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Organisation of the Body: Embryology > Limb development > Flashcards

Limb development Flashcards

1
Q

Limb - model system for human patterning formation

A 
Vertebrate limb = intricately patterned 
Obvious pattern/axes 
Dispensable 
Defects - easily recognisable 
(many rare genetic disorders affect limb patterning)
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2
Q

What is patterning?

A

Creation of specific structures according to a pattern (in specific locations)

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3
Q

Axes

A

Proximodistal: shoulder to fingertip
Dorsoventral: back to palm of hand
Anteroposterior - thumb to little finger

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4
Q

Proximodistal patterning

A

Development of limb bud
5 week period, week 4-8
Upper limb develop 1-2 days ahead of lower
End limb development - upper and lower almost synchronised

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5
Q

Initiation of limb bud development (P-D)

A

Cont. proliferation of parietal lateral plate mesoderm, stimulate by FGF 10
Upper limb bud: appear in lower cervical region at 24 days
Lower limb bud: appear in lower lumbar region at 28 days

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6
Q

Structure of limb bud

A 
Outer ectodermal (epithelial) cap 
Inner mesenchymal core - derive from parietal lateral plate mesoderm (bones + CT of limbs)
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7
Q

Apical ectodermal ridge (P-D patterning)

A

Ectoderm along distal border of limb bud thickens = AER at dorsal-ventral boundary of limb bud
Induced by underlying somatic mesoderm
Essential for outgrowth of limb

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8
Q

FGF feedback loop

A

FGF10 express in lateral plate mesoderm, induce AER, release FGF4 + 8, maintain FGF10 = continue outgrowth of limb bud (maintain progress zone)

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9
Q

Undifferentiated/progress zone

A

AER exert inductive influence on adjacent mesenchyme - cause remain popn. undifferentiated rapidly proliferating cells

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10
Q

Proximodistal development of limb

A

Limb grows, cells farther from AER start differentiating (cartilage + muscle)
Develop proximodistally to 3 components:
Stylopod, zeugopod, autopod

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11
Q

Stylopod

A

Humerus and femur (most proximal)

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12
Q

Zeugopod

A

Radius, ulna + tibia, fibula

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13
Q

Autopod

A

Carpals, metacarpals, digits/ tarsals, digits/ metatarsals (most distal)

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14
Q

Week 6

A

Mesenchymal core form 1st hyaline cartilage models
Terminal portion of limb bud flattens = hand plates/ foot plates (separate from proximal segment by circular constriction)
Second constriction - divide proximal portion into 2 segments
Cont. outgrowth influence by 5 segments of AER
Condensation of mesenchyme = cartilaginous digital rays
Death tissue between rays
Complete set of cartilaginous models (end week 6)

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15
Q

Digit formation

A

Cell death separates AER into 5

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16
Q

Limb rotation: Week 7

A

Upper - 90 degree laterally
Ext. muscles on lateral, posterior surface, thumbs lateral
Lower - rotate 90 degree medially
Ext muscles on anterior surface, big toe medial
Produce spiralling dermatomes

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17
Q

AER experiments (chicks): P-D patterning

A

AER = diff. programming abilities depending on stage limb development
(1) Remove AER:
Early - Stylopod only
Mid - S + Z
Late S + Z + A
(2) Replace AER w/ bead of FGF = restore normal limb
(3) Remove AER + progress zone from early limb development (S + Z) and transplant into late = S + Z + S + Z + A
(4) Remove AER + PZ from late and transplant to early = truncated limb (e.g. S + Z + hand)
FGF = signal and patterning

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18
Q

Thalidomide

A

May affect cells in progress zone, not know how long in contact w/ AER
As limb grows out, cells drop out of PZ and differentiate - thalidomide may disrupt
Result = phocomelia (absence/ deformity long bones, intestines + cardiac anomalies)

19
Q

Patterning signalling centres

A

Patterning needs 1+ spatially localised signalling centres
Multiple centres specify position
Limited range signal molecules used and reuse in developmental patterning
Cells respond in cell-type specific (CTS) way to signals, active CTS transcription program
Cells already partially specialised - allow limit range of signals for wide range of cell types
Develop higher eukaryotes need establish many signal centres

20
Q

HOX genes

A

Transcription factors
High functional conservation of homologues (same relative positioning) in craniocaudal axis
Express at different points along body axis
Multiple copies in humans - reduce affect of mutations, redundancy
As limb bud grows, functional domains progressively restricted:
HOX9/10 express in stylopod
HOX11 = Zeugopod
HOX13 = autopod

21
Q

HOX11 knock in mouse

A

Show broad expression throughout distal limb bud mesenchyme early in development
Expression rapidly restricted to zeugopod

22
Q

Regenerate tail stump of balloon frog

A

Treat w/ retinoic acid
Induce expression of HOX genes
Regenerate legs

23
Q

Control of anteroposterior patterning

A

Zone of polarising activity (ZPA): cluster mesenchymal cells at posterior border of limb
Secrete Shh signal (originally thought retinoic acid)

24
Q

Sonic hedgehog (Shh)

