Fertilization and Implantation

Question 1

When does fertilization occur?

Answer 1

~day 15-16 of the menstrual cycle

Question 2

where does the sperm bind on the oocyte?

Answer 2

zona pellucida

Question 3

what triggers the exocytosis of the acrosome (acrosomal reaction)?

Answer 3

a rise in Ca2+ inside the sperm

Question 4

what is the purpose of the hydrolytic enzymes that are housed within the acrosomal cap?

Answer 4

they locally dissolve the zona pellucida

Question 5

what is the cortical reaction?

Answer 5

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

Question 6

what causes the cortical reaction?

Answer 6

a rise in Ca2+ concentration inside the oocyte

Question 7

what specifically induces the completion of the oocyte’s second meiotic division and the formation of the second polar body?

Answer 7

the rise in Ca2+ levels inside the oocyte (with fertilization)

Question 8

where must both the sperm and the oocyte travel to for fertilization to occur?

Answer 8

to the ampulla of the oviduct

Question 9

about how many sperm actually reach the ampulla of the oviduct?

Answer 9

~200

Question 10

what helps to move the sperm through the cervix/uterus?

Answer 10

myometrium contractions that are a result of increased estrogen near the ovulation period

Question 11

what hormone actually causes down regulation to prevent immune cells from attacking the egg or the sperm?

Answer 11

progesterone

Question 12

What is sperm capacitation?

Answer 12

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

Question 13

where do incapacitated sperm bind?

Answer 13

to the epithelial cells of the oviductal isthmus

Question 14

why do sperm bind to the epithelial cells of the oviductal isthmus?

Answer 14

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

Question 15

what is associated with sperm capacitation and chemical signals from the oocyte?

Answer 15

hyperactivation of sperm

Question 16

what does hyperactivation of sperm involve and why is it important?

Answer 16

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

Question 17

for fertilization, 3 barriers must be breached. What are these barriers?

Answer 17

the expanded cumulus (corona radiata), the zona pellucida, and the plasma membrane of the oocyte

Question 18

what is the predominant composition of the cumulus cell matrix?

Answer 18

hyaluronic acid

Question 19

how do sperm digest the cumulus cell matrix (aka the hyaluronic acid)?

Answer 19

via membrane bound hyaluronidase

Question 20

what zone pellucida receptor do sperm contain?

Answer 20

ZP3 receptors, which are available after capacitation

Question 21

what does binding of the ZP3 receptor on sperm to the ZP3 glycoprotein trigger?

Answer 21

the acrosome reaction

Question 22

what happens during the acrosome reaction?

Answer 22

the inner sperm plasma membrane fuses with outer acrosomal membrane to release contents of the acrosomal vesicle

Question 23

what do the enzyme in the acrosomal vesicle do?

Answer 23

digest the zona pellucida

Question 24

how does the sperm bind to the oocyte’s plasma membrane?

Answer 24

sperm possess a protein called Izumo, which binds to the Izumo receptor on the oocyte

Question 25

what happens once the sperm binds to the izumo receptor on the oocyte's plasma membrane?

Answer 25

the entire sperm enters the egg during fusion, and the sperm DNA instantly de-condenses

Question 26

what forms around the sperm's DNA once inside the oocyte's plasma membrane?

Answer 26

a pronucleus

Question 27

what does the cortical reaction prevent?

Answer 27

polyspermy

Question 28

how does the completion of meiosis II begin?

Answer 28

by the breakdown of MAPK proteins

Question 29

when does egg activation occur?

Answer 29

as the sperm DNA condenses

Question 30

what initiates the first embryonic cleavage?

Answer 30

the fusion of the pronuclei (signifies the beginning of embryonic development)

Question 31

what is cleavage?

Answer 31

cell division without growth

Question 32

when does the embryo reach ~ the 16 cell stage morula?

Answer 32

day 3

Question 33

when is the embryo considered an early blastocyst?

Answer 33

~day 4

Question 34

when does implantation occur?

