iii. early cell differentiation - csus.edu
TRANSCRIPT
Cell Differentiation in the Early Embryo
• Shortly after fertilization, the cleavage of the egg into cells allows different fates to begin to develop in different cell populations
• Different species use different strategies to determine the fate of individual cells and groups of cells during cleavage
• Remember: differentiation means choosing a different set of proteins to express from the DNA that you (and everyone else) inherited
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III. Early Cell Differentiation
A. Differentiation in individual cells is all about choosing which DNA to Express
B. Levels of Commitment to DifferentiationA. Staged CommitmentB. SpecificationC. Determination
C. Three Major Strategies of SpecificationA. AutonomousB. ConditionalC. Synctitial
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B. Levels of commitment to any certain fate...
• Much the same idea as competence....
– A cell may be capable of differentiating to a certain cell fate: “competent”
– It may have gone far enough to be able to adopt that fate in a neutral part of the embryo: “specified”
– It may have gone far enough to be able to adopt that fate in an antagonistic part of the embryo: “determined”
– It may express all of the known characteristics of that fate: “terminally differentiated”
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Compare these terms to terms of “potency”
• Many overlaps between competence and potency....– A cell capable of differentiating to all fates:
“totipotent” = complete competence
– A cell capable of differentiating to fewer fates: “multipotent” = partial competence = specified?
– A cell capable of differentiating to a single fate: “single potency” = determined
– A cell expressing that single fate:“terminally differentiated”
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C. Specification of Limited Potential in the Early Embryo
• Three Basic Strategies
– Autonomous Specification: you become the cell types dictated by the egg cytosol you get
– Conditional Specification: you become the cell types dictated by the signals you receive
– Synctitial Specification: autonomous specification without the membranes
• Most organisms use a combination of strategies one and two
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Autonomous (mosaic) specification
A cell doesn’t need contact with other cells to fulfill its fate
Sea Squirts
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Conditional specification
Sea Urchins
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Conditional specification
What matters is the signals in any given location: if cells are transplanted they adopt the fate of the new position
If cells are removed, others adopt their fate
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• Most signals in the cleavage stage embryo reach most or all of the cells
• They tend to form gradients from high to low from the point of their release
• In a pure “Conditional Specification” different cell fates would result from different concentrations along the gradient
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• In the “Combination Specification” a greater complexity would form as signal gradients interacted with cells that got different starting material from the egg
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Gilbert’s Flag deal....
Pure “Conditional Specification”
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Gilbert’s Flag Deal...
The “ComboSpecification” The gradient is
specifying tothe cells but they each havetheir own history
If you transplantthem to a newplace, they willbecome those cell types but will display theirhistorical nature
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Part Figure II.10 Syncytial specification in Drosophila melanogaster
Insects reproducetheir nuclei duringcleavage withoutmaking new cell membranes
Their nucleithen expressproteins intothe commoncytosol basedon what TF’sthat regioninherited fromthe egg cytosol.
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IV. Gastrulation
A. Background Information
B. Invertebrates1. Sea Urchins2. Snails3. Tunicates4. C. Elegans5. Drosophila melanogaster
C. Vertebrates1. The Frog2. Zebrafish3. The Chick Embryo4. Mammals
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Gastrulation is a developmental process that takes the organism from the blastula stage through the gastrula stage
Structure Process Structure
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Gastrulation is the formation of the three germ layers
Terminology of cell movements in gastrulation:
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Gastrulation is the formation of the three germ layers
Terminology of cell movements in gastrulation:
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Gastrulation in the sea urchin
blastula
gastrula (pluteus larva)
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Gastrulation in the sea urchin
SkeletogenicMesenchyme
InvaginatingEndoderm
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Gastrulation in the sea urchin
Non-skeletogenicMesenchyme
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Figure 5.15 Entire sequence of gastrulation in Lytechinus variegatus (Part 3)
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Figure 5.16 Ingression of skeletogenic mesenchyme cells
Ingression from the epithelium:Snail TF causes micromeres to downregulate e-cadherins.
