Difference between revisions of "Bones - Anatomy & Physiology"

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<big><center>[[Musculoskeletal System - Anatomy & Physiology|'''BACK TO MUSCULOSKELETAL ANATOMY AND PHYSIOLOGY''']]</center></big>
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==Overview==
 
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[[Image:Horse Skeleton.jpg|thumb|right|250px|Horse Skeleton - Copyright Nottingham]]
 
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[[Image:Growth plate.jpg|right|thumb|250px|<small><center>Growth plate (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
 
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[[Image:Growth plate closer.jpg|right|thumb|250px|<small><center>Growth plate magnified (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
 
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[[Image:Aspinall Slide1.JPG|thumb|right|250px|<small>Image from [http://www.elsevierhealth.co.uk/veterinary-nursing/spe-60136/ Aspinall, The Complete Textbook of Veterinary Nursing], Elsevier Health Sciences, ''All rights reserved''</small>]]
 
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Bone comprises the structure of the skeletal system and provides lever arms for locomotion. Bone also plays important roles in maintaining mineral homeostasis, as well as providing the environment for [[Haematopoiesis - Overview|hematopoesis]] in [[Bone Marrow|marrow]].
 
  
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'''Short bones''' - Endochondral ossification continues in the diaphysis, until only the cartilage rim remains.
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Bone comprises the structure of the skeletal system and provide lever arms for locomotion.  Bone also plays important roles in maintaining mineral homeostasis as well as providing the environment for hematopoesis in marrow.
 +
==Development of Bone and Cartilage==
 +
*Osteogenesis
 +
**'''Intramembranous Ossification'''
 +
***Forms the '''flat''' bones of skull and mandible
 +
***No cartilaginous precursor: mesenchyme forms bone directly
 +
****Mesenchyme condenses, differentiates to pre-osteoblasts, then osteoblasts
 +
****Osteoblasts synthesize osteoid (collagen and proteoglycans)
 +
****Mineralization and Bone Spicule formation (spicules produce spongy bone)
 +
****Appositional Growth: laid down on the exterior (vs interior) surface of developing bone
 +
***Increasing association with blood vessels
 +
***Bone Marrow formed by mesenchyme between bone and blood vessels
 +
**'''Endochondral Ossification'''
 +
***Responsible for embryonic bone formation as well as growth in length
 +
***Via cartilaginous precursor:
 +
****Mesenchyme condenses to form bone outline
 +
****Core cells differentiate to chondroycytes and begin secreting cartilage matrix
 +
****Peripheral condensation forms perichondrium
 +
****Interstitial (length) and Appositional (width) growth
 +
****Central cells in developing diaphysis mature and hypertrophy
 +
****Matrix surrounding most mature chondrocytes calcifies
 +
****Perichondrium differentiates to osteoblasts
 +
***Capillary invasion to central core, forming trabecular bone
 +
*Bone Growth and Remodeling
 +
**Short bones: endochondral ossification continues in diaphysis until only cartilage rim remains
 +
**Long bones: secondary center of ossification develops in epiphyses
 +
***'''Epiphyseal Growth Plate''': transverse disc of cartilate between the two ossification centers, allows longitudinal growth to continue to maturity, then closes
 +
[[Image:Growth plate.jpg|right|thumb|100px|<small><center>Growth plate (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
 +
[[Image:Growth plate closer.jpg|right|thumb|100px|<small><center>Growth plate magnified(Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
 +
***Cartilage of epiphyseal growth plate is divided into: (from right to left on the magnified image)
 +
**** - Resting (reserve) zone
 +
**** - Proliferative zone
 +
**** - Hypertrophic zone
  
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'''Long bones''' - A secondary center of ossification develops in the epiphyses.
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**'''Primary''' bone has not yet been '''remodelled'''
−
 
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***Contains less mineral and more randomly arranged collagen fibers, trabecular organization
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See [[Bone & Cartilage Development - Anatomy & Physiology|Bone and Cartilage Development]]
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**Remodelling occurs by '''Haversian canals'''
−
 
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***Osteoclasts dig out canal, followed by blood vessel invasion
−
 
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***Concentric lamellae laid down
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==Bone Growth==
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***Phased resorption happens concurrently
−
 
 
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===Length===
 
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The '''epiphyseal growth plate (EGP, physis)''' is a transverse disc of cartilage between the two ossification centers and allows the bone to continue to grow in length after birth. It then closes. The EGP is a region of continuous chondroblast differentiation and maturation and can be divided into five zones:
 
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# '''Germinal Zone''' - chondroblast division at a low rate, and self renewal.
 
