Embryology 2 - Fertilization And The First Two Weeks Of Development

Front 1

What is sperm-zona binding?

Back 1

Sperm cell membrane has a receptor for ZP3 (located on the zona pellucida), and this is species-specific.

* This triggers the acrosome reaction

Front 2

What is acrosome reaction?

Back 2

The outer part of the acrosome membrane fuses with the sperm cell membrane. The acrosome releases the enzymes to penetrate the zona pellucida.

Front 3

What is sperm-egg binding?

Back 3

The sperm cell membrane has a ligand that binds to the receptor on the secondary oocyte membrane. This is NOT species-specific.

Front 4

What are the acrosomal enzymes?

Back 4

Hyaluronidase & acrosin accelerates the zona penetration

Front 5

What happens to the sperm after the sperm-egg binding?

Back 5

The sperm cell membrane and organelles enter the secondary oocyte but they are degenerated.

Front 6

What is ZP3 made up of?

Back 6

gluco-protein

Front 7

What is the postacrosomal segment?

Back 7

The part of the sperm cells that has sperm cell membrane. This part attaches to the oocyte membrane since the outer part of the sperm cell membrane has been lost during acrosome reaction.

- This is not species-specific

Front 8

When is the meiosis II completed?

Back 8

When the sperm binds to the egg; this triggers the meiosis II completion.

Front 9

What happens when the sperm binds to the oocyte membrane?

Back 9

- the oocyte completes the meiosis II

- the oocyte membrane release cortical granules, which triggers the zona reaction.

Front 10

What is zona reaction?

Back 10

Zona reaction: cortical granules damages the ZP3 so that is it no longer recognizable by the sperm cells & ** the zona becomes rigid; the sperm cells cannot enter anymore (block to polyspermy)

Front 11

What happens after the sperm cell enters the secondary oocyte?

Back 11

- Within the first 12 hours, the sperm cell head swells and form male pronucleus with 23 chromosomes.

- secondary oocyte completes the meiosis II -> female pronucleus and second polar body

Front 12

Describe the formation of zygote

Back 12

1. DNA duplication of female pronucleus and male pronucleus

2. They come together at the mitotic spindle (intermingling chromosomes)

3. 23 chromosome + 23 chromosome = 46 diploid zygote is formed and it is ready for the mitosis

Front 13

How are 3 second polar bodies are formed?

Back 13

- meiosis II of secondary oocyte -> 1 second polar body

- division of the first polar body -> 2 second polar bodies

Front 14

how does polyspermy occurs?

Back 14

- when zona pellucida does not exist OR ZP3 is not destroyed OR zona pellucida did not become rigid

- This results in an extra pronuclei

Front 15

What is cleavage?

Back 15

A series of RAPID mitotic division that results in a progressively larger number of progressively smaller cells.

Front 16

What happens in the first two weeks after fertilization?

Back 16

1. the zygote moves from the ampulla (site of fertilization) to the implantation site in the uterus

2. primitive yolk sac, amniotic sac and bilaminar embryo are formed

Front 17

Implantation:

Back 17

a complex interaction between maternal and embryonic tissues

Front 18

When does the first cleavage occurs and how does the subsequent divisions occur?

Back 18

- the first cleavage occurs 24 hours after the second polar bodies are produced

- The subsequent divisions do not occur at the same time but occurs at intervals of 12-24 hours

Front 19

What kind of cells do cleavage result?

Back 19

- blastomeres -> they are totipotent at the first time but it gets restricted subsequently.

- a ball of blastomere cells form morula (during compaction after 8-cell stage)

Front 20

What is compaction?

Back 20

This occurs after 8-cell stage. A ball of blastomere cells form a morula, in which the blastomere cells are TIGHTLY PACKED together to maximize the contact between them

- During compaction, trophoblast and inner cell mass are formed

Front 21

What are trophoblast and inner cell mass responsible for?

Back 21

Trophoblast: responsible for implantation and forming placenta (no contribution to embryo formation)

Inner cell mass: responsible for forming an embryo, amniotic membrane and primitive yolk sac membrane.

Front 22

What happens 4 days (32-cell stage of blastomere) after the fertilization?

Back 22

- The fluid is filled within the zygotic cell, forming a blastocyst cavity

- The inner cell mass is pushed aside.

Front 23

When does the blastocyst stage occur?

Back 23

- 4 days after the fertilization (after 32-cell stage of blastomere)

Front 24

When does the implantation occurs?

Back 24

- 5-6 days after the fertilization

Front 25

How is the implantation initiated?

Back 25

- the trophoblast release enzymes to degenerate the zona pelucida so that the embryo hatches out of the zona pelucida

Front 26

Where does the implantation occurs?

Back 26

- in the edometrium of the uterus (epithelium and connective tissues lie there)

** The endometrium is what being shed during parturition or menstruation

Front 27

When is the zona pelucida degenerated?

Back 27

5-6 days after the fertilization; before the implantation

Front 28

What is implantation?

Back 28

When the trophoblast cells line with the endometrium of the uterus (the innermost layer)

Front 29

Describe the process of implantation

Back 29

1. the trophoblast cells fuse with the endometrium of the uterus, which initiates the division of trophoblast cells.

