Lecture 33: Fertilization and Implantation

Question 1

When and where does fertilization happen?

Answer 1

Day 15-16 of menstrual cycle

Ampulla of oviduct

Question 2

How is the egg transported to the ampulla?

How is the sperm transported to the ampulla?

Answer 2

• LH surge > follicle rupture > ovum and corona radiata ejected to the peritoneum and swept to the oviduct by fibriae
• sperm enter the uterus via vagina > myometrium contractions propel the sperm to the egg

Question 3

How does incapacitated sperm “wait” for the egg?

Answer 3

Incapacitated sperm has ability to bind itself to the epithelial cells of the isthmus

*unbound before capacitation

Question 4

How is capacitation initiated?

Answer 4

exposure to female tract environment > plasma coatings on the head are removed

Question 5

What changes does capacitation induce on the sperm?

Answer 5

-wash inhibitory factors away, weaken acrosome by losing cholesterol, increase membrane permeability to Ca2+ to increase sperm motility (whip like motion of flagella)

Question 6

What barriers does the sperm have to penetrate to enter the egg?

Answer 6

• cumulus (corona radiata)
• zona pellucida
• plasma membrane of oocyte

Question 7

How does the sperm penetrate the corona radiata?

Answer 7

uses hyaluronidase digesting the hyaluronic acid which makes up most of the corona radiata

Question 8

How does the sperm penetrate the zona pellucida?

Answer 8

-now exposed Z3 receptor on the sperm binds Z3 protein in the egg > triggers acrosome reaction (fusion of sperm plasma membrane with acrosome membrane + zona pellucida digestion)

Question 9

How does the sperm penetrate the ovum plasma membrane?

Answer 9

-Izumo protein on sperm binds to Izumo receptor on oocytes > sperm DNA enters the plasma membrane and the male pronucleus forms

Question 10

How is polyspermy prevented?

Answer 10

once the first sperm and the egg fuse cell membranes > Ca2+ is released, Z proteins block additional binding and zona pellucida serves as a barrier

Question 11

What happens to the egg after sperm enters?

Answer 11

Activated > completes meiosis II to get to haploid and releases the 2nd polar body > remaining DNA forms the female pronucleus and merges with the male pronuclei to start cell division

Question 12

Describe the process of embryonic cleavage

Answer 12

• cell division into blastomeres without changing embryo size
• morula at day 3

Question 13

How is the blastocyst formed from the morula?

Answer 13

outer morula cells adhere to each other via desmosomes and tight junctions forming the trophectoderm > increase in Na+ and osmosis so fluid enters and forms the blastocoele
-remaining cells become the Inner cell mass (embryo proper)

Question 14

How does blastocyst hatching happen?

Answer 14

trophoblasts secrete proteases that digest ZP

proper timing important for implantation

Question 15

What is the role of hCG?

Answer 15

Trophoblasts and syncytiotrophoblasts produce hCG > spike saves the corpus luteum
-stimulates blastocyst secretion of autocrine growth factor that stimulates trophoblast and placenta growth

Question 16

What happens during the apposition stage of implantation?

Answer 16

-trophoblasts contact the endometrium crypt > ICM will rotate and attach itself to the side of the endometrium (orients itself)

Question 17

What happens during the attachment stage of implantation?

Answer 17

-trophoblasts adhere to the endometrial epithelium > initiation of decidualization (changes) of the endometrial stroma

Question 18

What events occur during the invasion stage of implantation?

Answer 18

endometrial epithelial cells degrade > trophoblasts fuse with endometrium and forms syncytiotrophoblasts > these penetrate up to the stroma

Question 19

What are the characteristics and functions of syncytiotrophlasts?

Answer 19

1. adhesive proteins such as cadherins and integrins
2. able to break down the EcM via proteases and hydrolytic enzymes
3. produces hCG, and also makes their own progesterone at 10 weeks
4. has phagocytotic and bidirectional transfer functions between the embryo and endometrium

Question 20

How do syncytiotrophoblasts invade the endometrium?

Answer 20

degrades matrix using metalloproteinases and angiogenic factors > its fingerlike projections reach maternal blood supply

Question 21

What events occur during decidualization?

Answer 21

-endometrial stroma increased vascular permeability, changed intracellular matrix composition and endometrial cell morphology to accommodate implantation

Question 22

Characteristics of the decidua that forms after decidualization

Answer 22

• made of enlarged and glycogen filled decidual cells > forms a sheet with adhesive junctions that makes embryo “stick”
• also secretes signals that prevent myometrium invasion (prevent hemorrhaging)

Question 23

Ectopic pregnancy

Answer 23

• implantation somewhere else other than uterine fundus (commonly at oviduct)
• No decidua at site so implantation is not controlled and can rupture and cause hemorrhage

Question 24

What initiates placentation?

Answer 24

spaces in the syncytiotrophoblasts appear at day 9 > fills with secretions, maternal blood etc. for nutrient transfer

Question 25

How does the placenta develop?

Answer 25

syncytiotrophoblasts proliferation in the spaces (primary villi) > mesenchyme for extraembryonic mesoderm ivade to form chorionic membrane (secondary villi) > mesenchymal cells form fetal blood vessels (tertiary villi)