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# '''Proliferation Zone''' - chondroblast division at a fast rate.
 
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# '''Maturation Zone''' - chondroblast differentiation into chondrocytes.
 
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# '''Hypertrophic Zone''' - chondrocytes enlarge and then die.
 
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# '''Dead cells''' become the matrix for bone growth.
 
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===Girth===
 
−
 
 
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The '''periosteum''' lines the outer surface of the diaphysis of the bone. It has a cellular inner layer, consisting of osteoblasts, and a fibrous outer layer. The '''endosteum''' lines the inner surface of the diaphysis of the bone. It contains osteoblasts ''and'' osteoclasts. Bone growth occurs by a shift in the equilibrium between osteoblasts and osteoclasts. Osteoblasts on the outside in the periosteum add bone to increase the bone's diameter. Osteoclasts on the inside in the endosteum remove this bone to maintain the bone diameter. When osteoclasts start removing less bone, or osteoblasts start adding more bone, the girth increases.
 
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−
 
 
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==Bone Remodeling==
 
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[[Image:Haversian system.jpg|right|thumb|250px|<small><center>Haversian system (Courtesy of RVC Histology images)</center></small>]]
 
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'''Primary''' bone has not yet been '''remodelled'''. It contains less mineral and more randomly arranged collagen fibers, known as trabecular organization. Remodelling of bone occurs by '''haversian canals'''. The osteoclasts dig out the canal longitudinally through the bone connecting with bone marrow cavity and periosteum. They are followed by anastamosing and branching blood vessels. Concentric bony lamellae are laid down with rows of interconnected lacunae containing '''osteocytes''' within. The osteocytes communicate by channels in young bones. Phased resorption happens concurrently.
 
−
 
 
−
'''Haversion canals + lamellae + osteocytes = Haversion system'''
 
  
 
==Types of Bone==
 
==Types of Bone==
  
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1. '''Long Bone'''
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*'''Long Bone'''
−
 
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**Found in the limbs and act as levers for locomotion
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Found in the limbs and act as levers for locomotion. An elongated '''diaphysis''' (shaft) and two '''epiphyses''' (ends), each of which encases a center for ossification.
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**An elongated '''diaphysis''' (shaft) and two '''epiphyses''' (ends), each of which encases a center for ossification
−
 
 
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2. '''Short Bone'''
 
−
 
 
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Found in places of articulation, such as the carpus and tarsus. All dimensions are relatively equal, generally signifying one center of ossification.
 
−
 
 
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3. '''Flat Bone'''
 
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Found in the skull, pelvic girdle, and scapula. It expands in two directions, with a broad surface for attachment of large muscle masses and protection of underlying structures.
 
−
 
 
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4. '''Irregular Bone'''
 
−
 
 
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Found in the vertebrae.
 
−
 
 
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5. '''Sesamoid Bone'''
 
  
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Eg. Patella and navicular bone. Found within tendons, where they change direction over prominences that would otherwise cause damage. They form '''synovial joints''' with major bones with which they are in contact. They also serve to displace tendon from the axis of the joint, increasing the leverage exerted by the muscle.
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*'''Short Bone'''
 +
**Found in places of articulation, such as the carpus and tarsus
 +
**All dimensions are relatively equal, generally signifying one center of ossification
  
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6. '''Splanchnic Bone'''
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*'''Flat Bone'''
 +
**Found in the skull, pelvic girdle, and scapula
 +
**Expand in two directions, with a broad surface for attachment of large muscle masses and protection of underlying structures
  
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Develop in soft organs remote from skeletal connection, eg. '''os penis'''.
+
*'''Sesamoid Bones'''
 +
**Eg. Patella, Navicular bone
 +
**Found within tendons, where they change direction over prominences that would otherwise cause damage
 +
**Form '''synovial joints''' with major bones with which they are in contact
 +
**Also serve to displace tendon from the axis of the joint, increasing leverage exerted by the muscle
  
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7. '''Pneumatic Bone'''
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*'''Splanchnic Bones'''
 +
**Develop in soft organs remote from skeletal connection: eg. '''os penis'''  
  
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Excavated to contain air spaces, such as the skull in the instance of paranasal sinuses, and the post-cranial skeleton of birds.
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*'''Pneumatic Bones'''
 +
**Excavated to contain air spaces, such as the skull in the instance of paranasal sinuses, and the post-cranial skeleton of birds
  
