Ryan Lecture 2 Flashcards
Fertilized egg —>
Blastula —> gastrula
What happens after fertilization - blastula formation
Cleavage occurs in almost all species
Extremely rapid mitotic divisions
Divides zygotic cytoplasm into numerous smaller cells = blastomeres
No increase in cytoplasmic vol
What is mid blastula transition
Slow down in rate of mitosis
All proteins/mRNA used = from oocyte/sperm
Onset of zygotic transcription - exception is mammals
When is blastula formed
End of cleavage
Variety of types = blastula, blastodisc, blastocyst (depends on Shape but all the same)
Describe xenopus blastula
Ectodermal = animal cap
Blastocoel = vegetal cap
Mesoderm, endoderm
Describe mouse blastula
Te = extraembryonic
Icm = embryo proper
Blastocoel = large, filled with fluid
Describe human (rabbit, chick) blastula
Flat blastodisc
Cavity becomes fluid filled
Epiblast = embryo
Hypoblast
Replaced by future cells of embryos, flat layered of cells
How many cells does human blastodisc/blastocyst have
70-100cells
When does human blastodisc formation begin
~5 days post fert
Describe early embryogenesis in drosophila- gen
Syncytial blastoderm - mitosis w/o cytokinesis
One nucleus, initial yolks cytoplasm (30 mins) —> 70 mins
Describe early embryogenesis in drosophila - first important thing
Nuclei move to periphery during 10th mitosis division
Cells surround yolks mass in early drosophila embryo
Describe early embryogenesis in drosophila - second important thing
After 13th division
Cell membranes form to create cellular blastoderm = single layer of cells around a yolky core
Mitotic divisions =synchronized, no longer at mid blastula transition, size of embryo stays the same
Dorsal, ventral and ant and post = many Nuclei important
Describe early embryogenesis in drosophila - third important thing
Cycle 14= mid blastula transition, zygotic transcription and asynchronous divisions, gastrulation begins
Cellularization = compartmentalize nucleus to single cell
Describe single cell to blastula In xenopus - gen
Fertilized egg —multiple rounds cell Division—> blastula, hollow sphere of cells (blastocoel)
Describe single cell to blastula In xenopus - features
Animal cap gives rise to embryo proper
Localization of proteins even at one cell stage
Important info that affects dev of cells, and axis of embryo
Describe single cell to blastula In zebrafish
Cleavage and blastoderm formation
Single cell - zygote to 512 cell, - now cell not that much bigger than single cell embryo
Describe single cell to blastula In mouse - gen
Totipotent stage =egg—>rotational cleavage—>4 cell stage —> morula
Pluripotent stage = compacted morula (tight junctions) —> blastocyst (te-extraembryonic, icm=embryonic relatively small compared to blastocoel cavity)
What is truly the most important time in your life
It is not birth, marriage, or death, but gastrulation
What is gastrulation
Series of cell movements and migration involving entire embryo
Extensive cell movements that establish final spatial relationship of the 3 germ layers = ectoderm, mesoderm, endoderm (also sometimes 4th=neural crest cells)
Species specific differences in pattern of gastrulation but only a few basic types movements
Describe the germ layers - gastrulation generally
Cells that wil form endoderm and mesoderm are brought to inside - MOUSE EXCEPTION
Ectoderm (skin and nervous system, surface and neuro ectoderm) spread over outside surface
Describe intercalaton - gastrulation cell movements
Rows of cells move between one another, creating an array of cells that is longer, in one or more directions (or another shape of cell)
Like zipper merge
Describe convergent extension - gastrulation cell movements
Cells intercalate in highly directional manner
Causes embryo to elongate
Lateral —> midline side in embryo
Longer, more directional group of cells
Correlated with planar cell polarity, now which direction is ant and post and where neighbours are
What does convergent extension depend on
Non canonical want/pcp (planar cell polarity) pathway
Describe model showing convergent extension
Bead model
Move in and spread out
Cluster of cells more likely to stick togetehr
Describe convergent extension in an Embryo
Happens during time of lots of mitosis
Dorsal side,rod like structure formed
Ball of cells = somites
Movement to middle gives embryo shape and structure
Describe invagination - gastrulation cell movements
