Revised Study Guide for the First Hour Exam


Some special embryological words: Learn to talk like a pro.

Polar bodies: Small cells formed on oocytes by the two meiotic divisions.

Animal pole: The place on an embryo surface where the polar bodies formed.
(In most species, yolk is less concentrated here. Frog eggs rotate with animal pole upward.) I know people who think that yolk density and distribution is what defines the animal pole, but they are wrong. It's the location of the polar bodies, and the oocyte nucleus)

Vegetal pole: The side of an oocyte or early embryo 180 degrees opposite the animal pole.

Blastomere: Any cell of a cleavage-stage or blastula-stage embryo
(Why not just call them early embryonic cells? It's traditional.)

Blastula: A stage of development of a sea urchin or an amphibian in which a hollow cavity has formed inside the embryo (with no connection to the outside)

Blastocoel: The hollow (water filled) cavity inside a blastula.

Gastrulation: Active movement of cells from the surface into the interior of a blastula.

"Early gastrula": An embryo in which gastrulation has just begun.
     Not many cells have moved into the interior, yet.

Guess what is meant by "mid-gastrula" and"late-gastrula" stages.

Invagination: inward folding inward of part of a epithelial sheet of cells
(Caused by contraction of the concave ends of the cells; & there are other theories)

Sea urchin gastrulation occurs by invagination. Human lungs form by invagination of part of the archenteron.

Archenteron: The future digestive tract, formed during gastrulation.

Blastopore: The opening into the archenteron.

    In sea urchin embryos, the blastopore forms at the vegetal pole.

    In frog and salamander embryos, the blastopore forms below the equator, on the side of the embryo that will become the posterior.

Stomodeum: A small invagination that becomes the mouth. Connects to the archenteron. In sea urchin gastrulas, it forms near the animal pole.

Ingression: Movement of epithelial cells out of an epithelium, and change to becoming mesenchymal. Partly caused by weakening of cell-cell adhesion, and increased adhesion to extracellular fibers on the side of the epithelium toward which the cells move.

For example, the primary and secondary mesenchyme cells of sea urchin embryos are formed by ingression.

In bird, reptile & mammal embryos, gastrulation is by ingression instead of by invagination.

Involution: A variation of invagination (in frog & salamander gastrulation) in which epithelial cells "roll" into the interior of the embryo. The blastopore is a curved slit, when formed this way. As this slit elongates, its ends curve downward, and eventually meet each other, forming a circle ("yolk plug stage").

Epithelial fusion Some examples are the fusion of the archenteron with the stomodeum and the fusion of one neural fold with the other neural fold.

Cavitation (Of masses of mesenchymal cells, to form epithelial tubes, or sheets), Reorientation of cells to form an epithelial sheet surrounding a water-filled cavity. The blastocoel is formed by cavitation. Teleost fish form their neural tube by cavitation, but other vertebrates form their neural tubes by folding.


Thought questions:

a) Would you expect the first polar body to be haploid, or diploid?
[answer: diploid]

b) What about the ploidy of a second polar body?
[answer: haploid]

c) Suppose that a sperm fused with a second polar body; might it then develop into some part of the person who develops from that oocyte? (explain your reasoning, pro or con.) answer: Yes, it might become part of the person's body. It is haploid and if it fuses with a sperm it would become diploid.

d) By what evidence might you be able to detect that part of a person's body developed that way? (hint: You wouldn't actually need to see it happen.)
[one eye blue, the other eye brown, etc., or other genetically controlled properties in different parts of the body]

e) If the spindle of a meiotic division (of an oocyte) got moved away from the animal pole, then the cleavages would divide to oocyte into more equal parts (i.e. either one or both polar bodies would be much bigger, even as big as a quarter or half the oocyte). What if such abnormal polar bodies developed as parts of the body?
Then they could differ genetically.

Why decide differentiated cell type by such active rearrangements as invagination, ingression and other "morphogenetic movements"?
Nobody knows for sure. It might be because forces influence cell differentiation.


What function is served, do you guess, by keeping extra sets of chromosomes until so late?
[answer: transcribing lots of RNA]
All four sets continue transcribing RNA to be stored in the oocyte.

Is there really a certain "moment of conception" at which time each individual becomes genetically unique?

[answer: If there were such a moment, it would be after fertilization, because meiosis isn't finished until after fertilization. ]
This is a matter of opinion, but the fertilized human oocyte is triploid until the second meiotic division is completed.

