MA 6 Flashcards
1
Q
What makes Xenopus a good model organism
A
-large eggs with defined animal and vegetal halves that are suitable for transplantation experiments
-easily microinjected, making them good for genetic studies
-used to study gastrulation by removing and transplanting tissues
-high fecundity- lots of eggs in each clutch
-external fertilization
2
Q
Cortical Rotation
A
- outer part of the embryo rotates while the inner part of the egg remains in place (molecules inside move relative to one another)
-reveals the grey crescent
-location of the grey crescent is defined by the point of sperm entry
-grey crescent is on the dorsal side of the embryo
3
Q
Experiment: expose embryos to UV light
A
- prevents cortical rotation.
-greater amounts of UV light decrease the amount of cortical rotation
-anterior structures are missing
-embryo becomes ventralized, no more dorsal structures. This means that cortical rotation is necessary for localizing dorsalizers.
4
Q
How does cortical rotation occur?
A
-sperm centriole enters the egg upon fertilization
- this allows the MTs to become organized.
-MT tracks separate the outside of the embryo from the inside, then provide a track for the cortical cytoplasm to rotate on.
5
Q
cleavage in amphibians
A
-Unequal radial holoblastic cleavage
-first cleavage is partway through the embryo
-second cleavage begins while the first one is happening
-additional cleavages in the animal pole compared to the vegetal pole, this is because the vegetal cells have a lot of yolk which makes it harder for them to divide. vegetal pole has larger cells as a result
6
Q
As a result of cleavage…
A
- 6000 cells
- No volume change
- No growth of individual cells- embryo is focused on mitosis
-more membrane throughout the cell
7
Q
Blastocoel
A
-space for cells to move through during gastrualtion
-divides the animal and vegetal cells into two distinct populations. This prevents interactions and allows the two populations to have unique characteristics.
8
Q
experiment: centrifugation of embryos
A
- increases the amount of cortical rotation
-induces a second axis
-embryos have two heads
9
Q
morula
A
- embryo is 16-64 cells
10
Q
blastula
A
- embryo is 128 cells
11
Q
Mid Blastula Transition
A
-cell divides enough so that there is a certain amount of DNA. Protein gets titered by DNA, and this induces the MBT.
-shift from using maternal stored mRNA to zygotic DNA
-mitosis slows down
-new genes are being transcribed
-cells become different from one another based on location and signaling molecules.
-cleavages become asynchronous
-cells become motile and start moving around
12
Q
experiment: treatment with Actinomycin
A
-actinomycin blocks transcription of mRNAs
-boost in proteins from the translation of maternal mRNAs still occurs.
-but, translation of zygotic genome does not happen
-less protein in treatment group than control once the MBT happens
13
Q
Goal of gastrulation
A
-get the germ layers into positions that will mimic their final functions
14
Q
What is the site of gastrulation in xenopus
A
-dorsal lip of the blastopore
-location is determined by the site of fertilization
15
Q
What are some early events of gastrulation
A
-cells begin to roll in to the embryo
-dorsal lip becomes bigger and extends around the embryo
-yolky endodermal cells get sucked into the embryo
-outside cells become ectoderm
-neural plate forms
16
Q
fate of vegetal cells
A
endoderm
17
Q
fate of animal cells
A
ectoderm
18
Q
cells opposite the site of sperm entry/ the Spemann organizer become…
A
- neural ectoderm
-notochord mesoderm
-head endoderm
19
Q
cells beneath the blastocoel become…
A
mesoderm
20
Q
overview of gastrulation
A
- cells move, forming the gut
-begins at the dorsal lip of the blastopore
-neurula folds up forming the neural tube
-defines the A/P axis
21
Q
5 Key movements during gastrulation
A
-epiboly
-vegetal rotation
-invagination
-involution and migration
-convergent extension
22
Q
epiboly
A
thinning and spreading of the animal cap cells over the vegetal hemisphere
-powered by proliferation and radial intercalation
-cells on the outside compress forming a thinner layer of cells
-amount of the embryo covered increases
-ectodermal cells move down, around and into the lip
23
Q
vegetal rotation
A
vegetal cell asymmetrically press up against the inner blastocoel roof on the dorsal side
- vegetal cells migrate and crawl along the blastocoel.