 
==Composition of Bone==
 
==Composition of Bone==
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[[Image:Bone histo.jpg|right|thumb|250px|<small><center>Histological structure of bone (Courtesy of RVC Histology images)</center></small>]]
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[[Image:Bone histo.jpg|right|thumb|100px|<small><center>Histological structure of bone (Courtesy of RVC Histology images)</center></small>]]
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[[Image:Bone micro structure.jpg|right|thumb|250px|<small><center>Microscopic bone (Courtesy of RVC Histology images)</center></small>]]
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[[Image:Bone micro structure.jpg|right|thumb|100px|<small><center>Microscopic bone (Courtesy of RVC Histology images)</center></small>]]
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Bone is a hard, highly specialised connective tissue . It consists of interconnected cells embedded in a calcified, collagenous matrix. It is a living, dynamic, responsive tissue, growing and remodelling throughout life.
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*Bone is a hard, highly specialised connective tissue  
−
 
+
*Consists of interconnected cells embedded in a calcified, collagenous matrix
−
Bone is composed of:
+
*Living, dynamic, responsive tissue, growing and remodelling throughout life
−
 
 
−
===Matrix===
 
−
 
 
−
'''Osteoid'''- An organic, uncalcified, homogeneous substance that stains light pink with H&E . Osteoid consists of type I collagen (90%), which resists tension, and bone-specific proteins (10%), including; Osteonectin, Osteopontin and Osteocalcin.
 
−
 
 
−
'''Mineral'''- An inorganic component that provides rigidity and consists of a crystalline lattice of calcium phosphate and calcium carbonate. It also contains Mg, Mn, Zn, Cu, Na, F. It accounts for 65% of bone.
 
−
 
 
−
===Cells===
 
−
 
 
−
'''Osteoblasts'''
 
−
 
 
−
A single layer of mesenchymal cells which synthesize bone extracellular matrix (osteoid). When active, they appear plump and cuboidal, with a basophilic cytoplasm. The cell membranes are rich in alkaline phosphatase (ALP). The cells are possibly involved in pumping calcium across membranes. They are promoted by growth factors and have receptors for [[Calcium#Parathyroid Hormone (PTH)|PTH]]. They contract in response, which provides space for osteoclasts to attach. When they are inactive, there is less cytoplasm, so they become flattened.
 
−
 
 
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'''Osteocytes'''
 
−
 
 
−
Osteocytes are embedded in their own matrix; reside within '''lacunae''' and are interconnected via channels forming '''canaliculi'''. Canaliculi create connections to form a huge neural-like junctional organization. They contact osteoblasts and each other with cytoplasmic processes and reach through canaliculi in the mineralised bone matrix. Osteocytes regulate the composition of bone fluid.
 
−
 
 
−
'''Osteoclasts'''
 
−
 
 
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Osteoclasts are giant (multinucleate [[Monocytes|monocytes]]) cells. Histologically, they are large, often multinucleated cells. They have an acidophilic cytoplasm. They sit in the bone surface depression known as, '''howship's lacuna'''. Osteoclasts respond to [[Calcium#Calcitriol (Activated Vitamin D3)|vitamin D]] by increasing their numbers and activity (parathyroid independent). Osteoclasts act to resorb the bone extracellular matrix (ECM). They firstly dissolve mineral followed by collagen, using the brush border. They do <u>not</u> have receptors for PTH, but do have receptors for [[Calcium#Calcitonin|calcitonin]].
 
−
 
 
−
Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from the stromal fibroblastic system ('''osteoprogenitor cells'''). Osteoclasts are derived from the haematopoietic system.
 
  
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==Organisation of Bone==
+
Bone is comprised of:
 +
*Matrix:
 +
**'''Osteoid''': Organic component
 +
***Uncalcified, homogeneous substance, Stains light pink with H&E
 +
***"Type I Collagen" (90%), which resists tension
 +
***Bone-specific proteins(10%): Osteonectin, Osteopontin, Osteocalcin
 +
**'''Mineral''': Inorganic component, provides rigidity
 +
***Crystalline lattice of calcium phosphate and calcium carbonate
 +
***Also contains Mg, Mn, Zn, Cu, Na, F
 +
***Accounts for 65% of bone
  