Answer 34

around day 6-8

Question 35

what are the individual cells called in a blastocyst?

Answer 35

blastomere

Question 36

what do the mitotic divisions maintain in the embryonic cleavage?

Answer 36

2N (diploid) complement

Question 37

what are blastomeres known to be?

Answer 37

totipotent

Question 38

what is blastocele development?

Answer 38

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

Question 39

what do the cells from the inner morula eventually form?

Answer 39

the embryo proper

Question 40

what are the cells from the inner morula known to be?

Answer 40

pluripotent (if you split in half, we could have identical twins)

Question 41

as the embryo continues to grow and prior to the implantation of the embryo, what needs to happen?

Answer 41

the blastocyst needs to hatch- the embryo needs to hatch from the zona pellucida

Question 42

how does blastocyst hatching occur?

Answer 42

the trophoblasts secrete proteases that digest the zona pellucida

Question 43

what does an inability to hatch from the zona pellucida result in?

Answer 43

infertility

Question 44

what does premature hatching from the zona pellucida result in?

Answer 44

abnormal implantation

Question 45

following the hatching from the zona pellucida, what does the blastocyst begin synthesizing and secreting?

Answer 45

molecules that promote the maintenance of pregnancy, promote implantation and placental development

Question 46

what is hCG secreted by?

Answer 46

trophoblasts and syncytiotrophoblasts following implantation

Question 47

when is hCG measurable in blood?

Answer 47

~8 days post ovulation

Question 48

what is the hCG structure closely related to?

Answer 48

LH

Question 49

what does hCG prevent?

Answer 49

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)

Question 50

what does it mean when it is said that hCG is an autocrine growth factor?

Answer 50

it stimulates trophoblast growth and development and it stimulates placental growth

Question 51

what is considered the first "stage" of implantation?

Answer 51

apposition- contact between the endometrium and the trophoblasts (typically occurs in a crypt of the endometrium)

Question 52

once apposition occurs, where does the inner cell mass of the blastocyst rotate?

Answer 52

near the endometrial epithelium

Question 53

what is the second stage of implantation, following the apposition?

Answer 53

attachment-the trophoblast cells adhere to the endometrial epithelium

Question 54

what does the stage of attachment initiate?

Answer 54

changes in the endometrial stroma (decidualization)

Question 55

what is the result of decidualization?

Answer 55

increased vascular permeability, changes in the intracellular matrix, and the cells take on a more stromal cell morphology

Question 56

what happens after decidualization/ attachment?

Answer 56

invasion-degradation of the endometrial epithelial cells

Question 57

what happens during invasion?

Answer 57

trophoblast fusion and formation of syncytiotrophoblasts

Question 58

what do syncytiotrophoblasts express?

Answer 58

adhesive surface proteins (cadherins and integrins)

Question 59

how do the syncytiotrophoblast break down the extracellular matrix during invasion?

Answer 59

they secrete matrix metalloproteases and other hydrolytic enzymes

Question 60

what is the source of secretion of hCG?

Answer 60

syncytiotrophoblasts

Question 61

what is decidualization a response to?

Answer 61

maternal stromal cells responding to invasion and progesterone

Question 62

what happens to the endometrial stroma during decidualization?

Answer 62

it is transformed into enlarged and glycogen-filled decidual cells

Question 63

what does the decidua form and what is its purpose?

Answer 63

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

Question 64

where is the most common site for ectopic implantation?

Answer 64

oviduct

Question 65

What does not occur during an ectopic pregnancy?

Answer 65

no decidualization- so invasion is not controlled and can rupture the tissues and cause hemorrhage

Question 66

when does placentation typically occur?

Answer 66

~ day 9

Question 67

what forms the primary villi?

Answer 67

proliferation of the syncytiotrophoblasts and cytotrophoblasts to lacunae

Question 68

what forms the secondary villi?

Answer 68

mesenchyme cells from the extraembryonic mesoderm invade the villi

Question 69

what forms the tertiary villi?

Answer 69

eventually the mesenchymal cells form fetal blood vessels de novo