-these tissues give rise to the gut
24
Q
Bottle cell formation and ingavination
A
localized apical constriction at the dorsal blastopore lip creates anisotropic forces that foster invagination
- bottle cells form as a result of apical constriction
-forms the dorsal lip of the blastopore
25
involution and migration
tissues roll in and pulls along the future mesoderm
26
convergent extension
medial to lateral intercalation of cells on the midline that drivers anterior to posterior extension
27
Steps of gastrualtion
1. Bottle cells initiate invagination
2. Dorsal lip forms, cells start to pull in
3. Mesoderm moves along the roof of the bastocoel, epiboly of ectoderm wraps cell around while other tissues move in. Blastocoel gets displaced by the developing archenteron. ventral lip of the blastopore gets displaced by the developing archenteron
4. tissues move all the way across the embryo. Blastocoel gets smaller as the archenteron gets larger, Blastocoel gets displaced to the opposite side of the dorsal lip.
5. tissues move completely around
6. some of the mesoderm moves around forming various organs. A/P axis is fully defined.
28
collective cell migration
-multiple rows of cells follow one another in a cluster
-leading row pushes out lamellipodia that connects to fibronectin lining the roof of the blastocoel
-b-catenin is essential for keeping the cluster of cells together
-integrins facilitate binding between cells and the ECM
-tension allows for the pulling of cells and creates a gap between the cells and the roof of the blastocoel.
29
experiment: KO E-cadherin in the embryo
-maternal messages in the embryo are knocked out
-no cadherins means that cells no longer connect to one another
- no organization of cells or formation of the blastocoel
-gastrulation does not occur
30
experiment: inject the cell with the cell binding portion of fibronectin
- integrin binding sites become saturated
-improper binding
-gastrulation is not able to occur and the ectoderm gets stuck on the outside of the embryo.
-cells are non involuted
31
yolk plug
remaining patch of endoderm
-at this point: all endodermal precursors have been brought into the interior of the embryo
-ectoderm envelopes the surface
-mesoderm is between the two layers
-gastrulation is mostly complete
32
mesoderm layer becomes...
notochord
33
IMZ
involuting marginal zone.
-undergoes convergent extension
-both ends extend at the same time
-helps induce the A/P axis
34
what are the two functions of the vegetal cells
A. differentiate into endoderm
b. induce cells immediately above them to become mesoderm
35
Axis formation in xenopus
-maternal VegT mRNA gets translated
-VegT turns on Nodal
-Nodal activates Eomes
-Eomes is a TF that tells these cells to become mesoderm
-Eomes and Nodal turns on VegT in a positive feedbalc loop
-Veg1 turns on Wnt
36
Sox17
-induce endodermal cells.
37
Vg1
dorsal mesoderm
38
VgT
mesoderm and endoderm
39
migration and involution of the mesoderm induces...
