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 which helps expose the molecules that can bind to zona pellucida of oocyte
Where can incapacitated sperm bind in the female tract to extend their lifespan and increase the probability that the sperm will be in the oviduct when the egg is ovulated?
Bind to the epithelial cells of the isthmus of the oviduct
How does sperm change when it under capacitation?
Flagella of sperm become hyperactivated.
Changing from wave-like to whip like motion increases mobility and helps propel the 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 & how does the sperm get through this?
Hyaluronic acid
Sperm digests it via hyaluronidase
What proteins make up the zona pellucida
ZP1- ZP4
What receptors do sperm contain that helps them get through the zona pellucida
ZP3 receptors
What is acrosome reaction
Binding of ZP3 receptors to ZP3 triggers the enzymes in the acrosome vesicle to digest the zona pellucida, allow the sperm to enter the holes created in the zona pellucida.
What protein do sperm possess that reacts with a receptor on the plasma membrane of the egg?
Izumo protein binds with Izumo receptor on the oocyte
The fusion of the sperm and egg triggers a cortical reaction. Describe this process.
PREVENT POLYSPERMY
1) Release of calcium
2) Alteration of ZP proteins to prevent binding of additional sperm
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 (trophoblast)
-supplies embryo with nourishment and later forms the major part of the placenta
What do inner cells of the morula form?
Inner cell mass (embryoblast) –> 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 specifically Syncytiotrophoblasts
What is the function of hCG?
- Prevents involution of the corpus luteum
- Leads to increased secretion of progesterone and estrogen
- prevents menstruation
- stimulates trophoblast and placental growth
What are the stages of implantation
Apposition
Attachment
Invasion
What occurs in apposition stage of implantation
Typically occurs in a crypt in the endometrium. The embryoblast rotates near the endometrial epithelium and there is contact between endometrium and trophoblast.
What happens in the attachment stage of implantation
Decidualization
Changes in endometrial stroma after implantation
- Increased vascular permeability
- Change in intracellular matrix composition
- Change in stromal cell morphology
*improves interaction b/t fetal and maternal placenta
What happens in the invasion stage of implantation ?
- 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 a decidua?
The name given to the endometrium when it is ready for implantation of embryo
-has adhesive junctions that prohibits embryo to migrate anywhere, especially myometrium
What is ectopic implantation?
Implantation of embryo outside the fundus of uterus
- Mostly in oviduct
- No decidualization
- Can cause tissue rupture and hemorrhaging
When does placentation start?
Day 9. Spaces begin to appear 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) Tertiary Villi: Mesenchymal cells form fetal blood vessels de novo
