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3 > Limb development and bone formation > Flashcards

Limb development and bone formation Flashcards

1
Q

Skull origin

A

Mesenchyme surrounding the brain

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2
Q

Viscerocranium

A

facial bones surrounding neck viscera
forms through endochondral and intramembranous ossification
forms from mesenchyme of head (neural crest ectoderm - ectosoderm)

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3
Q

Neurocranium

A

skull bones

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4
Q

Base of skull

A

endochondral ossification

formed from somites

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5
Q

Flat bones of skull

A

Intramembranous ossification
Forms from mixture of head mesenchyme and somites
Induced by neural tissue
No brain formation - lack of skull formation

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6
Q

Limb formation

A

begins in 4th week, vulnerable period: 5.5-7 wks
Position controlled by homeobox genes
1)Two paddle-shaped limbs grow outwards on either side
Core of dense mesenchyme surrounded by epithelium
2) Differentiation begins with condensation of mesenchyme into cartilage, then eventually bone - occurs simultaneously with muscle differentiation
3) Tips of limbs become paddle-shaped in week 6

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7
Q

Mesenchyme of limbs

A

primarily from myotomes of somites, some lateral plate cells, some neural crest cells

  • neural crest cells: pigment and Schwann cells
  • lateral plate cells - cartilage and bone
  • myotomal cells - muscles
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8
Q

Epithelium of limbs

A

called apical ectodermal ridge (AER)
AER interacts with mesenchyme and causes it to continue growing, mediated through the release of several fibroblast GF’s

Loss or damage to AER: cause amelia/meromelia

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9
Q

Digit formation

A

Week 6 - apoptosis between digits to form fingers and toes
Controlled by activity of retinoic acid on zone of polarizing activity (ZPA) to release Sonic hedgehog and bone morphogenetic proteins

Improper gradient of ZPA/sonic hedgehog —> polydactyly or syndactyly

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10
Q

AP axis of limbs

A

present from time of limb bud formation

determined by sonic hedgehog genes

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11
Q

DV axis of limbs

A

determined later in development

Determined by Wnt7

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12
Q

PD axis of limbs

A

maintained by Wnt7

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13
Q

Limb rotation

A

elbow rotates so that it points posteriorly

Hind limb rotates so that knee joint points anteriorly

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14
Q

Limb innervation

A

Forelimbs grow out from cervical region of spinal cord
Hind limbs grow out from lumbar region of spinal cord
Week 5
Only neurons that find the appropriate target survive

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15
Q

Limb blood supply

A

Each somite has segmental artery
Arm - brachial`
Leg - profunda femoris

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16
Q

Somites

A 
42-44 in total
4 occipital
8 cervical
12 thoracic
5 lumbar
5 sacral
8-10 coccygeal
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17
Q

Parts of somites

A

Dermatome
Myotome
Sclerotome - becomes bone

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18
Q

Vertebrae formation

A

Each forms from fusion of sclerotome of 4 somites
Myotome connects vertebral discs
Mesoderm left between vertebrae forms annulus fibrosus
Notochord forms nucleus pulposus of vertebral disc and centrum of vertebrae

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19
Q

Myotome division

A

Myotome splits into two groups in thorax and abdomen
Epaxial divison - back and neck muscles, innervated by dorsal rami of spinal nerves
Hypaxial division - trunk and limb muscles, innervated by ventral rami of spinal nerves

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20
Q

Muscle development

A

Most use mesenchymal cell precursors from somites

1) begins in week 4-5
- muscle cell precursors form and undergo migration to face, septum transversum, trunk, developing limb buds
2) week 5-6
- muscle cell begins differentiation
- fusion of myoblasts –> formation of multinucleated myotubes
- begin to synthesize actin and myosin
3) week 9 - month 5
- nuclei migrate to outside of myotube
- actin and myosin organized into contractile elements
- primary myotubes form without nerve cell involvement
- secondary myotubes require nerve cell involvement

