8- Fertilisation And The Luteal Phase

Question 1

How does sperm get coagulated and how is the coagulation reversed?

Answer 1

Prostatic and seminal vesicle secretions comprise seminal fluid which coagulates - prevents loss, later liquefies.

Movement through cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris.

Question 2

Describe the movement of sperm through the cervix

Answer 2

Cervical mucus is less viscous in the absence of progesterone (due to the increase in oestrogen) allowing sperm to pass.

Sperm can inhabit cervical crypts which may form a reservoir (allows for the slow release of sperm, increased chances of fertilisation). Some evidence of thermotaxis, but mechanism not yet elucidated.

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

Question 3

Describe the sperms journey to the egg

Answer 3

Passage through uterus not well understood, currents set up by uterine or tubal cilia may have a role.
Chemo-attractants released from the oocyte cumulus complex may attract the sperm.
Sperm become hyperactivactivated. Forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels.

Question 4

What are CatSper channels?

Answer 4

They are calcium sperm channels
They cause an influx of Ca2+ ions to hyperactivate the sperm

Question 5

What is capacitation and how is it achieved?

Answer 5

Biochemical rearrangement of the surface glycoprotein and changes in membrane composition must occur before the acrosome reaction can occur.

Capacitation is partly achieved by removing the sperm from the seminal fluid, also uterine or tubal fluid may contain factors which promote capacitation.

Question 6

What is the acrosome reaction and when does it occur?

Answer 6

Acrosin bound to the inner acrosomal membrane digests the zona pellucida so the sperm can enter.
Acrosome reaction occurs in contact with the zona –cumulus complex; the acrosomal membrane on the sperm head fuses releasing enzymes that cut through the complex.

Question 7

Describe ovulation

Answer 7

LH spike (triggers an inflammatory cascade) causes resumption of meiosis and ovulation. Converts the primary oocyte to secondary oocyte plus 1st polar body.

Basement membrane breaks (due to the inflammatory response) so blood pours into the middle.

Oocyte cumulus complex extruded out and caught by fimbrae of uterine tube.

Theca and granulosa become mixed and the empty follicle is known as the corpus luteum. It produces progesterone in the luteal phase.

Question 8

Describe the role of progesterone in the menstrual cycle

Answer 8

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 9

Describe the role of oestrogen in the menstrual cycle

Answer 9

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

Question 10

What is the difference between the luteal phase and the secretory phases of the menstrual cycle?

Answer 10

Luteal phase: in the ovary
Secretory phase: in the endometrium

Question 11

Describe the demise of the corpus luteum

Answer 11

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 ( increase in FSH/LH, decrease in progesterone, endometrium cannot be maintained) causes inter-cycle rise in FSH.

Cell death occurs, vasculature breakdown, CL shrinks. Over time it becomes a corpus albicans.

Question 12

What happens to rescue the corpus luteum during pregnancy?

Answer 12

CL rescued in pregnancy by hCG from embryo binding to its LH receptors. CL continues to produce progesterone and maintain endometrium

Question 13

What is the difference in hCG and LH structure?

Answer 13

They have different beta chains

Question 14

What does the menstrual cycle achieve?

Answer 14

Selection of a single follicle and oocyte.

Regular spontaneous ovulation.

Correct haploid number of chromosomes in the oocyte by completing meiosis I and beginning meiosis II.

Cyclical changes in the cervix and uterine tubes, to enable egg transport and sperm access.

Preparation of the endometrium of the uterus to receive the fertilised egg.

Support of the implanting embryo and endometrium by corpus luteum progesterone.

Initiating a new cycle if fertilisation does not occur.

Question 15

Describe the structure of the oocyte at ovulation

Answer 15

Cumulus oophorus
- Protect egg, derived from granulosa cells.

Corona radiata
- Innermost layer of cumulus cells in contact with the ZP. Formed by granulosa cells adhering to the oocyte before it leaves the ovarian follicle

Zona Pellucida
- Secreted by egg. Becomes impenetrable after fertilisation

1st Polar body
- 1st meiotic division completed.

Question 16

Describe the process of sperm binding and penetration

Answer 16

There are 4 steps involved in this process

1. The acrosome reaction occurs in contact with the zona-cumulus complex. Sperm penetrate cumulus and bind to ZP.
2. Sperm enzymes cut through ZP and sperm fuses with plasma membrane.
3. Sperm taken in by phagocytosis. Phospholipase Zeta activated by basal Ca2+ inside egg. PIP2 → DAG + IP3
   Causes release of intracellular Ca2+ leading to large Ca2+ spike
4. Cortical reaction as wave of Ca2+ sweeps around egg…release of proteases, peroxides and hyaline prevents polyspermy.

Question 17

What is polyspermy?

Answer 17

Many sperm entering the egg

Question 18

What is syngamy?

Answer 18

The process of the fusion of two cells or their nuclei during fertilisation
happens between gametes

Question 19

Describe the process of syngamy?

Answer 19

After meiosis I the oocyte has 23X chromosomes, but 2 copies of each chromosome arranged as sister chromatids.

Entry of the sperm causes an increase in Ca2+ via phospholipase Zeta from sperm.
Ca2+ causes the completion of meiosis II expelling the second polar body & cortical reaction.

The sperm nuclear membrane breaks down, the chromatin decondenses and chromosomes separate

4-7 hours after sperm penetration the two sets of haploid chromosomes become surrounded by distinct membranes forming two pronuclei.

These haploid structures synthesise DNA in preparation for the first mitotic division.

The pronuclei fuse and the mitotic metaphase spindle forms with the chromosomes assuming their position at its equator.

Mitosis is completed and the one cell zygote becomes a two cell embryo.

Question 20

Describe early embryo development

Answer 20

The fertilized egg has 2 pronuclei. This is the first sign of fertilization.

The developing embryo contains 6-8 cells 3 days after fertilization

Five days after fertilization it is called a blastocyst and differentiates into inner cell mass, blastocoel and trophoblast.

Question 21

Summarise fertilisation

Answer 21

Sperm enters oocyte causing Ca2+ waves.
Cortical granules fuse with zona pellucida to block polyspermy
Completion of MII expulsion of 2nd polar body.

Nucleus of sperm transformation to male pronucleus
Female pronucleus forms.

Pronuclei come together, membranes break down alignment of chromosomes on spindle…MITOSIS