Lecture 35: Fertilization and Implantation

Question 1

When does fertilization occur in the menstrual cycle?

Answer 1

Around days 15-16

Question 2

Where does fertilization occur?

Answer 2

Ampulla of oviduct

Question 3

What happens during ovulation?

Answer 3

Ovum and surrounding corona radiata ejected into peritoneum -happens after LH surge

Question 4

A sperm count of what can be considered as infertility?

Answer 4

<20 million/ejeculation

Question 5

What type of contractions help the sperm move through the cervix and uterus?

Answer 5

Myometrium contractions -result as increased estrogen near ovulation

Question 6

Describe sperm capacitation.

Answer 6

When sperm enters the female tract, seminal plasma coating and surface molecules are removed which helps expose the molecules that can bind to zona pellucida of oocyte

Question 7

Where can incapacitated sperm bind in the female tract to extend their lifespan and increase the probability that the sperm will be in the oviduct when the egg is ovulated?

Answer 7

Bind to the epithelial cells of the isthmus of the oviduct

Question 8

How does sperm change when it under capacitation?

Answer 8

Flagella of sperm become hyperactivated.

Changing from wave-like to whip like motion increases mobility and helps propel the sperm to penetrate outer layers of egg to reach plasma membrane

Question 9

What are the three barriers the sperm must breach to fertilize the egg?

Answer 9

1) Corona radiata
2) Zona pellucida
3) Plasma membrane of oocyte

Question 10

What is the corona radiata primarily made up of & how does the sperm get through this?

Answer 10

Hyaluronic acid

Sperm digests it via hyaluronidase

Question 11

What proteins make up the zona pellucida

Answer 11

ZP1- ZP4

Question 12

What receptors do sperm contain that helps them get through the zona pellucida

Answer 12

ZP3 receptors

Question 13

What is acrosome reaction

Answer 13

Binding of ZP3 receptors to ZP3 triggers the enzymes in the acrosome vesicle to digest the zona pellucida, allow the sperm to enter the holes created in the zona pellucida.

Question 14

What protein do sperm possess that reacts with a receptor on the plasma membrane of the egg?

Answer 14

Izumo protein binds with Izumo receptor on the oocyte

Question 15

The fusion of the sperm and egg triggers a cortical reaction. Describe this process.

Answer 15

PREVENT POLYSPERMY

1) Release of calcium
2) Alteration of ZP proteins to prevent binding of additional sperm
3) Release of hyaluronic acid, proteoglycans, proteinases
4) Zona pellucida forms a physical barrier

Question 16

What triggers the completion of meiosis II in the egg?

Answer 16

Calcium release

• breaks down MAPK proteins
• releases second polar body

Question 17

What initiates the first embryonic cleavage?

Answer 17

Fusion of male and female pronuclei

Question 18

What is cleavage?

Answer 18

Cell division without growth -goes from 1 cell –> 2 –> 4 –> 8 –> 16

Question 19

When does the embryo reach a 16 cell morula?

Answer 19

Day 3

Question 20

When does implantation happen?

Answer 20

Days 6-8

Question 21

What is the potency of a blastomere?

Answer 21

Totipotent -stem cells can give rise to any of the cell types found in an embryo as well as extra-embryonic cells (placenta).

Question 22

What do outer cells of the morula form?

Answer 22

Trophectoderm (trophoblast)

-supplies embryo with nourishment and later forms the major part of the placenta

Question 23

What do inner cells of the morula form?

Answer 23

Inner cell mass (embryoblast) –> pluripotent embryo proper

Question 24

What do trophoblasts do?

Answer 24

Secrete proteases that digest zona pellucida

-if not done, can be sign of infertility

Question 25

What secretes hCG?

Answer 25

Trophoblasts specifically Syncytiotrophoblasts

Question 26

What is the function of hCG?

Answer 26

• Prevents involution of the corpus luteum
• Leads to increased secretion of progesterone and estrogen
• prevents menstruation
• stimulates trophoblast and placental growth

Question 27

What are the stages of implantation

Answer 27

Apposition

Attachment

Invasion

Question 28

What occurs in apposition stage of implantation

Answer 28

Typically occurs in a crypt in the endometrium. The embryoblast rotates near the endometrial epithelium and there is contact between endometrium and trophoblast.

Question 29

What happens in the attachment stage of implantation

Answer 29

Decidualization

Changes in endometrial stroma after implantation

• Increased vascular permeability
• Change in intracellular matrix composition
• Change in stromal cell morphology

*improves interaction b/t fetal and maternal placenta

Question 30

What happens in the invasion stage of implantation ?

Answer 30

• Endometrial epithelial cells are degraded
• Syncytiotrophoblasts protrudes through basement membrane to reach endometrial stroma

Question 31

What is the role of syncytiotrophoblasts?

Answer 31

1) Express adhesive surface proteins
2) Breaks down ECM
3) Secretes hCG
4) Produces progesterone independent of corpus luteum
5) Phagocytosis and bidirectional transfer of gases, nutrients, and wastes

Question 32

What is a decidua?

Answer 32

The name given to the endometrium when it is ready for implantation of embryo

-has adhesive junctions that prohibits embryo to migrate anywhere, especially myometrium

Question 33

What is ectopic implantation?

Answer 33

Implantation of embryo outside the fundus of uterus

• Mostly in oviduct
• No decidualization
• Can cause tissue rupture and hemorrhaging

Question 34

When does placentation start?

Answer 34

Day 9. Spaces begin to appear within syncytiotrophoblasts

Question 35

Describe the process of placentation?

Answer 35

1) Primary Villi: proliferation of syncytiotrophoblasts and cytotrophoblasts to lacunae
2) Secondary Villi: mesenchyme cells from extraembryonic mesoderm invade villi
3) Tertiary Villi: Mesenchymal cells form fetal blood vessels de novo