur
,tibia
,fibula - SHORT
- usually cube shaped
- contain bone marrow but no marrow cavity
carples
tarsals - FLAT
-they are thin, flat, with no marrow cavity
examples- sternum
scapula
ribs
skull bones
4 IRREGULAR
-have complex shapes
-have no marrow cavity
include
pelvic bones
mandible
facial
- SESAMOID
they are the bones that develop in tendons
where there is considerabla pressure tension friction.
example- patella
On basis of development
- Endochondrial
Formed by -
Replacement of hyaline cartilage with bony tissue
This type of ossification occurs in bones of trunk and extremities - Intramembranous
Formed by - ** replacement of sheet like connective tissue with bony tissue**
This occurs in flat bones
On Basis of microscopic structure
IMMATURE. MATURE
1.Also called-
Woven. Lamellar
2. Collagen fibrils
Intertwined. Orderly arrange
3. Enriched in
BAG75, BSP. OSTEOCALCIN
4. Interfibrillar space
More. Less
5. Hematoxyphillic eosinophilic
- Rate if deposition-
Rapid, therefore slow, therefore
More number of less osteocyte
Oesteocytes are entrapped
entrapped** - Osteocytes-
Isodiametric. Flat and oblate - Mineral density
Lower higher - Water content
Higher. Lower - Mineralisation by
* Matrix vesicles collagen fibrils* - Osteroclasts
**Remove bone in portions
completely
Mature bone is divided into-
COMPACT. CANCELLOUS
1.also called
Cortical. Spongy
2.
Tightly packed. Honeycomb
3.
Has osteons haversian canals….
4.
Form a solid. Hasbonytrabaculare
Mass
Composition of bone
Composed of
60% inorganic
25% organic
5% water
- The inorganic component is composed of
hydroxyapatite crystals
Low CA/P RATIO
Thin plate like shape - The organic component is also known as osteoid
Collagen is the major component mainly type1 (>95%) with typeV (<5%)
They provide structural integrity
Help resist fractures
The alveolar bone contains
type 1 5 3 12 collagen
The sharpey fibres contains
Type 1 3 collagen
The fibroblasts produce
Type 3 & 12 during periodontal ligament formation
Noncollagenous proteins-(10%)
-most are endogenous proteins produced by bone cells
And others such as albumins are derived from sources such as blood, and are incorporated into the Matrix during oesteosynthesis
Some other important proteins are
-Matrix proteins-
- osteocalcin
- osteopontine
- osteonectin
- BSP
-proteoglycans
-bigycans
-BMP
- decorin
-Growth factors
-PDGF
- FGF
- IGF
-
OSTEOBLASTS
-cuboidal/flat cells
- mononuclear cells
- derived from osteoprogenitor cells of mesenchymal origin.
- periosteum serve as a important reservoir of osteoblasts.
-basophillic ( due to abundance of RER)
-abundance of synthetic organelles
- the constituents of bone matrix
(Enter it’s lumen
Carried by transfer vesicles
Enter Golgi complex
Where they are stored in form of granules
These granules r released on the surface, form bone
The noncollagenous proteins also deposited help in regulating mineral deposition.)
