Specification: Zebrafish & Chicken (lec 15/16)

Question 1

In frogs which pathways must be activated or inhibited for the formation of:

1) Brain
2) Spinal cord neurons
3) Epidermis

Answer 1

Brain: Inhibition of both Wnt and BMP pathways

Spinal cord neurons: Activation of Wnt and absence of BMPs

Epidermis: Activation of both Wnt and BMP pathways

Question 2

In frogs, the ____ gradient specifies posterioranterior polarity, and the _____ gradient specifies dorsal-ventral polarity.

Answer 2

Wnt; BMP

Question 3

Why do we study animal development using Zebrafish?

Answer 3

1) Small vertebrates (easy to store)
2) Eggs laid outside of the body (fertilization and development occurs externally)
3) Embryos are transparent (easy to visualize)
4) Development occurs quickly
5) Breeding occurs all year rounds

Question 4

What kind of cleavage do Zebrafish have?

Answer 4

Discoidal (telolecithal –> meroblastic)

Question 5

In zebrafish embryogenesis, _______ ____ plasm is evident at the 2-cell stage along the division plane and the dorsal-ventral axis is evident at the ____ stage.

Answer 5

Primordial germ; shield

Question 6

(T/F) The fates of the zebrafish embryonic cells are mapped.

Answer 6

True! They are autosomal!

Question 7

Fill in the blanks regarding zebrafish blastula:

1) The animal pole contains the _______, while the ventral-dorsal region contains the ______, and the vegetal pole contains the __________.

2) The notochord cells are found in the ______ region, while blood cells and somite muscle cells are found in the _______ region.

Answer 7

1) Ectoderm; mesoderm; endoderm

2) Dorsal; ventral

Question 8

(T/F) During epiboly in zebrafish, the vegetal cells move to the animal pole.

Answer 8

False!

During epiboly, the animal cells (ectoderms) move to the vegetal pole (endoderms).

Question 9

Briefly answer the questions regarding zebrafish development.

1) What happens to the cells at the border during epiboly?

2) What happens when the cortical cytoskeleton is prevented from contracting?

3) How can the mesodermal genes be expressed even when the cortical cytoskeleton is not contracting?

Answer 9

1) The cells at the border undergo structural changes and INVOLUTE. As they do, MESODERMAL GENES are activated.

2) When the cortical cytoskeleton is prevented from contracting, the animal cap cells (ectodermal) remain ectodermal and do not involute (do not become mesodermal).

3) If the animal cap cells are pulled by a magnetic field, the mesodermal genes are expressed.

Question 10

What mesodermal gene is expressed during involution in zebrafish?

Answer 10

Mesodermal gene NO TAIL is expressed by stretching and subsequent involution.

It is blocked by the lack of involution!

Question 11

______ ____ in zebrafish is the functional equivalent of the organizer of amphibians.

Answer 11

Embryonic shield

*both specify the dorsal-ventral axis

Question 12

(T/F) As the cells are undergoing epiboly and involution, the cells start to converge and extend towards the ventral region, forming the embryonic shield.

Answer 12

False!

As the cells are undergoing epiboly and involution, the cells start to converge and extend towards the DORSAL region, forming the embryonic shield.

*most of these cells express no tail gene and thus become mesodermal cells

Question 13

1) What happens when you implant the embryonic shield into the ventral region of another zebrafish embryo?

2) How else can the effect be achieved?

Answer 13

1) Development of a secondary axis!

2) The same effect can be achieved by activating β-catenin in embryos at sites opposite (ventral) where the embryonic shield (dorsal) will form.

Question 14

______ is found in the dorsal region of the zebrafish embryo and inhibits _______ (found in the ventral region).

Answer 14

Chordin; BMP

Question 15

In zebrafish, both nodal and BMP operate through the downstream actions of phosphorylated _____.

Nodal promotes _________ and _______ specification, while BMP induces _______ cell fates for all germ layers.

Answer 15

Smad

Endoderm; Mesoderm

Ventral

Question 16

Which gene inhibits nodal?

Answer 16

Lefty!

