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ANAT 381 > Ryan Lecture 5 > Flashcards

Ryan Lecture 5 Flashcards

1
Q

Describe compartment

A

A module of embryo that consists of a group of adjacent cells that do not mix with cells from neighbouring compartments
Display same gene expression profile - same molecular/genetic address, express similar patterns of gene

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2
Q

Describe compartment boundary

A

Border/region between 2 compartments that cells do not cross - stay with adjacent cells
Boundaries may be invisible = defines compartments, molecular boundaries

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3
Q

Describe 1970s discovery

A

Devleopemnt of techniques to create genetic mosaics in flies
Revealed boundaries that did not correspond with morphological landmarks = expressing genes in diff sub domains

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4
Q

Describe clones of cells within drosophila wing

A

Clones of cells within a compartment have jagged borders = proliferate across
Clones of some cells have straight border - at a-p boundary =does not align with wing vein, cells do not cross line

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5
Q

What are the 2 critical functions that boundaries perform

A

Prevent intermingling of cells = maintain population fo cells, cannot migrate
Provide positional info to flanking cells

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6
Q

Describe how to visualize compartment boundaries

A

Lineage marker
Shows that cells do not mix across boundary

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7
Q

Describe cell adhesion properties at compartment boundaries

A

Extra cellular proteins secreted by ecm =
Qualitative = diff sides secrete diff molecules, cells recognize other cells as diff
Quantitative = express diff levels of molecules

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8
Q

Describe the effect of diff amounts of E-cadherin = compartments and boundaries

A

Qualitative, p cad =red, e cad = green
E-cadherin = tighter
When p cad > e cad = cels move towards inside
When p cad = e cad = more intermixed
When p cad < e cad = red on outside, excludes red, so green matches wth cells that have higher affinity (green)
LEVEL OF PROTEIN CASUES SEGREGATION

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9
Q

Forming compartments and boundaries = 1

A

FIELD Of cells becomes subdivided by their interpretation of a morphogenetic gradient
Cells on left see low level morphogenetic
Cells on right see high level morphogen, cells =turn on diff patterns of gene expression

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10
Q

Forming compartments and boundaries = 2

A

Morphogen gradient induces transcription factors, adhesion/affinities
Response causes repression of that signal in cells

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11
Q

Forming compartments and boundaries = 3

A

Subdivisions maintained and refined by local cell cell interactions - short range signalling
Gradual refinement by feedback
Expresses one or other
Interface between= where border forms

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12
Q

Forming compartments and boundaries = 4

A

2 distinct populations = leads to formation of specializes cells at borders = boundaries
Cells are diff= recognize cells that look like them and cells that do not, not some level or types of factors

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13
Q

Forming compartments and boundaries = 5

A

Boundary influences surrounding cells - long range signalling - to regulate growth and patterning
Forms organizer region sometimes
Emits signals
Cells on either side respond diff bc diff patterns (tfs)

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14
Q

Forming compartments and boundaries = Gen

A

A= gradient, with threshold levels
B = inhibition
C= represses other cells = creates distinct domains

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15
Q

Where do organizers form

A

At compartment boundaries

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16
Q

What do organizers do

A

Communicate info to neighbouring compartemts by releasing signalling molecules or their inhibitors
Long range signaling
Short range signalling
Initial = morphogen gradient across cells, differential responses, at interface = set up new organizer region, new long and short range signalling

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17
Q

Give 3 ex’s of developmental compartments

A
  1. Drosophila = stripes in embryo, hox genes and ap compartments, wing
  2. Somites
  3. Vertebrate cns and hind brain (fore and midbrain)
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18
Q

Describe drosophila pair rule genes - developmental compartments

A

Stripes of eve and ftz
Quickly defined info
14 stripes of expression

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19
Q

Describe drosophila pair rule genes - gen

A

Stripes of pair rule genes turn on segment polarity genes
Components of wnt and shh signaling
Segment = contains eve and ftz
Parasegemnt = eve or ftz + GAP
(Eve and ftz come on in response to earlier patterning)
Wg = turn on in gap, wingless, wnt fam
En = posterior to wg, turn on engrailled

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20
Q

Describe drosophila pair rule genes - specifics

A

Parasegment = anterior end of one stripe to anterior end of next stripe
Wingless expressed between stripes eve and ftz
Engrailed expressed in cells expression eve and ftz but only 1 cell/stripe; 14 rows
Cells expressing engrailed is the posterior end of each segment

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21
Q

Describe drosophila pair rule genes - interaction between engrailed and wingless

A

Wingless diffused = signal
Sees wingless signal
Cells competent to respond
Releases shh and binds to patched receptor =
Turned on another morphogen = now have repressive effects on each other