- the osteoblasts also contain
-Acting
-Myosin
-Cytoskeletal proteins
Maintenance of cell shape
Attachment
Motility
FACTORS FAVOURING OSTEOBLASTS FORMATION -
1. transcription factors
- wnt pathway
- cbfa1(core binding factor alpha 1)/RUNX-2( runt related transcription factor)
- osterix
- osteocalcin
- BSP
- type 1 collagen
- Local and systematic factors
-
BMPs
(2,3,4,6) -
Cell growth factors
- FGF, IGF(1&2), TGF- beta, PDGF -
hormones
-PTH
-insulin
-growth hormone
-vitamin D3 - cytokine- IL6
FACTORS LIMITING OSTEOBLASTS FORMATION
TNF- alpha
Prolonged treatment with glucocorticoids
FUNCTIONS
- formation of new bone
- exhibits high level of alkaline phosphatase
- role in bone remodelling
- mineral metabolism
- mineralization of osteoid
BONE LINING CELLS- Oesteocytes
after the function of osteoblasts, they either gets entrapped in the bone matrix and become osteocytes OR the remain on the surface as lining cells OR they die by apoptosis
OSTEOCYTES
- the number of osteoblasts that become osteocytesdepend on the rapidity of bone formation
(Woven bone shoes more entrapped osteocytes for this same reason)
Lifespan - 25years
(Which exceed that of osteoblasts which is only 3months)
Within the bone matrix the osteocytes reduce in size and creates spaces around it- osteocytic lacuna
These appears oval/flattened
Narrow extension of the lacuna orm channels - canaliculi
In which present are osteocytic processes
Which contains microfilaments, SER
it’s distal end contact the process of adjacent cells (gap junctions)
The canaliculi penetrate the bone matrix thus allowing diffusion of nutrients gases and waste products
Osteocytes- sense change in environment and send signals that affect the bone remodelling
These signals maintains cell integrity and cell vitality
Failure can lead to sclerosis and death of bone 🍗 🦴
Inactive osteocytes processellipsoid cell body
The nucleus is oval
With thin faint basophillic stain
Few organelles
Old osteocytes
- lysosomes
- retract their process from canaliculi
- when dead their canaliculi and lacunae amg get plugged with debris
Their death leads to resorption of bone by osteoclasts
OSTEOCLASTS
- lie in howships lacunae
- large cells with closely packed nucleus
(Osteoclasts with more nuclei resorb bone faster than osteoclasts with less nuclei) - they are variable in shape (due to their motility)
- their cytoplasm shoes the presence o facid phosphatase containing vesicles and vacuoles
(The presence of acid phosphatase distinguish it from other multinucleated giant cells)
FORMATION
1.HEMATOPOIETIC STEM CELL IN BM
👇(stimulation of SCF, IL3, IL6)
2. CMPs(common myeloid progenitors)
👇 GM-CSF
👇(granulocyte/macrophage colony stimulating factor)
👇
3. GMPs
👇(Stimulation by M-CSF)
4. osteoclast progenitors
👇(Express)
5.
RANK. & C-fms interacts with
interacts. M-CSF
with
RANKL
👇
6. FULLY FUNCTIONAL OSTEOCLASTS
OPG inhibits RANK nad RANKL
Leads to inhibition of osteoclasts formation
REGULATION
1. FACTORS FAVOURING
-transcription factor
-RANK
-local and systematic factors
-hematopoietic factors**
-M-CSF
-cytokine- lL1.6.11. & TNF-alpha
- hormones
-D3
PTH
PGE2
GLUCOCORTICOIDS
FACTORS LIMITING
-differentiation
OPG
-local and systematic factors
-growth factors**
TGF-beta, IGF1 IGF2
-hormones
GLUCOCORTICOIDS
PTH
PGE2
CALCITONIN
ESTROGEN
-cytokine
(4.10.12.13.18)
IFN- gammma
Pharmacological
-bis-phosphates
INTRAMEMBRANOUS OSSIFICATION
-
direct formation of bone within the high highly vascular sheets of condense primitive mesenchyme.
-this process occurs in the flat bones of this skull and the clavicles.
-it begins approximately towards the end of second month of gestation
- FORMATION OF BONE MATRIX.
At the site where bone will form
⬇️
Presence of loose mesenchyme
(Widely separated, stellate cells with interconnected cytoplasmic process)
- CENTRE OF OSTEOGENESIS capillaries grow into the mesenchyme
⬇️
Cells at centre become round,
Basophillic, with thick interconnected cytoplasmic process.