Question 17

(T/F) Amniote vertebrates develop eggs on land.

Answer 17

True

Question 18

1) What are the functions of these in amniote vertebrate embryo:
a) Amnion
b) Allantois
c) Blood vessels

2) How is this arrangement modified in mammals?

Answer 18

a) The amnion provides a “water sac” in which the embryo develops

b) The allantois stores wastes

c) Blood vessels of the chorion exchange gases and nutrients from the yolk sac

2) In mammals, this arrangement is modified such that the blood vessels acquire nutrients and exchange gases via a PLACENTA joined to mother’s uterus rather than from the yolk sac.

Question 19

What kind of cleavage occurs in chicken?

Answer 19

Discoidal

Question 20

(T/F) In zebrafish both the epiblast and the hypoblast form the embryo. In chickens, the epiblast forms the embryo but the hypoblast forms the extra embryonic membrane.

Answer 20

True!

Question 21

Cleavage occurs in the _____ ______ in chick embryos.

Answer 21

Area pellucida

Question 22

After a lot of cleavage occurs in the chick embryo, there is ______ of cells in the posterior end aka ______’s ______.

Answer 22

Thickening; Koller’s sickle

Question 23

What is the primitive streak in chick embryo? How is it formed?

Answer 23

Primitive streak is homologous to amphibian blastopore.

Once Koller’s sickle forms, the cells extend toward the anterior region of the area pellucida forming the primitive streak.

Question 24

What is the Hensen’s node in chick embryo?

Answer 24

Hensen’s node is homologous to blastopore lip of organizer in amphibians.

It is also homologous to the embryonic shield of zebra fish.

Question 25

What happens after the formation of the primitive streak in chick embryo?

Answer 25

After forming the primitive streak, the cells extend more to form the primitive groove, where the cells will migrate into the embryo (allows for gastrulation).

Then, the primitive groove regresses and forms dorsal regions.

Question 26

(T/F) Primitive groove starts regression through the posterior pole which leads to gastrulation in chick embryo.

Answer 26

True!

Question 27

In the chick embryo, the cells migrating through Hensen’s node travel anteriorly to form the _________ ____ and ________; those migrating through the next anterior region of the streak travel laterally but converge near the midline to make _______ and _______; those from the middle of the streak form ______ ______ and _______ _____ _________.

Answer 27

Prechordal plate; notochord

Notochord; somites

Intermediate mesoderm; lateral plate mesoderm

*more posterior: more mesodermal fate

Question 28

(T/F) Various pathways including Hedgehod, Retinoic acid, Notch, Wnt, and FGF signalling play a role during axis extension and cell differentiation in chick embryos.

Answer 28

True!

Question 29

How is the chick anterior-posterior axis specified? Explain.

Answer 29

Specification of the chick anterior-posterior axis by GRAVITY.

ROTATION in the shell gland results in the lighter components of the yolk pushing up one side of the blastoderm. The more ELEVATED region becomes the POSTERIOR of the embryo.

Question 30

Fill in the blanks regarding the model for generating left-right asymmetry in the chick embryo:

1) On the left side of the Hensen’s node, _______ _______ activates ________, which stimulates ________ to induce the expression of ________. In the presence of this molecule, ________ gene is activated and this protein is active in the various organ primordia and specifies which side will be the left.

2) On the right side of the embryo, ______ is expressed along with its receptor ______. This activates _______, a protein that blocks expression of the gene for ________. In the absence of this molecule, Nodal is not activated and Pitx2 is not expressed.

Answer 30

1) Sonic hedgehog (Shh); Cerberus; BMPs; Nodal; Pitx2

2) Activin; IIa; Fgf8; Cerbeus

Question 31

Describe the left-right symmetry in the developing humans. When is it acquired? What organs are involved?

Answer 31

There is also left-right symmetry in the developing humans which is acquired by week 11, when the liver moves to the right and the heart moves to the left.

The symmetrical veins of the heart also regress differentially to form the superior and inferior venae cavae, which connect only to the right side of the heart.

The right lung branches into three lobes, while the left lung (near the heart) forms only two lobes.