Lecture 23 - Fertilization and Pregnancy

Question 1

What is fertilization?

Answer 1

The fusion of 2 haploid gametes, which will produce a zygote and all the necessary processes needed for this process to occur

Question 2

Describe step 1 of fertilization (oocyte at ovulation)

Answer 2

Successful fertilization occurs within a day of ovulation, not long after the oocyte enters the uterine tube. Some of the ejaculated sperm can reach the uterine tube within a few hours of ejaculation, and can remain viable for 5-7 days in the female reproductive tract

Question 3

Describe step 2 of fertilization (fertilization and oocyte activation)

Answer 3

Enzymes in the acrosome (tip) of the sperm are used to break down the layers of follicular cells that make up the corona radiata, and when one sperm breaks the oocyte membrane, the oocyte is activated

Many sperm are required to break through the oocyte membrane, just for one sperm to penetrate the oocyte

Question 4

Describe steps 3 and 4 of fertilization (zygote formation)

Answer 4

After a sperm has entered the ovum, two haploid pronuclei combine their chromosomes, forming a zygote. As soon as the chromosomes mingle, the first mitotic cell division occurs, around 24 hours after the point of fertilization

Question 5

Describe steps 5 and 6 of fertilization (cleavage)

Answer 5

Cleavage begins immediately after the zygote is formed. During cleavage, the zygote starts a series of repeated mitotic cell divisions, increasing its cell numbers without growing in size; this is considered pre-embryo

Question 6

Blastomere definition

Answer 6

Daughter cells produced during cleavage

Question 7

Morula definition

Answer 7

solid ball of cells

Question 8

What is a blastocyst and how is it formed?

Answer 8

A hollow ball of cells with a fluid-filled inner cavity and an inner mass of cells that will become the embryo

Around the time the morula enters the uterus, it sheds the zona pellucida and undergoes cell rearrangements, creating a blastocyst

Question 9

When does implantation occur and what is its process?

Answer 9

It occurs when the blastocyst fuses with the functional layer of the endometrium. The trophoblast cells (outer layer of blastocyst) drive the erosion of the endometrial epithelium, lodging the blastocyst within the endometrium

Question 10

Define cytotrophoblast

Answer 10

Has individual cells with plasma membranes

Question 11

Define syncytiotrophoblast

Answer 11

Many nuclei is sharing one cytoplasm

Question 12

What changes are involved during the implantation process?

Answer 12

The trophoblast cells divide rapidly, and separate into 2 layers which surround the inner cell mass (ICM) and begin to provide it with nutrients from the maternal tissue – cytotrophoblast and syncytiotrophoblast

Question 13

What does the ICM form during implantation?

Answer 13

The ICM forms 2 layers of cells between 2 new cavities - the amnion and the yolk sac

Question 14

When does gastrulation occur and what is its process?

Answer 14

It occurs following implantation and creates 3 distinct germ layers for the developing embryo

During gastrulation, cells from one layer of the ICM migrate into the space between the 2 cell layers, forming the 3 germ layers, all of which have different properties and will give rise to different organs

Question 15

Name the 4 extra-embryonic membranes that form around the developing embryo

Answer 15

1. Amnion - creates the amnionic cavity the embryo develops within
2. Yolk sac - creates a pouch involved in collecting and distributing nutrients
3. Chorion - separates the blastocoele from the trophoblast
4. Allantois - will form part of the umbilical cord

Question 16

Name 2 methods used for prenatal genetic testing or screening

Answer 16

1. Chorionic villous sampling (CVS) samples the chorion villi
2. Amniocentesis will sample the amniotic fluid

Question 17

What is the placenta and what are its 3 functions?

Answer 17

It is a foetal organ that acts as an interface between the foetus and the pregnant person

1. Nutrient and oxygen exchange between foetal and maternal bloodstream
2. Removal of CO2 and other wastes from foetal blood
3. Secretes hormones into both bloodstreams

Question 18

Describe the structure of the placenta

Answer 18

Chorion is a major component of the placenta and forms the chorionic villi that invade the endometrium and break down its blood vessels, forming an area of pooled blood that exchanges with foetal blood

Trophoblast layers surround each villus, and are responsible for hormone secretion by the placenta

Question 19

What maintains the production of ovarian progesterone during early pregnancy?

Answer 19

The syncytiotrophoblast layer of the embryo releases human chorionic gonadotropin (hCG) to maintain the production of ovarian progesterone

It’s similar to LH such that signals the corpus luteum to survive and keep secreting progesterone

The presence of hCG in maternal blood and urine is the first reliable and detectable sign of pregnancy

Question 20

True or false? Before the end of the first trimester, placental cells take over the secretion of progesterone and estrogen

Answer 20

True - the placental trophoblast tissues begin to synthesize larger amounts of progesterone, which maintains the integrity of the endometrium. It also begins to synthesize estrogens as pregnancy progresses

Question 21

Define medical abortion

Answer 21

Involves taking a combination of medicines to end the pregnancy by causing the uterine lining to be shed

Question 22

How does the PR-antagonist, mifepristone, lead to the termination of a pregnancy?

Answer 22

It is a synthetic steroid derivative that’s able to prevent progesterone from activating the progesterone receptor. Blockage leads to the softening of the cervix and the disintegration of the endometrium, detaching the placenta and embryo