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What is amniocentesis?
The process of removing cells from the amniotic fluid in order to test the fetus for genetic disorders ie. Downs Syndrome.
When does gastrulation begin?
Day 14/15, completed by the end of week 3
The conversion of the BILAMINAR (epiblast and hypoblast) embryo into three germ layers (ectoderm, mesoderm, endoderm)
What is the primitive streak?
- groove forming in the epiblast
- forms at the caudal end of the embryo
- sets up the primary axis of the embryo
- primitive node= most anterior/cranial portion of the streak
Explain which cells become each of the three germ layers.
- The first set of ingressing cells at the primitive streak displace the hypoblast to form the endoderm.
- The next set of cells that ingress from the epiblast become mesoderm.
- As the endoderm and mesoderm form, the epiblast is now called the ectoderm.
What is slug?
- Slug is a DNA binding transcription factor that regulated gene expression in cells at the primitive streak - the net effect of this gene expression is loss of adhesion between cells at the primitive streak and their transformation into mesoderm (transformation is termed epithelial-mesenchymal transformation).
- Good example of how differentiation occurs in gastrulation
What is the fate of the mesoderm?
Middle cell layer of the embryonic disc and precursor to the muscles, bones, lympatic system, spleen, blood cells, heart, lungs, reproductive and excretory systems.
What is the fate of the ectoderm?
- Top layer of the embryonic disk.
- Will later form; skin, hair, lenses of the eyes, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary and mammory glands, and all parts of the nervous system
What is the fate of the endoderm?
Inner cell later of the embryonic disc from which will form the lining of the lungs, the tongue, tonsils, urethra and associated glands, bladder, and the digestive tract.
What is the notochord?
- A mesodermal structure
- Made from the last cells to migrate through the primitive streak
- Induces the formation of the neural tube
What are the three differentiated regions of the mesoderm?
- Paraxial, intermediate and lateral plate
- The lateral plate mesoderm froms Somatopleure (dorsal) and splanchnopleure (venral)
Describe somites and explain which tissue gives rise to them.
- Somites are brick-like blocks that line the neural tube.
- The paraxial mesoderm differentiates into the somites - gives rise to axial musculature and axial skeleton.
- No somites in the future brain region.
Define induction and explain why neurulation is an example of this.
- A process whereby one cell or group of cells influences the developmental fate of another.
- Notochord releases short-range-acting molecules that induce formation of the central nervous system by signaling the ectoderm directly above it to thicken and form the neural plate.
Explain how the neural tube is formed.
- Notochord provides a signal that says 'this side is ventral'
- Neural plate buckles inward, forming the neural tube
- The neural folds fuse dorsally and seal off the neural tube.
Explain what patterning the neural tube means.
- Bone morphogenetic protein (BMP) is secreted from the dorsal end of the neural tube/neural folds (ectoderm) while Sonic hedgehog (Shh) is secreted ventrally from the notochord. This produces a dorsal and ventral gradient of morphogens (high BMP dorsally, low BMP ventrally, low Shh dorsally, high Shh ventrally).
- The relative amount of BMP and Shh in neural tube cells determines which transcription factor gets expressed in that cell. This leads to a grid of different transcription factor expression in the neural tube.
- Each transcription factor domain has progenitor cells that will eventually turn into neurons - each domain will give rise to a specific type of neuron.