fertilization and fetal development

Question 1

FSH

Answer 1

secreted from anterior pituitary gland-stimulates follicular growth, stimulates estrogen secretion(from developing follicles)

Question 2

estrogen

Answer 2

secreted in ovaries(depends on follicle)-development of endometrium, stimulates LH secretion (follicular phase), inhibits LH and FSH(luteal phase)

Question 3

LH

Answer 3

secreted from anterior pituitary-surge causes ovulation, development of corpus luteum, stimulates progesteron secretion

Question 4

Progesterone

Answer 4

secreted in ovaries(corpus luteum)-thickening of endometrium, inhibits LH and FSH (luteal phase)

Question 5

Follicular Phase

Answer 5

FSH stimulates growth of several follicles
Dominant follicle secretes estrogen
Estrogen inhibits growth of other follicles (and FSH)
Estrogen stimulates development of endometrium

Question 6

Ovulation

Answer 6

A surge in LH causes ovulation (egg release)

Rupturing of follicle creates a corpus luteum

Question 7

Luteal Phase

Answer 7

Corpus luteum secretes progesterone (and estrogen)
Progesterone stimulates development of endometrium
Estrogen and progesterone inhibit FSH and LH
Corpus luteum degrades over time
When corpus luteum degrades, progesterone levels drop
Without progesterone, endometrium cannot be maintained
Endometrium is sloughed away (menstruation)
No longer inhibited, FSH can start menstrual cycle again

If fertilisation of egg occurs, the zygote releases a hormone (hCG) which maintains the corpus luteum

Question 8

chemotaxis

Answer 8

The sperm is attracted to the egg due to the release of chemical signals from the secondary oocyte (chemotaxis)

Question 9

capacitation

Answer 9

When the sperm enters the female reproductive tract, biochemical changes to the sperm occur in the final part of its maturation (capacitation)

Question 10

zona pellucida

Answer 10

To enter the egg membrane, the sperm must penetrate the protective jelly coat (zona pellucida) surrounding the egg via the acrosome reaction

Question 11

polyspermy

Answer 11

To prevent other sperm from penetrating the fertilised egg (polyspermy), the jelly coat undergoes biochemical changes via the cortical reaction

Question 12

Describe the process of fertilisation, including the acrosome reaction, penetration of the egg membrane by a sperm and the cortical reaction

Answer 12

When the sperm enters the female reproductive tract, biochemical changes to the sperm occur in the final part of its maturation (capacitation)
The sperm is attracted to the egg due to the release of chemical signals from the secondary oocyte (chemotaxis)
Fertilisation generally occurs in the oviduct (fallopian tube)
To enter the egg membrane, the sperm must penetrate the protective jelly coat (zona pellucida) surrounding the egg via the acrosome reaction
The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg
To prevent other sperm from penetrating the fertilised egg (polyspermy), the jelly coat undergoes biochemical changes via the cortical reaction
The cortical granules release enzymes that destroy the sperm-binding proteins on the jelly coat
Now fertilised, the nucleus of the secondary oocyte completes meiosis II and then the egg and sperm nuclei fuse to form a diploid zygote

Question 13

role of hCG in early pregnancy

Answer 13

The endometrium is a blood-rich environment in which an implanted zygote can grow and it is sustained by the hormone progesterone
If progesterone levels aren’t maintained (i.e. the corpus luteum degenerates), then the endometrium will be sloughed away (menstruation)
A fertilised zygote develops into a blastocyst that secretes human chorionic gonadotrophin (hCG)
hCG maintains the corpus luteum post-ovulation so that the blastocyst can remain embedded in the endometrium and continue to develop
Gradually the placenta develops and produces progesterone (at around 8 - 10 weeks), at which point the corpus luteum is no longer needed

Question 14

early embryo development up to the implantation of the blastocyst

Answer 14

After fertilisation, the zygote undergoes several mitotic divisions to create a solid ball of cells called a morula (at around 4 days)
Unequal divisions beyond this stage cause a fluid-filled cavity to form in the middle - this makes a blastocyst (at around 5 days)
The blastocyst consists of:
An inner mass of cells (this will develop into the embryo)
An outer layer called the trophoblast (this will develop into the placenta)
A fluid filled cavity (called the blastocoele)
These developments all occur as the developing embryo is moving from the oviduct to the uterus
When the blastocyst reaches the uterus, it will embed in the endometrium (implantation)

