Conception and fertility (7.3)

Question 1

Describe the transport of gametes to the ampullary-isthmic junction and the changes during transport

Answer 1

Vagina: Semen are expelled onto the external Os. Semen undergoes coagulation (dependent upon coagulating enzymes and fibrinogen-like substrate) - allows the semen to be retained within the vagina and buffers against the acidic vaginal fluids. Coagulum dissolves after 20 - 60 minutes.

Cervix: Cervical fold/crypts may be utilised by spermatozoa. Cervical mucus may vary according to the hormone levels (↑ oestrogen → thin & watery) (↑ progesterone → thick and viscous)

Uterus: Movement is dependent upon own motility, uterine contracts (orgasmic) and uterine cilia

Uterine tube: Temporarily bind to epithelial cells BUT detach and re-acquire motility at ovulation allowing travel to the ampullary-isthmic junction. Chemoattractant dependent.

Question 2

Describe the phases of fertilisation, including the role of the acrosome and the prevention of polyspermy

Answer 2

1. Spermatozoal capacitation: Membrane cholesterol content is decreased (allows for increased permebailty to calcium) and flagellum undergoes hyperactivation

2. Digestion of oocyte outer cumulus cells: The spermatozoal acrosome secretes hyaluronidase to digest the cells and expose the zona pellucida.

3. ZP3 binding, triggering the acrosomal reaction: The acrosome swells and fuses with the plasma membrane, allowing exocytosis and content release. Acrosomal vesicles contain digestive enzymes which allow for the degradation of the zona pellucida.
4. Spermatozoa reach the perivitelline space
5. Spermatozoal ZP2 binding
6. Spermatozoal PLC triggers a rise in intracellular Ca²⁺. This allows for the release of cortical granules. The cortical enzymes digest the ZP2 receptors, preventing attachment to the zona pellucida and ultimately polyspermy.

Question 3

Describe the pre-implantation changes to the zygote

Answer 3

• Oocyte is suspended in metaphase II of meiosis 2 (continued from interphase I following ovulation)
• Calcium inhibits cytostatic factor, allowing the oocyte to complete meiosis 2
• Polar body generated

Syngamy: Fusion of the male and female gametes

1. The nuclear membrane of the sperm breaks down (highly condensed chromatin → filamentous strands)
2. Membranes surround the haploid chromosomes, creating pro-nuclei. The chromosomes move centrally and synthesise DNA
3. The pro-nuclei membranes are degraded and the chromosomes line up
4. Anaphase and telophase proceed
5. A cleavage furrow forms as the 1 cell zygote becomes a 2 cell conceptus

Question 4

Describe the process of implantation and the prolongation of the corpus luteum

Answer 4

Implantation:

• Zygote undergoes a series of cleavage events: Morula (day 3/4), blastocyst (day 5/6)
• Attachment, day 6, follows the shedding of the zona pellucida and exposure of the adhesive trophoblast cells. Following attachment primary decidualisation is triggered.
• Syncytiotrophoblast cells, secrete proteases, invade the endometrium of the uterus, allowing implantation of the blastocyst. The endometrium reforms over the site of invasion.

Prolongation of the corpus luteum:

• Syncytiotrophoblasts secrete hCG which acts to maintain the corpus luteum.
• This is acheived through the binding of hCG to LH receptors on luteal cells, allowing continued release of progesterone
• hCG also prompts secretion of relaxin