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22
Q

Describe drosophila pair rule genes - gradient

A

Level of signals = determines if turn n bristle or not
Signal = engrailed makes shh, shh and wingless repress activity of cells responding to ether
Shh = high ant, low post
Wg = low ant, high post

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23
Q

Describe vertebrate segments - allow for

A

Repetition to form
Makes them diff - if have ribs or nah

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24
Q

What does segmentation do

A

Provides a developmental mechanism for evolution of increasingly sophisticated structures, species specific, has diff patterns

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25
Q

What are somites

A

Masses of mesoderm formed from presomitic mesodermal
Repetitive structures along a-p axis
Anterior somites older than posterior somites

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26
Q

Describe somite formation rate

A

1 pair of somites formed every 90 mins in chick, 55 pairs in total in chick

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27
Q

Describe somite differentiation - gen

A

Differentiate to give rise to dermis, skeletal muscle, and vertebrae
Presomitic paraxial mesoderm proliferates, as ages differentiates =
Hypaxial dermomyotome, layer outer
Inner layer = myotome
Sclerotome and myotome give rise to diff tissues

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28
Q

Describe transverse section through trunk of chick embryo on days 2-4

A

2 day = epithelialized, young
3 day = dermomyotome, sclerotome
Late 4 day = epithelial layer undergoes EMT transition as they migrate away,
Gives rise to diff structures

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29
Q

Describe somitogenesis in zebrafish embryo

A

Continuous process as embryo grows and extends in a-p axis

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30
Q

When is mesoderm formed

A

At end of grastrulation
Paraxial endosperm, gives rise to somite
Intermediate mesoderm = kidneys, and gland
Lateral plate mesoderm = l and r patterning, asymmetric gene expression

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31
Q

Describe mesoderm derivatives

A

Paraxial mesoderm —> somite —> sclerotome (cartilage), syndetome (tendons), myotome (skeletal muscle), endotome (endothelial cells, dorsal aorta), dermatome (Dermis, skeletal muscle)

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32
Q

Describe mesoderm formation

A

At end of gastrulation
BMP expressed bilaterally at lat plate mesoderm
In Center = notochord, high chord in (bmp antagonist)

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33
Q

What is important for parasail mesoderm specification

A

BMP signalling
Molecules = chordin - notochord, paraxis - somites, pax2 - intermediate mesoderm
Exp = transplant noggin secreting cells (same effect as chordin) in to lat plate mesoderm
Result = somites
Why = need low level bmp for somites to form, restricts somites to paraxial mesoderm

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34
Q

What else is also important for generating presomitic mesoderm

A

Tailbud
Cells here highly proliferative
Psm = presomitic mesoderm
Nmp = neuromesodermal progenitors (bipotential = cells contribute to neural tube and mesoderm - psm)
DMZ = dorsal marginal zone, has nmp cells

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35
Q

What is important for paraxial mesoderm differentiation

A

Tbx6
Expressed in psm
Get smaller as embryos grows
Most posterior end dorsal mrgainal zone
Wnt —> t —> tbx6 —> d2i
Tbx6 downstream wnt and t
Important for paraxial mesoderm patterning

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36
Q

Describe mouse tbx6 knockout

A

Paraxial mesoderm transformed into neural tube
Neural tube strained w/ sox2, mutant = ectopic neural tubes express sox2, lose identity as mesoderm cels and somite progenitors
Pax6 = tf expressed in DMZ of forming neural tube, mutant = also show dorsal ventral patterning with respect to pax6, ectopic neural tubes express px6 dorsally
Conclusions = TBX6 IN NMPS (neuromesodermal progenitors ) AND PSM (presomitic mesoderm) REQUIRED FOR SOMITE IDENTITY

37
Q

Describe somites in stage 6 chick embryo

A

Paired even and repeated
90 min cycle

38
Q

How do somites know when and where to form

A

Embryological clock measures dev
Rate of somite segmentation 1pr/90 min, must be coordinated with overall dev and occur in parallel on right an left sides
If move to cooler temp, somites take longer to form?

39
Q

Describe clock wavefront model of somite formation- when and where

A

Cooke and zeeman 1976
Intersection of system to regulate where a boundary will form (wavefornt) and when a boundary should form (clock)
Refer to wave front as differentiation front

40
Q

Describe role of retinoids acid in paraxial mesoderm development

A

High ant, low post

41
Q

Describe role of fgf8 in paraxial mesoderm development

A

High post, low ant

42
Q

Describe role of retinoids acid and fgf 8 in paraxial mesoderm development - gen

A

Antagonistic signals along the a-p axis pattern NMPS during paraxial mesoderm dev
Antagonists important in positioning of somites
Tbx6 represses sox2

43
Q

Describe role of retinoids acid and fgf 8 in paraxial mesoderm development - specifics

