Fertilization Impantation

Question 1

What are the three layers that the sperm has to penetrate to fertilize the egg?

Answer 1

Outermost layer = Cumulus Cell matrix (corona radiate)

middle = Zona Pellucida

innermost = Plasma membrane (Oolemma)

Question 2

Fertilization occurs at what day?

what does fertilization allow for?

Answer 2

Day 15 or 16.

allows for recombination of genetic material

initiation of events before embryonic development

Question 3

fertilization happens mostly where?

sperm and egg travel where once fertilization happen?

Answer 3

ampulla of the uterine tube.

fundus of the uterine cavity.

Question 4

What are the 2 steps of fertilization?

Answer 4

capacitation (step 1)

Acrosome reaction (step 2)

Question 5

Why does capacitation need to happen?

Answer 5

in the male genitalia track there’s a high level of cholesterol which hardens and inhibits the acrosome from using its proteolytic enzymes so it keeps the sperms at bay, not allowing the sperm to be activated.

Question 6

Once the sperm enters the female genital tract, what does the uterus do?

2 things

what does the effect of calcium have on the sperm? (2) things

Answer 6

uterine and Fallopian tube fluids wash away inhibitory factors (cholesterol) on the sperm

the acrosome membrane that was hardened will now be permeable to calcium –> this calcium allows the tail to move faster and make it faster… also activates the sperm to release its enzymes

Question 7

What is the acrosome reaction?

Answer 7

the release of the enzymes in the acrosome to break through the 3 layers of the ovum to release its DNA inside

Question 8

What is the outermost layer of the egg and what breaks it down?

Answer 8

cumulus cell matrix

made up of hyaluronic acid.

Question 9

What is the second layer of the egg? what is it made of?

Answer 9

Zona Pellucida

ZP1, 2, and 3

ZP1 is structural.. it cross links ZP2 and 3

ZP2 –> blocks polyspermy

ZP3 initiates acrosome reaction.

Question 10

After capacitation, what receptor does the sperm now have?

no capacitation?

what does it use this for?

Answer 10

ZP3

no ZP3

it needs it to break through the second layer.

it uses the receptor to bind to ZP3.. once its bound, it allows the sperm to release all its enzymes which break apart the zona pellucida.

Question 11

What happens to the sperm once it gets through the zona pellucida?

then what happens?

Answer 11

it loses its ZP3 receptor

it now can bind to ZP2

Question 12

Once the sperm binds to ZP2, what happens?

Answer 12

the ovum recognizes this and says “hey, one of the sperm got through, so we can’t let any more enter”

so the binding of ZP2 (from the released ZP3) is what causes the Egg to not allow any more sperm to enter

Question 13

once the first sperm hits the plasma membrane, what happens?

Answer 13

it just goes through, and it’ll release its DNA so it can be bound to the female ovum’s DNA

Question 14

During the times of the month when the reproductive tract Is not releasing the egg, what is the female reproductive tract trying to do?

Answer 14

the female reproductive tract is trying to get pregnant.

if it knows its not the time of the month, its going to allow the body to hold onto sperm in a “sperm” reservoir, which is located in the oviductal isthmus..

sperm just hang out and bind to the epithelium. It’s not activated until the right time.

Question 15

after fertilization, what’s the first reaction to happen?

Answer 15

Cortical reaction

Question 16

what is the cortical reaction?

Answer 16

when the egg releases granules and calcium to prevent polyspermy.

ZP2 is going to bind to the released ZP3.. this is going to change ZP2 to ZPF.

that hardens the entire zona pellucida which prevents any sperm from entering the egg.

Question 17

What does the release of calcium during the cortical reaction do?

the egg is now active. what does it do then?

Answer 17

stimulate the egg to go through meiosis 2

the egg is finally activated and the sperm DNA can condense and there will be fusion of the male and female pronuclei to result in the first embryonic cleavage or the result of a Zygote.

Question 18

The first week of embryonic development.. what are the 2 stages?

Answer 18

1: compaction and blastocele development
2: blastocyst hatching

Question 19

compaction?

Answer 19

occurs with the morula which is around day 3.

the morula is a 16 cell mass

(day 4) –> what happens is the outer cells of the morula is going to increase cell to cell adhesions via tight junctions and desmosomes to really bring the cells together.

what it results in is an outer layer of trophoblasts which ends up making the extra embryonic structures (placenta) and it results in an inner cell mass which will end up being the embryo proper (or the actual embryo)

Question 20

while it compacts and forms the two distinct structures what else does it increase?

Answer 20

sodium and osmosis to form the blastocele

Question 21

embryo proper is what?

what about the morula?

Answer 21

pluripotent.. so it has the potential to be endoderm, mesoderm, or ectoderm

morula are totipotent

Question 22

Day 5 is what?

what happens?

Answer 22

late blastocyst –> hatching!

the entire structure will hatch from the zona pellucida.

