Fertilization

Question 1

What is fertilization?

Answer 1

fusion of male and female gamete cells

Question 2

Where does fertilization normally occur?

Answer 2

in the ampullary region of he uterine tube (Fallopian tube, oviduct)
^this is a broadening at the very start of the uterine tube adjacent to the ovary

Question 3

What happens to the sperm head in order for fertilization to occur?

Answer 3

glycoproteins that surround the sperm head are removed in a process called capacitation (within female reproductive tract)

Question 4

What is capacitation?

Answer 4

the process in which glycoproteins that surround the sperm head are removed

Question 5

What is the tissue outside the egg called?

Answer 5

cumulus oophorus

Question 6

The outer layer of the tissue surrounding the egg (cumulus oophorus) which contains associated granulose cells, is called what?

Answer 6

corona radiata

Question 7

The inner glycoprotein layer of the tissue (cumulus oophorus) surrounding the egg is called what?

Answer 7

zona pellucida

Question 8

The ability to penetrate the layers outside the egg is controlled by what?

Answer 8

acrosomal enzymes released from the tip of the sperm

Question 9

After penetrating the layers outside the egg, what does the sperm do?

Answer 9

the sperm membrane fuses with the oocyte membrane

• this triggers a change in membrane properties that acts as a block to polyspermy
• the membrane fusion event enables the sperm head and tail to enter the ovum

Question 10

As the sperm membrane fuses with the oocyte membrane and the sperm head and tail enter the ovum, the ovum undergoes cortical reaction. What is cortical reaction?

Answer 10

when the membrane of the ovum separates from the zona pellucida to form a perivitelline space

• this is another step to prevent entry of a second sperm
• digestive enzymes are released into the perivitelline space

Question 11

When does the second meiotic division of the egg take place?

Answer 11

as the sperm fuses to the ovum
*one set of maternal chromosomes is expelled in the second (or third?) polar body, leaving the female pronucleus in the ovum

Question 12

What is the female pronucleus?

Answer 12

the haploid set of chromosomes remaining in the ovum

Question 13

After entering the ovum, what happens to the tail and head of the sperm?

Answer 13

• the tail degenerates

- the head enlarges (due to decompaction and remodeling of chromatin) to from the male pronucleus

Question 14

What is the zygote?

Answer 14

the fertilized egg

*formed when the male and female pronucleus fuse to form a single diploid nucleus

Question 15

The zygote undergoes cell division into smaller cells to contribute to what?

Answer 15

contribute to the embryo or to extraembryonic tissues

Question 16

The zygote undergoes reductional division. What is this?

Answer 16

• the early mitotic divisions of the zygote that take place without intervening growth of the resultant daughter cells
• the single fertilized egg produces a larger number of smaller cells

Question 17

During early cell divisions of the zygote into smaller cells (2-cell to 16-cell), all cells are equivalent and closely associated with each other. The 8-16 cell stage is called what?

Answer 17

the morula (‘mulberry’) stage

Question 18

When does the morula begin to organize into a blastula (or blastocyst)? And what is this?

Answer 18

between the 32 and 128 cell stage
- the cells of the embryo begin to form a sphere surrounding a fluid-filled central space called the blastocele or the blastocyst cavity

Question 19

When the embryo contains about 100 cells, what happens?

Answer 19

there is a division into inner and outer cell masses

• the inner cell mass will become the embryo itself
• the outer layer of cells will produce the trophoblast layer (most cells from the cytotrophoblast, which is a population of cells with one nucleus each)

Question 20

During the early cleavage stages, the embryo moves through the uterine tube to the uterus. At about 5-6 days (blastocyst stage), what happens to the embryo?

Answer 20

it attaches to the wall of the uterus and begins the process of implantation

Question 21

What mediates attachment of the embryo to the uterine wall?