2. When the trophoblast cells divide, the daughter cells become multi-nucleated -> syncytiotrophoblast

3. the cellular part of the trophoblast cells are cytotrophoblast

4. Syncytiotrophoblast brings the rest of embryo into the endometrium of the uterus.

Front 30

What is the source of hCG hormone and what does hCG do?

Back 30

Syncytiotrophoblast; hCG signals the corpus luteum not to be degenerated

Front 31

When does the embryo migrate from the fertilization site to the implantation site?

Back 31

- While cleavage, compaction and formation of blastocyst occur

Front 32

What contributes to the movement of embryo through the uterine tube?

Back 32

- beats of cilia, epithelial cells of the uterine tube

- contraciton of smooth muscle of uterine tube wall

Front 33

How should embryo prevent oneself from ectopic pregnancy?

Back 33

- keeping zona pelucida until the embryo reaches the appropriate implantation site. This is because trophoblasts implant whenever they make contact with epithelial layer of the uterine wall.

Front 34

During the first two weeks after the fertilization, what events occur and when do they occur?

Back 34

1. Cleavage - 24 hours after the second polar body production

2. Compaction - after 8-cell stage

3. Blastocyst formation - 4 days after the fertilization (32-cell stage)

4. Hatching (day 5) & Implantation (day 6) - 5-6 days after the fertilization; zona pelucida is disrupted by the enzyme released by trophoblast (hatching)

Front 35

What is ectotic pregnancy? What kinds of ectotic pregnancies are there?

Back 35

The ectotic pregnancy is when the zona pelucida is taken off from the trophoblast before the embryo arrives at the right site for implantation

- placenta preavia

- tubal pregnancy (most common)

- peritoneal pregnancy

Front 36

HOw often does ectotic pregnancy occur?

Back 36

1 in 100; 90% of them are tubal pregnancy

Front 37

What isn't uterine tube be the place for implantation?

Back 37

- the uterine tube cannot be enlarged for fast growth of embryo

Front 38

What is the possible reason for the ectotic pregnancy?

Back 38

- this may be due to scarring caused from uterine tube damage (salpingitis)

Front 39

What is placenta praevia? What is the possible treatment for this?

Back 39

- the placenta blocks the internal os of the cervix. So, this causes excessive bleeding during the labour.

- treatment: C-section so that the surgeon do not interrupt the placenta (this is what provides the fetus with oxygen)

Front 40

What is peritoneal pregnancy?

Back 40

- Implantation occurs in the peritoneal (outside of the uterus).

- This causes a stoned child.

Front 41

What is the tubal pregnancy? What are the treatments for it?

Back 41

- The implantation occurs in the uterine tube. This is because the uterine tube is incapable of hypertrophy (expansion of the organ - uterus and cervix can do this). The tube can burst 6-7 weeks after fertilization.

- 90% of ectopic pregnancy

- Treatment: medical vs. surgical (salpingectomy & salpingostomy)

Front 42

What are the salpingectomy and salpingotomy?

Back 42

Salpingectomy: the surgery that is being taken to treat tubal pregnancy. The surgeon removes the uterine tube and the fetus and ends the pregnancy; this is more common

Salpingostomy: the surgeon takes out the fetus only. But this is risky because the possible remnant of the fetus can develop in to malignant tumor cells. Also, the tube can be scarred, which may not be functional later.

Front 43

In what kind of pregnancy, the patient get -ve result on the pregnancy test?

Back 43

tubal pregnancy - when the fetus dies before syncytiotrophoblast release hCG to corpus luteum

Front 44

Why does tubal pregnancy occur?

Back 44

The fertilized zygote have 3 days to travel down from fertilization site to the uterus for implantation. However, due to abnormality (genital or acquired) of uterine tube slows down the movement of the fertilized zygote.

- Then the fertilized zygote implants in the tube.

Front 45

What occurs simultaneously with the process of implantation?

Back 45

Formation of bilanminar embryo: the inner cell mass converts into the bilaminal embryonic disc.

- bilaminar embry: two-layered structure (epiblast and hypoblast)

Front 46

What is the difference between epiblast and hypoblast?

Back 46

They are derived from the inner cell mass and they are components of bilaminar embryo:

- Epiblast faces the amniotic cavity -> this is what becomes embryo

- Hypoblast faces the blastocyst cavity; forms primitive yolk -> this turns into endoderm; consequently, hypoblast does not have a direct contribution to formation of embryonic tissue.

Front 47

Which cells make the amniotic membrane?

Back 47

- inner cell mass

Front 48

What are formed while the implantation takes place?

Back 48

- bilaminar embryonic disc - epiblast and hypoblast

- yolk sac and yolk sac membrane

- amniotic cavity, amniotic membrane

Front 49

Percentage of miscarriage

Percentage of aneuploidy

Back 49

20%

60% of miscarriage

Front 50

Fertilization age vs gestational age

Back 50

- Fertilization age: the age starting from the day of fertilization

- Gestational age: the age starting from the first day of the last menstruation (2 weeks older than fertilization age)

Front 51

Day of ovulation?

Back 51

Day 14 in the 28-cycle

Front 52

On which day is hCG released from syncytiotrophoblast?

Back 52

It takes 5-7 days to get down to uterus for implantation, so it is on day 21.