A

Contribute to specification of anteroposterior axis
May induce TBF-beta signals
Morphogen = diff. affect on cell fate at diff concentrations

25
Q

ZPA mirroring (experimental)

A

ZPA on posterior side
Transplant another ZPA onto anterior = mirror image duplication (creates opposing Shh concentration gradient to original)

26
Q

How do we know Shh is the signal from ZPA? (experimental)

A

Put Shh gene into cells, replace ZPA w/ cells, see if mirror image duplication occur
Experiment w/ concentrations by changing no. cells

27
Q

Dorsoventral patterning (experimental)

A

Dorsal ectoderm = signalling centre

Chicks: remove, rotate, replace dorsal ectoderm = invert axis

28
Q

Wnt7a knock out mouse (dorsoventral patterning experimental)

A

Ventral structures on both sides (pads)

Wnt7a - induce activity of Lmx1 (TF)

29
Q

Lmx 1 mutation

A

Nail-patella syndrome:
No patella
Nails not fully formed

30
Q

Tbx transcription factors (experimental)

A

Tbx5 - specification of forelimb
Tbx4 - specification of hindlimb
Express both in same limb = 1 limb bud forms fore and hind limb
Put Tbx4 into forelimb of chick = become hindlimb
Mutate Tbx 3 and 5 = meromelia (partial limb loss)

31
Q

Bone development

A

Mesenchyme in bud condenses, cell differentiate to chondrocytes
Week 6 = first hyaline cartilage models from by chondrocytes
Endochondral ossification begin at end embryonic period: start at primary ossification centres diaphysis (long bone), progress to end of cartilaginous model
At birth: diaphysis = completely ossified, epiphyses (ends) = cartilaginous
Secondary ossification centres form in epiphyses, temporary cartilaginous plate between diaphyseal and epiphyseal ossification centres = epiphyseal plate - allow bones to grow after birth
Endochondral ossification cont. both sides of plate
Bone reach full length - epiphyseal plate disappear, epiphyses join w. shaft

32
Q

Development of different types of bone

A 
Long bone: epiphyseal plate at each extremity
Smaller bone (e.g. phalanges): plate at one extremity 
Irregular bone (e.g. vertebrae): several primary and secondary ossification centres
33
Q

Hypo-secretion of growth hormone

A

Pituitary dwarfism

34
Q

Endocrine control of postnatal longitudinal bone growth

A

Growth hormone - secrete by anterior pituitary gland

35
Q

Hyper-secretion of growth hormone

A

Giantism - XS longitudinal bone growth, originate in childhood before epiphyseal plates ossify

36
Q

Limb musculature

A

Derive from dorsolateral cells of somites (myotome), migrate into limb to form muscle
Muscle components initially segmented according to somite cells derive from
Elongate limb bud: muscle tissue split to flexor (ventral) and extensor (dorsal) components
+ splittings/ fusions = 1 muscle form from +1 original segment
Complex pattern of muscle determine by CT derived from lateral plate mesoderm

37
Q

Segmental positioning of limbs

A

Upper limb bud: opp. lower 5 cervical + upper 2 thoracic segments
Lower limb bud: opp. lower 4 lumbar + upper 2 sacral segments

38
Q

Innervation of limbs

A 
Buds form, ventral primary rami (branch from spec. spinal nerve) penetrates mesenchyme
Form plexus (brachial = upper, lumbosacral = lower)
Then form dorsal/ventral branches derived from spec. spinal segment 
Branches in retrospective divisions unite = large dorsal/ ventral nerves: radial nerve = combine dorsal segmental branches (supply extensor), ulnar, median nerves = combine ventral branches (supply flexor muscles) 
Nerve enter limb bud and immediately establish contact w/ differentiated mesodermal condensation: need early contact between nerve and muscle cells for functional differentiation, innervate nearest unoccupied territory of muscle, rostral nerves innervate proximal muscle first, subsequent nerves move distally along limb to innervate unoccupied muscle
39
Q

Spinal nerves

A

Important for differentiation/ motor innervation of limb muscles AND sensory innervation of dermatomes
Original dermatomal pattern change growth/rotation of extremities

40
Q

Limb abnormalities

A

Genetic + environmental factors
Reduction defects
Duplication defects
Malformation/ dysplasia

41
Q

Reduction defects (limb abnormalities)

A 
Amelia = complete absence 1+ extremity 
Meromelia = parts missing 
Ectrodactyly = absence digit
42
Q

Duplication defects (limb abnormalities)

A

Polydactyly = + 5 fingers (often lack proper muscle connections)

43
Q

Malformation/ dysplasia (limb abnormalities)

A

Syndactyly = malformed digits (fail remove mesenchyme between digits)

44
Q

Cleft hand and foot (clinical)

A

Patterning defect
Rare genetic cause (e.g. Wnt10B)
Abnormal cleft between 2nd and 4th metacarpal bones
3rd metacarpal + phalangeal bones almost always absent
Thumb/index + 4th/5th fingers = fused

Organisation of the Body: Embryology (4 decks)

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