Flashcards in Week 1 (Days 1-7) - HY Deck (52):
Where does fertilization occur?
In the ampulla of the uterine tube.
Where does the sperm bind and what reaction does it elicit?
1. The sperm binds to the zona pellucida of the secondary oocyte arrested in metaphase of meiosis II.
2. Triggers the ACROSOME reaction --> release of acrosomal enzymes (acrosin).
What is the function of the acrosomal enzymes?
To help sperm penetrate the zona pellucida.
What does penetration of the zona pellucida elicit?
The cortical reaction.
What is the cortical reaction?
The cortical reaction is the release of lysosomal enzymes from cortical granules near the oocyte cell membrane that changes the oocyte cell membrane potential and inactivates sperm receptors on the zona pellucida.
What is called "the polyspermy block"?
The changes due to the cortical reaction --> render the secondary oocyte impermeable to other sperm.
Does polyspermy block work well?
Not very well --> Diandric triploidy is quite common.
What is diandric triploidy?
An embryo with 3 sets of chromosomes, two of which come from the father.
What are the events after the fusion of sperm and secondary oocyte cell membranes?
1. Nuclear contents + centriole pair of the sperm enter the cytoplasm of the oocyte.
2. Sperm nuclear contents form the male PROnucleus.
3. The tail and mitochondria of the sperm degenerate --> mtDNA is of maternal origin.
4. The oocyte loses its centriole pair during meiosis.
5. Sperm centriole pair produce a microtubule organizing center (MTOC) --> Cardinal feature of human embryogenesis.
What process comes next after the events of fusion of sperm and secondary oocyte?
The secondary oocyte completes meiosis II --> forming mature ovum.
What is the nucleus of the ovum?
The FEMALE pronucleus.
What does the term "syngamy" describe?
The successful completion of fertilization --> formation of zygote.
What must take place for syngamy to occur?
The male and female pronuclei fuse and cytoplasmic machinery for proper cell division exists.
What is the life span of the zygote?
A few hours --> its existence terminates when the 1st cleavage division occurs.
What is the cleavage?
A series of mitotic divisions of the zygote, where the plane of the 1st mitotic division passes through the area of the cell membrane, where the polar bodies where previously extruded.
What does holoblastic cleavage mean?
Cells divide completely through their cytoplasm.
What does asymmetrical cleavage mean?
The daughter cells are unequal in size --> one cell gets more cytoplasm than the other.
When does asymmetrical cleavage occur?
At least during the first few cell divisions.
What does asynchronous cleavage mean?
Means only one cell will divide at a time.
Generally the largest daughter cell will divide next at least during the first few cell divisions.
What does the process of cleavage eventually form?
A blastula --> consisting of cells called blastomeres.
What forms a morula?
A cluster of blastomeres (16-32 blastomeres).
Up to what stage are blastomeres totipotent?
Up to 8-cell stage (each blastomere can form a complete embryo by itself).
What does the term "totipotency" mean?
A stem cell that can differentiate into every cell within the organism, including extraembryonic tissues.
What must happen for a blastocyst to form?
Occurs when fluid secreted within the morula forms the blastocyst cavity.
How is the inner cell mass of the blastocyst cavity called?
What is the result of the inner cell mass?
Embryoblasts are pluripotent or totipotent?
What does the term pluripotency mean?
A stem cell that can differentiate into ectoderm, mesoderm, and endoderm.
How is the outer cell mass of the blastocyst cavity called?
What part is eventually the outer cell mass of the blastocyst formation?
Of the placenta.
What must degenerate for implantation to occur?
The zona pellucida must degenerate for implantation to occur.
Where does the blastocyst implant?
Within the posterior superior wall of the uterus.
Where does the blastocyst implant during the secretory phase of the menstrual cycle?
Within the functional layer of the endometrium.
To what 2 blasts does the trophoblast differentiate?
What must happen for an ectopic tubal pregnancy (ETP) to occur?
Blastocyst must implant within the uterine tube due to delayed transport.
What is the MC site of an ETP?
The ampulla of the uterine tube.
Mention a common site for ectopic abdominal pregnancy.
The rectouterine pouch (pouch of Douglas).
Mention 3 conditions that frequently predispose to ETP.
1. Chronic salpingitis
3. Post-operative adhesions
In what women is ETP frequently seen?
To what may ETP lead?
1. Uterine tube rupture
if salpingectomy is not performed.
An ETP must be Dx from what?
2. Aborting intrauterine pregnancy
3. Bleeding corpus luteum of a normal intrauterine pregnancy
What are the main clinical signs of ETP?
1. Abnormal uterine bleeding
2. Unilateral pelvic pain
3. Incr. hCG --> But LOWER than original expected with uterine implantation pregnancy.
4. Massive 1st trimester bleed.
What is the basis if the monozygotic twining?
The molecular mechanisms that establish twin embryogenesis are active in the morula --> responsible for the morula splitting.
"Twinning causes the splitting, not vice versa."
What are the two possible routes to which can twinning morula travel?
1. Monochorionic twins
2. Dichorionic twins
What determines whether the monozygotic twins will be monochorionic or dichorionic?
1. If splitting occurs AFTER the differentiation of the trophoblast --> Monochorionic.
2. If splitting occurs BEFORE --> Dichorionic.
What is the main event of the dizygotic (fraternal) twinning?
The secondary oocyte completes meiosis II, but does not form a secondary polar body --> The DNA that would have sequestered in a 2nd polar body forms another female pronucleus.
--> 2 separate entities within the zona pellucida containing a female and a male pronucleus.
What is characteristic about the morula of the dizygotic twinning?
The morula is a chimera consisting of an assortment of cells with 2 different genotypes.
What happens in conjoined (siamese) twinning?
Occurs exactly like monozygotic twins except that there is incomplete "splitting" of the inner cell mass --> exact molecular mechanisms are not clear.
What is characteristic of all conjoined twins (except parasitic twins)?
They are symmetrical.
What are the MC types of conjoined twins?
1. Thoraco-omphalopagus --> fusion from upper chest to lower chest.
2. Thoracopagus --> fusion from upper chest to lower abdomen.
3. Omphalopagus --> fusion at lower chest.
4. Craniopagus --> fusion of skulls
What happens with parasitic twins?
Asymmetrically conjoined --> one twin is small and dependent on the larger twin.