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21
Q

Formation of synovial joints

A

Mesenchyme in centre of developing limb condenses and releases bone morphogenetic protein –> causes mesenchyme to develop into cartilage, then bone

Noggin: secreted by regions that form synovial joints, antagonizes BMP
- apoptosis –> formation of fluid-filled space, becomes the synovial joint

Surrounding mesenchyme condenses into ligaments of joint capsule and tendons

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22
Q

Intramembranous ossification

A

Develops directly from mesenchyme
does not use cartilage model
For flat bones in skull and face, mandible, clavicle

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23
Q

Stages of Intramembranous Ossification

A

1) Aggregation
2) Osteoblast trapping
3) 3D network of spongy bone
4) remodelling

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24
Q

Aggregation - IM ossification

A

1) mesenchymal cells migrate and aggregate
2) differentiate into osteoprogenitor cellsa nd osteoblasts
3) osteoblasts produce osteoid
- proteoglycans and type I collagen
- also produce alkaline phosphatase, which induces mineralization by causing ppt of calcium and phosphate salts

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25
Q

Osteoblast trapping - IM ossification

A

1) Osteoblasts get trapped in mineralized matrix they produce
2) Calcification of matrix and formation of spicules

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26
Q

3D network of spongy bone - IM ossification

A

1) mesenchyme condense to form periosteum
2) initially - osteoid is laid down in random arrangement –> woven bone, will be remodelled later
3) Osteoblast produce more bony matrix
- generate a lattice network
- appositional growth (growth outwards)
- vascularization of spongy bone brings in bone marrow and osteoclasts

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27
Q

Remodelling - IM ossification

A

Osteoclasts remodel bone

  • trabecular just deep to periosteum gets remodelled to compact bone collar
  • trabeculae on inside of bone gets remodelled to spongy bone
  • formation of collar of woven bone, later replaced by lamellar (compact) bone
28
Q

Endochondral ossification

A

cartilage model precursor

used by bones of extremities, axial skeleton that bear weight

29
Q

Stages of endochondral ossification

A

1) development of hyaline cartilage model
2) bony collar development
3) chondrocyte death, blood vessel invasion
4) formation of primary ossification centre
5) growth of endochondral bone
6) secondary ossification centres form
7) skeletal maturity

30
Q

Development of hyaline cartilage model - EC ossification

A

1) aggregation and differentiation of mesenchymal cells into chondrocytes
2) Chondrocytes produce cartilage matrix - forms model of bone
3) Two types of growth will occur:
- interstitial: growth in length due to chondrocyte division
- appositional: growth in width due to chondrocyte differentiation from perichondrium, resulting in more cartilage deposition

31
Q

Bony collar development - EC ossification

A

1) perichondral region starts to produce osteoblasts instead of chondrocytes
2) Perichondrium becomes periosteum
3) Collar becomes an osteogenic area, developing a collar of bone around the diaphysis and transitioning from a perichondrium to periosteum

Formed through intramembranous ossification

32
Q

Chondrocyte death/BV invasion - EC ossification

A

1) formation of bony collar cuts of blood supply to cartilage in diaphysis –> hypertrophy
2) Hypertrophic chondrocytes produce AP - induce cartilage ECM to calcify and mineralize
3) calcification results in nutrient cut off to chondrocytes - death
4) Blood vessels then grow through bony collar into diaphysis of bone

33
Q

Formation of primary ossification centre - EC ossification

A

1) mesenchymal cells migrate along blood vessel and differentiate into osteoprogenitor cells in marrow cavity
2) Osteoprogenitor cells contact calcified cartilage plates, become osteoblasts and lay down osteoid
3) get mixed spicules of bone and cartilage (differentiate using Mallory stain)

