Fertilization and Implantation Flashcards
What is fertilization?
Occurs day 15-16 of menstrual cycle
Allows for recombination of genetic material
Initiation of events that begin embryonic development
sperm will break through the zona pellucida to enter the oocyte
Transportation of the gametes
Both sperm and oocyte must travel to the ampulla of oviduct for fertillization to occur
LH surge leads to follicular rupture
Ocum and surrounding corona radiata are ejected into the peritoneum
Oocyte is swept into the oviduct by fimbriae
300 million sperm enter the vagina near the uterus but only about 200 reach ampulla of oviduct
what is the sperm count that makes a male considered infertile, and what helps sperm reach the oocyte?
less than 20 million sperm in ejaculate means the male is infertile
Myometrium contractions as a result of increased estrogen near ovulation period helps to move the sperm through the cervix and the uterus
What is sperm capacitation?
when sperm enter the female tract, seminal plasma coatings along with some of the surface molecules are removed from the sperm head to expose molecules that will help penetrate the zona pellucida
capacitation leads to hyperactivation of the sperm
that involves a change in flagella movement
-form wave to whip like movements
-allows for detachment from epithelium of the oviduct
-increases mobility
-helps propel sperm through outer layers of egg
what do incapacitated sperm do in the female tract?
incapacitated sperm bind actively to epithelial cells of the oviductal isthmus
- become unbound when they are capacitated
- binding slows capacitation process
- binding extends sperm lifespan
- binding increases the probability the sperm will be in the oviduct when the egg is ovulated
what are the three barriers that must be breached by the sperm for fertilization
- The expanded cumulus (corona radiata)
- The zona pellucida
- The plasma membrane of the oocyte
how do the sperm break through the corona radiata and the zona pellucida
Radiata cell matrix is predominantly hyaluronic acid and the sperm will digest it with its membrane bound hyaluronidase
Zona pellucida is comprised of glycoproteins ZP1, ZP2, ZP3, ZP4
Sperm contain ZP3 receptors after capacitation and binding of ZP3 receptors will trigger the acrosome reaction
- inner sperm plasma membrane fuses with outer acrosomal membrane to release contents of acrosomal vesicle
- the enzymes in the acrosomal vesicle digest the zona pellucida
- sperm can enter the holes created in zona pellucida
how doe the sperm get pass the plasma membrane of the oocyte?
Sperm posses a protein called Izumo
-binds to izumo receptor on the oocyte
Entire sperm enters the egg during fusion, sperm DNA will instantly de-condenses and a membrane called pronucleus forms arowund the DNA
what is the cortical reaction? what is it preventing?
Fusion of the sperm and the egg triggers the cortical reaction
- release of Ca+
- alteration of ZP proteins to block binding of additional sperm
- Hyaluronic acid and other proteoglycans, proteinases, released and hydrated
- zona pellucida forms a physical barrier
all this to prevent polyspermy
Egg activation and the zygote process?
Ca+ release stimulates the completion of meiosis II in the egg
- breakdown of MAPK proteins
- 2nd polar body released
Egg activation occurs as sperm DNA condenses
Pronucleus forms around female chromosomes
Male and female chromosomes replicate as pronuclei move together
Fusion of the pronuclei initiates the first embryonic cleavage
- signifies beginning of embryonic development
what are the steps of the week 1 embryonic development and at what day are they at each stage?
all steps are undergoing cleavage and not so much actual growth of the zygote and therefore the embryo will stay about 100um
embryo reaches 16 cell stage morula at day 3
early blastocyst at day 4
and will implant around days 6-8
what is embryoonic cleavage?
individual cells are the blastomeres
Mitotic divisions maintatin the 2N (diploid) complement
Cells will become smaller
the blastomeres are totipotent (can give rise to any cell type even a full embryo)
Embryo will reach 16 cell morula by day 3
What is compactation and blastocele development of the embryo?
Outer cells of morula increase cell cell adhesions via desmosomes and tight junctions
- form trophoectoderm
- increase Na+ transport and osmosis to form a blastocele
Inner cells become inner cell mass (ICM)
- will form embryo proper
- puripotent (can giver rise to most all cell types not full embryo though)
what is the process of blastocyst hatching
Hatching of embryo from zona pellucida prior to implantation
Trophoblasts secrete proteases that digest the zona pellucida
Inabillity to hatch can result in infertillity
premature hatching can result in abnormal implantation
what does the blastocyst secrete?
blastocyst will begin synthesizing and secreting molecules that promote maintenance of pregancy, promote implantation and placental development
Secretion of immunosuppressive and immunoregulatory factors:
-platelet activating factor, hCG, early pregnancy factor, immunosuppressive factor, pGE2, IL-1a, IL-6, INF-a, lukemia inhibitory factor and colony stimulating factor
