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lecture 14 Flashcards

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1
Q

what are examples of tissue morphogenesis

A

epithelial tube formation, branching morphogenesis, convergent extension and axis elongation

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2
Q

what is tissue morphogenesis controlled by

A

combo of intracellular signaling, proliferation, and active forces

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3
Q

what does intracellular signaling control

A

when and where these forces are created within the tissue

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4
Q

what is the biggest factor in tissue morphogenesis

A

active forces

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5
Q

what are active forces

A

forces produced by the material itself to change its own shape

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6
Q

why do we call embryonic tissue active material

A

because it can change its own shape during developmente

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7
Q

what two things happen in tube formation/convergent extension

A

flat surface of embryo invaginates and deepens to form primordial spinal cord/neural tube

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8
Q

what else happens

A

embryo elongates at one axis

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9
Q

when is this invagination/elongation process happening

A

as we go from gastrula stage to neurula stage

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10
Q

describe elongation

A

tube begins to form; embryo begins to get long at one axis and shorter in another axis

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11
Q

what is convergence

A

cells are coming together in one direction and elongating in another direction

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12
Q

what is convergent extension

A

flow of material towards the tube, towards the middle as it begins to elongate in perpendicular direction

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13
Q

what do many organs undergo during development

A

branching morphogenesis

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14
Q

describe branching morphogenesis in salivary glands

A

goes from embryonic nub, through cell divisions + clefting & branching, get grape like clusters of epithelial cells that secrete saliva down ducts into oral cavity

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15
Q

what do salivary glands undergo

A

rounds of clefting and branching –> go from single bud to a elaborate, highly branched epithelia

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16
Q

describe branching morphogenesis

A

branching splits a little, widens and forms a discrete cluster (we get distance b/w adjacent branches), repeats again and again to get smaller clusters of epithelial cells

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17
Q

what does basememt membrane separate

A

separates epithelial cells from region outside of organ

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18
Q

does basement membrane maintain its barrier while epithelial cells do their thing

A

yea duh

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19
Q

what is critical for organ development

A

branching morphogenesis

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20
Q

what is formed thru branching morphogenesis

A

salivary glads, organs like kidneys, lungs, blood vessels

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21
Q

what shapes tissues

A

physical forces

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22
Q

how do we measure physical forces in tissues

A

laser ablation, dual micropipette aspiration, traction force microscopy, atomic force microscopy

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23
Q

what does laser ablation measure

A

measures tension along cell-cell junctions thru recoil velocity

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24
Q

what is tension in a tissue reflective of

A

contractile activity within the cells making up that tissue

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25
Q

what can you do through cadherin junctions

A

trigger positive feedback loop to get a massive amount of actin contractility applied to junctions

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26
Q

how can we detect a cell that’s hella contractile or tense vs. one that isn’t

A

look at cells thru a microscope, label cadherin

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27
Q

what next (laser ablation)

A

use laser to break connections b/w cells; more tension cell is under, more it recoils

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28
Q

what happens if you cut connections in a cell w/ no tension

A

it doesn’t move

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29
Q

give an example of cell that’s under a lot of tension

A

developing drosophila embryo under hella tension

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30
Q

what does dual micropipette aspiration do

A

measures strength of cell-cell junctions

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31
Q

what is another relevant physical force

A

strength of adhesion; how strong are two cells bound to each other

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32
Q

what does strength of adhesion reflect

A

actin contractility

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33
Q

what does number of cadherins interacting w/ each other correlate w/

A

stronger adhesion between cells

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34
Q

how do we see how strong cells are sticking together

A

move cells around, let them stick and adhere

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35
Q

what is indirect way of measuring adhesion strength

A

how far you have to move the pipettes before you break the bonds

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36
Q

what does traction force microscopy do

A

measures force generated by cell-matrix adhesions (tensile stress)

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37
Q

what are traction forces

A

forces that strengthen and grow adhesions

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38
Q

what do traction forces do / help with

A

apply to extracellular environment, help pull the cell forward or remodel the extracellular environment

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39
Q

what else do traction forces do

A

align collagen fibers; or in wounds grab onto fibronectin that’s spilled out of blood serum, unfold it to reveal additional binding sites to promote adhesion during a wound healing response

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40
Q

how do we measure traction forces

A

flexible pillars; measure how much pillars deflect

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41
Q

describe how we measure traction forces

A

plate cells on flexible gell, put small beads, let cells attach & adhere to gel and apply traction forces to it

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42
Q

what is the distance the beads move when you kill the traction forces related to

A

how high those traction for es were

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43
Q

what happens if gel is being stretched by an enormous amount

A

beads are gonna move greater distance as the gel relaxes

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44
Q

basically what is it

A

beads move cuz of traction forces; stop the traction forces, beads will relax and go back to where they were before

