Neural Tube Formation Flashcards
Where does gastrulation begin in the fertilized frog egg?
Side opposite the point of sperm entry.
What is the animal cap?
Roof of the blastocoel in the animal hemisphere of the amphibious egg.
What does the blastocoel do in terms of the animal cap?
Prevents premature contact between vegetal and animal cap cells, keeping the animal cap cells undifferentiated.
Outline the five steps of Xenopus gastrulation.
- Epiboly
- Vegetal rotation
- Bottle cell formation and invagination
- Involution and cell migration
- Convergence and extension
Describe the epiboly step of frog gastrulation.
The thinning and spreading of the animal cap cells over the vegetal hemisphere.
Describe the vegetal rotation stage of frog gastrulation.
Vegetal cells asymmetrically press up against the inner blastocoel roof on the doral side.
Describe the bottle cell formation and invagination stage of frog gastrulation.
Localized apical constriction at the dorsal blastopore lip creates anisotropic forces that foster invagination.
Describe the involution and cell migration stage of frog gastrulation.
Leading edge of invaginating cells crawls up onto the blastocoel roof.
Describe the convergence and extension stage of frog gastrulation.
The targeted medial-to-lateral intercalation of cells on the midline drives anterior-posterior axis elongation.
What is the notochord?
A transient mesodermal rod that plays an important role in hindgut formation and nervous system patterning.
What portion of the frog egg becomes the ectoderm?
Animal hemisphere blastomeres.
What portion of the frog egg becomes the endoderm?
Vegetal hemisphere cells
What portion of the frog egg becomes the mesoderm?
Equatorial cells.
What are the two major functions of the vegetal cells?
- Differentiate into endoderm
- Induce the cells immediately above them to become mesoderm.
Describe the experiment conducted by Hans Spemann and colleaugues.
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.
Describe the results of Spemann’s experiment.
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.
What did the results of Spemann’s experiment indicate?
Showed that the dorsal lip of the blastopore is the only tissue in the early gastrula whose fate is self-determined.
What did Spemann refer to as the organizer?
Dorsal lip cells, notochord and endomesoderm.
Why did Spemann refer to the dorsal lip cells and their derivatives as the organizer?
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- The induce the host’s ventral tissues to change their fates to form a neural tube and dosal mesodermal tissue
- The organized the host and donor tissues into a secondary embryo with clear anterior-posterior and dorsal-ventral axes.
What is the default fate of the ectoderm?
To become neural tissue
How is the ectoderm induced to become epidermal tissue?
Certain parts of the embryo induce the ectoderm to become epidermal tissue by secreting BMPs.
What are BMPs?
Bone morphogenetic proteins.
How is the default fate of ectoderm maintained?
Organizer tissue scretes BMP inhibitors, protecting the ectoderm and enabling it to develop into neural tissue.
List the three major BMP inhibitors secreted by the organizer.
Chordin
Follistatin
Noggin
What is neurulation?
The process of forming the neural tube.
What are the three major responsibilities of the vertevrate ectoderm?
- One part forms the neural plate
- Another part becomes the epidermis
- The area between the above two is the neural crest
What does the neural plate become, and what is this a precursor to?
Forms the neural tube, the precursor of the central nervous system.
What makes up the central nervous system?
Brain and spindal cord.
What process does neurulation directly follow?
Gastrulation
Describe the process of neurulation.
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.
What happens to the cells at the apex of the neural folds during fold fusion?
The cells delaminate and become neural crest cells.
Give examples of common birth defects caused by failures in the neurulation process.
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Anencephaly
Spina bifida
How common are neural tube defects?
1/500 live births
50% of neural tube defects are preventable with what supplement?
Folic acid
Describe the molecular basis for separation of the neural tube from the surface ectoderm.
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.