(These cells will differentiate into osteoblasts) - OSTEOBLASTIC DIFFERENTIATION
The osteoblasts will secrete bone matrix
After their activity they will get in trapped In The Matrix to form osteocytes - CALCIFICATION OF MATRIX
Matrix begins to calcify and crystal formation occurs. - FORMATION OF WOVEN BONE
The first small mass of newly formed bone matrix is an irregular shaped spicule
These bonus spickles gradually lengthen and into long and anastomosing structures called trabeculae which extends in radial pattern
⬇️
Form spongy bone
⬇️
Trabaculae enclose blood vessels
⬇️
Woven bone 🦴 - APPOSITIONAL GROWTH
the osteogenic cells on the surface of spicule and trabeculae are always in a superficial repeating process again and again this is called as apposational growth
As the trabeculae increase in width due to appociational growth neighbouring capillaries are incorporated to provide nutrition to the osteocytes In The Deep layers
⬇️
7. BONE REMODELING
New bone deposited on one surface and resorbed on the other leading to trabaculary modelling
This remodelling maintains the shape and size of bones throughout life
- FORMATION OF COMPACT BONE
Due to continuous APPOSITIONAL growth and bone remodeling the CANCELLOUS bone is converted into compact bone - FORMATION OF HAVERSION SYSTEM
Narrow canal slide by ostogenic cells formed
⬇️
Enclosed blood vessels
⬇️
Lamella of bone added to the Bony wall of spaces in cancellace bones
⬇️
Haversian system FORMATION
ENDOCHONDRIAL OSSIFICATION
- replacement of cartilaginous model by bone
- occurs in the extremities of bones
- FORMATION OF CARTILAGINOUS MODEL
-at the site where the limb will develop later
The mesenchyme cells condense differentiate into chondrioblasts,
⬇️
Hyaline cartilage model
(Surrounded by perichondrium)
No osteoblasts r produce as the differentiation is taken place in avascular region - GROWTH
increase in length - interstitial growth
Increase in width - APPOSITIONAL growth - FORMATION OF BONE COLLAR AND PERIOSTEUM
- capillaries grow into the perichondrium
⬇️
Cells in perichondrium differentiation to osteoblasts
⬇️
Bone collar
At this stage perichondrium is reffer to as periosteum
- RESORPTION OF MINERALIZED CARTILAGE MATRIX
due to vacularization of midsection of cartilage model
⬇️
Chondrioblasts resorb the MINERALIZED matrix.
⬇️
The bone collar holds the shaft together which has been weakened by disintegration of cartilage.
- FORMATION OF PERIOSTEAL BUD
periosteal capillaries with osteogenic cells invade the calcifed cartilage at middle and supply the interior
⬇️
The osteogenic cells and the vessels comprises the periosteal bud.
⬇️
The capillary grow into the cartilage model and initiate the development of primary ossification centre
⬇️
CANCELLOUS bone 🍗
⬇️
Periosteal bud - osteoblasts
⬇️
Deposite bone matrix
⬇️
Network of mixed spicules
(Primary spongiosa) - FORMATION OF MEDULLARY CAVITY
osteoclasts break down the newly formed spongy bone and open up a MEDULLARY CAVITY in the centre of the shaft(diaphysis) in which hematopoietic stem cells entre giving rise to myeloid tissue
The two ends of the bone (epiphysis) - FORMATION OF SECONDARY OSSIFICATION CENTRE
-same as that of primary
-no medullary cavity(epiphysis)
- when the secondary ossification completed the hyaline cartilage remain at two places
- articular surface (articular cartilage)
- epiphysis and diaphysis in between
(Epiphyseal plate)
MINERALIZATION
Mineralization is the process of deposition of insoluble calcium salts in a tissue
MECHANISM OF MINERALIZATION.
Under normal conditions, there is insufficient concentration of available calcium and phosphate ions in blood and tissue fluid
for calcium phosphate to crystallize or precipitate spontaneously (unsaturated state).
The critical factor is the local (Ca++) x.
