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Question 1

In which portion of the frog's life-cycle would a frog appear most similar to a mammal?

a) The fertilized eggs of frogs and mammals are similar in size and look nearly identical.

b) A frog blastula is very similar to a mammalian blastocyst.

c) The events of gastrulation in frogs are highly similar to gastrulation and extraembryonic membrane formation in mammals.

d) Late in organogenesis, at the phylotypic stage, the body plans of frogs and mammals are remarkably similar.

e) Adult frogs are essentially indistinguishable from adult humans.

Question 2

The notochord is a ___ structure in vertebrate embryos that lies under the ___, and is flanked by ___.

a) mesodermal, neural tube, somites

b) endodermal, mesoderm, the gut

c) ectodermal, neural tube, mesoderm

d) mesodermal, ectoderm, endoderm

e) endodermal, epidermis, blastocoel

Question 3

During gastrulation in Xenopus, the blastocoel:

a) becomes the gut.

b) is filled with endodermal cells and disappears.

c) is filled with mesoderm and disappears.

d) does not change, but develops into what is called the archenteron.

e) is displaced, and its original location becomes an endoderm lined cavity, the archenteron, which is a precursor to the gut.

Question 4

The process in which the three germ layers form is called:

a) fertilization.

b) cleavage.

c) gastrulation.

d) organogenesis.

e) metamorphosis.

Question 5

During gastrulation in Xenopus, the future mesoderm and endoderm move inside the embryo through the "blastopore"; in contrast, in chickens, gastrulation involves cells moving inward through the:

a) blastoderm.

b) yolk.

c) cleavage furrow.

d) primitive streak.

e) gut.

Question 6

In mammalian development, the embryo will form from which population of cells?

a) The blastocyst.

b) The inner cell mass.

c) The trophectoderm.

d) The blastocoel.

e) The extraembryonic membranes.

Question 7

Maternal factors are:

a) energy rich proteins and lipids packaged into the vegetal pole.

b) hormonal influences encountered in the uterus in mammals.

c) mRNAs and proteins packaged into the egg by the mother, which act to control early development in the embryo.

d) genes contributed to the embryo by the maternal haploid genome.

e) components of the cortical granules which prevent polyspermy.

Question 8

The role of maternal factors (such as Vg-1, Xwnt-1, and VegT) packaged into the vegetal region of the Xenopus oocyte is:

a) in establishing the future dorso-ventral body axis.

b) to allow the vegetal region to divide during cleavage.

c) to make the vegetal region denser, so that the egg always floats with its animal pole upward.

d) to specify the future animal-vegetal body axis.

e) to provide energy and nutrients for the developing embryo.

Question 9

Between fertilization and the first cleavage division, actin filaments pull the cytoplasm of the frog egg toward the point of sperm entry in a process called:

a) cortical rotation.

b) dorsalization.

c) blastulation.

d) gastrulation.

e) determination.

Question 10

In frogs, maternally packaged Xwnt-1 leads to nuclear localization of _____ in the _____, which in response signals to adjacent cells to become the _____.

a) β-catenin, Nieuwkoop center, Spemann organizer

b) nodal, signalling center, shield

c) Vg-1, vegetal region, Nieuwkoop center

d) sonic hedgehog, limb bud, posterior

e) GFP, mesoderm, muscle and bone

Question 11

Remarkably, Vg-1 and wnt are used in both frogs and chicks to establish a body axis, although in chicks this is the antero-posterior axis instead of the dorso-ventral axis. What is the region of the chick embryo that is thus analogous to the Nieuwkoop center?

a) Area pellucida.

b) Blastoderm.

c) Primitive streak.

d) Posterior marginal zone.

e) Koller's sickle.

Question 12

Although the vertebrate body plan outwardly displays a mirror-image symmetry with regard to left and right, the internal organs are not symmetrical. How does this left-right asymmetry develop?

a) The two cells of the Nieuwkoop center (at the 32-cell stage, see Figure 3.32) signal each other and become either left or right.

b) Maternal factors that determine left and right are packaged into the egg, just as dorsalizing factors are.

c) Differential release of Ca²⁺ ions on the left side leads to expression of Nodal and Pitx2 on the left.

d) Gravity determines the left-right axis: that side which is down becomes right and that side which is up becomes left.

e) No special mechanism is required, since the formation of an antero-posterior axis and a dorso-ventral axis automatically determines left and right.

Question 13

What is the difference between "fate" and "specification"?

a) If cells are transplanted from their normal region in an embryo to a different region in a recipient embryo, such cells will alter their fate, but not their specification.

b) Cell fate describes the allocation of cells to the germ layers, ectoderm, mesoderm, or endoderm, whereas specification describes the exact tissues that each cell will ultimately become.

c) The fate map of an embryo does not change during development -- the fate map of an egg is the same as the fate map of a late blastula -- whereas the specification map of an embryo changes continually as the embryo's development proceeds.

d) The specification state of a cell is determined by transplantation of cells to recipient embryos, whereas the fate of a cell is determined by the culture of those cells in an artificial medium.

e) The fate of a cell is determined by labelling that cell and following it during normal development, whereas the specification state of a cell is determined by culturing a cell in an artificial medium and observing what tissues form from it.