−
The normal progression of bone is from woven bone to lamellar bone, even in pathology, except for [[Craniomandibular Osteopathy|canine craniomandibular osteopathy]] and [[Hypervitaminosis D|hypervitaminosis D]], where lamellar bone is replaced by woven bone.
+
*Cells:
  
−
===Woven bone===
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**'''Osteoblasts''': single layer of mesenchymal cells which synthesize bone extracellular matrix (osteoid)
 +
***When active, appear plump and cuboidal, with basophilic cytoplasm
 +
****Cell membranes are rich in alkaline phosphatase (ALP)
 +
****Possibly involved in pumping calcium across membranes
 +
****Promoted by growth factors
 +
****Have receptors for [[Bones - normal#Bone resorption|PTH]]: contract in response -> provide space for osteoclasts to attach
 +
***When inactive - less cytoplasm -> flattened
  
−
Woven bone consists of coarse collagen fibres. It is later removed by osteoclasts and replaced by lamellar bone. In adults, it is a sign of a pathological condition (e.g. fracture, inflammation, neoplasia).
+
**'''Osteocytes''': osteoblasts embedded in their own matrix; reside within '''lacunae''' and are interconnected via channels forming '''canaliculi'''
 +
***'''Canaliculi''' create connections to form a huge neural-like junctional organization
 +
***Contact osteoblasts and each other with cytoplasmic processes
 +
***Reach through canaliculi in mineralised bone matrix
 +
***Regulate composition of bone fluid
  
−
===Lamellar bone===
+
**'''Osteoclasts''': giant (multinucleate monocytes) cells which act to resorb bone ECM
 +
***Histologically:
 +
****Large, often multinucleated cells
 +
****Acidophilic cytoplasm
 +
***Derived from haematopoietic stem cells
 +
***Sit in bone surface depression - '''Howship's lacuna'''
 +
***Respond to vitamin D by increasing their numbers and activity (parathyroid independent)
 +
***Responsible for bone resorption
 +
****Firstly dissolve mineral followed by collagen, using brush border
 +
****Do <u>not</u> have receptors for PTH
 +
****Have receptors for [[Bones - normal#Bone resorption|calcitonin]]
 +
****Involute their brush border in response
  
−
Lamellar bone consists of orderly layers, which are much stronger than woven bone. There are fine collagen fibres in concentric or parallel laminae. There are two main types of lamellar bone:
+
**Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from stromal fibroblastic system ('''osteoprogenitor cells'''); osteoclasts from haematopoietic system
  
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1. '''Compact bone (cortical)'''
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==Organization of Bone==
 +
*Normal progression is from woven bone to lamellar bone, even in pathology, except for [[Bones - hyperplastic and neoplastic#Craniomandibular osteopathy|canine craniomandibular osteopathy]] and [[Bones - metabolic#Hypervitaminosis D|hypervitaminosis D]], where lamellar bone is replaced by woven bone
  
−
A sheath covers the external surface of long bone. It is thicker in the shaft and thins over the epiphyses. It is composed of thin lamallae in a series of concentric tubes arranged around small central canals (collectively known as an '''osteone'''). It forms 80% of the total bone mass and consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate.
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*'''Woven bone''':
 +
**"Random weave" which is only a normal feature in the foetus
 +
**Coarse collagen fibres
 +
**Later removed by osteoclasts and replaced by lamellar bone
 +
**In adults it is a sign of a pathological condition (e.g. fracture, inflammation, neoplasia)
  
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2. '''Cancellous bone (spongy or trabecular)'''  
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*'''Lamellar bone''':
 +
**Orderly layers which are much stronger than woven bone
 +
**Fine collagen fibres in concentric or parallel laminae
  
−
Stacks of parallel or concentrically stacked sheets arranged as rods, plates, and arches. In vertebrae, it forms flat bones and forms the hematopoeitic center of epiphyses of long bones. It contains no Haversian systems.
+
**Two main types:
 +
***'''Compact bone (cortical)'''
 +
****Sheath covers external surface of long bone, thicker in shaft and thins over epiphyses
 +
****Comprised of thin lamallae in a series of concentric tubes arranged around small central canals (collectively known as an '''osteone''')
 +
****Forms 80% of total bone mass
 +
****Consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate
 +
****Forms the shell of long bone shafts - contain [[Haversian systems]]
  
−
===Laminar bone===
+
***'''Cancellous bone (spongy or trabecular)'''
 +
****Stacks of parallel or concentrically stacked sheets arranged as rods, plates, and arches
 +
****In vertebrae, flat bones and forms the hematopoeitic center of epiphyses of long bones
 +
****Contains no Haversian systems
  
