After Midterm 1 - Blastulation, Gastrulation and Body-Axis Formation in the Chick

Question 1

In the chick, cell division occurs in a small island of cells in the animal pole, floating on top of the giant yolk, called the _________.

Answer 1

blastoderm or blastodisc

Question 2

At the very beginning, the blastoderm of the chick is multi-layered. Then the inner layers shed off/die, leaving only a single layer called the ________

Answer 2

epiblast.

Question 3

The space between the epiblast and the yolk, prior to the formation of the hypoblast, is referred to as the ____________

Answer 3

Subgerminal space

Question 4

What is the area pellucida?

Answer 4

The portion of the blastoderm that contains the single-cell layer epiblast and is the inner portion of the blastoderm, if you’re looking down at the embryo as a circle.

Question 5

What is the area opaca?

Answer 5

The cells surrounding the periphery of the area pellucida, which are several layers thick (the inner cells have not shed/died off), not one-cell layer thick like the area pellucida.

Question 6

After the formation of the hypoblast, the space between the hypoblast and epiblast is the ________

Answer 6

blastocoel.

Question 7

True or false. Before even gastrulation begins, the dorsal-ventral axis and the anterior-posterior axis of the chick is already established

Answer 7

True

Question 8

Where is the posterior marginal zone (PMZ) of the chick blastoderm located?

Answer 8

It’s located in one place where the area opaca meets the area pellucida.

Question 9

What is the koller’s sickle?

Answer 9

A structure that forms at the anterior-most edge of the posterior marginal zone.

Question 10

The hypoblast is formed from two sources. Explain

Answer 10

The hypoblast is formed from primary and secondary hypoblast cells.

Primary hypoblast cells are epiblast cells that have delaminated and formed islands of cells called hypoblast islands.

Secondary hypoblast cells form and extend from the Koller’s sickle.

The hypoblast is formed as secondary hypoblast cells extending from the Koller’s sickle joins hypoblast islands together.

Question 11

How do you know where is anterior and where is posterior of the chick embryo before gastrulation?

Answer 11

Where the Koller’s sickle forms is the posterior end, and the opposite end of the blastoderm is the anterior end.

Question 12

Where is the wnt-signalling pathway, and therefore beta-catenin, localized in the chick embryo?

Answer 12

Localized in the Koller’s sickle.

Homologous in function to the micromeres in the sea urchin and the organizer/cells of the dorsal blastopore lip of the xenopus

Question 13

What structure is it whose formation tells us that the chick embryo has begun gastrulation?

Answer 13

the primitive streak

Question 14

How does the primitive streak form?

Answer 14

Cells at the Koller’s sickle, in between the epiblast and hypoblast layer, will begin to extend via convergent extension from posterior to anterior, and these cells extending in between the epiblast and hypoblast forms the primitive streak.

Question 15

What’s the middle of the primitive streak called?

Answer 15

The primitive groove. The primitive streak is the whole structure, including the ridges around the primitive groove.

Question 16

What happens at the primitive “groove”?

Answer 16

Epiblast cells begin to go through first a small invagination, following by those cells currently in the primitive groove undergoing ingression via EMT. These ingressing epiblast cells will travel into the blastocoel.

Question 17

The cells that go through EMT in the chick embryo primitive groove are ______cells

Answer 17

epiblast cells

Question 18

Does the primitive streak elongate across the entire area pellucida?

Answer 18

No, but it does extend along the majority of the area pellucida.

Question 19

The primitive streak of the chick is analogous FUNCTIONALLY to the ________ in other model embryos, and STRUCTURALLY/MORPHOLOGICALLY to the _________ of the Xenopus embryo

Answer 19

Functionally, the primitive streak acts like the blastopore

Morphologically, the primitive STREAK is like an extended blastopore lip.

Question 20

Is the process of EMT that happens in chick gastrulation similar to EMT in sea urchin PMCs?

Answer 20

No. Different proteins and receptors are involved, but the steps are the same (lose affinity to x, gain affinity for y).

Question 21

What happens to the first set of epiblast cells that ingress into the blastocoel in the chick? I’m referring to the cells at the leading edge of the primitive groove? What germ layer do these cells become?

Answer 21

The cells that first ingress displace the hypoblast layer at the bottom of the blastocoel completely.
These cells become endoderm.

Question 22

What happens to the rest of the epiblast cells (not at the leading edge) that ingress into the blastocoel of the chick? What germ layer do these cells become?

Answer 22

These cells stay in the blastocoel and give rise to mesoderm.

Question 23

What happens to the epiblast cells that do not ingress/go into the primitive groove?

Answer 23

These cells undergo epiboly, and form the ectoderm of the chick

Question 24

Describe the migration of cells in chick gastrulation after the formation of the primitive groove, in full.

Answer 24

The cells that first ingress/go through EMT from the primitive groove displace the hypoblast layer at the bottom of the blastocoel completely. These cells later become endoderm.

Cells that ingress/go through EMT afterwards remain in the blastocoel and give rise to mesoderm.

Cells that do not ingress/go through EMT via the primitive groove undergo epiboly and form the ectoderm of the embryo.

Question 25

The very anterior edge of the fully-formed primitive streak is called the __________.

Answer 25

Hensen’s node

Question 26

Why are the cells that go through ingression, and become mesoderm, at the Hensen’s node, particularly of interest?

Answer 26

These cells give rise to the prechordal plate and the notochord, similar to how some cells in the organizer region of the Xenopus give rise to the prechordal plate (induces midbrain formation) and the notochord.

Question 27

The Hensen’s node is functionally similar to what structure in the xenopus and why?

Answer 27

The organizer/cells of the dorsal blastopore lip. Because the mesoderm that forms after EMT at the Hensen’s node gives rise to the notochord and prechordal plate, just like a subset of organizer cells do in the xenopus.

Question 28

Pretend you are making a fate map of the cells that become mesoderm in the chick. After they ingress through the primitive groove, describe what direction they go and what structures they form. Do this for three regions of the primitive streak:

a) The anterior-most region, the Hensen’s node
b) Just posterior to the Hensen’s node
c) At the middle of the primitive streak

Answer 28

a) cells that ingress at the Hensen’s node move anteriorly (↑) and after they become mesoderm, they will form the prechordal plate and the notochord.
b) cells that ingress just posterior to the Hensen’s node first travel laterally (to the sides) and then anteriorly, to give rise to mesoderm that will become the somites. These cells also help with the formation of the notochord. (↑←→↑)
c) cells that ingress at the middle of the primitive streak can go in one of three directions, they can travel posteriorly (↓), they can travel laterally (←→) or they can travel anteriorly (↑), giving rise to the intermediate and lateral plate mesoderm.

Question 29

Describe where we find neural ectoderm and epidermal ectoderm based on the primitive streak?

Answer 29

The ectoderm closest to the primitive streak, surrounding it, become neural ectoderm.

The ectoderm further away become epidermal ectoderm.

Question 30

Once the primitive streak has fully formed, the Hensen’s node begins to migrate from anterior to posterior to regress and shorten the primitive streak.

a) What happens to a region after the Hensen’s node has passed it?
b) What going on in regions the Hensen’s node hasn’t reached yet?

Answer 30

a) That region is now undergoing neurulation!

b) Regions the Hensen’s node hasn’t reached yet are still going through gastrulation.

Question 31

What is one feature of chick embryos that is unique to them that we haven’t seen in any other model embryo?

Answer 31

Neurulation can happen while other regions are still under gastrulation. These steps can happen simultaneously!