Incidentally, sea urchin oocytes really do become haploid before fertilization. Also jelly-fish.
But all other phyla are like vertebrates. Many are like dogs in which fertilization occurs before both meiotic divisions.

A) List 5 important differences between sea urchins versus mammal development.

B) List 12 important similarities

See the notes from the first lecture (January 11) for the answers to these questions.

Fertilization occurs when, relative to fertilization?

    C) In human oocytes?
    [answer: before the second meiotic division is finished]

    D) In dog oocytes
    [answer: before either meiotic division is completed]

    E) In Jelly-fish oocytes
    [answer:after the second meiotic division is complete, as in echinoderms]

    F) Name one other phylogenetic group of animals that are like jelly-fish in this respect.
    [answer: echinoderms]

G) What are the three primary germ layers?
hint: E, M, E.

H) What are two organs that develop from each of these, in mammals?
[ectoderm - brain, outer layer of skin
mesoderm - muscles, skeleton, kidneys
endoderm - lung, liver, pancreas]

I) Describe at least three active cell rearrangements.
[answer: Invagination, ingression, cavitation, ...]

J) Sketch cell arrangement in sea urchins at the 4 cell stage.

K) Sketch cell arrangements in a mammal embryo at the 4 cell stage.

L) Do mammal embryos sometimes not have a 4 cell stage? Explain why they might not.
[answer: Cleavages are not synchronous.]

M) Sketch cell arrangements at the blastula and blastocyst stages.

N) Blastocysts are a stage of embryonic development in...

    just sea urchins?
    just vertebrates?
    just mammals?
    just humans?
    most multicellular animals?
[answer: just mammals]

O) Blastulas are a stage of embryonic development in…

    just sea urchins?
    just vertebrates?
    just mammals?
    just humans?
    most multicellular animals?

[answer: many multicellular animals, but not mammals or teleosts or insects]

P) Morphogenetic cell movements occur during the development of? just sea urchins? etc.
[answer: all multicellular animals]

Q) Ectoderm, mesoderm and endoderm occur in? just sea urchins? just vertebrates, etc.
[answer: they all occur in embryos of nearly all multicellular animals]

R) Hox genes? Are they just in sea urchins, just in vertebrates, or what?
[answer:nearly all multicellular animals; maybe not sponges]

S) Colinearity, of locations of gene transcription versus locations of genes on chromosomes?
? Urchins, vertebrates, mammals, humans, most animals?
[answer: most animals]

T) What is the difference between holoblastic versus meroblastic cleavage?
[in holoblastic cleavage the cleavage furrow goes all the way through; in meroblastic cleavage, the furrow goes only partway through because of the yolk]

U) Rapid voltage changes occur in nerve cells, muscle cells and what other kind of cell?
[answer: oocytes]

V) If a human oocyte were simultaneously fertilized by two sperm, instead of one, then how many complete sets of chromosomes would it then have?
[answer: 4]

Y) Would it surprise you if hox genes were discovered in Paramecia, or other kinds of one celled animals? Would this change current beliefs in any important way?
[answer: yes; somewhat]

Z) What effect can mechanical forces sometimes have on undifferentiated stem cells?
[answer: change differentiation]

Suggest what this might explain in normal development.
Invent some way to take advantage of this effect for medical purposes.


Questions marked with a * are not easy, but you will gain a lot from thinking carefully about them. I won't ask many such difficult questions.

A) List and briefly describe (which should include drawing a sketch of each) at least four morphogenetic cell movements.

B) Describe and sketch several specific embryological example of invagination.

The following are nine examples of invagination:
Infolding of the archenteron in echinoderm embryos; or in amphibian embryos; or the infolding of the stomodeum of echinoderms, or amphibians, or mammals, or birds; infolding of the neural tube in embryos of amphibians, birds or mammals.

C) Describe and sketch specific embryological examples of epithelial fusion. Fusion of the archenteron with the stomodeum, in embryos of echinoderms and amphibians. Fusion of the neural folds with each other

D) Describe and sketch several specific embryological example of ingression. In echinoderm embryos, formation of the primary mesenchyme (which secretes the skeleton) and the secondary mesenchyme (which differentiates into muscle cells); formation of the neural crest in vertebrates

F) Which of these morphogenetic movements consists of epithelial cells becoming converted into mesenchymal cells? [all cases of ingression, primary mesenchyme formation, etc.]