Sheet of cells (epthelial sheet) bends inwards
Apically constrict, on one side
Get narrow and move into liumen = gives pop of cells = mesoderm or endoderm
Depends when they move in
Describe gastrulation in drosophila
Same outcome - 3 germ layers
Slightly diff process- happens through invagination
Describe gastrulation in drosophila - mesoderm
Derived from 1000 cells at Ventral midline that fold inwards to form ventral furrow
Describe gastrulation in drosophila - endoderm
Invaginates as 2 pockets at anterior and posterior ends of ventral furrow
Describe stage 4 drosophila embryo
Syncytial stage
Just before cellularization
Describe gastrulation in drosophila - invagination
Ventral view
Becomes mesoderm
2 pockets at ends = endoderm, ant and post
Overlying ectoderm will close up around
Describe invagination during drosophila gastrulation
Endoderm = more triangular in shape
Describe involution - gastrulation cell movements
Partially important in xenopus
Process by which an epithelial sheet rolls inwards to form an underlying layer
Dorsal lip of blastopore - cells involute, and move along surface of embryo, in opp dir
What is required for involution
Initial turn requires apical constriction
Dorsal leading edge - head-mesoderm = push in against cells that become later ectoderm = happens In dorsal blastopore lip
Bottle cells = change shape as involution happens
Describe process of involution - needed steps
Movement of nuclei - to basal side helps change shape - and apical actomyosin complex undergoes contraction to buckle epithelium- constriction, like hoodie
Describe amphibian gastrulation
Involution
Blastula —> gastrula —> neurula
Neurula = only future ectoderm on surface = surface ectoderm, neural plate ectoderm
Cells involute at dorsal blastopore lip
Describe xenopus cell movements during gastrulation - dorsal view of gastrulation
Cells move
Cells left on surface = forms start of neural tube and other cells will be the surface ectoderm
Describe xenopus cell movements during gastrulation - vegetal view of involuting cells
Surface ectoderm eventually covers neural tube - 15h elapsed time
Describe xenopus cell movements during gastrulation - internal view of gastrulation
See movement and involution - as come through dorsal lip of blastopore and move up and around that surface
Describe epiboly - gastrulation cell movements
Process in which a sheet of cells spreads by thinning movements of cells over another later of cells
Cells change shape
Especially important in zebra fish
Cell movement - by thinning
Can see in early xenopus
What does epiboly do in zebrafish
Movement important = over surface of yolk - brings yolk yo indifferent of embryo so can nourish embryo
Describe epiboly of zebrafish over time
30% epiboly, 4hr, embryo spreads over surface = c shape, epiboly cells gradually move down surface
—> 75% epiboly, 7.55 hr
—> 90% epiboly, 9hrs = stage embryo by how much these cells have moved down
1cell—>100% epiboly, as epiboly movement = cells contribute to grow on surface
Describe ingression - gastrulation cell movements
Individual cells leave epithelial sheet and become freely migrating mesenchyme
Similar to epithelial to mesenchymal transition
In most blastodisc shaped embryos = humans, rabbits, chicken and mouse, but mouse do not form blastodisc at this point
Fall out and migrate down - but lose connections with neighbours = lose cell cell tight junctions
Describe EMT (epithelial to mesenchymal transition) - stage 1
Initially = attached to neighbour via cell attachments =
Epithelial cell layer, epiblast = junctions complexes intact (tj, adherens, gap junctions)
Basement membrane intact
Describe EMT (epithelial to mesenchymal transition) - stage 2
Junctional complexes begin to breakdown
Induced
Describe EMT (epithelial to mesenchymal transition) - stage 3
Junctional complexes break down
Basement membrane breaks down
Ingresssing cell takes on mesenchymal phenotype - so cell can slip down
Describe EMT (epithelial to mesenchymal transition) - stage 4
Cells leaves epithelial layer of epiblast
Becomes mesenchymal
Will contribute to mesoderm or endoderm
Migrate out, cells that remain in epithelial layer = become ectoderm
Are all germ layers mesenchymal
NOOOOO
Any germ layer can be epithelial and any germ layer can be mesenchymal = refers to cell state
Describe gastrulation in Chick embryos - ingression
