Fertilization and Implantation Flashcards
What day c/t the menstrual cycle does fertilization occur?
Day 15-16
*allows for recombination of genetic material
Where does fertilization occur?
ampulla of oviduct
What leads to follicular rupture so oocyte can be swept into the oviduct by fimbriae for fertilization?
LH surge
How does an oocyte get into the oviduct from the ovary during ovulation for potential fertilization?
ovum and corona radiata released into peritoneum via LH surge, picked up by fimbriae and swept into oviduct
What million sperm/ejaculate is considered infertile?
less than 20
Out of the 300 million sperm that enter the vagina and uterus, how many reach the ampulla of the oviduct?
200
What helps sperm reach the ampulla in terms of contractions?
Myometrium contract from cervix to fundus to help sperm move up to oviduct
What hormone causes myometrial contractions that help sperm reach oviduct?
increased ESTROGEN near ovulation period
Generally describe sperm capacitation
Sperm are coated with surface molecules in epididymis, more seminal proteins added via semen, all removed in uterus to expose binding molecules for zona pellucida
Why do incapacitated sperm bind to the epithelial cells in the isthmus of the ovidcut?
- It slows capacitation process
- –>release when capacitated
- extends sperm lifespan by several days
- increases probability they will be in the oviduct when the egg is ovulated/released into oviduct
What occurs with hyperactivation of sperm during capacitation?
changes in flagella motion d/t chemical signals from oocyte
- ->wave like to whip like
- ->NECESSARY for sperm to detach from isthmus epithelium
- ->increases mobility
- ->helps propel sperm through outer layers of egg to the plasma membrane
What barriers must be breached on the egg for fertilization to occur?
- corona radiata/cumulus
- zona pellucida
- plasma membrane of oocyte
What is the corona radiata matrix primarily made of?
hyaluronic acid
–>digested by sperm hyaluronidase
What glycoproteins compose the zona pellucida?
ZP1
ZP2
ZP3
ZP4
What zona pellucida receptors are on sperm after capacitation?
ZP3
What triggers the acrosome reaction?
binding of ZP3 receptor on sperm to ZP3 on egg
What is the acrosome reaction?
-inner sperm PM fuses with outer acrosomal membrane to release contents of sperm
—>digest zona pellucida so sperm can enter
What is the next barrier for sperm to get through after zona pellucida/acrosome reaction?
plama membrane of oocyte
How does sperm enter the egg during fusion of PM?
sperm has protein called IZUMO that binds recepter on oocyte
–>sperm enters, DNA decondenses and pronucleus forms around it
What is the cortical reaction?
fusion of sperm and egg releases Ca++
–> alters ZP proteins to block additional sperm
–> hyaluronic acid and other proteoglycans released
–> ZP forms physical
barrier
What is the purpose of the cortical reaction?
prevent polyspermy
What triggers the cortical reaction?
fusion of sperm and egg
What stimulates completion of meiosis II after cortical reaction?
Ca++
–>breakdown of MAPK proteins
–>2nd polar body released
Describe egg activation
+ as sperm DNA condenses
- ->pronucleus forms around female chromosomes after finished meiosis II
- ->M and F DNA replicate as the pronuclei move together
- ->Fusion of pronuclei + first embryonic cleavage
What is considered the beginning of embryonic development?
fusion of M and F pronuclei
What day does the embryo reach morula (16 cell)?
3
What day does the embryo reach early blastocyst?
4
What day does the blastocyst implant?
6-8
What cells are considered totipotent?
blasomeres
—>individual cells in morula
What do outer cells of morula become?
increase cell-cell adhesion via desmosomes and tight jxns
–>form Trophoectoderm
*increase sodium transport and osmosis to form the blastocele
What do the inner cells of the morula become?
inner cell mass
–>forms embryo
What cell is pluripotent?
inner cell mass of morula
What happens to the zona pellucida prior to implantation?
hatches from embryo via:
–>trophoblasts secrete proteases to digest ZP
What can inability of zona pellucida to hatch cause?
infertility
What can premature hatching of zona pellucida cause?
abnormal implantation
What is the fxn of blastocyst in terms of maintenance of pregnancy?
synthesizes and secretes molecules to:
- promote maintenance of pregnancy
- promote implantation and placental development
–>also secretes immunosuppressive factors
What secretes hCG?
trophoblasts
syncytiotrophoblasts
When is hCG measurable in blood after ovulation?
8 days post ovulation
What structure is hCG related to?
LH
What is the fxn of hCG?
prevents involution of corpus luteum
- –>prevents menstruation
- –>leads to increased secretion of progesterone and estrogen
- autocrine fxns
- ->stimulates trophoblast growth and development
- –>stimulates placental growth
What are the autocrine fxns of hCG?
- stimulates trophoblast growth and development
- stimulates placental growth
What hormones does hCG stimulation affect?
increased progesterone and estrogen
What are the stages of implantation?
1: apposition
2: attachment
3: invasion
Describe apposition phase of implantation
Contact between crypt in endometrium and trophoblasts
–>inner cell mass rotates to face endometrial epithelium
Describe attachment phase of implantation
Trophoblasts adhere to lumibal endometrial epithelium
–>interact with surface proteins
+ decidualization
- ->increased vascular permeability
- ->intracellular matrix composition
- ->stromal cell morphology
Describe invasion phase of implantation
Degradation of endometrial epithelial cells
Trophoblast fusion and formation of syncytiotrophoblasts
–>protrude through basement membrane to reach endometrial stroma
What are the three main fxns of syncytiotrophoblasts?
Adhesive
–>express cadherins and integrins to bind uterine ECM
Invasive
–> breakdown extracellular matrix
Endocrine
–>hCG
What do syncytiotrophoblasts secrete?
hCG
–>maintains corpus luteum= progesterone secretion
When do syncytiotrophoblasts become highly steroidogenic?
10 weeks
–>produce progesterone for maintenance so no longer need corpus luteum
What are other fxns of syncytiotrophoblasts that occur during implantation and placentation?
- phagocytosis
- bidirectional placental transfer of gases, nutrients and wastes
What is decidualizaiton?
response of maternal stromal cells to invasion and progesterone
Describe decidualization
endometrial stroma transformed into enlarged and glycogen-filled decidual cells
- -> prepare for implantation
- -> has adhesive jxns that limit migration of embryo
- -> secretes signals to prevent embryo from invading myometrium
Is there decidualization in ectopic pregnancies?
NO