-Establishment of the AP axis
-induces formation of the neural tissue and the forebrain
-induces tissue to become posterior brain structures
40
primary embryonic induction
first set of inductive events that happens in the developing embryo
-induction where the progeny of dorsal lip cells induce the dorsal axis and neural tube
41
experiment: VgT KO
entire embryo becomes epidermis, dorsalizers are no longer there to induce dorsal phenotypes
42
Spemann's experiment: nuclear equivalence
-Use of a baby hair to separate out one nucleus from the rest
-development proceeds normally but one newt is smaller than the other
-each nucleus has enough information to encode an entirely functional embryo (conditional specification)
43
grey crescent
gives rise to cells that initiate gastrulation. Crucial for development
44
experiments: first cleavage does not bisect the grey crescent
- one embryo, containing the grey crescent develops normally. it has all of the elements needed to induce dorsal structures
-the other embryo does not contain the grey crescent, this one is completely epidermis
45
experiments: Spemanns transplantation experiments
-early cells express conditional development. Later, gastrula cells exhibit autonomous development
-transplanting tissue in early gastrula, tissue is conditionally specified
-presumptive neural tissue becomes epidermis
-transplanting later, tissue becomes what it is autonomously specified to be
-presumptive neural tissue becomes neural tissue
46
What is the first tissue to become autonomously specified
Spemann organizer
47
Spemann Organizer
-only part of the embryo that forms a second axis when transplanted
-Above the dorsal lip of the blastopore and derived from the grey crescent
-contains mesoderm layers that later form somites
-induces formation of neural and dorsal tissues
-organizes D/V and A/P axis
48
Experiment: transplantation of the Spemann Organizer
-transplanted into an embryo in early gastruation
-induces a second site of gastrulation
-second set of neural tube, lumen of the gut, notochord resulting in two conjoined embryos
49
Niewkoop Center
dorsalmost vegetal cells that are capable of inducing the Spemann organizer
-b-catenin is a key signaling molecule in determining where this is
-necesary and sufficient in forming the D/V axis
50
where is b-catenin localized
dorsal side of the embryo
51
How does b-catenin localize
A. Formation of parallel arrays of MTs extend along the vegetal hemisphere along the future D/V axi
b. cortical rotation
C. Dsh becomes localized at the dorsal position of the embryo- it gets placed there by being transported by kinesin (moves towards the positive end of the MTs)
-before fertilization, b-catenin is found all throughout the embryo
52
b-catenin induces the A/P axis by...
initiating the movement of the involuting mesoderm
53
Dsh and GP
initiate the inactivation of GSK3 and the protection of b-catenin
54
Wnt11
Amplifies the signal and stabilizes the GbP and Dsh and organizes them to protect b-catenin
55
b-catenin
associates with other TFs and gives them new properties
56
GSK3
phosphorylates b-catenin and targets it for degredation by the proteosome of the ventral side
GSK3
57
GSK3 is inactivated by...
Dsh, GbP and Wnt11
58
experiment: inject a broken GSK3
-no more degredation of b-catenin
-b-catenin is present all throughout the embryo
-induces the formation of a second axis
59
b-catenin is also used for
-ovary induction
-induction of the micromeres during gastrulation
-axis formation in xenopus
60
High levels of Nodal and b-catenin...
induce the organizer
61
High levels of nodal and low levels of b-catenin...
ventral mesoderm
62
nodal is upregulated by...
overlap of b-catenin and vg1
63
when is b-catenin upregulated?
up until gastrulation- localized in both the niewkoop center and the Spemann organizer
64
What does the organizer do?
a. self differentiates into mesoderm
b. dorsalizers surrounding mesoderm form somite cells
c.dorsalizes ectoderm and induces the formation of the neural tube
d. initiates the movements of gastrulation
65
How does the organizer do its job?
-no positive inducer for neural tissue
-neural tissues form by blocking the molecules that induce formation of the epidermis
-neural tube is the default. Forms when protected from bMP molecules.
66
bmp
powerful ventralizer, induces the formation of the ectoderm as well as dorsal mesoderm.
67
experiment: KO bmp
only neural tissue forms, many small heads
68
noggin
-one of the genes found in a mutant screen.
-dorsalizes an embryo that is ventralized by UV light
-too much noggin forms an embryo that is completely dorsal
-antagonizes bMP4 and bMP2 and allows for the induction of neural tissued
69
chordin
binds to bmp4 and bmp2 and prevents complexing with their receptors
70
Follistatin
inhibits bmps
71
when does neurulation begin?
during gastrulation
72
what three groups does the ectoderm differentiate into
A. outer ectoderm
b. neural crest cells
C. neural tube
73
outer ectoderm becomes...
mostly epidermis
74
neural crest cells become...