Question 15

blastocys

Answer 15

Unequal divisions beyond the morula (4days) cause a fluid-filled cavity to form in the middle - this makes a blastocyst (at around 5 days)
blastocyst consists of:
An inner mass of cells (this will develop into the embryo)
An outer layer called the trophoblast (this will develop into the placenta)
A fluid filled cavity (called the blastocoele)
These developments all occur as the developing embryo is moving from the oviduct to the uterus
When the blastocyst reaches the uterus, it will embed in the endometrium (implantation)
(hollow in the middle)

Question 16

Structure and Function of Placenta

Answer 16

The placenta is a disc-shaped structure that nourishes the developing embryo
It is formed from the development of the trophoblast upon implantation and eventually invades the uterine wall
The umbilical cord connects the fetus to the placenta and maternal blood pools via open ended arterioles into intervillous spaces (lacunae)
Chorionic villi extend into these spaces and facilitate the exchange of materials between the maternal blood and fetal capillaries
Nutrients, oxygen and antibodies will be taken up by the fetus, while carbon dioxide and waste products will be removed
The placenta is expelled from the uterus after childbirth

Question 17

Hormonal Role of Placenta

Answer 17

The placenta also takes over the hormonal role of the ovary (at around 12 weeks)
Estrogen stimulates growth of the muscles of the uterus (myometrium) and the development of the mammary glands
Progesterone maintains the endometrium, as well as reduces uterine contractions and maternal immune response (no antibodies against fetus)
Both estrogen and progesterone levels drop near time of birth

Question 18

fetus is supported and protected by the amniotic sac and amniotic fluid

Answer 18

The fetus develops in a fluid-filled space called the amniotic sac
Amniotic fluid is largely incompressible and good at absorbing pressure, and so protects the child from impacts to the uterine wall
The fluid also creates buoyancy so that the fetus does not have to support its own body weight while the skeletal system develops
Finally, amniotic fluid prevents dehydration of the tissues, while the amniotic sac provides an effective barrier against infection

Question 19

materials are exchanged between the maternal and fetal blood in the placenta

Answer 19

Maternal Blood to fetal blood-Oxygen, glucose, lipids, water, minerals, vitamins, antibodies, hormones, amino acides
Fetal blood to maternal blood-carbon dioxide, urea, hormones, water.

Question 20

process of in vitro fertilisation

Answer 20

In vitro fertilisation refers to fertilisation that occurs outside the body (‘in vitro’ = ‘in glass’)

Stop normal menstrual cycle (with drugs)
Hormone treatments to develop follicles (FSH to stimulate follicle growth ; hCG for follicle maturation)
Extract multiple eggs from ovaries
Sperm selected, prepared (capacitation) and then injected into egg via intra-cytoplasmic sperm injection (ICSI)
Fertilisation occurs under controlled conditions (in vitro)
Implantation of multiple embryos into uterus
Test for pregnancy is conducted to see if implantation was successful

Question 21

ethical issues associated with IVF

Answer 21

Advantages of IVF

Chance for infertile couples to have children
Genetic screening of embryos could decrease suffering from genetic diseases
Spare embryos can be stored for future pregnancies or used for stem cell research

Disadvantages of IVF

IVF is expensive and might not be equally accessible to all
Success rate is low (~15%) and therefore stressful for couples
It could lead to eugenics (e.g. gender choice)
Often leads to multiple pregnancies which may be unwanted, unable to be budgeted for and involves extra birth risks
Issues concerning storage and disposal of unused embryos (right to life concerns)
There are cultural and religious objections to embryo creation by such means
Inherited forms of infertility may be passed on to children

Question 22

Advantages of IVF

Answer 22

Chance for infertile couples to have children
Genetic screening of embryos could decrease suffering from genetic diseases
Spare embryos can be stored for future pregnancies or used for stem cell research

Question 23

Disadvantages

Answer 23

IVF is expensive and might not be equally accessible to all
Success rate is low (~15%) and therefore stressful for couples
It could lead to eugenics (e.g. gender choice)
Often leads to multiple pregnancies which may be unwanted, unable to be budgeted for and involves extra birth risks
Issues concerning storage and disposal of unused embryos (right to life concerns)
There are cultural and religious objections to embryo creation by such means
Inherited forms of infertility may be passed on to children