34
Q

Growth of endochondral bone - EC ossification

A

1) Diaphyseal marrow cavity enlarges, pushes up toward epiphyses
2) get distinct zone of cartilage at each end = epiphyseal cartilage
3) Epiphyseal growth plate - division between diaphyseal cavity and cartilage
- site where avascular cartilage is converted to vascularized bnoe
- continues until skeletal maturity
- responsible for growth in length of bones

35
Q

Formation of 2ndary ossification centre - EC ossification

A

1) develop in the same way as primary ossification centre
2) Bone laid down on calcified spicules, leaving primary spongy bone - cartilage is resorbed
3) Happens shortly after birth
- 2ndary ossification centres appear in proximal eiphysis
- same process as in diaphysis
- formation delayed after primary

36
Q

SKeletal maturity - EC ossification

A

1) proliferation of cartilage in epiphyseal plate continues until skeletal maturity achieved
2) deposition of bone until cartilage is gone
3) epiphysela and diaphysela marrow cavities become confluent
4) epiphyseal closure/elimination of epiphyseal growth plate
5) only remaining cartilage is bone found on articular surfaces

37
Q

Growth plate layers

A

1) zone of reserve cartilage
2) zone of proliferation
3) zone of hypertrophy
4) zone of calcified cartilage
5) zone of resorption

38
Q

Zone of reserve cartilage

A

1
Closest to epiphysis
normal, resting chondrocytes
no proliferation/active matrix production

39
Q

Zone of proliferation

A 
2
Actively dividing chondrocytes
cells get larger
organize into columns
cells actively produce collagen and matrix proteins
actual lengthening
40
Q

Zone of hypertrophy

A

3
Accumulate glycogen granules in cytoplasm
secrete type 1 and type X collagen
synthesize AP

41
Q

Zone of calcified cartilage

A

4
hypertrophied chondrocytes degenerate
cartilage matrix becomes calcified
calcified cartilage –> scaffold for new bone formation

42
Q

Zone of resorption

A

5
Closest to diaphysis
in contact with marrow cavity
small blood vessel invasion - brings osteoprogenitor cells
Undergo differentiation to osteoblasts
Begin deposition of bone onto calcified cartilage
get mixed spicules, eventually replaced by spongy bone

43
Q

Bone modeling

A

How bone attains its adult shape
Growth in length: endochondral ossification - proliferation of cartilage at growth plates
- occurs at long bone epiphyses
Growth in girth: periosteal growth, at long bone diaphysis

Metaphyseal inwaisting - adding length and manipulating circumference to maintain bone shape
Modified by envelopes and attachments - tendons, capsules

44
Q

Bone metabolizing unit

A

Unit bone where bone formation is coupled to bone resorption
Consists of osteoblasts, osteoclasts and osteocytes arranged in:
- osteons in cortical bone
- trabeculae in spongy bone

45
Q

Formation of osteons

A

Form when capillaries invade cortical bone/woven bone
Cutting cone model:
- osteoclasts move through dense bone down axis of diaphysis, degrading bone and producing a tube of empty space
- osteoblasts follow and lay down concentric lamellae

in immature woven bone - primary osteon
in matural cortical bone - secondary osteon

46
Q

Wolff’s law

A

bone will adapt to the load it is placed under
stress causes strain on bone - mixture of tensile and compressive stress
- tensile: osteoclast activity
- compressive: osteoblast activity

47
Q

Types of bone healing

A

Primary: no motion at fracture site
Secondary: motion at fracture site

48
Q

Primary bone healing

A

Due to stress fractures, reduction of fractures surgically

  • 20% direct contact between fragments
  • no fracture callus
  • gap filled with woven bone
  • cutting cones then cross fracture site to create new osteons
49
Q

Secondary bone healing

A

1) Inflammation
2) Repair
3) Remodelling

50
Q

Inflammation - 2ndary bone healing

A

1) begins immediately, continues ofr a couple weeks
2) Hematoma develops
3) Necrotic tissue is absorbed
4) Bleeding becomes source of progenitor cells
5) granulation tissue forms at site of fracture
6) proliferation of osteoblasts and fibroblasts