-mesenchymal cells, neurons pigment, facial cartilage
75
neural tube becomes...
brain and CNS
76
convergent extension in neurulation
convergent extension allows for elongation of the embryo. This occurs in a defined direction
77
primary neurulation
- cells surrounding the neural plate direct it to proliferate, invaginate, and separate from the ectoderm, forming a hallow tube
- notochord directs the overlying ectoderm to form the neural tube
-creates the neural tube and neural crest and leaves behind the epidermis
78
Steps of neruation
1. elongation/ folding of the neural plate
-neural plate is created from elongated dorsal ectodermal cells
-cells begin to fold upward forming the MHP and neural groove
2. Convergence of folds and formation of the DLHPs
-epidermal cells have a pushing force that push cells towards the midline
-wedges, apical constriction helps with bending
3. Closure: cells at dorsal most portion of the neural tube become neural crest cells
-separation due to different kinds of cadherins
-n-cadherins in neural cells
-e-cadherins in epithelium
-cell cell interactions and microfilaments are crucial for neurulation
79
secondary neurulation
neural tube forms a solid chord of cells that sinks into the embryo
80
neural tube closure in mammals
multiple points of closure in humans rather than one point of closure
81
spina bifida
failure of the end of the neural tube to close
82
anacephaly
anterior of the neural tube does not close
-brain is exposed to amniotic fluid and degrades
83
minor issues in closure 3
cleft lip/cleft pallette
84
environmental impacts on NTD
folic acid is added to bread and prenatal vitamins because low folate levels cause epigenetic changes and mutations that cause neural tube defects
85
mammalian eggs
-smallest in the animal kingdom, difficult to manipulate experimentally
-hard to visualize
-produced in smaller numbers
-mouse eggs mostly used in experiments
86
steps of fertilization in mammals
-early follicles have stored eggs that are arrested in meiosis
-hormones stimulate the release of the egg into the fallopian tube- fimbrae facilitate this movement
-eggs begin to accumulate stored mRNAs and nutrients
- egg moves down the fallopian tube, this is where fertilization occurs.
- fertilization allows the egg to complete meiosis
-cleavage begins as the embryo moves down the fallopian tube and into the uterus
- egg hatches from zona pellucida and implants into the uterus
87
zona pellucida
outer membrane that prevents the egg from hatching prematurely
-premature hatching results in ectopic pregnancy, or implantation into the uterus and can be life-threatening if left untreated
88
Cleavage in mammals
-rotational holoblastic
-first cleavage is meridional
-second cleavage has one meridional plane and one equatorial plane
- cleavages go all the way through the embryo
89
how is mammalian cleavage different from other model organism we've studied?
-asynchronous cell divisions start early, so the embryo can have an odd number of cells at any point
-mammalian zygotic genome becomes active earlier- it is needed during cleavage and gastrulation
-less reliance on stored maternal mRNAs
-some of the slowest cleavages in the animal kingdom
90
totipotent
cells are capable of producing any cell type - seen in early cells`
91
pluripotent cells
cells that can form many types of cells
-we see this in the ICM, these cells can form many types of cells, but cannot differentiate into trophoblast
92
early 8 cell stage
cells are loosely arranged
93
compacted 8 cell stage
-E-cadherins are expressed
-tight junctions begin to form
-embryo becomes compacted
- inside of the sphere is not sealed off, but gap junctions allow small molecules to pass through
94
16 cell stage
-morula
-cells split between the inside and the outside and begin to develop unique characteristics based on where they are positioned
-no internal cavity yet
95
blastocyst
-cavitation- the blastocoel forms
-outer trophectoderm and ICM form
96
trophectoderm
these cells become extraembryonic tissues that are critical for supporting the growth of the embryo
-chorion/ fetal portion of the placenta
-has integrins that allow attachment to the wall of the uterus
-proteases that are able to degrade the uterine lining and allow for implantation
97
Inner Cell Mass (ICM)
cells that become the embryo, yolk sac, allantois, and the amnion
-also supports the trophoblast by secreting cells that help it divide
98
Overview of implantation
1. Hypoblast cells delaminate from the ICM and line the boastocoel
2. loss of membranes, syncytium forms, trophoblast forms villi (cyto and syncytial trophoblast and ingression into uterine tissue
3. amniotic cavity is filled with amniotic fluid and surrounds the embryo
epiblast separates into embryonic epiblast and amniotic ectoderm
4. yolk sac forms from the hypoblast and the blood vessels begin to provide the embryo with nutrients from the mother
99
syncytial trophectoderm
placental barrier between maternal and fetal blood that allows for exchange of nutrients and gasses
-facilitates in implantation by production of enzymes and apoptotic factors
100
cytotrophectoderm
-internal of the syncytiotrophoblast and external to the blastocyst.