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Formed on the periosteal surface of diaphysis. It accommodates rapid growth of large dogs and farm animals. Plates of woven bone from within the periosteum. As it forms, it fuses with the bone surface.
+
*'''Laminar bone'''
 +
**Formed on periosteal surface of diaphysis
 +
**Accommodates rapid growth of large dogs and farm animals
 +
**Plates of woven bone from within the periosteum
 +
**Concentric plates
 +
**As it forms, it fuses with the bone surface
  
 
==Periosteum and blood supply==
 
==Periosteum and blood supply==
−
 
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*'''Periosteum''':
−
===Periosteum===
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**Specialised sheath of connective tissue covering bone except at the articular surfaces
−
 
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**Loosely attached except at tendon insertions and boney prominences (associated with major blood vessels penetrating bone)
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The periosteum is the specialised sheath of connective tissue covering bone, except at the articular surfaces. It is loosely attached, except at tendon insertions and bony prominences. Histologically, there is an outer layer which is fibrous for support, and an inner layer that is osteogenic. It consists of a rich supply of nerves and lymph vessels, including nutrient, metaphyseal and periosteal arteries. The normal flow of blood from the medulla to the periosteum is due to higher pressures in the medulla. Young animals have a greater blood supply.
+
**Histologically:
−
 
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***Outer layer - fibrous for support
−
The '''endosteum''' lines the marrow cavity. The '''medullary cavity and cancellous interstitium''' is for bone marrow storage and production. [[Bone Marrow - Anatomy & Physiology#Red marrow|'''Red marrow''']] is a richly vascularized, gelatinous tissue with hematopoeitic properties found abundantly in young animals and [[Bone Marrow - Anatomy & Physiology#Yellow marrow|'''yellow marrow''']] has been converted from red marrow by fat infiltration, causing hematopoeitic properties to dwindle.
+
***Inner layer - osteogenic
 +
**Rich supply of nerves and lymph vessels
 +
**Nutrient, metaphyseal, periosteal arteries
 +
**Normal flow of blood from medulla to periosteum due to higher pressures in medulla
 +
**Young animals have greater blood supply
 +
*'''Endosteum''' lines the marrow cavity
 +
*'''Medullary Cavity and Cancellous Interstitium''': bone marrow storage and production
 +
**[[Bone Marrow - Anatomy & Physiology#Red marrow|Red Marrow]]: richly vascularized, gelatinous tissue with hematopoeitic properties found abundantly in young animals
 +
**[[Bone Marrow - Anatomy & Physiology#Yellow marrow|Yellow Marrow]]: fat infiltration converts red marrow to yellow, causing hematopoeitic properties to dwindle
  
 
==Biomechanics of Bone==
 
==Biomechanics of Bone==
 +
*'''Wolff's Law''': Bone architecture adapts in response to the loads applied upon it according to mathematical laws
 +
**'''Load''': the external force placed on a structure, F
 +
**'''Strain''': the proportional change in the structure's dimensions
 +
**'''Stress''': the internal forces resisting the change in dimension caused by the load
 +
*The normality of bone architecture and appearance is directly related to its loading history
 +
*Cells use strain as a stimulus to adjust mass and architecture according to load
 +
*Resorption and osteogenesis happen concurrently to maintain bone integrity
  
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Bone growth and maintenance of normal structure are directly related to mechanical forces. Mechanical forces generate bioelectrical potentials (piezoelectricity), these potentials strengthen bone. Inactivity reduces the potentials, causing bone loss.
+
==Structure and Function of Cartilage==
−
 
+
*Function: resist compression, provide resilience and support at sites where flexibility is desired
−
===Wolff's Law===
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*Structure:
−
 
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**'''Chondrocytes''': reside within '''lacunae''' within ECM synthesizing '''matrix'''
−
Bone architecture adapts in response to the loads applied upon it according to mathematical laws;
+
**Type II Collagen (except fibrocartilage)
−
 
+
**Proteoglycans with associated glycosaminoglycans
−
'''Load''': the external force placed on a structure, F.
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***Continually turned over
−
 
+
***The most vulnerable component of cartilage
−
'''Strain''': the proportional change in the structure's dimensions.
+
***Decresed proteoglycan -> loss of lubrication -> collagen disruption -> frays, clefts, fibrillation, ulcers, exposure of bone, [[Musculoskeletal terminology|eburnation, +/- osteophytes and joint mice]]
−
 
