Reproduction: Fertilisation and the luteal phase

Question 1

Why is the ejaculate initially coagulated?

Answer 1

Prevents loss of sperm and stops the sperm form being able to move around too much until it reaches the cervix

Question 2

What happens to the ejaculate as a result of contact with the cervical mucus?

Answer 2

Seminal fluid, abnormally morphological sperm and cellular debris all removed from ejaculate so only sperm cells continue journey through cervix

Question 3

What effect does absence of progesterone have on the cervical mucus? What does this mean for passgae of sperm through cervical mucus?

Answer 3

• Cervical mucus is less viscous in the absence of progesterone
• This allows sperm to pass through it into uterus

Question 4

Where exactly within the uterine tube does fertilisation take place?

Answer 4

Takes place in the ampulla of the uterine tube

Question 5

What mechanisms are there that help the sperm travel through to the Oocyte within the uterine tube?

Answer 5

• Currents set up by uterine cilia may help sperm passage through uterine tube
• Chemo-attractants released from the cumulus oocyte complex (COC) may attract the sperm

Question 6

What happens to the sperm as they get close to the egg?

Answer 6

Sperm cells become hyperactivated - tail beats faster causing them to swim faster

Question 7

What causes the sperm cells to become hyperactivated as they get closer to the egg?

Answer 7

• Increased Ca²⁺ influx via specialised calcium channels called CatSper channels on the sperm cell surface
• Increased Ca²⁺ influx may be mediated by local factors released from oocyte

Question 8

What is Sperm capacitation?

Answer 8

Maturation of the spermatozoa that allows them to be able to fertilise an oocyte

Question 9

What changes occur to the sperm during capacitation?

Answer 9

• Biochemical rearrangement of the glycoproteins on the sperm
• Changes in sperm head membrane composition

Question 10

How is the corpus luteum formed once ovulation of dominant follicle occurs?

Answer 10

• After ovulation theca and granulosa cell layers of leftover follicle become mixed causing corpus luteum to be produced

Question 11

Explain how the corpus luteum is destroyed if pregnancy doesn’t occur

Answer 11

• If no pregnancy occurs the corpus luteum experiences cell death, its vasculature breaks down and it shrinks and folds in on itself.
• Also losses secretory function
• Due to these processes it becomes the corpus albicans

Question 12

What is the corona radiata and how is it formed?

Answer 12

• Innermost layer of cumulus cells in contact with the the Zona pellucida
• Formed by granulosa cells adhering to the oocyte before it leaves the dominant follicle

Question 13

Explain the process of the acrosome reaction

Answer 13

1. As sperm cell reaches cumulus oocyte complex acrosomal membrane releases the hyaluronidase enzyme
2. Hyaluronidase digests culumus cells of corona radiata
3. Acrosomal membrane then fuses with adhesion molecules on zona pellucida
4. Inner acrosomal membrane releases acrosin which digests the zona pellucida
5. This allows sperm to be taken into the oocyte via phagocytosis

Question 14

What occurs as a result of the sperm cell being taken into the oocyte?

Answer 14

• Once sperm taken into oocyte the sperm head membrane and the oocyte membrane fuse
• This fusion causes the activation of Phospholipase Zeta within sperm cell membrane
• This leads to release of intracellular Ca²⁺ within the oocyte

Question 15

Why does the fusion of sperm head and oocyte membranes lead to the activation of phospholipase Zeta?

Answer 15

Due to the presence of basal Ca²⁺ inside the oocyte

Question 16

What does the large increase in intracellular Ca²⁺ as a result of the activation of phospholipase Zeta lead to?

Answer 16

Leads to the cortical reaction

Question 17

What happens during the cortical reaction?

Answer 17

• Intracellular rise in Ca²⁺ causes cortical granules to relelase their contents
• Cortical granules release proteases, peroxides and hyaline
• This causes degradation of the adhesion molecules on the zona pellucida
• This prevents anymore sperm cells from entering the oocyte

Question 18

Apart from the cortical reaction what else does the increase in intracellular calcium in the oocyte cause?

Answer 18

• Causes the secondary oocyte to complete meiosis II thus forming the mature oocyte
• Also causes the second polar body formed during meiosis II to be expelled from the oocyte membrane

Question 19

After the cortical reaction, what happens to the sperm nucleus within the oocyte cytoplasm?

Answer 19

• Sperm nuclear membrane breaks down
• Chromatin decondenses
• Chromosomes separate

Question 20

Explain how the male and female pronuclei are formed?

Answer 20

Two sets of haploid chromosomes become surrounded by distinct membranes forming two pronuclei.

Question 21

After the pronuclei are formed what happens to them and what is produced as a result?

Answer 21

The pronuclei fuse to form diploid zygote

Question 22

After fusion of pronuclei to form diploid zygote what process occurs?

Answer 22

• DNA replication occurs before mitosis begins
• Mitotic spindle forms
• Chromosomes line up at centre of the cell
• Mitosis is completed to produce embryo (2 cells) from zygote (1 cell)