Neural Tube Formation

Question 1

Where does gastrulation begin in the fertilized frog egg?

Answer 1

Side opposite the point of sperm entry.

Question 2

What is the animal cap?

Answer 2

Roof of the blastocoel in the animal hemisphere of the amphibious egg.

Question 3

What does the blastocoel do in terms of the animal cap?

Answer 3

Prevents premature contact between vegetal and animal cap cells, keeping the animal cap cells undifferentiated.

Question 4

Outline the five steps of Xenopus gastrulation.

Answer 4

1. Epiboly
2. Vegetal rotation
3. Bottle cell formation and invagination
4. Involution and cell migration
5. Convergence and extension

Question 5

Describe the epiboly step of frog gastrulation.

Answer 5

The thinning and spreading of the animal cap cells over the vegetal hemisphere.

Question 6

Describe the vegetal rotation stage of frog gastrulation.

Answer 6

Vegetal cells asymmetrically press up against the inner blastocoel roof on the doral side.

Question 7

Describe the bottle cell formation and invagination stage of frog gastrulation.

Answer 7

Localized apical constriction at the dorsal blastopore lip creates anisotropic forces that foster invagination.

Question 8

Describe the involution and cell migration stage of frog gastrulation.

Answer 8

Leading edge of invaginating cells crawls up onto the blastocoel roof.

Question 9

Describe the convergence and extension stage of frog gastrulation.

Answer 9

The targeted medial-to-lateral intercalation of cells on the midline drives anterior-posterior axis elongation.

Question 10

What is the notochord?

Answer 10

A transient mesodermal rod that plays an important role in hindgut formation and nervous system patterning.

Question 11

What portion of the frog egg becomes the ectoderm?

Answer 11

Animal hemisphere blastomeres.

Question 12

What portion of the frog egg becomes the endoderm?

Answer 12

Vegetal hemisphere cells

Question 13

What portion of the frog egg becomes the mesoderm?

Answer 13

Equatorial cells.

Question 14

What are the two major functions of the vegetal cells?

Answer 14

1. Differentiate into endoderm
2. Induce the cells immediately above them to become mesoderm.

Question 15

Describe the experiment conducted by Hans Spemann and colleaugues.

Answer 15

Used differently pigmented embryos of two species of newt.
The darker dorsal lip was transplanted into an early lighter embryo, in a region fated to become ventral epidermis.

Question 16

Describe the results of Spemann’s experiment.

Answer 16

The dorsal lip tissue invaginated as normal, disappearing beneath vegetal cells. This pigmented tissue then differentiated into the notochord and other structures that normally form from the dorsal lip.

Question 17

What did the results of Spemann’s experiment indicate?

Answer 17

Showed that the dorsal lip of the blastopore is the only tissue in the early gastrula whose fate is self-determined.

Question 18

What did Spemann refer to as the organizer?

Answer 18

Dorsal lip cells, notochord and endomesoderm.

Question 19

Why did Spemann refer to the dorsal lip cells and their derivatives as the organizer?

2

Answer 19

1. The induce the host’s ventral tissues to change their fates to form a neural tube and dosal mesodermal tissue
2. The organized the host and donor tissues into a secondary embryo with clear anterior-posterior and dorsal-ventral axes.

Question 20

What is the default fate of the ectoderm?

Answer 20

To become neural tissue

Question 21

How is the ectoderm induced to become epidermal tissue?

Answer 21

Certain parts of the embryo induce the ectoderm to become epidermal tissue by secreting BMPs.

Question 22

What are BMPs?

Answer 22

Bone morphogenetic proteins.

Question 23

How is the default fate of ectoderm maintained?

Answer 23

Organizer tissue scretes BMP inhibitors, protecting the ectoderm and enabling it to develop into neural tissue.

Question 24

List the three major BMP inhibitors secreted by the organizer.

Answer 24

Chordin
Follistatin
Noggin

Question 25

What is neurulation?

Answer 25

The process of forming the neural tube.

Question 26

What are the three major responsibilities of the vertevrate ectoderm?

Answer 26

1. One part forms the neural plate 2. Another part becomes the epidermis 3. The area between the above two is the neural crest

Question 27

What does the neural plate become, and what is this a precursor to?

Answer 27

Forms the neural tube, the precursor of the central nervous system.

Question 28

What makes up the central nervous system?

Answer 28

Brain and spindal cord.

Question 29

What process does neurulation directly follow?

Answer 29

Gastrulation

Question 30

Describe the process of neurulation.

Answer 30

Neural plate edges thicken move upward, forming neural folds and a U-shaped neural groove, which divides the future R/L sides of the embryo. The neural folds on the lateral sides of the neural plate migrate toward the midline of the embryo, eventually fusing to form the neural tube beneath the overlying ectoderm.

Question 31

What happens to the cells at the apex of the neural folds during fold fusion?

Answer 31

The cells delaminate and become neural crest cells.

Question 32

Give examples of common birth defects caused by failures in the neurulation process. | 2

Answer 32

Anencephaly Spina bifida

Question 33

How common are neural tube defects?

Answer 33

1/500 live births

Question 34

50% of neural tube defects are preventable with what supplement?

Answer 34

Folic acid

Question 36

Describe the molecular basis for separation of the neural tube from the surface ectoderm.

Answer 36

Cells that will become the tube initially express E-cadherin. This stops as the tube folds and N-cadherin begins to be synthesized. Adhesion is lost.