alveolar bone Flashcards
name the 4 cell types in bone and the % of mature alveolar bone they comprise
1) bone lining cells
2) osteoclasts = <1%
3) osteocytes = >90%
4) osteoblasts = <5%
name the origin of
a) bone lining cells
b) osteoclasts
c) osteocytes
d) osteoblasts
a) mesenchymal
b) haemopoietic
c) mesenchymal
d) mesenchymal
mesenchymal = from (ecto) mesenchymal stem cells called osteoprogenitor cells
haemopoietic = derived from blood monocytes / macrophages from hematopoietic progenitor cells in bone marrow
what are osteoclasts and what is their role
- resorbing surfaces of alveolar bone
- multinucleated giant bone resorbing cells
- specialised organ of resorption adjacent to bone surface (ruffled border)
- rich in acid phosphatase
- ruffled / brush border
what do osteoclasts lay in
HOWSHIPS LACUNAE (resorption concavities on resorbing surfaces)
what is the border structure of an osteoclast when actively degrading bone
RUFFLED / BRUSH BORDER
- adjacent to bone surface its degrading
- cell membrane deeply folded
- composed of many microvilli so large SA for resorption
how does bone resorption occur once osteoclast has been activated
2 stages
1) mineral phase removed / dissolved
2) remaining organic matrix removed
describe the steps in osteoclast bone resorptive action
1) osteoclasts attach to bone matrix @ surface of bone
2) create sealed acidic microenvironment by proton pump secreting protons across ruffled border = pumps h+ to create extracellular lysosome + dissolve bone mineral (like HAP)
3) exposes organic matrix in resorbing lacuna + its degraded by proteases (ie cathepsin K) (ruffled border pumps enzymes to degrade collagenous matrix)
4) endocytosis of organic degradation products @ ruffled border
5) degradation products transported in transport vesicles, released by exocytosis at membrane opposite ruffled border
what do osteoclasts do when under osteoblast control
express receptors for hormones
what are osteocytes and what do they represent
- lie within bone itself in lacunae + surrounded by calcified bone matrix
- represent osteoblasts trapped in bones organic matrix
- induce osteoclast activation
- receptors for parathyroid hormone
- secrete sclerostin
- role in local degradation of bone, affecting mineral + organic matrix
what are osteocytes in constant communication with
THEMSELVES, OSTEOBLASTS + BONE LINING CELLS through their cell processes ie via fine canniculi
what are the 2 functions of osteocytes
1) induction of osteoblast activation
2) primary mechano-sensors in bone = detect strain ie mechanical strain in bone
what system do osteocytes lie around
HAVERSIAN SYSTEM (osteon)
- osteocyte lacunae and numerous interconnecting canniculi around central haversian canal
- osteocytes sit in their lacunae in concentric rings of bone matrix (lamellae) around central Haversian canal and their long processes lie in canniculi (small channels)
what is a haversian system (osteon)
basic unit of compact bone
runs longitudinally
what are osteoblasts and what is their role
- specialised connective tissue cells (layer of them prominent as cuboidal cells on bone surfaces where there is active bone formation - line surface of forming bone)
- numerous cell contacts between cell membranes of adjacent cells + in contact w underlying osteocytes (gap junctions allow cell-cell comms)
- secrete + mineralise organic matrix (are encorporated into bone matrix as osteoCYTES)
- receptors for several hormones ie parathyroid hormone; affect bone by altering secretion of cytokines + growth factors
what do osteoblasts secrete
1) formative components of bone
2) molecules controlling its own activity
3) molecules w controlling influence in activating bone resorbing cells (osteoclasts)
4) osteoid (organic matrix of bone which becomes calcified after deposition) as several specific proteins = osteoid layer is PALE staining next to dark stained mineralised matrix
5) building blocks of collagen type 1 (lie parallel to bone surface in matrix)
6) cytokines + growth factors (regulate cell function + bone formation)
name an osteoblast function also carried out by osteocytes
mechano-sensing
- osteoblasts also detect mechanical strain on bone
describe the cells which bone lining cells, osteoclasts and osteoblasts are derived from
MESENCHYMAL STEM CELLS
- reside in bone marrow and region of proliferating cells adjacent to osteoblast layer in periosteum
- osteogenic precursors (in pdl + bone forming tissue) = associated with small blood vessels
- ectomesenchymal stem cells = similar properties but arise from neural crest cells
what is osteoid in bone formation
- osteoid = bone-like
- newly deposited UNmineralised bone matrix
- layer of it covers surfaces where active bone formation is occuring
- produced by osteoblasts (which form a well defined layer @ its surface)
