Sexual Reproduction

Seite 1

Standard Level Biology: Human Reproduction

Human Sexual Reproduction is the process of fertilisation between a male gamete and a female gamete. Making half of both genetic makeups into a zygote. Sexual dimorphism and physiology is determined by the hormones of the zygote.

Testosterone in the male determines the development of male genitalia during embryonic development as well as secondary sex characteristics during puberty and the sex drive of the male.

The structure of both sex organs enables the release of gametes on top of which the uterus and vagina provide a defensive and sturdy environment for the embryo.

The menstrual cycle times the ovulation of a female for possible fertilisation. Optimum timing for implantation of a fertilised egg is when the endometrium is highly vascular. If fertilisation does not occur during ovulation the endometrium will simply fall off resulting in blood excretion.

The hypothalamus regulates the menstrual cycle, producing the hormone GnRH. The hormone triggers the production of a follicle stimulating hormone and a luteinizing hormone in the pituitary glands. These two hormones then trigger the ovaries which creates a negative feedback of progesterone and oestrogen into the hypothalamus. Oestrogen also creates the endometrium increase of vascular tissues.

In vitro fertilisation is within the fallopian tube and takes place 24-48 hours post ovulation. The zygote will divide through mitosis and within several days it will travel through the tube to the endometrium of the uterus. The embryo is now divided plentifully and will implant in the vascular tissue.

Reasons for the inability to fertilise may stem from low sperm count, impotence, abnormal ovulation, blocked fallopian tubes.

Today IVF procedures exist to make fertilisation inevitable. Women are injected with FSH for 10 days, ensuring the development of multiple Graafian follicles within her ovaries. Several oocytes (eggs) are harvested surgically and semen obtained from the male counterpart are mixed with the oocytes in separate containers. Upon microscopic observation it is apparent which fertilisations have succeeded and which have not. Up to 2-3 fertilised eggs are placed back into the woman's uterus, maximum, where they will implant into the vascular tissue resulting in at least one healthy embryo. This procedure is expensive and does have failure risks. Other fertilised eggs that were not implanted may be frozen for further use upon failure.

There are ethical issues concerning this method.