-breaks down the endometrial cells and allows finger like projections to penetrate through and pull the embryo until it is completely covered
101
epiblast
pluripotent primary lineage that will form the definitive germ layers of the embryo
102
embryonic epiblast
single celled epithelium which generates the embryo through gastrulation
103
amniotic ectoderm
epithelial layer that lines the inner roof of the amniotic cavity. derived from the epiblast before gastrulation begins
104
yolk sac
not used by humans but remains due to shared ancestry with avians
105
somites
segmental axial structures of vertebrate embryos that give rise to vertebral column, ribs, skeletal muscles, and subcutaneous tissues
106
gastrulation in mammals
-initiated at the SMO - implantation of the SMO can induce gastrulation in other tissued
-forms the primitive streak and the primitive groove
-cells move into the groove and down into the deeper layers of the embryo
-hypoblast cells get displaced by the epiblast cells
-hypoblast cell eventually form the endoderm
-EMT occurs as the cells begin to move around
-cells between the two layers move around
107
what happens of the SMO is duplicated
conjoined twins- the point at which they are conjoined depends on the site of the duplication
108
formation of the notochord
mesodermal cells form tight connections with the overlying mesoderm. This anchors the folding neural tube
-formation of the neural tube is initiated by the movement of the dorsal mesoderm and signaling of dorsalizers that repress the formation of epidermal ectoderm
109
What are the 3 responsibilities of the vertebrate ectoderm
1. become the neural plate
2. become the epidermis
3. become the neural crest, forming the peripheral nervous system
110
Development of the placenta
blood from the mother can now be released into the placental space, allowing for the exchange of nutrients and oxygenated/ deoxygenated blood.
-formed from the chorion, the trophoblast and blood vessel containing mesoderm
111
choronic villi
-contains capillary beds from the baby
-connects the blood supply from the fetus to the mother
-exchange of deoxygenated blood and waste
112
monozygotic twins
come from one egg
113
twins- less than 5 days
-each embryo has its own blastocyst and trophoblast
-this causes the development of two chorions and two amnions
114
twins- days 5-9
blastocyst is formed already,
-one trophectoderm between two babies, but 2 ICMs
-this leads to the development of 2 amnion within one chorion
115
twins- after day 9
-one amnion and one chorion
-concern for conjoined twins because conjoined twins are normally formed during gastrulation
116
dizygotic twins
2 eggs, the mother ovulates two eggs instead of one, and both get fertilized
117
chorion
extraembryonic membrane and the portion of the placenta that enables the fetus to get oxygen and nourishment from the mother. Also secretes hormones that cause the mother's uterus to retain the fetus and produces regulators of the immune response so that the mother will not reject the embryo
118
amnion
thin membrane forming a closed sac about the embryo that contains fluid that protects it from shock or drying out
119
allantois
fetal membrane lying below the chorion in many vertebrates. Part of the placenta in mammals