- type I collagen fibrils arranged parallel to bone surface embedded in ground substance of proteoglycans, glycoproteins + other proteins
where and when does mineralisation take place in osteoid
- along linear mineralising front
- when 5-10um thickness is reached
what is the lag phase in bone formation
before deeper layer of osteoid matures sufficiently for mineralisation
deeper layers start to mineralise when 5-10um osteoid thickness is reached
what 2 incremental lines can be seen in bone
1) resting / mineralising line
2) reversal line
describe the resting / mineralising line
- smooth regular line w several running in parallel
- marks former surface of bone deposition
describe the reversal line
- irregular scalloped appearance
- fossilised line, outline reflects form positions of howships lacunae
- marks bone surface where resorption ‘reversed’ to formation (ie if resorption no longer required due to change in functional load on that area of bone)
- osteoclasts migrate away + osteoblasts form new bone on vacated surface
name the components of alveolar bone and the % of it that they make up
1) inorganic material = 60%
2) organic material = 25%
3) water = 15%
describe the inorganic, mineral material of alveolar bone
needle like impure HAP crystals (50nm wide, 8nm thick)
impregnate + surround collagen fibres (ie distributed in spaces between + on surface of collagen fibrils)
provide rigidity + resistance to compression
describe the organic material of alveolar bone
- 90% collagen type I = contributes to property of resisting loads + provides resilience (prevents fractures)
- 10% non-collagenous proteins (proteoglycan, glycoprotein, osteonectin, osteopontin, osteocalcin, growth factors, serum proteins)
- above all produced by osteoblasts
- trace amounts of collagen types III + V (esp in immature or healing bone)
what is organic matrix of alveolar bone
composite material of inorganic mineral and organic matrix
organic matrix = 90% collagen (mostly osteoblast secreted intrinsic collagen, EXTRINSIC collagen formed by pdl fibroblasts is inserted as sharpeys fibres)
- proteins derived from other sources (ie blood) inc cytokines + growth factors incorporated in bone matrix during development
what is alveolar bone
mineralised connective tissue which supports + protects teeth
what is bone remodelling
- bone can remodel + adapt to changing functional demands
- bone formation occurs @ same site following resorption
- no change in shape of bone bc of coordinated actions of bone cells
- resorption and formation at remodelling sites is under tight control to ensure balance between the 2 processes
list the 5 stages in the remodelling cycle and describe each one
1) resting state
2) resorption = due to recruitment, migration + activation of osteoclasts (by osteocytes)
3) reversal = resorption stops, apoptosis / migation away of osteoclasts
4) formation (longest) = osteoblast (remove unmineralised osteoid by releasing enzymes ie matrix metalloproteinases (MMPs)) recruitment, migration, differentiation, forms new bone in resorption site
phase triggered by factors produced by osteoclasts, mononuclear reversal cells, or released from resorbed bone matrix
5) resting = formation stops following mineralisation + surface is lined by flattened layer of cells
what is mature alveolar bone a combination of
what does this combination confer
- 80% compact (cortical bone) = forms a dense solid mass
- 20% cancellous bone = open meshwork / lattice arrangement of individual bony trabeculae that surround marrow, may be absent around some anterior teeth (allowing outer layer of alveolar plate + bone socket lining to fuse)
- confers max strength at min skeletal weight
what is the
a) inner layer
b) outer layer
of compact bone called
what lies between these 2 layers
where is the outer layer (compact bone) of this plate
a) inner / internal alveolar plate (lines tooth socket wall) = continuous w palatine process of maxilla in upper jaw + internal (medial) surface of body of the mandible in lower jaw so thicker on palatal side of upper teeth
b) outer / external alveolar plate = thicker on buccal of posterior teeth, variable round anterior teeth, thinner in incisor region
CANCELLOUS BONE between them
theres also an arbitrary boundary between alveolar process and body of the mandible
meets compact bone lining tooth socket at alveolar crest 1.5 to 2 mm below level of CEJ
what can we see on clinical radiographs and what is it
- lamina dura (cribiform plate of bundle bone)
- thin layer of compact bone which lines the tooth socket and gives attachement to principle fibres of pdl
reflects sieve-like appearance of vascular canals passing from alveolar bone into pdl
what is adult bone deposited in
3-5um thick lamellae (layers)
how are lamellae arranged in compact bone
2 major patterns
1) circumerential lamellae
- at external (periosteal) + internal (endosteal) surfaces