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45
Q

what is atomic force microscopy

A

measures the material properties of individual cells w/ functionalized probe

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46
Q

what is another characteristic of cells

A

whether they are soft or stiff

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47
Q

what are soft cells

A

highly metastatic cells

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48
Q

what cells are stiff

A

normal epithelial cells

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49
Q

describe atomic force microscopy

A

use small probes –> how much force microscope has to apply to deform the cell is directly related to how stiff the cell is

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50
Q

what happens if the cell has to apply a lot of force to dent the cell

A

it would be stiff

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51
Q

what happens if the cell doesn’t apply as much force to dent it

A

cellis soft

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52
Q

what changes in a cell to cause it to become rigid or stiff

A

pressure & rigid membrane / cytoskeleton

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53
Q

what is the final way to measure forces

A

pressure

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54
Q

describe micropressure system

A

stick needle into pressurized compartment

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55
Q

what happens to pressure as cells move

A

pressure goes up and down when they are migrating vs. non-migrating

56
Q

where do forces need to be generated to cause a planar field of cells to become invaginated (to form neural tube)

A

exactly where the tube is going to form

57
Q

describe how the force is applied

A

not uniform; it’s a gradient, most strong in the center and drops off towards the edges

58
Q

what is another example of how forces can participate in developmental processes

A

traction forces

59
Q

what is important in epithelial tube formation

A

cell-cell junction force generation; cadherins, alpha catenin unfolding to activate force where the tube is gonna form

60
Q

what is an example of how traction forces can be used to create tissue (tendon)

A

collagen; type 1

61
Q

what happens during development when a tendon needs to form

A

there’s a lot of collagen deposited

62
Q

how is collagen arranged

A

randomly in embryonic tissue

63
Q

what happens when an embryonic fibroblast gets in the right area

A

it’s able to begin to remodel the matrix

64
Q

what is collagen remodeling done in response to

A

signaling, growth factor, physical environment it’s embedded in

65
Q

what does extracellular matrix act as

A

blueprint/scaffold to create the tissue

66
Q

what is next step in tendon formation

A

embryonic fibroblasts remodel the matrix to form that blueprint (where traction forces come in)

67
Q

what happens when you add a cell to this otherwise randomly arrayed collagen fibers

A

become less uniform

68
Q

what is step 2 of developing tendon

A

cells align the collagen matrix in addition to depositing new matrix

69
Q

what does aligned matrix do

A

forms scaffold for additional cells to come in, deposit more matrix, ultimately forming a tendon

70
Q

what is the key point

A

traction forces align the randomly deposited matrix at earlier step of development to form blueprint for where a tendon is ultimately gonna form to attach the muscle to bone

71
Q

how are stress fibers created to generate the traction forces

A

integrins are receptors that interact w/ collagen;

72
Q

where are integrins

A

on cytoplasmic face of membrane they’re connected to actomyosin thru talin & vinculin

73
Q

how do we get more contractility

A

positive feedback can be triggered from outside or inside to unfold talin, get bigger, stronger adhesions generating more traction forces

74
Q

what does tissue remodeling depend on

A

coordination of actin-mediated contraction w/ cell-cell adhesion

75
Q

when we talk about cell-cell junctions what are we talking about

A

bundles of actomyosin filaments near apical surface of epithelial cells (on sides)

76
Q

when we talk about contractility that shapes epithelial cells

A

we talk about bundle of actin filaments across epithelial cells that are connected at cadherins

77
Q

what do adherens junctions do

A

respond to forces generated by actin cytoskeleton

78
Q

describe how cadherins bind to each other

A

cadherins homophilic binding to the partner cadherin on adjacent cell

79
Q

how are cadherins plugged into actomyosin filaments

A

through alpha catenin (NOT talin)

80
Q

what is alpha catenin

A

the force sensor

81
Q

what happens if something were to pull on the cell or from inside the cell on the adhesion

A

alpha catenin would unfold, provide additional binding sites for vinculin, bring additional actomyosin fibers strengthening the adhesions junctions and increase tension in that part of the tissue

82
Q

how are those stress fibers created

A

formins

83
Q

what are formins activated by

A

rhoA which nucleates long straight actin filaments that will be bundled into stress fibers

84
Q

what is the cross linker

A

alpha actinin

85
Q

how does myosin enter

A

rhoA is activated, activates ROCK/MLCK, phosphorylates myosin regulatory chains in neck region of myosin heavy chain –>binds actin, unfolds tail so it does bipolar filaments that slip into bundled by alpha actinin

86
Q

how does myosin generate force

A

myosin conformational cycle driven by ATP hydrolysis release and binding

87
Q

when we talk about branched actin network, what does it mean

A

Rac1 arp 2/3, 70* angles etc.

88
Q

what is needed in epithelial tube formation

A

something has to happen contractility wise to get that initial invagination, coupled with proliferation & other forces that get epithelial tube formed

89
Q

where is belt of actomyosin fibers located

A

towards apical surface (top of epithelial cells but still on the sides)

90
Q

how is belt arranged

A

continuous belt along the perimeter of the epithelial cell

91
Q

what happens when it becomes activated

A

everything contracts uniformly around the perimeter (purse-string mechanism), cell becomes narrower overallw

92
Q

is the contractility uniform ALL around the cell?