A

Retinoic acid high at anterior - raldh2 involved in RA synthesis
Fgf8 high at posterior
Mesp = expressed at boundary of low RA/low FGF front = indicates where next somite will form
Primarily expressed in psm

44
Q

What regulates epithelialization during somite boundary formation

A

Eph-Ephrin signalling

45
Q

Describe whole role of eph-ephrin signalling, steps 1-4

A
  1. Mesp restricted to anterior half somitomere minus 1
  2. Unregulated ephA4 in anterior half
  3. EphA4 upregulates binding partner ephrinB2 in presumptive posterior SO (somite about to form), post half
  4. Triggers epithelialization and formation of a boundary
46
Q

Describe role of eph-ephrin signalling, steps generally

A

S-1 mesp —> ephA4 then goes and to next half ant somite, and turns on ephrinB2 ahead
= ant s-1 and post s-0 somite epithelialization
Cells of psm = mesenchymal
But Turning on genes = epithelialization and detaches from psm and moves on

47
Q

When is ephA4 EXPRESSED

A

AS NEW somites form -chick embryos

48
Q

Describe where somites form

A

Opposing FGF/RA gradients —> mesp —> eph/ephrin

49
Q

Describe when somites forms

A

Only one boundary formed at a time

50
Q

What controls when somites form

A

Notch = time keeper
Juxtacrine signalling molecule

51
Q

Describe notch - time keeper of somites

A

Oscillates in segmentally define pattern
All species have at least 1 notch target gene that oscillates
Turns on and off v rapidly In cells
Waves of notch expression

52
Q

Describe notch - time keeper of somites NEGATIVE FEEDBACK

A

Notch target inhibits notch signaling
Inhibitor - target is unstable
When inhibitor gets degraded notch signalling comes back on
Waves of notch signaling creates clock

53
Q

How do waves of notch signalling connect to differentiation front

A

Targets of notch signalling = lunatic fringe, hairy (hairy enhancer of split) = genes that oscillate = important
Ant out of sync with post
Turns off and on in cells sequentially

54
Q

What is wave of genes expression due to - notch time keeper

A

Sequential activation of gene expression not due to movements of cells

55
Q

Describe whole somite formation process

A

1 cycle of notch/target takes ~90 minutes
Mesp - needs low RA and fgf, NOTCH target in anterior half repressed notch activity in anterior half…eph/ephrin.. met (mesenchyme to epithelial transition determines boundary)
High FGf at posterior prevents cells from being competent to respond to notch signal, so not form somites early, no mesp so no somites

56
Q

Describe compartments - somites and A-P axis

A

Reiterated compartments resemble one another but give rise to different derivatives, e.g. somites that form cervical vertebrae do not form ribs; but somites that form thoracic vertebrae do (determined by position along AP axis before somitogenesis)

57
Q

Recall = hox gene expression

A

Spatiotemporal colinearity of hox gene expression in psm
Layered on, turned on a-p, 3’earlier than 5’, ant post, identity of somites
IS CORRELATED WITH CHROMATIN REMODELING = genes come on at diff times, opens at 3’ end and then gradually progresses

58
Q

What do chicks have compared to mouse - vertebral pattern along a-p axis

A

Chicks have more cervical vertebrae

59
Q

What happens when you transplant presomitic mesoderm from region that would normally form thoracic vertebrae caudal (posterior) to the 1st somite in a younger embryo?

A

Transplantation exp
Psm of thoracic vertebrae from post, older embryo
Psm of cervical vertebra in chick embryo =
Gain of function, ant to posterior transformation
Has ribs - looks like thoracic, PSM ALREADY HAS PATTERNING INFO

60
Q

WHAT happens when transplant thoracic somites into cervical region

A

Ribs from on neck vertebrae, derived from thoracic somites
Carries a-p info already
Due to notch gene expresssion

61
Q

Transplant cervical somites into thoracic region

A

Ribs do not form on vertebrae derived from cervical somites
Chick embryo = flat, so easy to do on one side

62
Q

How do you stop making somites

A

Consider rate of oscillation vs rate of axis elongation (proliferation/wnt signal)
If axis elongation sustained in balance with clock rate = infinite #, if sustain and balance with wavefront would make get infinite somites
If axis elongation is less than clock rate = somite formation will use up PSM

63
Q

Is there a 1:1 relationship between somite number and vertebrae #

A

NAHHHH
Humans = 52 somites, 33 vertebrae
Mouse = 65 somites, 29 + 25 (tail) vertebrae
Snake = <500 somites, 100-300 vertebrae