Answer 21

selectin adhesion

• the trophoblast cells express L-selectin on their surface and this adheres to oligosaccharide molecules on the uterine wall
• additional adhesion events and invasive activity by trophoblast cells then allow implantation to occur

Question 22

When can infertility result?

Answer 22

• if the uterine tube becomes damaged or blocked due to inflammation or scarring (this may prevent access of the sperm to the egg or movement of the egg to the uterus)
• defects in cilia function

Question 23

What drives the fertilized egg/zygote towards the uterus?

Answer 23

ciliary function on the surface of cells of the uterine tube

Question 24

What is an ectopic pregnancy? How/where does it occur?

Answer 24

• extra-uterine pregnancy; may result if the zygote implants anywhere but the uterus
• most commonly in ampullary region of the uterine tube
• embryo usually dies after about 30 days
• may be associated with sever hemorrhaging (and risk to the life of the mother)

Question 25

What is suggested as for why approximately half of all zygotes fail to implant correctly?

Answer 25

thought to be due largely to chromosomal abnormalities that are incompatible with normal development
(perhaps failure of the ovum to correctly undergo second meiotic division during fertiliziation, i.e. resulting in 3n embryos)

Question 26

After successful implantation, approximately how many developing embryos are lost to spontaneous abortion?

Answer 26

20-25%

*many suffer from serious developmental abnormalities

Question 27

What are two ways of losing the zygote or embryo?

Answer 27

• failure of zygote to implant correctly

- spontaneous abortion

Question 28

What forms the syncytiotrophoblast population? What is this?

Answer 28

• a group of trophoblast cells close to the inner cell mass
• these are multinuclear cells that lead to invasion into the endometrium of the uterus and will eventually cover the cytotrophoblast

Question 29

What regulates the ability of trophoblast cells to fuse to form the cyncytiotrophoblast?

Answer 29

an ancient retroviral protein called SYNCYTIN

*this protein is also important for fusion of embryonic and maternal blood vessels in the placenta

Question 30

What begins implantation?

Answer 30

L-selectin mediated attachment of syncytiotrophoblast cells to the uterine wall

Question 31

After implantation has begun with L-selectin mediated attachment of syncytiotrophoblast cells to the uterine wall, what do the syncytiotrophoblast cells do?

Answer 31

• they digest their way into the uterine stroma (through uterine epithelium into uterine stroma) and wrap themselves around the cytotrophoblast during invasion

Question 32

After the embryo and membranes are buried in the uterine wall, what happens to the syncytiotrophoblast?

Answer 32

the syncytiotrophoblast layer surrounds the entire embryo

Question 33

2/3 of twins are dizygotic or fraternal. What does this result from?

Answer 33

these result from two separate ovulation and fertilization events and two separate implantations
*fraternal twins have a genetic component no more alike than any two siblings

Question 34

Why is the incidence of fraternal twins and multiple births increasing?

Answer 34

due to the ability of fertility drugs to induce multiple ovulations

Question 35

1/3 of twins are monozygotic or identical. What is this due to?

Answer 35

• this can be due to splitting of the fertilized egg during the very early cleavage stages
• alternatively, it can occur by formation of two inner cell masses within the blastocyst
• at a very low frequency, identical twins may fuse to form conjoined twins

Question 36

What are two blocks to polyspermy?

Answer 36

• fast block: change in membrane potential that prevents further sperm fusion events
• slow block: at the time of sperm entry, the ovum undergoes an event called the cortical reaction, this forms a space between the egg membrane and the zona pellucida- perivitelline space; digestive enzymes are released into the perivitelline space

Question 37

What are two ways of developing monozygotic/identical twins?

Answer 37

• splitting of the embryo during early cleavage stages (before 8 cells)
• formation of 2 inner cell masses

Question 38

What are some serious health consequences associated with premature birth?

Answer 38

• incomplete brain development
• respiratory problems
• vision, hearing problems
• jaundice (elevated bilirubin levels)
• multiple births are the largest single cause of prematurity
• assisted pregnancy is the largest single cause of multiple births