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Figure 5.17 Formation of syncytial cables by skeletogenic mesenchyme cells of the sea urchin
The syncytial cables are the site of calciumcarbonate deposition for the sea urchin skeleton
VEGF and FGF are chemotactic guides for mesenchyme.
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Figure 5.19 Invagination of the vegetal plate ___________________________________
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Figure 5.21 Mid-gastrula stage of Lytechinus pictus, showing filopodial extensions of non-skeletogenic mesenchyme
Without the filopodia and pulling of the non-skeletogenic mesenchyme the archenteron can only get 2/3 of the way!
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Figure 5.22 The imaginal rudiment growing in the left side of the pluteus larva of a sea urchin ___________________________________
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Gastrulation in molluscs
A very different strategy:Epiboly of the ectoderm surrounds the presumptive endoerm and mesoderm cells
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Figure 5.32 Formation of a glochidium larva by the modification of spiral cleavage
In fresh water clams, you either evolve or you can only spread downstream
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Figure 5.33 Phony fish atop the unionid clam Lampsilis altilis
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Gastrulation in the tunicate
Endoderminvagination
Mesoderminvolution
Ectodermepiboly
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Gastrulation is the formation of the three germ layers
Terminology of cell movements in gastrulation:
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Convergent extension of the tunicate notochord
Starts as 4x10sheet of cells
Tunicate notochord has 40 cells
They migrate to the midline End up asrow of 40
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Gastrulation in C. elegans
Starts early: 24-cell stage Most movements are single cells
Cell fusion is unique
186 cellsof skin fuseinto just 8 cells
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Gastrulation in Drosophila
• A little more complex from this point on!
– Occurs in a ~hard egg case
– 2 main series of steps:
• segregation of germ layers
• final migrations to finish the larva
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Gastrulation in Drosophila
Fate Map
1st mesoderminvaginates
about the same timethe head is distinguished
Mesoderm ultimatelyseparates and forms antube fully in the interior
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Gastrulation in Drosophila
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Figure 6.5 Schematic representation of gastrulation in Drosophila
Next theendoderminvaginatesat both endsof ventralfurrow
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Gastrulation in Drosophila
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Figure 6.4 Gastrulation in Drosophila (Part 2)
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Figure 6.5 Schematic representation of gastrulation in Drosophila
As the endoderminvaginates, the ectoderm andmesoderm extendand converge towrap around thedorsal side to formthe “germ band”
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• This finishes the first series of steps
• The second series involves:
– retraction of the germ band
– segmentation of the thorax and abdomen
– epibolic closure of the dorsal epidermis
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Figure 6.5 Schematic representation of gastrulation in Drosophila
Germ band retracts
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Figure 6.4 Gastrulation in Drosophila (Part 3)
fully extended mostly retracted
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Figure 6.6 Comparison of larval (left) and adult (right) segmentation in Drosophila (Part 1) ___________________________________
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Radial, Holoblastic Cleavage of a frog egg
Same basic design as the sea urchin
It is unequal because of the large amount of yolk in the vegetal end
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Gastrulation in Xenopus laevis
Adult cell fates are specified by position at the end of blastulation
Endoderm and ectodermstart in the outer layers.
Mesodermal precursorsstart in the inner layers.
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Step 1: Blastopore Formation
Bottle cells startgastrulation bychanging shapeand involuting.
They are surface cellsfrom the vegetal-animalmargin but do the samejob as the vegetal poleepithelium in sea urchins
Create the archenteron.
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Step 2: Endoderm and Mesoderm Involute up the Inside of Blastocoel
Once the bottle cells get movement startedthey become non-essential.
Endoderm and mesoderm migrate inward and upward with the endodermal cells in the lead
“Vegetal rotation” moves the deep endoderminto contact with inner blastocoel surface
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Figure 7.7 Surface view of an early dorsal blastopore lip of Xenopus ___________________________________
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Step 3: Establishment of the Dorsal Mesoderm
Convergent extension ofmesodermal cells allowsthem to assume an inter-mediate position
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Step 4: Elimination of the Blastocoel
Growth of archenteronand endodermal divisioneliminates it entirely
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Step 5: Establishment of Ventral Mesoderm
Ventral and lateral involution is slower
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Step 6: Epiboly of the Ectoderm
The “yolk plug” is made up of immobile endodermal cells that are covered by ectodermal cell division and epiboly.