When factors operate locally, to raise this ion product,
calcium phosphate separates out in the solid phase
Once the microcrystals have begun to form, they continue to grow and also catalyze further crystalli- zation of calcium phosphate even at sites where the Ca2+. Pi product does not exceed the plasma level
THEORIES OF MINERALIZATION
The two main mechanisms involved in mineralization are the booster mechanism and the seeding mechanism
The booster mechanism states-
that local increase in the level of calcium or phosphate causes supersaturation and the solute, i.e., calcium or phosphate precipitates out from tissue fluid to the site of mineralization.
the seeding mechanism, which states that-
certain substances like the collagen act as seed or nucleators to attract calcium and phosphorus ions
BASED ON THESE TWO MECHANISMS, THREE THEORIES HAVE BEEN PROPOSED.
(A) Alkaline phosphatase theory
: This theory is based on the booster mechanism
- The evidence for this theory is based on the observation that alkaline phosphatase was found always in areas of mineralization.
- This enzyme hydrolyses a broad range of organic phosphate containing substrates and increases the local inorganic phosphate concentration
This leads to supersaturation and precipitation of phosphatase ions from tissue fluid.
Alkaline phosphatase also removes inhibitors of mineralization like pyrophosphates.
The main drawback of this theory is that this enzyme is also found in areas which do not mineralize.
(B) Seeding theory:
This theory is based on the seeding mechanism.
The most important seed is collagen.
Other seeds or nucleators are phosphoproteins, glycosaminoglycans, and chondroitin sulfate.
Evidence for collagen as a seed comes from the
observation that the deposition of calcium and phosphate ions occur first in relation to the cross-banding of collagen.
Evidences against collagen include the collagens of soft tissue, which do not initiate mineralization and
occurrence of mineralization in the absence collagen in the highly mineralized enamel tissue.
(C) Matrix vesicle theory:
The matrix vesicle theory states that amorphous calcium phosphate is formed within the secretory(matrix) vesicle of the hard tissue matrix secreting cells like osteoblast.
The alkaline phosphate present within these cells serve to increase the phosphate ion content within the cell
and mitochondria releases calcium from its stores
leading to formation of amorphous calcium phosphate within the secretory (matrix) vesicle.
These are then released in nucleating sites within the collagen fiber. Thus, this theory incorporates the booster and the seeding mechanisms.
BONE RESORPTION
BONE RESORPTION
what?
Bone resorption involves dissolution of crystalline hydroxyapatite
followed by proteolytic cleavage of organic component of bone matrix,
Sequence of events of bone resorption
The first phase involves the formation of osteoclast
progenitors in the hematopoietic tissues
⬇️
followed by their exit from blood vessels
⬇️
formation of resting pre osteoclasts and osteoclasts in the bone itself.
⬇️
Osteoblasts play a major role
1. by retracting, to expose the mineral to the osteoclast
2. releasing a soluble factor that activates these cells.
⬇️
activated osteoclasts resorbing the bone
ALTERATIONS IN THE OSTEOCLASTS
Immediately before the resorption , the OSTEOCLAST undergo changes alterations are the
1.development of a RUFFLED border
2.sealing zone at the PLASMA membrane.
⬇️
. The ruffed BORDER CONSISTS of many infoldings of the cell membrane resulting in fingerlike projections of the cytoplasm
Thus, extensive surface is created well suited for an intensive exchange between the cell and bone
⬇️
At the periphery of the ruffled border, the plasma membrane is smooth and apposed closely to the bone surface.
The adjacent cytoplasm, devoid of cell organelles contains contractile actin microfilaments, surrounded by two vinculin rings. This region is called the clear (sealing) zone
⬇️
This zone serves to
1.attach the cell very closely to the surface of bone
2.creates an isolated microenvironment in which resorption can take place without diffusion of the hydrolytic enzymes produced by the cell into adjacent tissue.
⬇️
When osteoclasts arrive at the resorption site, they use the sealing zone to attach themselves to the bone surface.
