13 - Zygote to Embryo 1 Flashcards
The result of fertilization is the formation of a single diploid cell called ______.
A zygote.
Formation of the zygote initiations the process of _______?
Prenatal embryonic development.
We can collectively group the events of the first three weeks of embryonic development as __________?
Embryogenesis - the formation of an embryo.
Human embryonic development is traditionally divided into two periods called _____ and _____.
The embryonic period
The fetal period.
What occurs during the embryonic period?
Zygote (1 cell) becomes an embryo with a body plan, 3 layers, and 3 axis.
3 layers become 4 tissues, which become organs and organ systems.
Segmentation, head, link and trunk formation, and embryo folding occurs.
How long is the embryonic period?
8 weeks, the end of which gets you to an embryo.
Describe the fetal period of embryonic development?
Unborn baby - growth and maturation occurs.
Continued development of some systems.
What is the perinatal period? Why is this a clinically important period?
Starts during fetal period and ends after fetal period.
From 22 weeks to 1 month after birth.
This is usually when premature births occur.
What is the neonatal period?
The first month after birth (also perinatal period).
How is human embryonic development divided in the clinics?
Into three 12 week trimesters.
When does fertilization age start? What about the gestational age?
Begins at fertilization - this is the timeline that the embryo runs on.
Starts at last menstruation - two weeks later than fertilization age.
What does it mean when you say someone is 12 weeks pregnant?
That is their gestational age.
Their fertilization (embryo) is actually only ten weeks.
What occurs during the first week of embryonic developement?
Fertilization, cleavage, blastocyst formation, bilaminal embryonic disc.
Goes from one cell to solid ball of cells to fluid filled cyst with disk.
What is the next step after zygote formation?
Cleavage.
What is compaction?
Occurs after cleavage - it’s when the cells that divided in the cleavage stage loose their distinction.
Describe the cell division during the cleavage stage?
Rapid; every 12-24 hours with NO G1 or G2.
Controlled and asynchronous.
No cell growth so the ratio of cytoplasm to nucleus goes from 600:1 to 3:1
What controls the first cell division during cleavage? When is the embryo genome activated? What is needed next?
The maternal program.
Embryo genome activated at 2-4 (4-6?) cell stage.
New proteins needed for cleavage and blastocyst formation.
Are all cleavage-stage blastomeres (cells formed by cleavage of zygote) the same? What is this knowledge the basis for?
Yes.
Basis for cloning. Nucleus of an adult cell can be removed and put into an oocyte with no nucleus to clone whatever animal the nucleus came from.
In mammalian embryos, each blastomere up to about the 8 cell stage has the potential to form _____? Whose experiment determined this?
A complete embryo if isolated and allowed to develop - Spemann experiment.
But if it stars in place during development it only contributes to part of the embryo.
How does a blastocyst form? When does this occur?
Day 4/5
- 8 cell stage has two cell lineages: outer cells and inner cells
- Compaction occurs to create a morula, a solid ball of cells
- Cavity formation in uterine cavity creates a blastocyst:
outer cells become the trophoblast and inner cells become the inner cell mass (ICM)
The ______ becomes the first tissue of the embryo which is an ______ tissue. Describe the cells in this tissue.
Trophoblast.
Epithelium: cells joined by junction and form layers. Cells polarize. Cell layer lines cavity or covers surface.
What type of tissue does the trophoblast form? What does this require?
Extra-embryonic tissue: membranes that surround the embryo but don’t form the parts of the embryo itself.
Require paternal derived alleles (genomic imprinting)
What happens to the inner cell mass?
Most of these cells become part of the embryo.
ICM remodels into bilaminar disc containing the epiblast (primitive ectoderm) upper layer and the lower hypoblast (primitive endoderm) layer.
What shape are cells in the epiblast layer? What about the hypoblast layer?
Epiblast: taller, columnar-like
Hypoblast: flatter, squamous-like
What is the last stage of cleavage? What are the functions of this?
“hatching” from the zona pellucida.
ZP keeps blastomere together, prevents premature implantation, and is an immunological barrier.
What is regulation as it pertains to an embryo in the cleave stage?
The ability of an embryo/organ to produce a normal embryo/structure when parts have been removed or added.
How does the embryos ability for regulation change?
It decreases with age of the embryo: 0-8 cells are totipotent, while 16+ cells only ICM cells are pluripotent.
What is an example of regulation by an embryo?
You can destroy some blastomeres and still have normal development occur because the embryo can compensate for what is lost.
What is a chimeric embryo? Is it viable?
Embryo composed of cells from different zygotes that can be generated during cleavage.
Still possible to get a normal embryo.
How do blastomeres begin to exhibit cell diversity? What are factors that are believed to mediate cell diversity.
Certain blastomeres have different fates such as trophoblast, epiblast, and hypoblast.
Factors: cell position, number of cell divisions, cytoplasmic determinants, cell-cell interactions.
Describe the levels of potency during embryonic development?
Totipotent: has all developmental pathway options
Pluripotent: several developmental pathway options
Bipotent: 2 developmental pathway options
Unipotent: 1 option (terminally differentiated).
The term differentiate or differentiation are often used to describe all phases in the process of ____?
Limiting a cell’s potency.
What is a characteristic of terminally differentiated cells?
They have stable phenotypes and cell divisions result in daughter cells with the same phenotype.
Terminal differentiation does NOT mean it’s post-mitotic.
Define stem cells?
Differentiated cells which retain some pluripotent capabilities under appropriate influence.
Is gene expression during embryonic development random?
No. It is patterned and follows a specific plan.
Describe gene expression during embryonic development.
Results in recognizable morphological arrangements.
Occurs at different levels: molecular-organism
Occurs along axes
Controlled by several families of TFs.
What are homeotic mutations an example of? What is an example?
Abnormal patterning during development.
Differential expression of HOX genes control regional patterning in embryos.
Very rarely, division can occur after the formation of the embryonic disc. What does this result in?
Formation of conjoined (parasitic) twins.
How can multiple embryos be formed (ie twins) ?
If the ZP opens up and multiple embryos grow.
What are the stages in which twins can form? What percentage is each?
65% occur from divisions at the blastocyst stage: two inner cell masses instead of one. They have the same placenta but different amnions.
35% occur from divisions between 2-cell stage to morula. Independent embryos with the same genome. Separate placentas and amnions.
What are the three basic steps of development during week one?
- Fertilization: 2N restored, genetic sex determined, cleavage initiated
- Cleavage: multicellularity, blastocyst formation, first recognizable cell lineages (trophoblast, ICM), bilaminar disc formation.
- Implantation initiated (bridges week 1 and 2): blastocyst attachment to uterine endometrium