*G) The neural tube of teleosts forms by rearrangement of a solid rod of cells into a hollow epithelial tube. This process is called "cavitation". In what sense is it the reverse of ingression?
[ingression converts epithelial cells to mesenchymal cells]

H) What is at least one other example in which a solid mass of cells rearranges to form an enclosed cavity?
[answer: blastocoel formation]

I) What is a fundamental difference between the blastocyst of mammal embryos and the blastula of echinoderm (sea urchin) embryos?
[the whole blastula becomes part of the body in sea urchins, but only about 5% of a blastocyst becomes part of the body]

J) How similar are they? Make a sketch of each. How could you tell them apart?

K) About what percentage of the cells in a mammal blastocyst will become parts of the body of the animal?
[about 5%]

L) What structures are formed by the other 95% of the blastocyst cells?
[part of the placenta]

M) List at least five examples of embryological changes in shapes or positions of organs in which the driving force was long believed to be growth, in the sense of enlargement and/or mitotic division of cells. Diagrams of six examples are shown in this drawing [web link]. Other examples are elongation of capillaries and nerve axons.

*N) Suggest experiments by which you could test whether a an embryological process is directly caused by cell enlargement, or is caused by mitosis, or is caused by the exertion of traction by cells.

*O) Sketch from memory the diagram of the branching railway tracks leading to 'brain", "sensory nerves", "skin", "notochord", "dermatome", "myotome", "sclerotome", "kidneys", & "heart".

P) An exam question might consist of this branching pattern, with blanks instead of words, and you would be asked to fill in the blanks.,

Q) Argue true or false: "The concept of embryonic induction is that the metaphorical railway switches in this diagram are controlled by specific signaling chemicals secreted by nearby cells."

*R) Suggest possible reasons why geometric rearrangements of embryonic cells need to occur in order for the eventual 250 differentiated cell types to be located in the correct anatomical locations, relative to each other. (I am guessing there must be some reason. Why not just form the nervous system by differentiation of cells that were already located in the interior of the blastula, etc.?)

    ?Perhaps because movements are an effective way of sharpening the boundaries between organs?

    ?Perhaps because physical forces are used as signals that contribute to induction of cell differentiation?

    ?Perhaps migrations of cells cause them to become located next to a sequence of different cells, from which they get a series of different chemical signals, that (for some reason) they need to receive one at a time, in certain sequences?

    ?Please suggest some other possible reason.

T) Based on what you now know about some the normal cell rearrangements that occur during embryonic development, please invent some possible birth defects.
For example, what if the edges of neural tube failed to fuse with each other?

U) Chemical signals that can stimulate embryonic induction can also cause birth defects: Explain why you could have predicted this.
[They might cause extra organs to form in the wrong place]

V) Many scientists now hope to develop methods to re-grow damaged organs, starting with undifferentiated "stem cells". Explain why they will need to use chemicals that can cause embryonic induction.
[to form new organs]

W) Explain the difference between holoblastic cleavage versus meroblastic cleavage.
[in the latter, yolk prevents the cleavage furrow from cutting all the way through]

X) Compare amounts and distributions of yolk in oocytes of sea urchins, mammals, teleosts, amphibians, and birds.

Y) Imagine that you discovered oocytes of some unknown kind of animal, could you predict whether they would undergo holoblastic or meroblastic cleavage, based on size of the oocyte, amount of yolk, and/or un-eveness of yolk distribution? Please explain.
[If lots of yolk, densely packed into the oocyte, then meroblastic]

Z) Why does mammal gastrulation (i.e. gastrulation of mammal embryos) resemble bird gastrulation much more than mammal gastrulation resembles gastrulation of amphibian embryos?
[because mammals evolved from egg-laying mammals and from reptiles]


A) What are the advantages of zebra fish for research? List at least six.
You can suggest some that are not listed on the web page.

B) Please rank these advantages in order of importance, and explain your reasons.
Most important, second most important, least important?

C) Are there (Is there?) any advantages that is/are absolute "deal-breaker", in the sense that no animal or plant that lacks that (those) properties could be used as model research organisms?
[Can you keep the organism in captivity? Can you fertilize the oocytes? Can you raise embryos to adults?]

