Fertilization in humans + IVF

Describe the process of fertilization in humans

• Capacitation – biochemical changes which occur post ejaculation to improve sperm motility

• Acrosome reaction – the release of hydrolytic enzymes which softens the zona pellucida (jelly coat)

• Dissolution of nuclear membranes – Both sperm and egg lose their nuclear membranes (aligns homologous chromosomes for the first time)

Capacitation

Occurs after ejaculation, when chemicals released by the uterus dissolve the sperm’s cholesterol coat.

• This improves sperm motility (hyperactivity), meaning sperm is more likely to reach the egg (in the oviduct)

• It also destabilises the acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact

Acrosome Reaction

When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat.

Dissolution of nuclear membranes

• Once inside the egg, the sperm tail and the majority of the sperm's cytoplasm, including the mitochondria, are rapidly degraded by egg cell components.

  • (This ensures only the sperm's genetic material contributes to the formation of the zygote.)

• The mitochondria from the sperm are particularly targeted for destruction (which is the reason why mitochondrial DNA is maternally inherited)

Outline the events of joint mitosis following fertilization

The last stage is joint mitosis, often referred to as “the first mitotic division”. All the genetic material from the sperm and egg (in the form of condensed chromosomes) combine and undergo the first cell division to form a diploid (2n) zygote. 

Define in vitro fertilization (IVF)

• In vitro fertilisation (IVF) refers to fertilisation that occurs outside of the body (in vitro = "in glass").

• It involves using drugs to suspend normal ovulation (down regulation), before using hormone treatments to collect multiple eggs (superovulation).

IVF main steps

• down regulation

• superovulation

• fertilization

• implantation

IVF Checklist

• stimulate follicules: FSH

• Extract oocytes

• Collect sperm

• In vitro fertilization

• implantation

Down regulation

• Drugs are used to halt the regular secretion of FSH and LH – this in turn stops the secretion of estrogen and progesterone.

• By arresting the hormonal cycle, doctors can take control of the timing and quantity of egg production by the ovaries.

Superovulation

• Superovulation involves using artificial doses of hormones to develop and collect multiple eggs from the woman.

• The patient is firstly injected with large amounts of FSH to stimulate the development of many follicles.

• The follicles are then treated with human chorionic gonadotropin (hCG) – a hormone usually produced by a developing embryo.

  • hCG stimulates the follicles to mature.

  • The egg is then collected (via aspiration with a needle) prior to the follicles rupturing.

Fertilization

• The extracted eggs are then incubated in the presence of a sperm sample from the male donor.

• The eggs are then analysed under a microscope for successful fertilisation.

Implantation

• Approximately two weeks prior to implantation, the woman begins to take progesterone treatments to develop the endometrium.

• Healthy embryos are selected and transferred into the female uterus (or the uterus of a surrogate).

• Multiple embryos are transferred to improve chances of successful implantation (hence twins/triplets/etc. are far more common in IVF).

• Roughly two weeks after the procedure, a pregnancy test is taken to determine if the process has been successful.

Ethicality of IVF

• For

• most forms of infertility are due to environment and will not be passed on

• embryos that died don’t feel pain or suffer

• desire to be parents

• overcomes the unhappiness brought by infertility

Ethicality of IVF

• Against

• infertility may be passed on

• some embryos are denied chance of life

• unnatural process

• infertility should be accepted as the wills of god