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CT bone and osteogenesis Flashcards

1
Q

General functions of bone (6)

A

-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)

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2
Q

Main characteristic of bone tissue

A

a mineralised ECM (containing both inorganic and organic material)

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3
Q

Compare and contrast cartilage and bone

A

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

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4
Q

What is the organic portion of bone ECM composed of?

A

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

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5
Q

What is the inorganic portion of bone ECM composed of?

A

35% OF WEIGHT:

-crystals of hydroxyapatite (calcium and phosphate): arranged along collagen type 1 fibrs an keep bone stiff

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6
Q

What is the function of the existing multiadhesive GPs in matrix

A
  1. OSTEONECTIN: binds hydroxyapatite and collagen
  2. OSTEOPONTIN: binds hydroxyapatite and integrins
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7
Q

What is the function of the bone specific proteins of the osteoid

A
  1. OSTEOCALCIN: stimulates calcification
  2. MATRIX GLA: inhibits vascular calcification

!! they are vitamin K dependent proteins

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8
Q

What cell types are present in bone tissue (5)

A
  1. Osteocyte
  2. Osteoblast
  3. Osteogenic stem cells
  4. Osteoclast
  5. Bone lining cells
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9
Q

Periosteum vs endosteum

A

-periosteum: sheath outside bones that vascularises, innervates and provides cells that help osteogenesis

-endosteum: membrane lining the center of bones, containing BM

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10
Q

Osteoprogenitor cells description and location

A

-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

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11
Q

Osteoblasts decription and location

A

-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)

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12
Q

Differences between active and inactive osteoblasts

A

ACTIVE: basophilic cytoplasm, abundant RER/Golgi (secretory function), small cytoplasmic PAS+ granules

INACTIVE: flat cells instead of cuboidal

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13
Q

Osteocytes

A

-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

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14
Q

Bone lining cells

A

-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

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15
Q

What are the 3 functional stages of an osteocyte?

A

!! differ depending on the stage of osteocytic remodelling

  1. QUIESCENT: low RER/Golgi, calcified matrix layer is apposed to membrane
  2. FORMATIVE: more RER/Golgi, matrix deposition due to osteoid vesicles within lacunae, nucleus in basal portion of cell
  3. RESORPTIVE: abundant RER/Golgi, contain lysosomes

!! quiescent when dormant

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16
Q

Osteocyte remodelling definition

A

Use of mechanoreceptors to respond to physical stimulation and cause bone loss/gain

DECREASED stimulus: loss
INCREASED stimulus: gain

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17
Q

Osteocyte remodelling process under increase of mechanical stimuli (5 STEPS)

A
  1. Mechanical simulus input to the bone
  2. Interstitial fluid loss through canaliculi –> generates transient electrical potential
  3. Potential opens voltage gated channels of Ca in osteocyte membrane –> causes increase in conc of certain molecules that favour bone formation
  4. Opening of hemichannels to release substances
  5. IGFs also released to cause differentiation of osteogenic stem cells into osteoblasts

INCREASE IN BONE MASS

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18
Q

Osteocyte remodelling under decrease of mechanical stimuli

A
  1. Mechanical stimulus reduction sensed
  2. Release of MMPs (matrix metalloproteinases)
  3. Reverse remodelling of the pericanalicular space

DECREASE IN BONE MASS

19
Q

Osteoclasts description and location

A

-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

20
Q

3 specialised memberane layers of osteoclasts while ACTIVELY resorbing bones

A
  1. RUFFLED BORDER: direct concact with bone, membrane infoldings (increases area for exocytosis of enzymes and H+ secretion). Mitochondria and lysosomes.
  2. 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
  3. BASOLATERAL REGION: allows exocytosis of digested material
21
Q

Classification of bones

A

WOVEN (IMMATURE): either arranged in intertwined bundles (fetal skeleton) or as parallel bundles

LAMELLAR (MATURE): compact bone or spongy bone

22
Q

4 types of bone shapes

A
  1. long (femur)
  2. short (heel)
  3. flat (sternum)
  4. irregular (verterbrae)
23
Q

Location of spongy bone and compact bone in diff shaped bones

A
  1. LONG: outer shell of compact and inner epiphysis of spongy
  2. SHORT: outer compact surface, internal spongy area
  3. FLAT: layer of spongy bone between 2 layers of compact bone
24
Q

