Fertilization and implantation LO5

Question 1

Small window of time…

Answer 1

• The secondary oocyte can only survive for 12- 24 hrs following ovulation.
• Sperm will survive for up to 3 days in the female reproductive tract. So for fertilisation to take place, intercourse must occur on the day of ovulation or in the few days prior to ovulation.

Question 2

Capacitation

Answer 2

Capacitation thins the cap of the acrosome in readiness to release the enzymes it contains. Capacitation requires exposure to the secretions of the female reproductive tract and takes about 8-10 hours to occur. Sperm freshly deposited in the female reproductive tract are incapable of fertilization.
- destabilisation of the acrosomal sperm head membrane which allows it to penetrate the outer layer of the egg, and chemical changes in the tail that allow a greater mobility in the sperm.

Question 3

Steps to fertilisation.

Answer 3

1. The corona radiata surrounds the ovum. Once sperm reachs the ovum they must weave their way through the granulosa cells of the corona radiata to reach the zona pellucida.
2. Sperm bind to the zona pellucida. This causes a rise in calcium levels in the sperm resulting in the complete breakdown of the acrosomal membranes, releasing the acrosomal enzymes.
3. Having digested through the zona pellucida the sperm binds to special receptors on the oocyte membrane.
4. The sperm and oocyte membranes fuse. Sperm DNA is then released into the oocyte.
5. Release of sperm DNA into the ovum stimulates the cortical reaction. The cortical reaction is where calcium levels of the oocyte increase causing destruction of sperm receptors and hardening of the zona pellucida preventing further entry of sperm. This blocks polyspermy maintaining the normal complement of chromosomes within the fertilized egg (now called a zygote)

Fertilization usually occurs in the distal end of the uterine tube

Question 4

Embryonic development

Answer 4

• The zygote begins to undergo rapid cell division
• Many small cells are produced with a large surface area to volume ratio facilitating exchange of oxygen, nutrients and wastes. This large number of cells provide the necessary building blocks for the developing embryo.
• zygote is free floating in the uterine tube and beginning its journey towards the uterus.
• The zona pellucida provides some protection for the zygote as it traverses the female reproductive tract.
• This journey takes 5-7 days.
• As cell division continues a fluid filled cavity begins to form and the zona pellucida starts to break down. The zygote is now called a blastocyst and is ready for implantation.

Question 5

blastocyst

Answer 5

The blastocyst consists of the inner cell mass which becomes the embryo, a fluid filled cavity and a layer of trophoblast cells, which become the placenta. When developed the placenta will nourish the developing embryo, remove wastes and produce hormones.

Question 6

Implantation

Answer 6

• occurs about 6-7 days after fertilization.
• uterine cycle is about halfway through secretory phase.
• endometrium is most receptive. The blood supply is well developed and uterine glands are secreting glycogen rich mucous.
• the blastocyst buries itself into the endometrium and endometrial cells cover the blastocyst. Special proteins on the trophoblast cells attach to receptors on the endometrium facilitating this process. This process usually takes around five days.

Question 7

hCG

Answer 7

Human chorionic gonadotrophin signals to the corpus luteum in the ovary to continue to produce progesterone and oestrogen.
- maintains the endometrium.
- With maintenance of the corpus luteum menses does not occur and the embryo remains implanted in the endometrium with its rich blood supply and glycogen rich mucous secreting glands.
- Progesterone production must be maintained throughout pregnancy. Its contribution to maintenance of the endometrium, development of breast tissue and the development of the fetus is of vital importance. Embryonic hCG signaling to the corpus luteum performs this function for the first 8-12 weeks of pregnancy. Once the placenta is mature enough, it will take over the continued production of progesterone.