Cells that fall out and moves towards midline embryo = primitive groove
Once cells reach primitive groove= eat go and they migrate into place
Result =ectoderm, mesoderm, endoderm
Name important parts of chick embryo
Hesens node
Primitive groove
Epiblast
Blastocoel
Hypoblast
Migrating mesenchyme = will contribute to lateral plate mesoderm
Describe gastrulation movements in chick embryos - formation of primitive streak, dorsal view
Formed post to ant
Describe gastrulation movements in chick embryos -dorsal view
Cells stained with red dye
Get there and u turn = mostly on one side
Left and right patterning very early and carry info with them as they go forwards
How do human embryos gastrulate - describe
Flat blastodiscs - most similar to chick and rabbit embryos
3rd week post fert
~14 days —> 21 days, now have definite germ layers
Describe gastrulation in mouse
NOT FLAT Blastocyst
Icm forms structure - embryo proper
Endoderm and mesoderm come to outside of embryo - day 6.5
Different tho
What is most mammalian embryos gastrulation similar to
Birds/reptiles
Describe mouse and human - single cell thru gastrulation
Blastocyst stages similar, humans = more blastodisc shape
Early gastrula = diff
Late gastrula = MOUSE (tail end swings around and lateral parts ectoderm start to encompass embryo) HUMAN (tissues enclose yolk sac)
What is at the end of gastrulation
3 germ layers
Tissues gain memory as go through all this
Germ layers = also patterned with respect to d-v and a-p axes
Describe ectoderm
Outer cell layer
Epidermis, neural tissue, placodes (optic, optic, olfactory, hypophyseal)
Describe mesoderm
Middle cell layer
Demis, muscle, bone, kidney, gonads
Describe endoderm
Inner cell layer
Epithelial lining of gut, lung, liver, pancreas
Describe 4th germ layer
Neural crest cells
Arise at boundary of neural and non neural ectoderm during neural tube closure - after gastrulation has happened
Cells in peripheral and enteric nervous systems, muscle, craniofacial bones and cartilage, melanocytes, VERY DIVERSE
DESCRIbe neural crest cells and their derivatives
What is left after gastrulation = neural plate border
Will give rise to neural tube, brain and spinal cord - MEDIAL
Gives rise to surface ectoderm = LATERALLY
Cells migrate away = epi to mesenchymal transition, and migrates to diff parts of body as neural tube closes
How do neural crest cells become what they do
Migratory path= where they end up determines what they become
From from ectodermal region originally
Smooth muscle cells, osteoblasts, osteoclasts, adipocytes, chondrocytes, melanocytes, Schwann cells, neurons
Bones in head = diencephalic
Heart and components, vasculature and neurons in brain
What is an organizer
Group of cells capable of organizing neighbouring cells
Provide important info to surrounding cells
What can organizer do
Can induce ectopic cell fates in host tissue following heterotopic transplantation
Important for endogenous structures but can also re pattern cells that are competent in other regions
Describe Hilde mangold
Student, cut and paste experiment in amphibian embryos
Not awarded cause dead
Describe hilde mangolds experiment
Take pieces of early blastula embryo and transplant them into other parts
Normal = primary endogenous axis, neural tube, notochord, sortie and ganglia form
Then she used tissue derived from transplanted dorsal blastopore lip = no pigment and could induce whole other axis, regions of embryo can induce axis
Secondary induced axis = new tissues, can tell since og structures pigmented
What did hilde conclude basically
Dorsal blastopore lip = region where cells would pass by during gastrulation and get info
Transplantation of dorsal blastopore lip induces a secondary axis
Is it a fate mapping exp - hilde
It didn’t become something else but it did induce other cells to
Does this tell you about the potential of dorsal blastopore lip - hilde mangold
Inducing tissue
Then what will target become, or what will it induce =
WHAT IS POTENTIAL OF TISSUE WE ARE TRANSPLANTING
OR
WHAT IS POTENTIAL OF IT TO INDUCE SOMETHING ELSE
WHAT Happens when transplant embryonic shield of zebrafish
Embryonic shield = specialized region embryo
In sit exp - looking at shh
= gives 2 diff embryos - similar effect of hilde exp
What happens when transplant hensens node from chick embryos
Hensens node = ant end primitive streak
Can induce another embryo to dev
