Fertilisation & the Luteal Phase

Question 1

What state is ejaculated sperm in?

Answer 1

Ejaculated semen is coagulated

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

Question 2

What is the role of the cervical mucus?

Answer 2

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

Question 3

How is sperm able to move through cervical mucus?

Answer 3

Sperm passes into cervix

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

Question 4

How does sperm take breaks travelling to the ovum?

Answer 4

Sperm can inhabit cervical crypts which may form a reservoir. Some evidence of thermotaxis, but mechanism not yet elucidated

Question 5

How long does fertilisation take?

Answer 5

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

Question 6

Explain the passage of sperm to the uterus

Answer 6

Passage through uterus not well understood, currents set up by uterine or tubal cilia may have a role

Question 7

How is sperm attracted to the ovum?

Answer 7

Chemoattractants released from the oocyte cumulus complex may attract the sperm

Question 8

How do sperm travel along towards the ovum?

Answer 8

Sperm become hyperactive. Forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels

Question 9

What promotes sperm capacitation?

Answer 9

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

Question 10

What does sperm capacitation involve?

Answer 10

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

Question 11

Describe the acrosomal reaction

Answer 11

Acrosome reaction occurs in contact with the zona cumulus complex; the acrosomal membrane of the sperm head fuses releasing enzymes that cut through the complex

Question 12

How does the sperm enter the egg cell?

Answer 12

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

Question 13

What causes ovulation?

Answer 13

LH spike causes resumption of meiosis and ovulation

Question 14

What happens to the oocyte during ovulation?

Answer 14

Converts the primary oocyte to secondary oocyte plus 1st polar body

Question 15

Why does the basement membrane break?

Answer 15

Basement membrane breaks so blood pours into the middle

Question 16

How is the egg released for contact with sperm?

Answer 16

Follicular fluid contains the oocyte cumulus complex which is exuded out and caught by fimbrae of uterine tube

Question 17

Describe what happens to the theca and GC?

Answer 17

Theca and granulosa become mixed and the empty follicle is known as the corpus luteum

Question 18

What hormone does the corpus luteum produce?

Answer 18

It produces progesterone in the luteal phase

Question 19

What is the role of progesterone in the secretory phase?

Answer 19

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

Question 20

What is the role of oestrogen in the secretory phase?

Answer 20

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

Question 21

How long doe sthe corpus luteum last without fertilisation?

Answer 21

If fertilisation does not occur, CL has an inbuilt finite lifespan of 14 days

Question 22

Why does corpus luteum not last without fertilisation?

Answer 22

Fall in CL-derived steroids causes inter-cycle rise in FSH

Question 23

How does Corpus luteum disintegrate?

Answer 23

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

Question 24

How does corpus luteum get saved during pregnancy?

Answer 24

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

Question 25

What does the menstrual cycle achieve?

Answer 25

• Selection of a single follicle and oocyte.
• Regular spontaneous ovulation
• Correct haploid number of chromosomes in the oocyte.
• Cyclical changes in the cervix and uterine tubes, to
  enable egg transport and sperm access
• Preparation endometrium of uterus to receive fertilised
  egg
• Support implanting embryo and endometrium by corpus
  luteum progesterone
• Initiating a new cycle if fertilisation does not occur

Question 26

Describe the state of DNA replication at start of ovulation?

Answer 26

Egg started Meiosis I years ago in foetal uterus and underwent meiotic arrest until egg is ovulated

Question 27

Describe the division of the egg

Answer 27

When egg divides, it is split in half and all the cytoplasm remains with the used half. The polar body contains all unused DNA

Question 28

What is the zona pellucida?

Answer 28

A glycoprotein structure is a layer surrounding and secreted by the egg.
After fertilisation it becomes impenetrable and blocks second sperm trying to fertilise the egg

Question 29

What is the corona radiata?

Answer 29

Outside pellucida is the corona radiata - Innermost layer of cumulus cells contacting ZP secreted by GC. sperm has to penetrate this layer for successful fertilisation

Question 30

What is the Cumulus Oophorus?

Answer 30

The inner layers of granulosa cell

Question 31

Outline how the sperm binds and penetrates?

Answer 31

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 and causes further
   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 32

What is syngamy?

Answer 32

> The fusion of two cells, or of their nuclei, in reproduction

Question 33

Describe the DNA structure of the egg after meiosis I

Answer 33

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

Question 34

Descrie the effect of sperm entry in the zona pellucida?

Answer 34

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

Question 35

What happens to the sperm nuclear membrane?

Answer 35

The sperm nuclear membrane breaks down, the chromatin de-condenses and chromosomes separate.

Question 36

What happens in 4-7 hours after fusion?

Answer 36

The two sets of haploid chromosomes become surrounded by distinct membranes forming pronuclei. These haploid structures synthesise DNA in preparation for first mitotic division

Question 37

What happens to the pronuclei?

Answer 37

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

Question 38

What is the result of mitosis?

Answer 38

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

Question 39

What is the first sign of fertilisation?

Answer 39

The fertilized egg has 2 pronuclei

Question 40

Describe the cell count of the developing embryo

Answer 40

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

Question 41

What is a blastocyst?

Answer 41

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