+
**Hyaluronic acid
−
'''Stress''': the internal forces resisting the change in dimension caused by the load.
+
**75% Water
−
 
+
**Avascular: nutrients/waste move via diffusion
−
Cells use strain as a stimulus to adjust mass and architecture according to load. Resorption and osteogenesis happen concurrently to maintain bone integrity. This is mediated by two hormones:
+
**Perichondrium is composed of two layers:
−
 
+
***Fibrous: outer, dense irregular connective tissue
−
1. [[Calcium#Parathyroid Hormone (PTH)|'''Parathyroid hormone (PTH)''']]
+
***Chondrogenic: inner, flattened cells that differentiate to chondrocytes
−
 
+
*Types of Cartilage
−
Produced by <u>chief cells in the parathyroid glands</u> in response to <u>decreased</u> serum calcium. In response, osteoclasts increase in number and resorb mineralised matrix to increase Ca in blood.
+
**'''Hyaline Cartilage'''
−
 
+
***Most abundant in the body: glassy, translucent sheen
−
2. [[Calcium#Calcitonin|'''Calcitonin''']]
+
***Normally blue-white, smooth with moist surface, Turns yellow and becomes thinner in old age
−
 
+
***Found in nose, trachea, bronchi, ventral ends of ribs and sternal attachmont
−
Produced by <u>C-cells in the thyroid glands</u> in response to <u>increased</u> serum calcium. Inhibits osteoclasts.
+
***Surrounded by perichondrium
−
 
+
***At sites of articulation, providing resilient frictionless surface that resists compression
−
In neonates, bone growth predominates and modelling is important. In adults, the formation of bone is balanced by resorption - remodelling. It continues throughout life under the influence of hormones and mechanical pressure. Bone resorption may exceed formation in pathological states (hormonal, trauma, nutritional) or in old age and disuse.
+
***Found at epiphyseal growth plates
−
 
+
**'''Elastic Cartilage'''
−
==Links==
+
***Yellow appearance
−
Click here for information on [[Bone & Cartilage Development - Anatomy & Physiology|Bone and Cartilage Development]].
+
***found in auricular cartilage, larynx, eustacian tube, and epiglottis
−
 
+
***Surrounded by perichondrium
−
Click here for information on [[Bones - Pathology|bone and cartilage pathology]].
+
***Resiliance with added flexibility
−
 
+
**'''Fibrocartilage'''
−
 
+
***More collagen ('''Type I''') and less proteglycans than hyaline
−
{{OpenPages}}
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***Resists high tensional strain
−
[[Category:Bone and Cartilage - Anatomy & Physiology]]
+
***Often in transition with hyaline
−
[[Category:A&P Done]]
+
***Found in intervertebral discs, tendon/ligament attachment to bone, joint menisci, and articular surface of some joints (such as the temperomandibular)
 +
***NO perichondrium

Revision as of 10:28, 18 July 2008

BACK TO MUSCULOSKELETAL ANATOMY AND PHYSIOLOGY

Bone comprises the structure of the skeletal system and provide lever arms for locomotion. Bone also plays important roles in maintaining mineral homeostasis as well as providing the environment for hematopoesis in marrow.

Development of Bone and Cartilage

  • Osteogenesis
    • Intramembranous Ossification
      • Forms the flat bones of skull and mandible
      • No cartilaginous precursor: mesenchyme forms bone directly
        • Mesenchyme condenses, differentiates to pre-osteoblasts, then osteoblasts
        • Osteoblasts synthesize osteoid (collagen and proteoglycans)
        • Mineralization and Bone Spicule formation (spicules produce spongy bone)
        • Appositional Growth: laid down on the exterior (vs interior) surface of developing bone
      • Increasing association with blood vessels
      • Bone Marrow formed by mesenchyme between bone and blood vessels
    • Endochondral Ossification
      • Responsible for embryonic bone formation as well as growth in length
      • Via cartilaginous precursor:
        • Mesenchyme condenses to form bone outline
        • Core cells differentiate to chondroycytes and begin secreting cartilage matrix
        • Peripheral condensation forms perichondrium
        • Interstitial (length) and Appositional (width) growth
        • Central cells in developing diaphysis mature and hypertrophy
        • Matrix surrounding most mature chondrocytes calcifies
        • Perichondrium differentiates to osteoblasts
      • Capillary invasion to central core, forming trabecular bone
  • Bone Growth and Remodeling
    • Short bones: endochondral ossification continues in diaphysis until only cartilage rim remains
    • Long bones: secondary center of ossification develops in epiphyses
      • Epiphyseal Growth Plate: transverse disc of cartilate between the two ossification centers, allows longitudinal growth to continue to maturity, then closes
Growth plate (Image sourced from Bristol Biomed Image Archive with permission)
Growth plate magnified(Image sourced from Bristol Biomed Image Archive with permission)
      • Cartilage of epiphyseal growth plate is divided into: (from right to left on the magnified image)
        • - Resting (reserve) zone
        • - Proliferative zone
        • - Hypertrophic zone
    • Primary bone has not yet been remodelled
      • Contains less mineral and more randomly arranged collagen fibers, trabecular organization
    • Remodelling occurs by Haversian canals
      • Osteoclasts dig out canal, followed by blood vessel invasion
      • Concentric lamellae laid down
      • Phased resorption happens concurrently