Question 14

A fate map of a Xenopus blastula, just before gastrulation begins, shows (Figure 3.44) that the top portion of the embryo will become ectoderm (skin and nerve), the central portion will become mesoderm (bone, muscle, and blood), and the lowest portion will become endoderm (gut). How is it that the endoderm and mesoderm, shown on the outside in the fate map, end up on the inside in the embryo after gastrulation?

a) The endoderm ends up inside through a process that can be visualized as if one pokes their finger into the bottom of a soft ball until the lowest endodermal portion ends up deep inside, the marginal zone mesoderm also ends up inside, and the ectoderm now encloses the entire outer surface.

b) The endodermal cells begin first to move into the embryo through the blastopore, displacing the blastocoel and forming a gut; as gastrulation proceeds, the blastopore spreads sideways and the mesoderm follows the endoderm in, ending up between the endoderm and the ectoderm.

c) The ectodermal cells divide and spread down over the rest of the embryo, so that the endoderm ends up on the bottom, ventral, surface of the embryo, and the mesoderm ends up around the middle, just as is shown in the fate map.

d) The three germ layers adopt their final positions through, first, the growth of the mesodermal marginal zone down over the endoderm, followed by, secondly, the growth of ectoderm down and over the mesoderm.

e) The cells, having already been fated to form endoderm and mesoderm, migrate directly into the interior of the embryo and take up residence in appropriate places.

Question 15

What property of the mammalian embryo permits the production of chimeric mice?

a) The inner cell mass is highly regulative, so that extra cells derived from the inner cell mass of a different embryo are incorporated without causing defects.

b) If half of the cells of the inner cell mass are removed and replaced with cells from half of a different embryo's inner cell mass, the embryo will heal and develop normally.

c) The embryo can develop outside the womb, making surgical manipulations possible.

d) Blastomeres separated at the two-cell stage will regulate and each forms a normal embryo.

e) Genetically determined pigmentation differences are incompatible in the developing embryo.

Question 16

If cells from the animal pole of a frog blastula (animal cap cells) are placed into contact with cells from the vegetal hemisphere, after removing the cells fated to form mesoderm, what is the result?

a) The cells regulate to form a normal embryo.

b) Animal cap cells are induced to form mesodermal derivatives.

c) Vegetal hemisphere cells are induced by the animal cap cells to form mesodermal derivatives.

d) An embryo forms with only ectodermal and endodermal derivatives.

e) Both animal and vegetal cells lose all specification and an undifferentiated ball of cells results.

Question 17

The "mid-blastula transition" is the point in development when:

a) translation of maternal mRNA is initiated.

b) cell determination becomes fixed.

c) cell division in the embryo ends.

d) transcription of zygotic genes begins.

e) blastocoel formation occurs.

Question 18

What is the mechanism by which cells fated to be mesoderm form in the marginal zone of the Xenopus embryo?

a) Vegetal region cells migrate toward the animal pole and become mesoderm.

b) Cells in the marginal zone are fated to be mesoderm, but they do not become specified or determined as mesoderm until after they move internally during gastrulation.

c) Cells of the vegetal region are induced by their proximity to the animal region cells to become mesoderm.

d) Maternal factors packaged into the marginal zone act on these cells to instruct them to become mesoderm.

e) Secreted signals of the TGF-β family are released by the cells of the vegetal region and act on the adjacent cells of the marginal zone to instruct them to adopt a mesodermal fate.

Question 19

How is the Spemann organizer distinguished from the other mesodermal cells during induction by the cells of the vegetal region?

a) The cells of the Spemann organizer become different as they pass over the dorsal lip of the blastopore during gastrulation.

b) β-catenin, which has become localized to the nucleus in the Nieuwkoop center as a result of cortical rotation, turns on high levels of the TGF-β signalling molecule, Nodal, and this relatively high level distinguishes the adjacent cells of the Spemann organizer from the rest of the mesoderm.

c) The Spemann organizer is formed from cells of the Nieuwkoop center, which have migrated upward in the embryo.

d) BMP-4 signalling from ventral mesodermal cells toward the dorsal region specify the dorsal-most cells as organizer.

e) Signals from the animal region combine with signals from the vegetal region in the dorsal-most mesoderm, and specify those cells as the Spemann organizer.

Question 20

Patterning of the Xenopus mesoderm along the dorso-ventral axis involves an antagonistic relationship between chordin and BMP-4. Is there an analogy here to dorso-ventral patterning in the Drosophila embryo?

a) Yes, the Drosophila homolog of BMP-4, Decapentaplegic (Dpp), is antagonized by the Drosophila homolog of chordin, short gastrulation (Sog), in patterning of the dorso-ventral axis.

b) Yes, Drosophila also uses BMP-4 to specify ventral fates and chordin to specify dorsal fates.

c) Yes, the Drosophila Dorsal protein is identical to the vertebrate BMP-4 protein, and specifies the ventral region of the embryo.

d) No. the closest analogy from Drosophila would be the antagonism between Hunchback and Nanos, but neither of those are signalling molecules, and they act to pattern the antero-posterior axis.

e) No, insect development is totally different than that of real animals like frogs and chickens.

Oxford University Press, Online Resource Centre, Chapter 03.

Chapter 03

Instructions