*D) A certain species of sponge is listed on the internet as being a "model organism", although they do not reproduce in captivity and live only on the Great Barrier Reef, along north-eastern Australia.
Should we add the following new criterion for choosing model organisms: "In order to study them, scientists must go to beautiful tropical islands"?
This is meant to be humorous

*E) Would actual Zebras be as good as Zebra Fish as experimental animals? Why not?
[answer: No? Long life cycle, Large size; Lions eat them.]

F) Xenopus laevis is now being replaced by Xenopus tropicalis as a "model organism" for the reason that X. laevis turned out to be tetraploid. Figure out why tetrapoidy is such a disadvantage. (Hint: genetic manipulation is more difficult; imagine knockouts)

G) Teleost oocytes are spherical until fertilized, but cytoplasm then flows into a hemispherical bump at the animal pole. Suggest what force would be capable of causing this flow? You are welcome to suggest two or more possible forces, or sets of forces.

H) What and where is the "Yolk Syncytial Layer"?

I) What and where is the "Enveloping Layer"

J) What and where are the "Deep Cells"

K) Which of these develop into the body of the fish?
[answer: just the deep cells]

L) Draw cross-sections of teleost embryos at several stages of development?

*M) If you wanted to test whether embryonic induction is a normal part of teleost development, how would you try to do this?
[graft an embryonic structure to an abnormal location]

*P) Suppose that a researcher inserted a micropipette through the enveloping layer and sucked out about half of the deep cells (before gastrulation), and then let gastrulation and body formation continue as normally as it could, which of the following might happen?

    may ? A half-sized fish would develop, with all the organs half their normal sizes?
    [answer: probably]

    ? A tail-less fish would develop, in which the head and body were normal size, but no posterior organs developed?
    [answer: maybe]

    ? Gastrulation failed to occur, and no parts of the fish developed?
    [answer: possibly]

? Can you invent/imagine further results that might occur? (maybe headless fish?)?

What could each of these results tell us (or hint to us) about normal mechanisms of fish development?

*Q) When and where would you expect each of the hox genes to begin being transcribed to make their messenger RNAs? First the most 3-prime hox genes, then the hox gene next to it in the 5-prime direction, then the third one along the 3' - 5' axis?

*R) Design some experiments that would take advantage of one or more of the peculiarities of teleost embryos to help answer some fundamental biological question.

S) Suppose that teleosts have evolved a fundamentally different set of mechanisms to neurulate, gastrulate, or to control cell differentiation (as compared with the mechanisms evolved by mammals) then does that mean that discovery of the human mechanisms will be delayed or even prevented?
[Probably delay, because people will have mistaken preconceptions]

(In fact, teleost embryos form their neural tube by hollowing out a solid rod of cells, as compared with folding a flat sheet of epithelial cells, as occurs in all other vertebrates)

*T) Deep cells converge toward the body axis equally from right and left, adding themselves to the posterior end of the body (first head, then neck, then,,,, etc and finally tail. What might happen if you physically removed all or most of the deep cells to the left of the body axis, just before they had a chance to add themselves to the body?

And what could each possible result tell you about normal mechanisms of development?

Visualize an experiment in which deep cells from a teleost embryo were inserted into the blastocoel of a gastrulating salamander embryo.

*U) Compare this with Pröscholdt's discovery of induction

X) If conjoined twins developed from a teleost embryo, describe and sketch sequential stages by which such twinning could/would occur.

*Y) Could conjoined twins develop (in any kind of animal) in such a way that very different combinations of hox genes are transcribed in one twin than the other, at the location where they are merged? [probably, yes]

If this never happened, what would you interpret about the mechanism(s) that control which hox genes are expressed in different parts of the body?

*Z) Invent an imaginary sequence of early embryonic events, as different as possible from what actually occurs in teleosts, amphibians, birds or mammals.


*A) What are some testable predictions of the assumption that the reptile&bird pattern of development evolved first in egg-laying species? [I have just thought of one! Hint: I got the idea from Ichthyosaurs, which could not crawl up on beaches to lay eggs]

*C) What do primates have in common with rodents that could explain the similarity of implantation of their embryos?
[answer: Hide in trees or burrows. Mothers & newborns are less vulnerable.]

*M) Would it be possible for a species of reptile or bird to have identical twins?
[answer: yes]

N) Would it be possible for a species of reptile or bird to have conjoined twins?
[answer: they frequently do]

> P) Why can't you observe belly-buttons on birds?
Because feathers are not transparent. Birds do have them; even turtles & snakes do.