51
Q

Repair - 2ndary bone healing

A

begins within 2 weeks, for weeks-months

1) formation of bridging soft callus: non-ossified cartilaginous and fibrous tissu
2) replaced by a hard callus via endochondral ossification

52
Q

Remodelling - 2ndary bone healing

A

1) starts part way through repair phase, continues for years
2) bone assumes a more normal shape
3) woven bone replaced by laminar bone through Haversian remodelling
4) based on stress

53
Q

Influences on fracture healing

A

General: age, comorbidities (diabetes, vascular disease), nutrition, function, smoking

Mechanical: amount of energy/trauma, anatomical location, fracture stability, bone type (scaphoid and clavicle take a long time), bone loss

Biologic: soft tissue envelope health, infection, drugs (osteoporosis drugs, NSAIDs)

Biochemical/hormonal

54
Q

Metaphyseal healing

A

Cortex is thing - minimal external callus laid down
Tissue goes through similar staging but callus is formed within bone
Radiographs: sclerotic fracture line
Large external callus would interfere with joint function

55
Q

Diaphyseal healing

A

Cortex is thick - large external callus visible on radiographs

56
Q

Remodelling of childhood bone deformities

A

Potental greater with:

  • young children
  • fracture close to growth plate
  • deformity in the plane of mortion

rotational deformities do not remodel well

57
Q

Osteoporosis

A

Loss of bone density/imbalance in osteoclast and osteoblast activity

Role of estrogen

  • inhibits osteoblast apoptosis and increases lifespan –> increased osteoblast deposition
  • mediates oxiative stress on osteoblast
  • mediates RANKL-induced differentiation of osteoclasts, ensures there isn’t too much differentiation that would increase osteoclast activity
58
Q

Calcium-regulating hormones

A

Low Ca in blood:

  • PTH stimulates osteoclasts to resorb bone and liberate Ca
  • bind osteoblasts, stimulates osteoblasts to express RANKL and downregulate OPG expression

High Ca:

  • calcitonin stimulates osteoblasts to take calcium out of blood for deposition
  • binds osteoclasts, disrupts cytoskeleton and inhibits its resorptive ability
59
Q

Bone components

A 
inorganic (65%)
hydroxyapatite
calcium
phosphorus
magenesium
citrate
potassium
sodium
organic (35%)
type I collagen
PGs
multi-adhesive glycoproteins
bone-specific vitaminK dependent proteins
GHs and cytokines
60
Q

Periosteum structure

A

Outer layer: Fibroblasts, type I collagen, nerve, blood vessels
Inner: blood vessels, osteoprogenitor cells, osteoblasts

anchored to the bone by collagen fibers

61
Q

Endosteum

A

membranous lining covering the inner surface of compact bone and spongy bone
often 1 layer thick - osteoprogenitor cells, bone-matrix secreting cells, bone-lining cells
Osteoprogenitor + bone-lining cells = endosteal cells

62
Q

Fracture fixation

A

intramedullary nail fixation

doesn’t stabilize it completely - callus still forms

63
Q

Healing time

A 
fracture in metaphyseal: 6-8 weeks for a reasonable heal
2x in elderly
1/2 children
2x cortical bone
2x open fracture
2x smoker
2x non-compliant patient
64
Q

TGF beta

A

transforming growth factor

induce mesenchymal cells to produce type II collagen

65
Q

IGFII

A

stimulates type I collagen
cartilage matrix synthesis
bone formation

66
Q

PDGF

A

platelet-derived growth factor
released from platelets
attracts inflammatory cells to fracture site

67
Q

Hormonal influences on bone healing

A

TH/PTH - increase callus, affect remodelling
Cortisone - decreases callus proliferation
GH - increases callus volume
Calcitonin - effects unclear

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