Can also induce a regionally structured a-p neural axis
Leads to = tissues from host being induced to start new ap structure
Spinal cord - donor
Forebrain
Forebrain
Spinal cord - host
What is spemann mangold organizer
In amphibians
Sets up v early = at blastocyst stage
At dorsal blastopore lip
Describe molecules required for induction of spemann mangold organizer
B cate + VegT, Vg1 (both needed to form organizer)—> nodal related high —> organizer
VegT, Vg1 —> nodal related low —> ventral mesoderm (lower nodal signal)
Describe molecular signals required for patterning germ layers - ventral centre
Bmp4, bmp7
(Cv2, sizzled, Bambi, xlr, tsg)
Describe molecular signals required for patterning germ layers - spemann mangold organizer
Antagonists inhibit activity fo signalling molecules, dampens signal at that place
Chordin, noggin, follistatin, frzb1, sfrp2, crescent, dickkopf-1, cerberus
Describe the mutiple cell signalling inhibitors the organizer produces
Frzb1, dickkopf-1, crescent = inhibit wnts
Cerberus = inhibits xwnt-8, xnrs , bmps (Nodal related)
Chordin, noggin, follistatin = inhibit bmps
Inhibits of pathway = v imrpoatnt in organizers themselves
Name the major axes in early embryo - 3
Dorsal - ventral, back to front
Anterior - posterior, top to bottom
Left - right (int vs ext human body = look symmetrical but not inside)
What is an axis
Axis usually defines a direction or orientation
Asymmetrical - may be initially symmetrical
Name additional axes in older embryos
Proximal -distal = close to far
How does asymmetry - polarity happen
Unfertilized egg = single cell, usually radially symmetrical
Initial divisions after fert can generate functionally equivalent daughter cells
(Autonomous specification exception, bc first division asymmetric
Eventually cells must become different
Name the 2 ways cells can become different - cell divisions
Asymmetric cell division = localization of cytoplasmic determinants followed by asymmetric cell division = recall, spindle parallel to determination gradient
Asymmetric patterning through inductive signals = cell cell signalling within fields of identical cells (closer = more signal info) or across boundaries (groups of cells that can stick together)
Describe dorsal ventral axis in mammals
Forms from icm cells that are in contact with trophoblast
Icm = tells that undergo gastrulation
Dorsal axis, do not know much about it overall, but do know it form individual organs and spinal cord
Describe d-v axis in chick - gen
Formation depends on pH
Occurs at blastodisc stage
Describe d-v axis in chick - specificallly
Water and sodium ions transported from basic albumen (egg white above blastodisc) = basic through cells to acidic sub germinal cavity below in Hypoblast area
Creates a membrane potential diff
Side facing albumen becomes dorsal
Side facing yolk becomes ventral - got cells
RESULTS= in endodermal/future gut next to yolk, food source
Where is d-v axis initiated in drosophila
Syncytial blastoderm = before cellularization in single cell
Describe d-v axis formation in drosophila - gen protein
Depends on tf dorsal
Dorsal = product if maternal effect gene = mRNA deposited in egg from female/mom - around from beginning
Describe d-v axis in drosophila-specifics
Dorsal initially found throughout syncytial blastoderm -so present throughout cytoplasm
Dorsal protein comes nuclear only on future ventral side of embryo, dorsal regulates ventral phenotypes
How are many proteins named in drosophila
Based on phenotype caused when gene mutated
When dorsal mutated = causes dorsal phenotype
What happens of absence of dorsal - in drosophila
Or if dorsal doesn’t enter nucleus = embryo becomes dorsalized
Has 2 dorsal sides of Emrbyo
What happens if dorsal enters all the nuclei in drosophila
Embryo becomes centralized
2 ventral sides
Describe dorsal entering nucleus
Dorsal triggered to enter nuclei in regions of embryo where spatzle ligand binds its receptor (toll)
Ligand present in ventral superficial surface (only on ventral side) = binds to receptors on surface of future cells
Toll activation = Triggers another series of events = casues dorsal enter nucleus
Describe Nobel prize 1995 discovery
If mutate toll = no ventral phenotype
When does d-v axis patterning start for all species - name
At blastula stage in mouse
At blastoderm stage in chick
At blastoderm stage in drosophila
At fertilization in xenopus
