Fertilisation And The Luteal Phase

Question 1

Describe the coagulation of the seminal fluid

Answer 1

• Prostatic and seminal vesicle secretions comprise seminal fluid which coagulates
• This prevents loss and later liquifies
• Movement through the cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris

Question 2

Describe what happens when sperm passes into the cervix

Answer 2

• Cervical mucus is less viscous in the absence of progesterone allowing sperm to pass
• Sperm can inhabit cervical crypts which may form a reservoir
• Some evidence of thermotaxis but mechanism not elucidated
• Fertilisation typically occurs within 24-48 hours but sperm have been recovered alive after 5 days

Question 3

What is the journey like from the outside 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 becomes hyper activated
• Forceful tail beats with increased frequency and amplitude mediated by CA2+ influx via CatSper channels

Question 4

What is Sperm capacitation?

Answer 4

The process whereby sperm becomes capable of fertilising an egg

Question 5

How is Sperm Capacitation achieved?

Answer 5

• Partly achieved by removing the sleek from the seminal fluid
• Uterine or tubal fluid may also contain factors which promote capacitation

Question 6

What changes must occur before the acrosome reaction during Sperm capacitation?

Answer 6

• Biochemical rearrangements of the surface glycoprotein
• Changes in membrane composition must occur before the acrosome reaction can occur

Question 7

Where does the acrosome reaction occur?

Answer 7

Occurs in contact with the zona - cumulus complex

Question 8

What happens in the Acrosome reaction?

Answer 8

• The acrosome membrane on the sperm head fuses with the egg
• This releases enzymes that cut through the complex
• Acrosin bound to the inner acrosomal membrane digests the zona pellucida so the sperm can enter

Question 9

Explain how Ovulation occurs? (PART 1)

Answer 9

• LH spike causes resumption of meiosis and ovulation
• This converts the primary oocyte to secondary oocyte plus 1st polar body
• basement membrane then breaks so blood pours into the middle

Question 10

Explain how Ovulation occurs? (PART 2)

Answer 10

• Oocyte cumulus complex extruded out and caught by fimbrae of uterine tube
• Theca and Granulosa become mixed
• The empty follicle is known as the corpus Luteum.
• It produces progesterone in the luteal phase

Question 11

In the Luteal/Secretory phase, what does the Progesterone do?

Answer 11

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

Question 12

In the Luteal/Secretory phase, What does the estrogen do?

Answer 12

• Helps to maintain endometrium in the luteal phase
• (Causes proliferation in the follicular phase)

Question 13

When does the demise of the corpus Luteum occur?

Answer 13

• If fertilisation does not occur
• Corpus Luteum has an inbuilt finite lifespan of 14 days

Question 14

What happens during the demise of the Corpus Luteum? (PART 1)

Answer 14

• Regression of the CL is essential to initiate a new cycle
• A fall in CL derived steroids causes inter cycle rise in FSH
• Cell death occurs
• Vasculature breakdown

Question 15

What happens during the demise of the Corpus Luteum? (PART 2)

Answer 15

• Corpus Luteum shrinks
• Overtime 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 16

What does the Menstrual cycle achieve?

Answer 16

• Selection of a single follicle and oocyte
• Regular spontaneous Ovulation
• Correct haploid number of chromosomes in the oocyte by completing meiosis 1 and beginning meiosis 2
• Cyclical changes in the cervix and uterine tubes, to enable egg transport and sperm access

Question 17

What other processes does the Menstrual cycle achieve?

Answer 17

• Preparation of the endometrium of the uterus to receive the fertilised egg
• Support of the implanting embryo and endometrium by corpus Luteum progesterone
• Iniating a new cycle if fertilisation does not occur

Question 18

Oocyte at Ovulation
What is the role of the cumulus oophorus?

Answer 18

Protects eggs derived from the Granulosa cells

Question 19

Oocyte at Ovulation
What is the role of the Corona radiata?

Answer 19

innermost layer of the cumulus cells in contact with the ZP
- Formed by Granulosa cells
- Adhering to the oocyte before it leaves the Ovarian Follicle

Question 20

Oocyte at Ovulation
What is the role of the Zona Pellucida?

Answer 20

Secreted by egg, becomes impenetrable after fertilisation

Question 21

Oocyte at Ovulation
What is the 1st Polar body?

Answer 21

This is formed when the 1st meiotic division is completed

Question 22

Describe the processes involved in sperm binding and penetration (PART 1)

Answer 22

1. The acrosome reaction occurs in contact with the zona cumulus complex. Sperm penetrate cumulus and bind to ZP
2. Sperm enzymes digest through ZP and sperm fuses with plasma membrane

Question 23

Describe the processes involved in sperm binding and penetration (PART 2)

Answer 23

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 24

What is syngamy?

Answer 24

The fusion of two cells or their nuclei during reproduction

Question 25

Explain what occurs during Syngamy (PART 1)

Answer 25

1. After meiosis 1, the oocyte has 23x chromosomes but 2 copies of each chromosomes are arranged as sister chromotids
2. Entry of the spermatozoa causes an increase in Ca2+ via phospholipase Zeta from sperm.
3. Ca2+ causes the completion of meiosis 2 expelling the second polar body & cortical reaction

Question 26

Explain what occurs during Syngamy (PART 2)

Answer 26

4. The Aperm nuclear membrane breaks down, The chromatin condenses and chromosomes separate
5. 4-8 hours after the two sets of haploid chromosomes become surrounded by distinct membranes forming two pronuclei
6. These haploid structures synthesise DNA in preparation for the first mitotic division

Question 27

Explain what occurs during Syngamy (PART 3)

Answer 27

6. The pronuclei fuse and the mitotic metaphase spindle forms with the chromosomes assuming their position at its equator
7. Mitosis is completed and the one cell zygote becomes a two cell embryo

Question 28

Describe the 3 different stages of early embryo development

Answer 28

• Stage 1: The fertilised egg has 2 pronuclei. This is the first stage of Fertilisation
• Stage 2: The developing embryo contains 6-8 cells 3 days after fertilisation
• Stage 3: Five days after fertilisation, it is called blastocyst. It differentiates into inner cell mass, Blastocoel and trophoblast

Question 29

Check out fertilisation summary diagram