CT bone and osteogenesis Flashcards
General functions of bone (6)
-skeletal function (mechanical support)
-protects organs
-locomotion (connections to tendons)
-site of energy storage (lipids in yellow BM)
-hametopoiesis in RBM
-mineral storage (calcium for homeostasis)
Main characteristic of bone tissue
a mineralised ECM (containing both inorganic and organic material)
Compare and contrast cartilage and bone
SIMILARITIES (4):
-both are specialised CT
-both have ECM of inorganic and organic material
-Cells trapped within lacunae
-Posess a bilayer CT covering (perichondrium/ periosteum)
DIFFERENCES (2):
- bone has more abundant cells than ECM
-bone is highly vascularised
What is the organic portion of bone ECM composed of?
65% OF WEIGHT: CALLED THE OSTEOID:
-colalgen type 1 (90%)
BONE GROUND SUBSTANCE: all proteins that are not collagen (10% of osteoid):
-proteoglycan aggregates
-decorin and biglycan: regulate collagen assembly and bone mineralisation)
-Multiadhesive GPs: osteonectin and osteopontin
-Bone specific proteins: osteocalcins and matrix GLA protein
-GFs and cytokines: IGFs, TNF, TGF, PDGF, inteleukins
What is the inorganic portion of bone ECM composed of?
35% OF WEIGHT:
-crystals of hydroxyapatite (calcium and phosphate): arranged along collagen type 1 fibrs an keep bone stiff
What is the function of the existing multiadhesive GPs in matrix
- OSTEONECTIN: binds hydroxyapatite and collagen
- OSTEOPONTIN: binds hydroxyapatite and integrins
What is the function of the bone specific proteins of the osteoid
- OSTEOCALCIN: stimulates calcification
- MATRIX GLA: inhibits vascular calcification
!! they are vitamin K dependent proteins
What cell types are present in bone tissue (5)
- Osteocyte
- Osteoblast
- Osteogenic stem cells
- Osteoclast
- Bone lining cells
Periosteum vs endosteum
-periosteum: sheath outside bones that vascularises, innervates and provides cells that help osteogenesis
-endosteum: membrane lining the center of bones, containing BM
Osteoprogenitor cells description and location
-derived from mesenchymal cells
-required for osteogenesis
-differentiate into osteoblasts (triggered by the TF RUNX2, and stimulated by IGF1/2)
PROGENITOR –> PREOSTEOBLAST –> OSTEOBLAST
LOCATION: single layer under the periosteum and endosteum
Osteoblasts decription and location
-secretes osteoid of ECM (collagen and bone proteins)
-differentiation potential into osteocytes
-single layer lying apposed to forming bone
calcification (via secretion of matrix vesicles)
-can be active or inactive (which changes cell morphology)
-10/20% differentiate into osteocytes when surrounded by ECM, others undergo apoptosis, others become inactive (bone lining cells)
Differences between active and inactive osteoblasts
ACTIVE: basophilic cytoplasm, abundant RER/Golgi (secretory function), small cytoplasmic PAS+ granules
INACTIVE: flat cells instead of cuboidal
Osteocytes
-differentiated osteoblasts
-90/95% of bone cells
-enclosed within lacunae surounded by mineralised matrix
-have dendritic morphology
-cell process are enclosed with CANALICULI (gap junctions and hemichannels that allow communication with osteoblasts)
-contain 3 functional stages
Bone lining cells
-flat shaped osteoblasts that cover the bone surfaces where neither bone resorption nor formation occurs
-nucleus is thin and flat, low RER/Golgi
-activity is determined by the needs of teh bone upon change
What are the 3 functional stages of an osteocyte?
!! differ depending on the stage of osteocytic remodelling
- QUIESCENT: low RER/Golgi, calcified matrix layer is apposed to membrane
- FORMATIVE: more RER/Golgi, matrix deposition due to osteoid vesicles within lacunae, nucleus in basal portion of cell
- RESORPTIVE: abundant RER/Golgi, contain lysosomes
!! quiescent when dormant
Osteocyte remodelling definition
Use of mechanoreceptors to respond to physical stimulation and cause bone loss/gain
DECREASED stimulus: loss
INCREASED stimulus: gain
Osteocyte remodelling process under increase of mechanical stimuli (5 STEPS)
- Mechanical simulus input to the bone
- Interstitial fluid loss through canaliculi –> generates transient electrical potential
- Potential opens voltage gated channels of Ca in osteocyte membrane –> causes increase in conc of certain molecules that favour bone formation
- Opening of hemichannels to release substances
- IGFs also released to cause differentiation of osteogenic stem cells into osteoblasts
INCREASE IN BONE MASS
Osteocyte remodelling under decrease of mechanical stimuli
- Mechanical stimulus reduction sensed
- Release of MMPs (matrix metalloproteinases)
- Reverse remodelling of the pericanalicular space
DECREASE IN BONE MASS
Osteoclasts description and location
-multinucleated and very large cells
-responsible for bone degradation
-derived from haematopoietic progenitor cells (CMP) which then differentiate into monocytes and then osteoclasts.