- lamellae in parallel layers surrounding bony surfaces
2) haversian system (osteons)
- deep to above
- lamellae arranged as small concentric layers around central neurovascular (haversian) canal = together w 20 concentric lamellae = forms haversian SYSTEM
what is a consequence of remodelling on compact / cortical bone
interstitial lamellae
- found between adjacent concentric lamellae
- represent remnants of previous partially resorbed Haversian systems + old circumferential lamellae (exist due to bone remodelling)
how are lamellae arranged in cancellous bone
- they form trabeculae (surround marrow spaces)
what is marrow
in young bone
- HAEMOPOIETIC
- contains stem cells of mesenchymal type (make fibroblasts, osteoblasts, adipocytes, chondroblasts and myoblasts) + of blood cell lineage (make osteoclasts)
in older bonw
- yellow
- haemopoietic potential lost
- increased accumulation of fat cells (fatty marrow)
what are sharpeys fibres
- extrinsic fibres that insert into cribiform bone plate perpendicular to surface
- derived from principle fibres of unmineralised PDL
- less numerous but thicker than those at CEMENTUM surface
- between them osteoblasts lay down new bone matrix
what is formed by varying degrees of mineralisation of sharpeys fibres
1) embedded fibres = may remain unmineralised at centre OR be fully mineralised
2) transalveolar fibres = enter bone in mesiodistal plane + pass straight through to become continuous w smaller fibres from root of adjacent tooth
= represent attachment of PDL into cribriform plate
describe the gross anatomy of alveolar bones
- part of maxilla or mandible that supports + protects teeth
- arbitrary boundary at level of root apices of teeth
- forms the inferior part of the maxilla (where consists of 2 parallel plates of cortical bone - buccal + palatal alveolar plates between which lay sockets of individual teeth)
- forms superior margin of horseshoe-shaped body of the mandible (here consists of buccal + lingual alveolar plates joined by the 2 septa)
- alveolar process is area of bone to level of root apices
what is the
a) interdental septa
b) interradicular septa
a) lie between each socket
b) divide roots of multirooted teeth
what is woven bone
- immature newly formed bone, fracture repair
what is lamellar bone
- mature fine fibred adult bone, replaces woven bone
how does woven bone differ from lamellar
smaller diameter
larger, irregularly spaced osteocytes
random collagen fibre orientation (widely spaces)
more non-collagenous matrix
larger + more bone forming cells (30% of vol of tissue compared w 2% for adult)
rapid matrix mineralisation, formation + turnover
enriched in phosphoproteins +BSP (lamellar = osteocalcin rich)
describe bone functions which show how bone tissues work synergistically during health to serve multiple functions
- in jaws (support for + protection of teeth)
- systemically (hormonal factors control bone physiology, locally mechanical forces (inc tooth movement), growth factors + cytokines = regulatory functions)
- adaptation (dynamic tissue, adapts based on physiologic needs, bone adjusts mechanical properties according to metabolic + mechanical requirements)
- HAP (regulates bone elasticity stiffness + tensile strength, skeletal adaptation mechanism executed by bone remodelling)
- cortical bone (mechanical/protective role)
- cancellous (trabecular bone) (mineral metabolism)
how do non-collagenous proteins of the organic matrix act
1) various functions act during bone formation OR following their release during bone resorption 2) binding some bind to collagen others bind to collagen and mineral some bind bone cells to matrix
what % of each type of bone undergoes YEARLY remodelling
3% of cortical
25% of cancellous
what are trabeculae
- formed by bone layers or lamellae in cancellous
- Lamellar bone forming them has scanty lacunae containing osteocytes
- arrangement along stress lines helps bone withstand forces while maintaining minimal mass
- 3D = network of interconnecting struts
define haemopoietic
capacity to form blood cells + cells derived from their lineage
what are neurovascular bundles
interconnect w one another and endosteum and periosteum via Volkmann canals
what is the lifespan of osteoblasts
1 month
- after = 30% become embedded in organic matrix as osteocytes + remainder flatten (lining cells) or die
what processes do osteocytes have
dendritic cytoplasmic processes
- star-shaped
- extend into thin channels (canaliculi) in mineralised matrix + contact processes from adjacent cells to form a cellular network
explain the role of osteocytes as primary mechanosensors
- detect deformation of cell processes (deformation of bone following loading deforms the cell processes/membranes of osteocytes directly or indirectly through movement of tissue fluid in lacuna-canalicular system
- transmit this info to other cells
