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
-helps expose molecules that can bind to zona pellucida of oocyte

Question 7

Where can incapacitated sperm bind to the female tract to extend its lifespan?

Answer 7

Epithelial cells of the isthmus of oviduct

Question 8

How does sperm change when it under capacitation?

Answer 8

Changes in flagella motion (hyper-activated)

• whip like
• increased mobility
• propels 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?

Answer 10

Hyaluronic acid

Question 11

What glycoprotein receptor of the zona pellucida triggers acrosome reaction?

Answer 11

ZP3

Question 12

What protein does sperm possess that reacts with a receptor on the plasma membrane?

Answer 12

Izumo protein binds with receptor

-sperm can enter egg after this and form a pronucleus

Question 13

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

Answer 13

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

Question 14

What triggers the completion of meiosis II in the egg?

Answer 14

Calcium release

• breaks down MAPK proteins
• releases second polar body

Question 15

What initiates the first embryonic cleavage?

Answer 15

Fusion of male and female pronuclei

Question 16

What is cleavage?

Answer 16

Cell division without growth

-goes from 1 cell –> 2 –> 4 –> 8 –> 16

Question 17

When does the embryo reach a 16 cell morula?

Answer 17

Day 3

Question 18

When does implantation happen?

Answer 18

Days 6-8

Question 19

What is the potency of a blastomere?

Answer 19

Totipotent

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

Question 20

What do outer cells of the morula form?

Answer 20

Trophectoderm

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

Question 21

What do inner cells of the morula form?

Answer 21

Inner cell mass –> pluripotent embryo proper

Question 22

What do trophoblasts do?

Answer 22

Secrete proteases that digest zona pellucida

-if not done, can be sign of infertility

Question 23

What secretes hCG?

Answer 23

Trophoblasts

Syncytiotrophoblasts

Question 24

What is the function of hCG?

Answer 24

Increased secretion of progesterone and estrogen
-prevents menstruation

Autocrine growth factor
-stimulates trophoblast and placental growth

Question 25

Where exactly does implantation take place?

Answer 25

Posterior wall of uterine fundus

• crypt in endometrium
• contact between endometrium and trophoblasts

Question 26

What is decidualization?

Answer 26

Changes in endometrial stroma after implantation

• increased vascular permeability
• intracellular matrix composition
• stromal cell morphology

*improves interaction b/t fetal and maternal placenta

Question 27

What happens after implantation?

Answer 27

Invasion

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

Question 28

What is the role of syncytiotrophoblasts?

Answer 28

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 29

What is the decidua?

Answer 29

Endometrium ready for implantation of embryo

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

Question 30

What is ectopic implantation?

Answer 30

Implantation of embryo not in the fundus of uterus

• mostly in oviduct
• no decidualization
• can cause tissue rupture and hemorrhaging

Question 31

When does placentation start?

Answer 31

Day 9

-spaces within syncytiotrophoblasts

Question 32

Describe the process of placentation?

Answer 32

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