Q) If an indentation in the flexible shell of a reptile pushed directly on the anterior end of the primitive streak, then what sort of birth defect might result?
[answer: two heads]

R) Would you expect it to be possible for the same kind of birth defect to occur in a bird?
[answer: sure, why not - but less likely because egg shells are not flexible]

S) Is it possible for early mammal embryos to implant in the fallopian tubules or other tissues?
[answer: "Ectopic pregnancy" Occurs all too often.]

T) WHY is it possible for early mammal embryos to implant in the fallopian tubules or other tissues?
[answer: They can digest any other tissue!]

U) If a set of genetically identical triplets were born, with two of the three having been inside the same chorion and the third having been surrounded by a completely different chorion, then describe the events that must have happened to produce the bodies of the triplets.

V) If each of a pair of twins were implanted at different parts of a uterus, some distance apart, then what event must have occurred to produce this result?
[answer: two different blastocysts]

A) What fraction of babies are born with birth defects in the USA?
[answer: 5%]

*D) What are "septal defects"?
[answer: holes between the right and left sides of the heart]

E) If a certain protein or other chemical has the effect of inducing formation of a second neural tube when implanted into the blastocoel, why would you predict that this chemical might be a teratogen?
[yes; two brains would be abnormal]

F) For the purpose of testing chemicals to find out whether they might cause human birth defects, please design (and describe) a test using chicken eggs.

G) If mice were discovered that had a consistently high frequency (let's say 50%) of spina bifida or some other birth defect, suggest research projects to use these mice for the following purposes.

    1) To discover more chemicals (equivalent to folic acid) that could reduce the frequency of spina bifida if more of that chemical were in people's food.
    2) To discover chemicals or other treatments what might produce increased frequencies and seriousness of spina bifida.
    3) To discover new kinds of treatments for human babies born with spina bifida.
    4) Please try to invent some other use for such mutant mice... (I can think of two more)

K) Based on your knowledge of normal embryonic development , please invent or imagine a birth defect. Describe what goes wrong and what the symptoms would be?

M) Suppose that a frog was discovered in which part of the body is haploid: What might have gone wrong in meiosis that could have produced this result?
[answer: if the second polar body became part of the blastula]

N) Suppose that you had some way to cause development of sperm that have no DNA (or whose DNA has been damaged so badly that it can never replicate or make mRNA). If you fertilized a fish egg with these sperm, what would be the result?
[answer: haploid fish]

O) What if, in addition, you prevented the first cleavage division from occurring (in one of your oocytes that were fertilized with inactivated sperm), but then let all further cell divisions occur normally.
[Hint: What would be unusual about the genes in the resulting fish?]
[answer: Fish homozygous for ALL their genes! This has been done thousands of times, and is a standard method. A LOT easier than inbreeding.]

P) What are two different ways that you could produce triploid animals?
[answer: Fertilize with two sperm or resorb the second polar body]

Q) Imagine that an embryo's first polar body was as large enough to become one of the individual embryonic cells, what sort of detectable abnormality could this produce?
[answer: part of the body would be genetically different from the rest]

*S) Would you tend to guess that conjoined twinning results from

    (1) One embryo splitting into two, but with one part of the body not separating.
    (2) Two (originally separate) embryos adhering together but merging only parts of their bodies.
    [either could be true]

*T) What kind of evidence would/could help answer this question?

    (1) Would it help answer this question if a pair of identical twins were born sharing two different parts of the body? (in the sense of being joined at two separate places)

    (2) Or would this disprove both possible mechanisms? [Hint: No]

    (3) Does the Nobel Prize-winning discovery of Proshöldt and Spemann influence your conception of what sorts of process might be the cause of conjoined twins? [the cause could be induction of a second body]

V) In a few percent of identical twins, one of the twins has its aorta on the right side, and all its abdominal organs (stomach, spleen, intestine, chambers of the heart) are mirror images of the normal geometry of these asymetrical organs. This is called situs inversus viscerum, and can also occur in babies that are not twins. Please invent a possible cause, in general terms, it happens because development of these organs is partly controlled by...?

Z) If a pair of twins were conjoined head to back (which doesn't really happen), in what sort of spatial pattern would you expect hox gene expression to occur? Please make a sketch.






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