Question 23

recall that the zona pellucida was hardened during the cortical reaction

Answer 23

that hardened zona pellucida is still enveloping the blastocyst and that is a problem because if you have a hard envelope around the blastocyst that means that the embryo is trying to grow, but it can’t do so as it’s being restricted.

so it needs to hatch or leave in order to grow and develop

Question 24

If you have someone who is unable to hatch the blastocyst from the zona pellucida then what happens?

Answer 24

infertility. the embryo can’t grow and develop

Question 25

premature hatching?

Answer 25

ectopic pregnancy.

remember that fertilization happens in the ampulla and then it travels back and implants into the fundus of the uterus.

if you have a blastocyst that hatches too early from the zona pellucida while its still traveling along the oviduct to reach the uterus, it can very easily implant along the uterine tube.. which is bad.

Question 26

hCG?

Answer 26

when an ovum is released, corpus luteum is also released.

in the case that the egg becomes fertilized, the corpus luteum is prevented from going away from hCG because it sticks around to prevent menstruation. (don’t want to shed the uterus because that’s where the embryo is).. also increases levels of progesterone and estrogen to maintain the pregnancy.

it’s an autocrine growth factor to stimulate trophoblast growth and development and stimulate placenta growth.

Question 27

trophoblast differentiates to what?

Answer 27

cytotrophoblast and syncytiotrophoblast

Question 28

cytotrophoblast?

Answer 28

supporting cells for the syncytiotrophoblast

Question 29

syncytiotrophoblast?

Answer 29

increase invasion and degradation of the extracellular matrix of the endometrium

important because it allows implantation of the embryo into the uterus

also secretes hCG

Question 30

during the first 10 weeks, we need to make sure that we don’t lose the corpus luteum.. what maintains this? what secretes it?

Answer 30

hCG

syncytiotrophoblast

Question 31

as implantation progresses, what functions do syncytiotrophoblasts do?

Answer 31

phagocytosis and bidirectional placental transfer of gases, nutrients, wastes

Question 32

Implantation?

Answer 32

occurs because before the blastocyst implants its bathed in uterine secretion that allows for oxygen, nutrients, and other metabolic substances to be exchanged but as the blastocyst is growing and expanding, it needs some more intimate association with the uterus in order to get all of the things it needs.

so it results in implantation

Question 33

where does implantation occur?

what kind of process?

Answer 33

fundus of the uterus.

It’s an invasive process so the trophoblasts on the outer layer will actually adhere and migrate and invade into the endometrial layer of the uterus.

the result is the blastocyst will differentiate the outer trophoblast layer into syncytiotrophoblasts and cytotrophoblasts in order to break down the extracellular matrix of the endometrial stroma.

Question 34

this implantation process occurs in 3 steps.. what are they?

Answer 34

Acquisition
Attachment
Invasion

Question 35

Acquisition?

Answer 35

Acquisition –> when contact is made with the endometrium and the trophoblasts.

this is in the crypts of the endometrium. and the inner cell mass is rotated so it faces the endometrium epithelial cells

Question 36

Attachment?

Answer 36

trophoblast will adhere and surface proteins will interact with the epithelial cells and this will initiate changes in the endometrial stroma.

this process is known as decidualization.

Question 37

Deciduization.. what changes happen? (3)

Answer 37

increased vascular permeability

changes in the intracellular matrix composition..

changes in the stromal cell morphology

Question 38

Invasion?

Answer 38

you get degradation of the endometrial epithelial cells and the trophoblast fuses into the endometrium and it’s going to differentiate into syncytiotrophoblasts and cytotrophoblasts.

Question 39

After implantation occurs, the mother side needs to respond.. what is the response?

Answer 39

deciduization

Question 40

after deciduization occurs, what happens to the endometriuM?

Answer 40

it has decidual cells and it’s no longer called the endometrium.

it’s now called the “decidua”

Question 41

why is the decidua important?

Answer 41

it will participate in active exchanges of gas, nutrients, and waste for the embryo

it’ll protect the baby from the mother’s immune system.

it allows for a very controlled invasion of the trophoblast. –> you don’t want it to invade to outside of the uterus. you want a very controlled process.

Question 42

what are the spaces in the syncytiotrophoblasts? when does this happen?

how does it happen?

what do they do?

Answer 42

lacunae

day 9

syncytiotrophoblast invades into the endometrial stroma which causes the holes.

they’re filled with endometrial secretions, maternal blood, digestive matrix for basic nutrient transfer for the fetus.

Question 43

what are the three stages of the formation of the placenta?

Answer 43

primary, secondary, and tertiary villi.

Question 44

what happens during the primary stage of placentation?

Answer 44

you’re just getting proliferation of the syncytiotrophoblast and cytotrophoblast into the lacunae.

they are growing into the spaces so that nutrients can exchange between the maternal side and the fetal side.

Question 45

what happens during the 2ndary villi stage of placentation.?

Answer 45

you have mesenchyme cells from extraembryonic mesoderm which invades the villi.

Question 46

what happens during the tertiary villi stage of placentation?

Answer 46

these mesenchyme cells are going to form the fetal blood vessels and that’s what is really important in terms of bringing nutrients to the fetus and exchanging gases and stuff.