Reproductive Biology Revision Assignment

MELANIE PESCUD

10328705

FERTILISATION TO IMPLANTATION

For fertilisation to occur, the female must ovulate an egg each month around the time of intercourse. To do this she must have a normal menstrual cycle. In the first phase of the menstrual cycle a follicle grows in the ovary, while the lining of the uterus builds up to receive a fertilized egg. At mid-cycle when the egg is ready, the ovary releases the egg. This stage is ovulation and the best time to conceive.

Sexual intercourse must occur for fertilization to occur in sexually reproducing animals. During intercourse muscle contractions propel semen from the penis into the vagina. Sperm must be deposited at the cervix. Here it is engulfed in cervical mucus. Sperm travel up the uterine lining into the uterine tube. From 200 to 600 million sperm are deposited around the external os of the uterus and in the fornix of the vagina during sexual intercourse. About 200 sperm reach the fertilization site and only one can enter. Most sperm degenerate and are reabsorbed by the female genital tract.

Sperm must acquire the capacity to fertilise while in the female reproductive tract. The acrosome reaction must occur. During the process of fertilization, the initial interaction between male and female gametes is mediated by a sperm receptor, designated ZP3, which resides in the zona pellucida surrounding the oocyte. The sperm receptor function of the ZP3 molecule plays a key role in the first step of the fertilization process. Following sperm-oocyte binding, ZP3 triggers the sperm acrosome reaction that releases the protein machinery enabling a spermatozoon to penetrate the zona pellucida. A change in morphology of sperm head is also important in enabling binding to and subsequent penetration of zona pellucida. Sperm motility is also important as this allows the sperm to travel to the egg in the first place.

Fertilisation occurs in the oviduct. Initially this involves the fusion of gamete membranes, a block to polyspermy and the completion of the second meiotic division so that the secondary oocyte becomes the ovum. The block to polyspermy occurs when one sperm has entered the zona pellucida and a zona reaction occurs which changes the properties of the zona pellucida and makes it impermeable to other sperm by altering the composition of the extracellular glycoprotein coat and changing the plasma membrane. The fertilized egg is called a zygote after the formation and migration of the two pronuclei.

Fertilisation results in the restoration of diploid number of chromosomes so that a new unique individual is formed. Also primary sex is determined with the XX or XY chromosomal combinations.

The zygote divides to form more cells, while being pushed along the uterine tube. This is termed cleavage. Each cell is termed a blastomere. As the divisions progress there will be 4 cells by 40 hours and 12 to 16 cells after 3 days at which stage the zygote is called a morula. The morula enters the uterus at day 4 at which time the zona pellucida disappears. After four days the zygote has 100 cells and is called a blastocyst.

The blastocyst then develops fluid-filled spaces and these coalesce to form blastocoele. Next there is differentiation of the blastomeres into inner cell mass and trophoblast. The inner cell mass is what gives rise to the embryo, and the trophoblast gives rise to the placenta.

When the blastocyst reaches the uterine lining it is suspended for two days, implanting in the uterine wall by the sixth day after fertilization. This requires invasive action of the syncytiotrophoblast. This is the multi-nucleated outer layer of the trophoblast. The underlying mono-nucleated trophoblast is termed the cytotrophoblast.

The syncytiotrophoblast breaks down cells of the uterine lining (endometrium) and the maternal blood vessels, there is an invasion into the maternal arterial supply so that the endothelium is replaced by trophoblast cells, lacunae form within the syncytium. This will in turn lead to the formation of the early placenta. The endometrium, which is now termed the decidua, grows over the implantation site.

HORMONES INVOLVED

Oestrogen

· Associated with sexual behaviour in some species

· Regulates secretion of gonadotrophins

· Prepares uterus for spermatozoal transport and alters the cervical mucus ( higher oestrogen levels increase the water content of mucus and make it more conducive to sperm transport)

· Increases vascular permeability and tissue oedema

· Stimulates growth and activity of the mammary and endometrium

· Prepares endometrium for progesterone action

Progesterone

· Regulates secretion of gonadotrophins

· Prepares uterus for conceptus by stimulating the glandular epithelium to secrete a glycogen-rich material, the endometrium thickens and the glands become wide, tortuous, and saccular

· Maintains uterus during pregnancy by decreasing the smooth muscle contractions which could result in expulsion of the new embryo when it contacts the uterus. Progesterone also decreases uterine sensitivity to stimulation of the embryo moving freely throughout the uterus and touching the lining of the uterus. Without progesterone this stimulation could initiate contractions which would expel the embryo

Leptin

· Possible role in fetal and placenta growth and development

· May play an advantageous effect on oocyte and zygote development in the early stages after conception

hCG (human chorionic gonadotrophin)

· Stimulates the production of oestrogen and progesterone within the ovary

· hCG released very early in pregnancy and not present in any other time

· hCG is produced by the syncytiotrophoblast and maintains the activity of the corpus luteum in the ovary