64
Q

Describe snake vs mice

A

Snakes = long coil of somites

65
Q

What mechanisms/pathways can we alter to get more segments - snake case study

A

Tail boys elongation - more proliferation
Fgf, wnt, RA gradients
Notch delta signalling -rate of on/off gene expression
Eph-ephrin B2 epithelialization
Extra layers of patterning? Species specific things that overlay basic

66
Q

What questions can we ask- snake case study

A
  1. How could alterations in one of these mechanisms lead to changes in segment number to make the axis of a snake as compared to that of a mouse?
    2.What would be the ramifications to the other three mechanisms based upon your proposed change?
67
Q

What does data from corn snakes show - snake case study

A

They have 3 fold more oscillations in lunatic fringe expression in PSM compared with similar stage mouse embryos
More rapid oscillations

68
Q

What is fore vs hind limb

A

Fore = arms
Hind = legs

69
Q

Name all 3 limb axes

A

Proximal-distal
Dorsal-ventral
Anterior-posterior

70
Q

Describe proximal distal limb axis

A

Proximal = closest to the body - shoulder/pelvic girdle
Distal = tips of digits

71
Q

Describe dorsal ventral limb axis

A

Dorsal = back of limb/hand - top of foot
Ventral = front of limb/hand - bottom of foot
NAILS on dorsal side

72
Q

Describe anterior posterior limb axis

A

Mostly looked at relative to hand/foot
Anterior = thumb/big toe, digit 1
Posterior = pinky, little toe, digit 5

73
Q

Describe limb bud morphogenesis

A

Begins as outpouching of cells - limb buds
Limb bud = posterior lat plate mesoderm - skeletal precursors
Somite cells - muscles precursors
Overly ectoderm - outer layer, inside =mesoderms (lpm + somites)

74
Q

Do both hind and fore limb buds form at same time

A

Forelimb bud forms 1/2 days before hindlimb bud

75
Q

Describe transverse scanning electron micrograph through limb bud

A

Mesenchymal mesoderm cells
Thin ectoderm

76
Q

Describe limb bud days 9-11

A

Mouse
Limb buds continue to grow, get bigger
Day 33 in humans

77
Q

Describe limb bud E11

A

Apical ectodermal ridge - AER is now clearly visible = thicker ectoderm
Forms at dorsal ventral boundary

78
Q

Describe limb bud day 12

A

Limb bud becomes paddle shaped, flattens
Indentations that will become inter digit region are visible
Programmed cell death = apoptosis in inter digit region important to prevent webbing (some species need webbing tho like ducks)
Human = 36 days

79
Q

Describe digits growth in humans

A

Between days 48 and 56
Web at 48 has
By 56 days most webbing gone

80
Q

What is limb bud formed by

A

Out pouching of lateral plate mesoderm at the correct position along a-p axis in parallel
Cells from dorsal region lat plate mesoderm =undergo epithelial to mesenchymal transition

81
Q

How is position of outpouching determine of limb bud - exp 1 1918

A

Detwiler and Harrison = showed removing groups of cells blocked limb bud growth

82
Q

How is position of outpouching determine of limb bud - exp 2 1925 / 1933

A

1925 = hertwig transplanted groups of cells to new places and go a limb, depends on stage and patterning info that comes with it
1925/1933 = balinsky - showed that optic vesicle (ear), pituitary (thickened ectoderm that will become pituitary) and/or olfactory sac (lining of nose) could induce limb bud in inter limb region (note = all derived from ectodermal placode = thickened region)

83
Q

How is position of outpouching determine of limb bud - exp 3 1971

A

Rosenquist = marked groups of cells and watched where they go = fate mapping

84
Q

Describe role of RA and FGF8 along a-p axis - limb bud

A

Hox genes expressed along a-p axis regulate expression of RA and FGF
RA and FGF8 act as antagonistic signals to induce Tbx5 expression (homeobox gene)
FGF8 high in ant region
RA high in forelimb field
HOXC6 gene
Opposing gradients

85
Q

What determines position of forelimb bud

A

Tbx5

86
Q

Describe RA and FGF8 knockout exp in limb bud

A

Looking at rare = RA response element cloned upstream LacZ gene - turns blue
Rdh10 knockout = block RA expression, get extension of FGF ant expression = moves boundary
Also lose expression Tbx5
In situ hybridization for FGF8 expression - can see it is more expressed in heart

87
Q

What signals position of Lat plate mesoderm outgrowth

A

Fgf10 = 4 blocks expression, where outgrowth, turned on after Tbx5
If transplant fgf beads or cells = can induce limbs
If plant close to forelimb = look like wing
If plant in middle = chimera, both features
If plant closed to hind = looks like leg

88
Q

What can FGF do - limb buds

A

Induce ectopic limb buds
Can induce normally patterned bud, with digits
Chick embryo

ANAT 381 (24 decks)

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