Structure of lamellar compact bone

A

-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

25
2 methods used to study compact bone
1. 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 2. GROUND BONE: preservation of organic component, sample cut using saw and ground up until a suitable thickness !! LACUNAE AND CANALICULAE ARE VISUALISED IN BLACK
26
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
27
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
28
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
29
2 types of bone formation
1. intramembranous ossification: proliferation of existing mesenchymal cells into osteoblasts and osteocytes 2. endochondral ossification: formation of cartilage from mesenchymal cells and then replacement of that cartilage with bone
30
What are the types of bone growth?
1. Appositional growth: associated with increase in age, responsible for increase in diameter/ thickness of bones 2. Longitudianl/Epiphyseal growth: increases length and occurs in the epishysis of long bones
31
What are the types of bone remodelling?
1. Bone resorption via osteoclasts 2. Mineralisation via osteoblasts 3. Osteocytic remodelling (increase or decrease in mass depending on mechanical stimuli)
32
Process of bone resorption (6 STEPS)
1. 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 2. Osteoclast activated and attached to bone via podosomes to form Howships lacunae (resorption bays) 3. Secretion of H+ ions via ATP dependent proton pumps which break down H2CO3 into H+ and HCO3- via carbonic anhydrase 2 4. Digestion via release of lysosomal hydrolases (CATEPSIN K) and MMPs 5. Causes the osteoid (organic portion) to be degraded into amino acids and sugars 6. Products released into blood circulation
33
Process of bone mineralisation (4 STEPS)
1. Release of matrix vesicles by osteoblasts containing alkaline phosphatase enzymes (phosphate ion hydrolysis from other molecules) 2. Formation of nanocrystals of CALCIUM HYDROXYAPATITE --> surround the matrix vesicles 3. Full mineralisation of matrix vesicles that surround collagen fibers 4. Over time - full mineralisation and hardening of bone (and reupture of central matrix vesicles) !! 1/2: Vesicular phase 3/4: Fibrilar phase
34
Process of intramembranous ossification (7 STEPS)
1. Mesenchyme condensation into clusters 2. Differentiation into osteoblasts 3. Synthesis of osteoid by osteoblasts (just collagen and ground substance) 4. Differentiation into osteocytes to form ossification centers 5. Organisation into trabecular matrix and periosteum 6. Condensation of blood vessels intothe RBM 7. Development of compact bone superficial to trabecular bone
35
what bones undergo intramembranous vs endochondral ossification?
IM: most craniofacial bones + clavicle EC: other bones (especially long bones)
36
Process of endochondral ossification (11 STEPS)
1. mesenchymal stem cells form chondroblasts that synthesise cartilage matrix 2. Hyaline cartilage containing lacunae trapped chondrocytes forms the template (in the shape of the bone needed) 3. Region in the bone center becomes vascularised which changes the perichondrium into periosteum (hence cells go from chndrogenic to osteogenic) 4. Synthesis of osteoblasts forms periosteal bone collar around diaphysis 5. Calcification of matrix due to hypertrophy (swelling) of chondrocytes 6. Death of chondrocytes due to inhibition of nutrient diffusion - produces cavities in the bone 7. Blood vessels and hematopoietic cells reach cavities --> forms primary ossification center 8. Creation of spicule: mixture of calcified cartilage and calcified bone layers 9. Growth via APPOSITIONAL GROWTH 10. Secondary ossification center forms via same process in epiphysis 11. Development via LONGITUDINAL GROWTH !! in all cases the deposition process is: OSTEOBLAST - OSTEOID - IMMATURE - MATURE
37
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
38
Fracture repait in bones (4 STEPS)
1. Hematoma formed by deposition of blood clot at the site of fracture 2. Soft callus formation: fibrocartilagenous callus (hyaline & fibrocartilage) 3. Bony callus formation: mineralisation of fibroartilagenous callus into bony callus of immature bone 4. Bone remodelling: via osteoblasts and osteoclasts --> immature into mature (harder) bone
39
Definition and components of a bone remodelling unit
BRU: interaction os osteoclasts and osteoblasts to degrade/mineralise bone 1. CUTTING CONE: site of osteoclast action 2. CLOSING CONE: site of osteoblast action
40
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
41
Pathologies associated with high osteoclast activity
-osteoporosis -rheumatoid arthritis -paget disease
42
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
43
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

Histology (12 decks)

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