-Affected by factors released: M-CSF and RANKL
-regulated by calcitonin
-contain 3 specialised membrane domains
LOCATION: surface lining of trabeculae in resorption bays called HOWSHIPS LACUNAE
3 specialised memberane layers of osteoclasts while ACTIVELY resorbing bones
- RUFFLED BORDER: direct concact with bone, membrane infoldings (increases area for exocytosis of enzymes and H+ secretion). Mitochondria and lysosomes.
- SEALING ZONE: area on each side of the ruffled border that attaches osteoclast to the bone surface, site of resorption and degradation, ring arranged actin –> provides tight seal between plasma membrane and mineralised matrix of bone
- BASOLATERAL REGION: allows exocytosis of digested material
Classification of bones
WOVEN (IMMATURE): either arranged in intertwined bundles (fetal skeleton) or as parallel bundles
LAMELLAR (MATURE): compact bone or spongy bone
4 types of bone shapes
- long (femur)
- short (heel)
- flat (sternum)
- irregular (verterbrae)
Location of spongy bone and compact bone in diff shaped bones
- LONG: outer shell of compact and inner epiphysis of spongy
- SHORT: outer compact surface, internal spongy area
- FLAT: layer of spongy bone between 2 layers of compact bone
Structure of lamellar compact bone
-Haversian system composed of osteon units
-central haversian canal containing osteonal arteries
-Volkman canals: perforating canals through lamellae of osteons for communication
-osteocytes trapped in lacunae are present
-collagen fibers (orthogonally arranged per alternating lamellar ring)
4 layers of lamellae:
1. Concentric lamellae: cylinder lamellae surrounding haversian canal
2. Internal circumferential lamellae: border medullary cavity and are where trabeculae (of spongy bone) extend from
3. Outer circumferentia lamellae: adjacent to periosteum
4. Interstitial lamellae: within osteons, remnaments of previous concentric lamellae that have been remodelled - dont surround a haversian canal
2 methods used to study compact bone
- DECALCIFICATION: destruction of inorganic component via decalcification (putting bone in acid), tissue obstained is put through paraffin, stained with H&E
!!! USED TO VISUALISE ACIDOPHILIC COMPONENTS - GROUND BONE: preservation of organic component, sample cut using saw and ground up until a suitable thickness
!! LACUNAE AND CANALICULAE ARE VISUALISED IN BLACK
Structure of mature spongy bones
-lamellar trabeculae form network
-internal spaces filled with red bone marrow
-lined by endothelium
-osteocytes are connected by canaliculi
-receive their nutrietns from blood vessels in the BM (bcos trabaculae are very thin)
CROSS SECTION OF TRABECULAE:
lining cells are osteoblasts and osteoclasts
osteocytes found within lacunae between lamellae
!! NO OSTEONS OR HAVERSIAN CANALS
Structure of the periosteum
-dense CT
-contains vascularisation and innervation + lymph vessels
TRILAYERED:
-outer: anchored to bone surface, Sharpey fibers (collagen entering matrix for position stability)
-inner: contains blood vessels and fibroblasts
-deep: contains progenitor cells that diffferentiate into osteoblasts
Characteristics of woven bone
-immature
-contained mainly within fetal skeleton
-non lamellar
-random arrangement of cells
-higher proportion of bone cells compared to mature
-more ground substance in the matrix
-more intensely stained under H&E
2 types of bone formation
- intramembranous ossification: proliferation of existing mesenchymal cells into osteoblasts and osteocytes
- endochondral ossification: formation of cartilage from mesenchymal cells and then replacement of that cartilage with bone
What are the types of bone growth?
- Appositional growth: associated with increase in age, responsible for increase in diameter/ thickness of bones
- Longitudianl/Epiphyseal growth: increases length and occurs in the epishysis of long bones
What are the types of bone remodelling?