⬇️
The attachment of the osteoclast cell membrane to the bone matrix at the sealing zone is due to the presence of cell membrane proteins known as integrins.
REMOVAL OF HYDROXYAPATITE
The initial phase involves the dissolution of the mineral
component by the action of hydrochloric acid (HCI)
⬇️
The protons for the acid arise from the activity of cytoplasmic carbonic anhydrase II, which is synthesized in the osteoclast.
⬇️
The protons are then released across the ruffled border into the resorption zone by an ATP consuming proton pump.
⬇️
This leads to a fall in pH to 2.5-3.0 in the osteoclast resorption space.
DEGRADATION OF ORGANIC MATRIX
Organic constituents of bone tissue remain after the dissolution of mineralized component
⬇️
ProteoIstic enzymes are synthesized by osteoclasts, namely, cathepsin-K and MMP-9
The enzymes are synthesized in rough endoplasmic reticulum,
transported to Golgi complexes,
and moved to the ruffled border in transport vesicles
, and the contents of the same are released into extracellular compartments
Creating extra cellular lysosomes
As a result, a visible depression or howships lacunae is excavated into the bone.
Cathepsin-K degrades major amount of type I collagen and other noncollagenous proteins, which have been demineralized by the acidic environment of the resorptive bone.
MMP9 (collagenase B) is believed to be required for osteoclasts migration
MMP-13 degrades osteoid and exposes the sites for osteoclast attachment.
REMOVAL OF DEGRADATION PRODUCTS
Once liberated from bone, the free organic and inorganic particles of bone matrix are taken in or endocytosed from the resorption lacunae, across the ruffled border, into the osteoclast.
⬇️
These are then packed in membrane-bound vesicles
⬇️
These vesicles and their contents pass across the cell and fuse with functional secretory domain (FSD), in region of the basal membrane.
⬇️
Then the vesicles
released by exocytosis.
⬇️
Following resorption,
⬇️
osteoclasts undergo apoptosis.
Role of Tartrate-Resistant Acid Phosphatase in Bone Resorption( TRAP)
Tartrate resistant acid phosphatase (TRAP) enzyme plays a role in bone resorption inside and outside the osteoclast cell. :-
- It accumulates extracellularly in the bone matrix immediately adjacent to ruffled border of resorbing osteoclasts.
- Osteopontin, BSP, and osteonectin act as substrates for TRAP.
- Osteopontin
enables osteoclasts to adhere to bone surface by binding with integrin
⬇️
TRAP removes the phosphate group from osteopontin.
⬇️
This disrupts the attachment of osteoclast to the bone surface - It degrades phosphoproteins in bone matrix
- It also hydrolyses and liberates pyrophosphate from bone matrix which is an inhibitor of resorption.
- Intracellularly, it transports the matrix degradation products from ruffled border to FSD.
⬇️
It is then secreted out of cells together with matrix degradation products.
⬇️
Later, they leak into the circulation at a rate that corresponds to the amount of resorption activity being undertaken by the osteoclast.
⬇️
Hence, its assessment by immunoassay is useful for diagnosing and monitoring bone resorptive activity. - It binds to ẞ-2 microglobulin, a carrier protein, which clears it from the area of bone resorption.
- Like all other enzymes, it is metabolized in the liver and its metabolites are secreted in the urine.
BONE REMODELING
- What
Bone remodeling is a continuous physiological process
that occurs in a adult skeleton
in which bone resorption is followed by new bone formation,
Maintaining strength and structure.
Bone remodeling is performed by clusters of bone resorbing osteoclasts and bone forming osteoblasts
Which are arranged within a temporary anatomical structure known as basic multicellular unit (BMU)
An active BMU
consists of a leading front of bone resorbing osteoclasts forming the cutting cone
Reversal cells of un- clear phenotype follow the osteoclasts, covering the newly exposed bone surface and prepare it for deposition of re placement bone
Osteoblasts occupy the tail portion of the BMU, the closing cone, and secrete and deposit osteoid and direct its formation and mineralization into mature lamellar bone.