Types of Bone

  • Long Bone
    • Found in the limbs and act as levers for locomotion
    • An elongated diaphysis (shaft) and two epiphyses (ends), each of which encases a center for ossification
  • Short Bone
    • Found in places of articulation, such as the carpus and tarsus
    • All dimensions are relatively equal, generally signifying one center of ossification
  • Flat Bone
    • Found in the skull, pelvic girdle, and scapula
    • Expand in two directions, with a broad surface for attachment of large muscle masses and protection of underlying structures
  • Sesamoid Bones
    • Eg. Patella, Navicular bone
    • Found within tendons, where they change direction over prominences that would otherwise cause damage
    • Form synovial joints with major bones with which they are in contact
    • Also serve to displace tendon from the axis of the joint, increasing leverage exerted by the muscle
  • Splanchnic Bones
    • Develop in soft organs remote from skeletal connection: eg. os penis
  • Pneumatic Bones
    • Excavated to contain air spaces, such as the skull in the instance of paranasal sinuses, and the post-cranial skeleton of birds

Composition of Bone

Histological structure of bone (Courtesy of RVC Histology images)
Microscopic bone (Courtesy of RVC Histology images)
  • Bone is a hard, highly specialised connective tissue
  • Consists of interconnected cells embedded in a calcified, collagenous matrix
  • Living, dynamic, responsive tissue, growing and remodelling throughout life

Bone is comprised of:

  • Matrix:
    • Osteoid: Organic component
      • Uncalcified, homogeneous substance, Stains light pink with H&E
      • "Type I Collagen" (90%), which resists tension
      • Bone-specific proteins(10%): Osteonectin, Osteopontin, Osteocalcin
    • Mineral: Inorganic component, provides rigidity
      • Crystalline lattice of calcium phosphate and calcium carbonate
      • Also contains Mg, Mn, Zn, Cu, Na, F
      • Accounts for 65% of bone
  • Cells:
    • Osteoblasts: single layer of mesenchymal cells which synthesize bone extracellular matrix (osteoid)
      • When active, appear plump and cuboidal, with basophilic cytoplasm
        • Cell membranes are rich in alkaline phosphatase (ALP)
        • Possibly involved in pumping calcium across membranes
        • Promoted by growth factors
        • Have receptors for PTH: contract in response -> provide space for osteoclasts to attach
      • When inactive - less cytoplasm -> flattened
    • Osteocytes: osteoblasts embedded in their own matrix; reside within lacunae and are interconnected via channels forming canaliculi
      • Canaliculi create connections to form a huge neural-like junctional organization
      • Contact osteoblasts and each other with cytoplasmic processes
      • Reach through canaliculi in mineralised bone matrix
      • Regulate composition of bone fluid
    • Osteoclasts: giant (multinucleate monocytes) cells which act to resorb bone ECM
      • Histologically:
        • Large, often multinucleated cells
        • Acidophilic cytoplasm
      • Derived from haematopoietic stem cells
      • Sit in bone surface depression - Howship's lacuna
      • Respond to vitamin D by increasing their numbers and activity (parathyroid independent)
      • Responsible for bone resorption
        • Firstly dissolve mineral followed by collagen, using brush border
        • Do not have receptors for PTH
        • Have receptors for calcitonin
        • Involute their brush border in response
    • Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from stromal fibroblastic system (osteoprogenitor cells); osteoclasts from haematopoietic system

