III. Early Cell Differentiation ______

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 Differentiation ______A. Staged Commitment B. Specification C. Determination ______C. Three Major Strategies of Specification A. Autonomous B. Conditional ______C. 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” ______

______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 ______Sea Squirts ______

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A cell doesn’t need contact with other cells to fulfill its fate ______

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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 ______

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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 “Combo Specification” The gradient is ______specifying to the cells but they each have their own history ______

______If you transplant them to a new place, they will become those ______cell types but will display their historical nature ______

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Part Figure II.10 Syncytial specification in Drosophila melanogaster ______

Insects reproduce ______their nuclei during cleavage without making new cell membranes ______

Their nuclei then express ______proteins into the common cytosol based on what TF’s ______that region inherited from the egg cytosol. ______

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IV. Gastrulation ______

A. Background Information ______B. Invertebrates 1. Sea Urchins 2. Snails ______3. Tunicates 4. C. Elegans 5. Drosophila melanogaster ______C. Vertebrates 1. The Frog ______2. Zebrafish 3. The Chick Embryo 4. Mammals ______

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______Gastrulation is a developmental process that takes the organism from the blastula stage through the gastrula stage ______

______Structure Æ Process Æ 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 ______

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______gastrula (pluteus larva) ______

______Gastrulation in the sea urchin ______

______Skeletogenic Mesenchyme ______

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Invaginating ______Endoderm

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Gastrulation in the sea urchin ______

Non-skeletogenic ______Mesenchyme

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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 ______

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______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 calcium carbonate deposition for the sea urchin skeleton ______

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______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 ______

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______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 ______

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______A very different strategy: Epiboly of the ectoderm surrounds the presumptive ______endoerm and mesoderm cells ______

______Figure 5.32 Formation of a glochidium larva by the modification of spiral cleavage ______

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______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 ______

Endoderm ______invagination

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Mesoderm involution ______

______Ectoderm epiboly ______

______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 ______

Tunicate notochord has 40 cells ______

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Starts as 4x10 End up as They migrate to the midline sheet of cells row of 40 ______

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Gastrulation in C. elegans ______

Starts early: 24-cell stage Most movements are single cells ______

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Cell fusion is unique ______

______186 cells of skin fuse into just 8 cells ______

______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 mesoderm invaginates ______Mesoderm ultimately separates and forms an about the same time tube fully in the interior the head is distinguished ______

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Gastrulation in Drosophila ______

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______Figure 6.5 Schematic representation of gastrulation in Drosophila ______

Next the endoderm ______invaginates at both ends of ventral furrow ______

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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 ______

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As the endoderm ______invaginates, the ectoderm and mesoderm extend and converge to ______wrap around the dorsal side to form the “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 ______

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______Germ band retracts ______

______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 ______

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It is unequal because of the large amount of yolk in the vegetal end ______

______Gastrulation in Xenopus laevis ______

Adult cell fates are specified by position at the end of blastulation ______

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______Endoderm and ectoderm Mesodermal precursors start in the outer layers. start in the inner layers. ______

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Step 1: Blastopore Formation ______Bottle cells start gastrulation by changing shape ______and involuting. ______

They are surface cells from the vegetal-animal ______margin but do the same job as the vegetal pole epithelium in sea urchins ______Create the archenteron. ______

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Step 2: Endoderm and Mesoderm Involute up the Inside of Blastocoel ______

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Once the bottle cells get movement started ______they become non-essential.

Endoderm and mesoderm migrate inward and upward with the endodermal cells in the lead ______

“Vegetal rotation” moves the deep endoderm into contact with inner blastocoel surface ______

______Figure 7.7 Surface view of an early dorsal blastopore lip of Xenopus ______

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Step 3: Establishment of the Dorsal Mesoderm ______

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______Convergent extension of mesodermal cells allows them to assume an inter- ______mediate position

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Step 4: Elimination of the Blastocoel ______

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Growth of archenteron and endodermal division eliminates it entirely ______

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______Step 5: Establishment of Ventral Mesoderm ______

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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. ______

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______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 ______

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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 ______

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______Step 1: Epiboly of blastoderm over yolk driven by radial 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 to final positions under presumptive ectoderm ______

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Figure 7.43 Cell migration of endodermal and mesodermal precursors ______

Hypoblast formation causes Epiblast cells a thickening at the margin are multipotent called the “germ ring” ______

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Mesodermal cells Presumptive start heading to the endoderm vegetal pole, then spreads out ______reverse and head randomly (!) toward animal pole. ______

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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 ______

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The dorsal shield is an intercalation of epiblast and hypoblast that will give rise to the chordamesoderm. ______

______Figure 7.44 Convergence and extension in the zebrafish gastrula ______

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Figure 7.37 Zebrafish development occurs very rapidly (Part 2) ______

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Simultaneous convergence and extension of ______other epiblast cells bring neuronal progenitors to the dorsal midline to form the ‘neural keel’ ______

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Avian Gast rulat ion is dominated by the ______huge amount of yolk

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Forces it to become nearly planar rather ______than typical spherical ______

______By the time the egg is laid the epiblast has ~20,000 cells )

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______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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______Koller’s sickle is a thickening under the epiblast at the posterior end of the area pellucida ______3-4 hours ______The epiblast above appear to stay together and travel ahead as Henson’s node cells ______

The primitive streak 7-8 hours starts anterior of the ______sickle as cells increase in thickness forming the equivalent of the frog blastopore lips ______

______Mediolateral intercalation and convergent extension of the primitive streak ______

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______The first cells near the edge form endoderm, the next through form mesodermal structures ______

In avian embryos, all cells that go through undergo an epithelial to ______mesenchymal transition ______

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______15-16 hours The epiblast cells that go in through Hensen’s node become midline mesoderm, such as the notochord ______

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The primitive groove is equivalent ______to the frog blastopore itself ______

______19-22 hours ______

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The farther posterior you enter the groove, the wider ______your migration swings ______

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______Regression of the streak is accompanied by organogenesis which occurs anterior to posterior

23-24 hours ______

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4-somite stage ______

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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 ______

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This is the source of embryonic ______stem cells ______

The remainder ______is 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 blastocyst in the uterus induces development ______of both maternal and embryonic structures to establish a supply of nutrients, waste removal, shock absorption, etc. ______

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Mammalian gastrulation is very much like that of ______birds and reptiles, even though we don’t have any where 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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