At any given time, the processes of bone synthesis and bone breakdown go on simultaneously and the status of the bone represents the net result of a balance between the two processes. This phenomenon is called “coupling” of bone resorption and formation.
FUNCTION. The main functions of remodeling are to **prevent the accumulation of damaged and fatigued bone by
1. regenerating new bone,
2. allow the bone to respond to changes in mechanical forces, 3.facilitate mineral homeostasis
SEQUENCE OF EVENTS OF BONE REMODELING
(A) Quiescent phase (rest)
(B) Activation phase
detection of signal in form of mechanical strain/hormonal action by osteocytes
⬇️
translation of these signals into biological signals
⬇️
recruitment of monocyte-macrophage osteoclast precursors
⬇️
interaction of osteoclast and osteoblast precursor cells (RANKL on osteoblasts and RANK on osteoclasts)
⬇️
differentiation, migration and fusion of large multinucleated osteoclasts
(C) Resorption phase recruitment of osteoclasts precursors
⬇️
osteoclasts dissolve minerals followed by release of TGF, PDGF, IGF-1 and 2
⬇️
scalloped Howship’s lacunae formed (resorption tunnel/cutting cone)
⬇️
mononuclear cells (macrophages remove collagen remnants)
(D) Mineralization phase (incorporation of hydroxyapatite)
(E) Quiescent phase
REGULATORY FACTORS-
MEDIATORS
- PTH
- produced in response to hypocalcemia
stimulate bone resorption
& Bone formation by synthesis of IGF1 TGF-b - CALCITONIN
- produced when blood calcium rises
-inhibits bone resorption
- promotes calcium salt deposition in bone matrix thereforereducing blood calcium levels
-reduces the number of osteoclasts - VITAMIN D METABOLITE
- both - ESTROGEN
- bone formation
- increas the number of osteoblasts
- * increase OPG, resorption* - GH
-* increase the synthesis of IGF1 IGF2, which stimulate proliferation and differentiation of osteoblasts* - GLUCOCORTICOIDS
- * inhibits the synthesis of IGF1*
- suppress BMP2 and Cbfa1
LOCAL FACTORS
IL-1
- does not have direct action on the osteoclast,
but like PTH acts via the osteoblast. - It inhibits the apoptosis of osteoclasts.
TNF-a and TNF-B
- stimulate bone resorption in vitro.
- The bone resorbing effect is mediated through osteoblasts.
- TNF is also believed to inhibit bone collagen and non- collagenous protein synthesis.
Prostaglandins
- are local pathological mediators of bone destruction,
- where there is inflammation. Bacterial products, such as lipopolysaccharide, capsular material, lipoteichoic acids, and peptidoglycans
, may act as foreign antigens and induce monocytes, macro- phages, and then bone cells to produce prostaglandins and cytokines such as IL-1, leading to bone resorption.
IGF-1 and-2
- increase the number and function of osteoblasts
stimulating collagen synthesis.
- They mediate osteoblast- osteoclast interaction and participate in bone remodeling.
BMPs
- are highly abundant in bone tissue and participate in the formation of bone and cartilage
- They are the most important factors for osteoblast differentiation.
Markers of Bone Turnover
• The main serum markers of bone formation are
- **alka-line phosphatase (total),
- alkaline phosphatase (skeletal isoenzymes),
- osteocalcin,
- TRAP,
- ẞ2 microglobulin**.
• Serum 32 microglobulin
is a marker for high bone remodeling.
- It plays a role as bone-derived growth factor regulating osteoblasts and osteoclasts.
- It has been proposed as a bone remodeling biological marker in high bone turnover conditions,
especially vitamin D deficiency and secondary hyperparathyroidism.
The main markers of bone resorption present in the urine
- *calcium,
- urinary hydroxyproline*.