Organization of Bone

  • Woven bone:
    • "Random weave" which is only a normal feature in the foetus
    • Coarse collagen fibres
    • Later removed by osteoclasts and replaced by lamellar bone
    • In adults it is a sign of a pathological condition (e.g. fracture, inflammation, neoplasia)
  • Lamellar bone:
    • Orderly layers which are much stronger than woven bone
    • Fine collagen fibres in concentric or parallel laminae
    • Two main types:
      • Compact bone (cortical)
        • Sheath covers external surface of long bone, thicker in shaft and thins over epiphyses
        • Comprised of thin lamallae in a series of concentric tubes arranged around small central canals (collectively known as an osteone)
        • Forms 80% of total bone mass
        • Consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate
        • Forms the shell of long bone shafts - contain Haversian systems
      • Cancellous bone (spongy or trabecular)
        • Stacks of parallel or concentrically stacked sheets arranged as rods, plates, and arches
        • In vertebrae, flat bones and forms the hematopoeitic center of epiphyses of long bones
        • Contains no Haversian systems
  • Laminar bone
    • Formed on periosteal surface of diaphysis
    • Accommodates rapid growth of large dogs and farm animals
    • Plates of woven bone from within the periosteum
    • Concentric plates
    • As it forms, it fuses with the bone surface

Periosteum and blood supply

  • Periosteum:
    • Specialised sheath of connective tissue covering bone except at the articular surfaces
    • Loosely attached except at tendon insertions and boney prominences (associated with major blood vessels penetrating bone)
    • Histologically:
      • Outer layer - fibrous for support
      • Inner layer - osteogenic
    • Rich supply of nerves and lymph vessels
    • Nutrient, metaphyseal, periosteal arteries
    • Normal flow of blood from medulla to periosteum due to higher pressures in medulla
    • Young animals have greater blood supply
  • Endosteum lines the marrow cavity
  • Medullary Cavity and Cancellous Interstitium: bone marrow storage and production
    • Red Marrow: richly vascularized, gelatinous tissue with hematopoeitic properties found abundantly in young animals
    • Yellow Marrow: fat infiltration converts red marrow to yellow, causing hematopoeitic properties to dwindle

Biomechanics of Bone

  • Wolff's Law: Bone architecture adapts in response to the loads applied upon it according to mathematical laws
    • Load: the external force placed on a structure, F
    • Strain: the proportional change in the structure's dimensions
    • Stress: the internal forces resisting the change in dimension caused by the load
  • The normality of bone architecture and appearance is directly related to its loading history
  • Cells use strain as a stimulus to adjust mass and architecture according to load
  • Resorption and osteogenesis happen concurrently to maintain bone integrity

Structure and Function of Cartilage

  • Function: resist compression, provide resilience and support at sites where flexibility is desired
  • Structure:
    • Chondrocytes: reside within lacunae within ECM synthesizing matrix
    • Type II Collagen (except fibrocartilage)
    • Proteoglycans with associated glycosaminoglycans
      • Continually turned over
      • The most vulnerable component of cartilage
      • Decresed proteoglycan -> loss of lubrication -> collagen disruption -> frays, clefts, fibrillation, ulcers, exposure of bone, eburnation, +/- osteophytes and joint mice
    • Hyaluronic acid
    • 75% Water
    • Avascular: nutrients/waste move via diffusion
    • Perichondrium is composed of two layers:
      • Fibrous: outer, dense irregular connective tissue
      • Chondrogenic: inner, flattened cells that differentiate to chondrocytes
  • Types of Cartilage
    • Hyaline Cartilage
      • Most abundant in the body: glassy, translucent sheen
      • Normally blue-white, smooth with moist surface, Turns yellow and becomes thinner in old age
      • Found in nose, trachea, bronchi, ventral ends of ribs and sternal attachmont
      • Surrounded by perichondrium
      • At sites of articulation, providing resilient frictionless surface that resists compression
      • Found at epiphyseal growth plates
    • Elastic Cartilage
      • Yellow appearance
      • found in auricular cartilage, larynx, eustacian tube, and epiglottis
      • Surrounded by perichondrium
      • Resiliance with added flexibility
    • Fibrocartilage
      • More collagen (Type I) and less proteglycans than hyaline
      • Resists high tensional strain
      • Often in transition with hyaline
      • Found in intervertebral discs, tendon/ligament attachment to bone, joint menisci, and articular surface of some joints (such as the temperomandibular)
      • NO perichondrium
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