- Bone resorption via osteoclasts
- Mineralisation via osteoblasts
- Osteocytic remodelling (increase or decrease in mass depending on mechanical stimuli)
Process of bone resorption (6 STEPS)
- OSTEOCLASTOGENESIS: release of M-CSF and RANKL release by osteoblasts when a crack in bone is sensed –> promotes HSC to CMP to monocyte to active osteoclast
- Osteoclast activated and attached to bone via podosomes to form Howships lacunae (resorption bays)
- Secretion of H+ ions via ATP dependent proton pumps which break down H2CO3 into H+ and HCO3- via carbonic anhydrase 2
- Digestion via release of lysosomal hydrolases (CATEPSIN K) and MMPs
- Causes the osteoid (organic portion) to be degraded into amino acids and sugars
- Products released into blood circulation
Process of bone mineralisation (4 STEPS)
- Release of matrix vesicles by osteoblasts containing alkaline phosphatase enzymes (phosphate ion hydrolysis from other molecules)
- Formation of nanocrystals of CALCIUM HYDROXYAPATITE –> surround the matrix vesicles
- Full mineralisation of matrix vesicles that surround collagen fibers
- Over time - full mineralisation and hardening of bone (and reupture of central matrix vesicles)
!! 1/2: Vesicular phase
3/4: Fibrilar phase
Process of intramembranous ossification (7 STEPS)
- Mesenchyme condensation into clusters
- Differentiation into osteoblasts
- Synthesis of osteoid by osteoblasts (just collagen and ground substance)
- Differentiation into osteocytes to form ossification centers
- Organisation into trabecular matrix and periosteum
- Condensation of blood vessels intothe RBM
- Development of compact bone superficial to trabecular bone
what bones undergo intramembranous vs endochondral ossification?
IM: most craniofacial bones + clavicle
EC: other bones (especially long bones)
Process of endochondral ossification (11 STEPS)
- mesenchymal stem cells form chondroblasts that synthesise cartilage matrix
- Hyaline cartilage containing lacunae trapped chondrocytes forms the template (in the shape of the bone needed)
- Region in the bone center becomes vascularised which changes the perichondrium into periosteum (hence cells go from chndrogenic to osteogenic)
- Synthesis of osteoblasts forms periosteal bone collar around diaphysis
- Calcification of matrix due to hypertrophy (swelling) of chondrocytes
- Death of chondrocytes due to inhibition of nutrient diffusion - produces cavities in the bone
- Blood vessels and hematopoietic cells reach cavities –> forms primary ossification center
- Creation of spicule: mixture of calcified cartilage and calcified bone layers
- Growth via APPOSITIONAL GROWTH
- Secondary ossification center forms via same process in epiphysis
- Development via LONGITUDINAL GROWTH
!! in all cases the deposition process is: OSTEOBLAST - OSTEOID - IMMATURE - MATURE
What is the process of apositional growth?
-increases diameter of bone
-bone matrix is deposited within layers (circumferential lamellae) that are parallel to surface
-hence as the number of layers increases, the diameter increases
Fracture repait in bones (4 STEPS)
- Hematoma formed by deposition of blood clot at the site of fracture
- Soft callus formation: fibrocartilagenous callus (hyaline & fibrocartilage)
- Bony callus formation: mineralisation of fibroartilagenous callus into bony callus of immature bone
- Bone remodelling: via osteoblasts and osteoclasts –> immature into mature (harder) bone
Definition and components of a bone remodelling unit
BRU: interaction os osteoclasts and osteoblasts to degrade/mineralise bone
- CUTTING CONE: site of osteoclast action
- CLOSING CONE: site of osteoblast action
Endocrine control of bone remodelling
PTH: released from parathyroid when Ca levels are too low: promotes osteoclastogenesis for the resorption of bone and release of Ca into blood stream to increase to normal levels
CALCITONIN: released from C cells of thyroid when Ca levels are too high: inhibits effect of PTH on osteoclasts, so bone resorption is inhibited
Pathologies associated with high osteoclast activity
-osteoporosis
-rheumatoid arthritis
-paget disease
osteoporosis
-increase osteoclast activity which leads to increased resorption of bone
-thinner trabaculae
-weak bone so more frequent fractures
-can be type 1/2/3
paget disease
-misshapen bones and higher chances of fractures
-mainly occurs with increased age
-mineralisation occurs at a faster rate than absorption, but the mineralisation is faulty and causes mishapes and weaker structure