The remnant of the blastopore, where the endoderm and ectoderm meet, becomes the anus of the embryo.
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Figure 7.13 Epiboly of the ectoderm is accomplished by cell division and intercalation ___________________________________
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Zebrafish development occurs very rapidly
24-hours from 1 cell to vertebrate embryo!
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Figure 7.40 Discoidal meroblastic cleavage in a zebrafish egg
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Figure 7.41 Fish blastula (Part 2)
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Cell movements during zebrafish gastrulation
Step 1: Epiboly of blastoderm over yolk driven byradial intercalation of deep cells into outer layer
Step 2: Ingression and involution of epiblast (ectoderm)to form hypoblast (endoderm and mesoderm)
Step 3: Mesoderm and endoderm migrate across yolk tofinal positions under presumptive ectoderm
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Figure 7.43 Cell migration of endodermal and mesodermal precursors
Hypoblast formation causesa thickening at the margincalled the “germ ring”
Mesodermal cellsstart heading to thevegetal pole, thenreverse and head toward animal pole.
Presumptiveendodermspreads outrandomly (!)
Epiblast cellsare multipotent
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Step 4: Convergence and Extension of the Entire Gastrula
After the hypoblast is formed, many cells converge towards the dorsal side and extend towards the anterior
The dorsal shield is an intercalationof epiblast and hypoblast that willgive rise to the chordamesoderm.
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Figure 7.44 Convergence and extension in the zebrafish gastrula
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Figure 7.37 Zebrafish development occurs very rapidly (Part 2)
Simultaneous convergence and extension of other epiblast cells bring neuronal progenitorsto the dorsal midline to form the ‘neural keel’
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Avian Gastrulationis dominated by thehuge amount of yolk
Forces it to becomenearly planar rather than typical spherical
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)By the time the egg is laid the epiblast has ~20,000 cells
The blastocoel is the area between the epiblast and hypoblast
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• All of the cells of the avian embryo come from the epiblast
• The hypoblast gives rise only to extraembryonic structures, such as yolk sac and yolk stalk
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3-4 hours
7-8 hours
Koller’s sickle is a thickening under the epiblast at the posteriorend of the area pellucida
The primitive streakstarts anterior of thesickle as cells increasein thickness formingthe equivalent of thefrog blastopore lips
The epiblast aboveappear to stay togetherand travel ahead asHenson’s node cells
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Mediolateral intercalation and convergent extension of the primitive streak ___________________________________
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The first cells near theedge form endoderm,the next through formmesodermal structures
In avian embryos, allcells that go through undergo an epithelial to mesenchymal transition
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15-16 hours
The primitive groove is equivalentto the frog blastopore itself
The epiblast cells that go in through Hensen’s nodebecome midline mesoderm, such as the notochord
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19-22 hours
The farther posterior youenter the groove, the wideryour migration swings
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23-24 hours
4-somite stage
Regression of the streak isaccompanied by organogenesiswhich occurs anterior to posterior
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Figure 8.7 Chick gastrulation 24–28 hours after fertilization
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Figure 8.15 Development of a human embryo from fertilization to implantation
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The Inner Cell Mass will Form Embryo
This is the source of embryonicstem cells
The remainderis trophoblast
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Figure 8.21 Schematic diagram showing the derivation of tissues in human and rhesus monkey embryos
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Figure 8.22 Tissue formation in the early mammalian embryo
Implantation of the blastocystin the uterus induces developmentof both maternal and embryonicstructures to establish a supplyof nutrients, waste removal,shock absorption, etc.
Mammalian gastrulationis very much like that ofbirds and reptiles, even though we don’t have anywhere near the yolk content.
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Picture the blastocyst full of yolk..... ___________________________________
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Poor old Henson discovered this node as well but didn’t get the naming rights ___________________________________
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