Lecture 35: Fertilization and Implantation Flashcards
When does fertilization occur in the menstrual cycle?
Around days 15-16
Where does fertilization occur?
Ampulla of oviduct
What happens during ovulation?
Ovum and surrounding corona radiata ejected into peritoneum
-happens after LH surge
A sperm count of what can be considered as infertility?
<20 million/ejeculation
What type of contractions help the sperm move through the cervix and uterus?
Myometrium contractions
-result as increased estrogen near ovulation
Describe sperm capacitation.
When sperm enters the female tract, seminal plasma coating and surface molecules are removed
-helps expose molecules that can bind to zona pellucida of oocyte
Where can incapacitated sperm bind to the female tract to extend its lifespan?
Epithelial cells of the isthmus of oviduct
How does sperm change when it under capacitation?
Changes in flagella motion (hyper-activated)
- whip like
- increased mobility
- propels sperm to penetrate outer layers of egg to reach plasma membrane
What are the three barriers the sperm must breach to fertilize the egg?
1) Corona radiata
2) Zona pellucida
3) Plasma membrane of oocyte
What is the corona radiata primarily made up of?
Hyaluronic acid
What glycoprotein receptor of the zona pellucida triggers acrosome reaction?
ZP3
What protein does sperm possess that reacts with a receptor on the plasma membrane?
Izumo protein binds with receptor
-sperm can enter egg after this and form a pronucleus
The fusion of the sperm and egg triggers a cortical reaction. Describe this process.
1) Release of calcium
2) Alteration of ZP proteins to prevent polyspermy
3) Release of hyaluronic acid, proteoglycans, proteinases
4) Zona pellucida forms a physical barrier
What triggers the completion of meiosis II in the egg?
Calcium release
- breaks down MAPK proteins
- releases second polar body
What initiates the first embryonic cleavage?
Fusion of male and female pronuclei
What is cleavage?
Cell division without growth
-goes from 1 cell –> 2 –> 4 –> 8 –> 16
When does the embryo reach a 16 cell morula?
Day 3
When does implantation happen?
Days 6-8
What is the potency of a blastomere?
Totipotent
-stem cells can give rise to any of the cell types found in an embryo as well as extra-embryonic cells (placenta).
What do outer cells of the morula form?
Trophectoderm
-supplies embryo with nourishment and later forms the major part of the placenta
What do inner cells of the morula form?
Inner cell mass –> pluripotent embryo proper
What do trophoblasts do?
Secrete proteases that digest zona pellucida
-if not done, can be sign of infertility
What secretes hCG?
Trophoblasts
Syncytiotrophoblasts
What is the function of hCG?
Increased secretion of progesterone and estrogen
-prevents menstruation
Autocrine growth factor
-stimulates trophoblast and placental growth
Where exactly does implantation take place?
Posterior wall of uterine fundus
- crypt in endometrium
- contact between endometrium and trophoblasts
What is decidualization?
Changes in endometrial stroma after implantation
- increased vascular permeability
- intracellular matrix composition
- stromal cell morphology
*improves interaction b/t fetal and maternal placenta
What happens after implantation?
Invasion
- endometrial epithelial cells are degraded
- syncytiotrophoblasts protrudes through basement membrane to reach endometrial stroma
What is the role of syncytiotrophoblasts?
1) Express adhesive surface proteins
2) Breaks down ECM
3) Secretes hCG
4) Produces progesterone independent of corpus luteum
5) Phagocytosis and bidirectional transfer of gases, nutrients, and wastes
What is the decidua?
Endometrium ready for implantation of embryo
-has adhesive junctions that prohibits embryo to migrate anywhere, especially myometrium
What is ectopic implantation?
Implantation of embryo not in the fundus of uterus
- mostly in oviduct
- no decidualization
- can cause tissue rupture and hemorrhaging
When does placentation start?
Day 9
-spaces within syncytiotrophoblasts
Describe the process of placentation?
1) Primary Villi: proliferation of syncytiotrophoblasts and cytotrophoblasts to lacunae
2) Secondary Villi: mesenchyme cells from extraembryonic mesoderm invade villi
3) Mesenchymal cells form fetal blood vessels de novo