- affect osteoblast activity by changes in sclerostin (glycoprotein secreted by mature osteocytes which inhibits osteoblasts + bone formation) expression
- Mechanical loading of bone decreases sclerostin expression, leading to upregulation of growth factors that stimulate osteoblasts
what is the lifespan of osteoclasts
10-14 days (only recruited when required)
after = apoptosis
what are osteoclasts responsive to
2) locally acting factors produced by osteoblasts (ie RANKL (receptor activated nuclear factor kappa B ligand - protein imp in osteoclast formation, function + survival)
3) osteocytes ie sclerostin
4) systemic hormones ie parathyroid hormone (have receptors for)
5) reduced pH + reduced o2 (hypoxia) in microenvironment (bone tissue)
explain what RANKL (receptor activated nuclear factor kappa B ligand) is
- protein imp in osteoclast formation, function, survival, expressed on cell membranes of odontoblasts)
- driver for fusion of mononuclear preosteoclasts into multinucleated osteoclasts and their activation
- Osteoblasts express RANKL, binds to its receptor, RANK, on surface of mononuclear osteoclast precursors and mature multinuclear osteoclasts
Activation of the RANK receptor
1) induces signalling cascade
2) leading to differentiation + fusion of osteoclast precursor cells and promotion of survival + activity of mature osteoclasts
required for
1) differentiation of precursors into multinucleated osteoclasts
2) activation of mature osteoclasts to begin bone resorption
where may stem cells for osteoblasts in pdl be derived from
- perivascular cells in the ligament
- adjacent bone marrow
role of mature osteoblasts in the remodelling cycle
1) produce more osteoprotegerin (OPG) (soluble decoy receptor protein for RANKL = blocks interactions between RANKL+ RANK + interferes w osteoclast formation so osteoblasts essential for osteoclast differentiation and function) + less RANKL
2) reduction in RANK/RANKL interactions
3) inhibition of osteoclast activity
4) allowing osteoblasts to refill resorption lacuna
what are 2 clinical considerations relating to alveolar bone
1) bone retention + resorption
2) implants
explain consideration of bone retention + resorption
- Functional loading required to stimulate alveolar bone remodelling (maintain normal bone structure)
- Load-induced strains in alveolar bone need to be intermittent (not continuous) through loading cycles (like in mastication)
- osteogenic response = depends on size of load, freq + rate of application
molecular mechanisms
1) forces impinging on bone = transduced into bone resorption or deposition
2) osteocytes + surface layer of osteoblasts/bone lining have role
bone loss
1) associated w reduction in mechanical loads acting on bone
2) ie tooth loss due to extraction / trauma is followed by gradual resorption of residual alveolar ridge
why is retaining natural teeth beneficial for bone retention + resorption
- bc maintain alveolar bone mass
- when anterior teeth have fractured = retained roots used to support a removable denture (this much tensile functional loading through PDL helps retain some alveolar bone)
maintenance of alveolar bone for bone retention + resorption depends on…
- maintenance of the alveolar bone dependent on presence of functioning teeth
- if teeth lost = alveolar bone resorbed, mandible severely reduced + strength compromised
- so fracture may be a problem
- height of alveolar crest (where teeth still present ) - even here its reduced WITH the area distally where teeth absent - change in bone illustrates its plasticity
- bones continually remodel by synchronised formation + degradation (response to changes in loads applied) + effect changes in morphology during development
- net bone loss if loss of loading on bone via teeth degradation outstrips formation
explain the consideration of dental implants
IMPLANT SUPPORTED CROWN
1) can be independent of adjacent teeth
2) therapeutically attractive bc stimulates bone
3) induce inc in density in response to functional load
what is osseointegration of dental implants
union between implant and adjacent living bone
titanium based materials most commonly used in jaws
explain the healing process of dental implants
1) once implant in cancellous bone healing occurs via blood clot formation, bone modelling, bone remodelling
2) initial blood clot contains factors for successful osteogenesis (cytokines, growth factors, osteoprogenitor cells)
3) fibrin clot replaced by granulation tissue (mesenchymal cells + blood vessels proliferate in this collagen fibre rich tissue)
4) osteoblasts (migrating from adjacent bone marrow) invade granulation tissue
osteoid matrix = surrounds blood vessels in mesenchymal tissues after 1 week
In osteoid = HAP deposition leads to woven bone formation
5) subsequent weeks = woven bone replaced by lamellar bone + marrow
