Fertilisation and the Luteal Phase

Question 1

In what form is sperm when it is first ejaculated?

Answer 1

Coagulated to prevent loss - later liquifies

Question 2

What happens when sperm moves through the cervical mucus?

Answer 2

It removes seminal fluid, abnormally morphological sperm and cellular debris

Question 3

Why is sperm able to pass through the cervix more easily?

Where does sperm inhabit?

Answer 3

Cervical mucus is less viscous in the absence of progesterone allowing sperm to pass

Sperm can inhabit in cervical crypts which may form a reservour - some evidence of thermotaxis but mechanism not yet elucidated

Question 4

What is the most typical time fertilisation occurs?

Answer 4

Fertilisation occurs typically within 24-48 hours but sperm have been recovered alive after 5 days

Question 5

What factors facilitate the journey of the egg to the uterus?

Answer 5

Uterine or tubal cilia

Question 6

What attracts the sperm to the egg?

What happens to sperm on its journey to the egg?

Answer 6

Chemoattractants release from the oocyte cumulus complex to attract the sperm

Sperm will become hyperactivated forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels

Question 7

What is capacitation and acrosome reaction?

Answer 7

Sperm needs to undergo remodelling prior to fertillisation in order for fertillisation to be succesful - sperm must be kept out of the seminal fluid before being exposed to the oocyte

This process is called capacitation

CAPACITATION = physiological changes that sperm must undergo before it is able to fertilise the egg

• It is partly achieved by removing the sperm from the seminal fluid, also uterine or tubal fluid may contain factors promoting capacitation
• Biochemical rearrangement of the surface glycoprotein and changes in the membrane composition must occur before the acrosome reaction can occur

Question 8

Describe what occurs in the acrosome reaction?

Answer 8

Occurs in contact with the zona-cumulus complex

The acrosomal membrane on the sperm head fuses releasing enzymes that cut through the complex

ACROSIN - bound to the inner acrosomal membrane digests the zona pellucida so that sperm can enter

Question 9

What happens in the acrosomal reaction?

Answer 9

Acrosin binds to the inner acrosomal membrane digests the zona pellucida so that the sperm can enter

Question 10

What does the LH surge do?

Answer 10

• Requires resumption of meiosis and ovulation
• Converts primary oocyte to secondary oocyte plus 1st polar body

Question 11

What catches the oocyte cumulus complex?

Answer 11

The fimbrae of the uterine tube

Question 12

What is the corpus luteum?

Answer 12

Mixed theca and granulosa cells and the empty follicle

Question 13

What does the corpus luteum produce in the luteal phase?

Answer 13

Progesterone and oestrogen

Question 14

What is the role of progesterone in the luteal secretory phase?

Answer 14

Progesterone is secreted by the corpus luteum

• It makes the endometrium secretory and receptive to implantation
• Supresses cilia in uterine tubes once oocyte has already passed
• Makes cervical mucus viscous again to prevent further sperm penetration

Question 15

What is the role of oestrogen in the luteal/secretory phase?

Answer 15

Helps to maintain endometrium in luteal phase (causes proliferation in follicular phase)

Question 16

How does demise of the corpus luteum occur?

Answer 16

• If fertilisation does not occur, CL has an inbuilt finite lifespan of 14 days.
• Regression of CL essential to initiate new cycle.
• Fall in CL-derived steroids causes inter-cycle rise in FSH.
• Cell death occurs, vasculature breakdown, CL shrinks. Over time it becomes a corpus albicans
• CL rescued in pregnancy by hCG from embryo binding to its LH receptors. CL continues to produce progesterone and maintain endometrium.

Question 17

What is the purpose of menstruation?

Answer 17

1. Selection of a single follicle and oocyte.
2. Regular spontaneous ovulation.
3. Correct haploid number of chromosomes in the oocyte. (LH spike causes resumption of meiosis)
4. Cyclical changes in the cervix and uterine tubes, to enable egg transport and sperm access.
5. Preparation of the endometrium of the uterus to receive the fertilised egg.
6. Support of the implanting embryo and endometrium by corpus luteum progesterone.
7. Initiating a new cycle if fertilisation does not occur.

Question 18

What is the cumulus oophorus?

Answer 18

Protect egg, derived from granulosa cells

Question 19

What is the corona radiata?

Answer 19

Innermost layer of cumulus cells in contact with ZP.

Formed by granulosa cells adhering to the oocyte before it leaves the ovarian follcile

Question 20

What is the zona pellucida?

Answer 20

Secreted by the egg.

Becomes inpenetrable after the cortical reaction

Question 21

What is the 1st polar body?

Answer 21

Collection of chromosomes released from the primary oocyte after the first meiotic division

Question 22

Describe the sperm binding and penetration reaction

Answer 22

1. The acrosome reaction occurs in contact with the zona-cumulus complex.
2. Sperm penetrate cumulus and bind to the zona pellucida.
3. Sperm enzymes cut through ZP and sperm membrane come in contact with plasma membrane and fuse.
4. Sperm taken in by phagocytosis. Phospholipase Zeta (similar to PLC, exists in sperm membrane) activated by basal Ca2+ inside egg. PIP2 → DAG + IP3
5. Causes release of intracellular Ca2+ leading to large Ca2+ spike
6. This signals fertilisation in the egg
7. The Ca2+ release causes cortical granules to release their contents containing proteases, peroxides and hyaline which causes cross-linking and degradation of the adhesion molecules on the zona pellucida preventing further sperm from entering the egg preventing polyspermy
8. Ca2+ also triggers the presumption of meiosis II

Question 23

What happens when sperm enters the egg?

Answer 23

• Increase in phospholipase zeta from sperm
• Calcium causes completion of meiosis II expelling the second polar body and cortical reaction

Question 24

Describe the process of syngamy (fusion of two cells)

Answer 24

1. After meiosis I the oocyte has 23X chromosomes, but 2 copies of each chromosome arranged as sister chromatids
2. Entry of the sperm causes an increase in Ca2+ via phospholipase Zeta from sperm
3. Ca2+ causes completion of meiosis II expelling the second polar body and cortical reaction
4. The sperm nuclear membrane breaks down, the chromatin decondenses and chromosomes seperate
5. 4-7 hours after sperm penetration, two sets of haploid chromosomes become surrounded by distinct membranes forming pronuclei
6. These haploid structures synthesise DNA in preperation for the first mitotic division
7. The pronuclei fuse and mitotic metaphase spindle forms with the chromosomes assuming their position at its equator
8. Mitosis is completed and one cell zygote bcomes a 2 cell embryo

Question 25

What do the haploid structures during syngamy do?

Answer 25

They synthesise DNA in preparation for the first mitotic division

Question 26

Summarise the steps of fertilisation

Answer 26

• Sperm enters oocyte causing Ca2+ waves
  
  • Cortical granules fuse with zona pellucida to block polyspermy
• Female oocyte completion of meiosis II and expulsion of 2nd polar body
• Nucleus of sperm transformation to male pronucleus + female pronucleus forming
• Pronucleii will come together membranes break down, alignment of chromosomes on spindle
• = MITOSIS

Question 27

What is the first sign of fertillisation?

Answer 27

The fertilized egg has two pronuclei

Question 28

What happens 3 days after fertilization?

Answer 28

3 days

• Developing embyro contains 6-8 cells 3 days after fertilization

Question 29

On what day does the blastocyst form?

Answer 29

• Blastocyst forms Day 5 after fertilization.
• The cells will differentiate into trophoblast, blastocoel and inner cell mass (embryoblast)