Fertilization and Implantation Flashcards
What happens during capacitation? Why is it important?
- The sperm becomes hyper-activated; the motion of the flagella changes from wave-like to whip-like
- Important to help propel sperm though outer layer of egg to reach its membrane
What days do fertilization typically occur?
days 15-16 of menstrual cycle
Where do incapacitated sperm bind? Why is the binding important?
- to the epithelial cells of oviductal isthmus
- binding slows capacitation process and extends sperm lifespan, increasing probability that sperm will be in oviduct when egg is ovulated
What barriers must be breached for fertilization to occur? (3)
- corona radiata
- zona pellucida
- plasma membrane of oocyte
How does the sperm breach the corona radiata?
It has hyaluronidase that digests the hyaluronic acid that makes up this membrane
How does the sperm breach the zona pellucida?
Sperm contains a receptor for one of the glycoproteins (ZP3), binds to it causing the acrosome reaction so enzymes can digest the zona pellucida
What is the acrosome reaction?
When the inner sperm plasma membrane fuses with the outer acrosomal membrane to release contents of the acrosomal vesicle (enzymes that make holes in ZP)
How does the sperm breach the egg plasma membrane?
Has a protein called Izumo, and binds to Izumo receptor on oocyte, fusing with it
What does the fusion of the sperm and the egg trigger?
the cortical reaction
What happens during the cortical reaction? Why is it important?
- Calcium is released
- ZP proteins altered
- ZP solidifies, forming a physical barrier
THIS PREVENTS POLYSPERMY
What does calcium release from cortical reaction stimulate?
the egg to complete meiosis II
What happens during the first week of embryonic development? (2)
- embryo reaches morula stage (day 3) then early blastocyst stage (day 4)
- implantation occurs (days 6-8)
What is the potency of the blastomeres?
they are totipotent
What is the potency of the inner cell mass?
it is pluripotent
What must happen before the embryo can implant?
It must hatch from the zona pellucida
How does hatching occur?
trophoblasts secrete proteases that digest the ZP
What can happen if the egg hatches prematurely?
abnormal implantation
What does the blastocyst secrete/synthesize in preparation of maintenance of pregnancy?
- secretes immunosuppressive and immunoregulatory factors
What is hCG secreted by? (2)
- trophoblasts
- syncytiotrophoblasts
What is the function of hCG?
- prevents involution of corpus luteum, preventing menstruation and increasing secretion of progesterone and estrogen
- is an autocrine growth factor, stimulating trophoblast and placental growth
What are the stages of implantation? (3)
- apposition
- attachment
- invasion
What happens during the attachment stage?
Trophoblast cells adhere to endometrial epithelium, initiate decidualization
What changes happen during decidualization? (3)
- increased vascular permeability
- intracellular matrix composition changes
- stromal cell morphology
What happens during the invasion stage? (3)
- the endometrial epithelial cells degrade
- trophoblast fuses and syncytiotrophoblast is formed
- syncytiotrophoblast protrudes through the basement membrane and reaches endometrial stroma
What do the syncytiotrophoblasts do? (4)
- express adhesive surface proteins
- breaks down extracellular matrix
- secretes hCG
- makes steroids at 10 weeks
What is one thing that can happen if decidualization goes wrong? Why does it happen?
- postpartum hemorrhage
- this can happen if the signals that normally prevent the embryo from invading the myometrium fail
Where is the most common site of ectopic implantation?
Oviduct
What DOES NOT happen during ectopic implantation? What are the consequences?
- Decidualization doesn’t happen
- invasion isn’t controlled, ruptures tissues and causes hemorrhage
What happens during placentation? (3)
- lacunae form w/in syncytiotrophoblasts
- maternal capillaries broken
- lacunae fill with endometrial secretions, maternal blood, digested matrix for nutrient transfer
What day approximately does placentation occur?
Day 9
What happens in the primary villi?
syncytiotrophoblasts and cytotrophoblasts go to lacunae
What happens in the secondary villi?
mesenchymal cells from extraembryonic mesoderm invade villi
What happens in the tertiary villi?
mesenchymal cells form blood vessels