Fertilization and Implantation Flashcards
When does fertilization occur?
~day 15-16 of the menstrual cycle
where does the sperm bind on the oocyte?
zona pellucida
what triggers the exocytosis of the acrosome (acrosomal reaction)?
a rise in Ca2+ inside the sperm
what is the purpose of the hydrolytic enzymes that are housed within the acrosomal cap?
they locally dissolve the zona pellucida
what is the cortical reaction?
there is exocytosis of granules that lead to changes in the zona pellucida proteins causing the zona pellucida to harden and prevent the entry of other sperm
what causes the cortical reaction?
a rise in Ca2+ concentration inside the oocyte
what specifically induces the completion of the oocyte’s second meiotic division and the formation of the second polar body?
the rise in Ca2+ levels inside the oocyte (with fertilization)
where must both the sperm and the oocyte travel to for fertilization to occur?
to the ampulla of the oviduct
about how many sperm actually reach the ampulla of the oviduct?
~200
what helps to move the sperm through the cervix/uterus?
myometrium contractions that are a result of increased estrogen near the ovulation period
what hormone actually causes down regulation to prevent immune cells from attacking the egg or the sperm?
progesterone
What is sperm capacitation?
when sperm are exposed to the female tract environment, the seminal plasma coatings along with some of the surface molecules are removed- thus exposing portions of the molecules that can bind to the zona pellucida of the oocyte
where do incapacitated sperm bind?
to the epithelial cells of the oviductal isthmus
why do sperm bind to the epithelial cells of the oviductal isthmus?
it slows the capacitation process and extends sperm lifespan, which increases the probability of the sperm being in the oviduct when the egg is ovulated
what is associated with sperm capacitation and chemical signals from the oocyte?
hyperactivation of sperm
what does hyperactivation of sperm involve and why is it important?
it involves a change in flagella motion from wave-like to whip like; it is necessary for sperm to detach from the epithelium of the oviduct and it increases mobility; helps propel sperm through the outer layers of the oocyte
for fertilization, 3 barriers must be breached. What are these barriers?
the expanded cumulus (corona radiata), the zona pellucida, and the plasma membrane of the oocyte
what is the predominant composition of the cumulus cell matrix?
hyaluronic acid
how do sperm digest the cumulus cell matrix (aka the hyaluronic acid)?
via membrane bound hyaluronidase
what zone pellucida receptor do sperm contain?
ZP3 receptors, which are available after capacitation
what does binding of the ZP3 receptor on sperm to the ZP3 glycoprotein trigger?
the acrosome reaction
what happens during the acrosome reaction?
the inner sperm plasma membrane fuses with outer acrosomal membrane to release contents of the acrosomal vesicle
what do the enzyme in the acrosomal vesicle do?
digest the zona pellucida
how does the sperm bind to the oocyte’s plasma membrane?
sperm possess a protein called Izumo, which binds to the Izumo receptor on the oocyte
what happens once the sperm binds to the izumo receptor on the oocyte’s plasma membrane?
the entire sperm enters the egg during fusion, and the sperm DNA instantly de-condenses
what forms around the sperm’s DNA once inside the oocyte’s plasma membrane?
a pronucleus
what does the cortical reaction prevent?
polyspermy
how does the completion of meiosis II begin?
by the breakdown of MAPK proteins
when does egg activation occur?
as the sperm DNA condenses
what initiates the first embryonic cleavage?
the fusion of the pronuclei (signifies the beginning of embryonic development)
what is cleavage?
cell division without growth
when does the embryo reach ~ the 16 cell stage morula?
day 3
when is the embryo considered an early blastocyst?
~day 4
when does implantation occur?
around day 6-8
what are the individual cells called in a blastocyst?
blastomere
what do the mitotic divisions maintain in the embryonic cleavage?
2N (diploid) complement
what are blastomeres known to be?
totipotent
what is blastocele development?
the outer cells of the morula increase their cell adhesion molecules (desmosomes and tight junctions)- this forms a trophoectoderm, which allows for an increase in Na+ transport and osmosis to form a blastocele
what do the cells from the inner morula eventually form?
the embryo proper
what are the cells from the inner morula known to be?
pluripotent (if you split in half, we could have identical twins)
as the embryo continues to grow and prior to the implantation of the embryo, what needs to happen?
the blastocyst needs to hatch- the embryo needs to hatch from the zona pellucida
how does blastocyst hatching occur?
the trophoblasts secrete proteases that digest the zona pellucida
what does an inability to hatch from the zona pellucida result in?
infertility
what does premature hatching from the zona pellucida result in?
abnormal implantation
following the hatching from the zona pellucida, what does the blastocyst begin synthesizing and secreting?
molecules that promote the maintenance of pregnancy, promote implantation and placental development
what is hCG secreted by?
trophoblasts and syncytiotrophoblasts following implantation
when is hCG measurable in blood?
~8 days post ovulation
what is the hCG structure closely related to?
LH
what does hCG prevent?
the involution of the corpus luteum (prevents menstruation and leads to increased secretion of progesterone and estrogen which will aid in early maintenance of the pregnancy)
what does it mean when it is said that hCG is an autocrine growth factor?
it stimulates trophoblast growth and development and it stimulates placental growth
what is considered the first “stage” of implantation?
apposition- contact between the endometrium and the trophoblasts (typically occurs in a crypt of the endometrium)
once apposition occurs, where does the inner cell mass of the blastocyst rotate?
near the endometrial epithelium
what is the second stage of implantation, following the apposition?
attachment-the trophoblast cells adhere to the endometrial epithelium
what does the stage of attachment initiate?
changes in the endometrial stroma (decidualization)
what is the result of decidualization?
increased vascular permeability, changes in the intracellular matrix, and the cells take on a more stromal cell morphology
what happens after decidualization/ attachment?
invasion-degradation of the endometrial epithelial cells
what happens during invasion?
trophoblast fusion and formation of syncytiotrophoblasts
what do syncytiotrophoblasts express?
adhesive surface proteins (cadherins and integrins)
how do the syncytiotrophoblast break down the extracellular matrix during invasion?
they secrete matrix metalloproteases and other hydrolytic enzymes
what is the source of secretion of hCG?
syncytiotrophoblasts
what is decidualization a response to?
maternal stromal cells responding to invasion and progesterone
what happens to the endometrial stroma during decidualization?
it is transformed into enlarged and glycogen-filled decidual cells
what does the decidua form and what is its purpose?
an epithelia-like sheet with adhesive junctions that inhibit migration of the implanting embryo and a production of signals that prevent the embryo from invading the myometrium
where is the most common site for ectopic implantation?
oviduct
What does not occur during an ectopic pregnancy?
no decidualization- so invasion is not controlled and can rupture the tissues and cause hemorrhage
when does placentation typically occur?
~ day 9
what forms the primary villi?
proliferation of the syncytiotrophoblasts and cytotrophoblasts to lacunae
what forms the secondary villi?
mesenchyme cells from the extraembryonic mesoderm invade the villi
what forms the tertiary villi?
eventually the mesenchymal cells form fetal blood vessels de novo