A

no, other wise whole cell would shrinkw

93
Q

where is this contractiion

A

near the top; top gets narrower while the bottom stays relatively wide

94
Q

what does apical constriction lead to

A

tube formation and branching morphogenesis

95
Q

describe drawstring mechanism

A

top gets narrow, bottom stays relatively wide

96
Q

what is consequence of drawstring mechanism

A

narrower top, wider bottom –> exactly what it takes to get it deformed from flat to that first invagination

97
Q

what does purse string mechanism / apical constriction lead to

A

triggers a domino effect to get the full tube formation

98
Q

describe the shape of it

A

cone-like shape

99
Q

what does this process depend on

A

cadherin cell-cell junctions (connected to actomyosin filaments in a full belt along the perimeter of the cell (interconnected in adjacent cells)

100
Q

what happens when cells contract

A

you get non-uniform change in shape that leads to first invagination and first step on the way to epithelial tube formation

101
Q

what does tube formation require

A

proliferation and actin-mediated contraction

102
Q

how does that contractility get activated in that local zone? what changes in cells at cell-cell junctions to get the cells to become activated

A

(local) paracrine signaling activates rhoA, rhoA activates actomyosin contractility by applying a bit of force (increasing myosin a little bit), unfolds alpha catenin to get positive feedback loop

103
Q

what happens after positive feedback loop

A

all of a sudden cells are contracting at their apical surfaces, entire tissue is invaginating in that one little strip

104
Q

what does this process lead to

A

epithelial tube formation

105
Q

describe rhoA activity of other parts of embryo

A

lower rhoA activity

106
Q

what does convergent extension do

A

elongates developing tissues

107
Q

what is convergent extension

A

what happens as the embryo begins to deform form a round spherical object to a lengthening embryo

108
Q

where does convergent extension happen

A

all over the region of cell that’s converging and extending

109
Q

how does convergent extension lead to elongation of tissues

A

hundreds, thousands of cells in tissue are doing this continuously leading to flow in one direction and extension in the other

110
Q

what happens to blue cells

A

come together and form a connection

111
Q

what happens to green cells

A

get pushed apart in the opposite, perpendicular direction

112
Q

what happens to entire embryo as this occurs

A

shrinks top and bottom, elongates left and right

113
Q

what do cells do

A

come together to and bottom, go apart left and right

114
Q

what is the net result of convergent extension

A

macroscopic change to embryo where it begins to lengthen dramatically in one axis

115
Q

where does molecular machinery come into play

A

cell-cell junctions

116
Q

what happens to one set of cell-cell junctions

A

goes away, as the other set is being created

117
Q

why is one set of cell-cell junctions eliminated

A

to allow blue cells to be able to make contact w/ each other

118
Q

what is the final step

A

they form a new cell cell junction

119
Q

what happens to cell junctions

A

junction between gray cells is dissolved, junction between green cells forms

120
Q

what is end result

A

tissue shrinks in one axis, elongates in perpendicular axis

121
Q

what are the molecular mechanisms that drive this process

A

actomyosin contractility that begins to put tension on the junctions that need to go away, draw those cells together

122
Q

what else is it coupled with

A

endocytosis

123
Q

what happens as those junctions go away

A

cadherins and membranes are recycled into the cell, allows cell cell junctions to be erased

124
Q

what happens when Rac1 gets activated to expand the zone of contact

A

rhoA gets activated to strengthen and grow the cell-cell junctions

125
Q

what is final piece of the puzzle

A

as you’re getting cell-cell junctions formed, you’re gonna have exocytosis of cadherins up to the cell membrane to help form new cell-cell junctions

126
Q

what is overall process of convergent extension

A

junctions between cells go away

127
Q

what happens for these junctions to go away

A

actomyosin contractility is increased, and E-cadherin is endocytosed away from the cell

128
Q

how is contractility increased

A

downstream of RhoA, activates myosin 2, bipolar filament assembly, additional actomyosin filaments, etc.

129
Q

how is it endocytosed

A

clathrin-mediated endocytosis (clathrin coats, uncoats, rab effectors, v snares, t snares, etc.)w

130
Q

when is e-cadherin exocytosed

A

first step of convergent extension

131
Q

what happens as a result of endocytosis and contraction

A

cell-cell junctions shrink and other cells get closer and closer together

132
Q

how do they form cell-cell contacts

A

rac-dependent expansion, rho-dependent strengthening, secretion of E-cadherin to those newly formed junctions

133
Q

examples of tissue morphogenesis

A

tube formation, convergent-extension, branching

134
Q

what are significant factors in shaping developing tissues

A

physical forces (cell proliferation, material properties, active forces (pressure and contractility))

135
Q

how does tissue shaping/morphogenesis occur

A

signaling and physical forces work together

bio 211 (18 decks)

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