Reproduction

• Living organisms need to be able to reproduce


  • Like most other animals, humans carry out sexual reproduction, which involves the joining together of two gametes the sperm and the egg (ovum)
• The male reproductive system

  1. Urethra - Tube through which the sperm leave the penis

  2. Prostate gland - Adds fluid to nourish the sperm

  3. Penis - Organ that introduces sperm into vagina

  4. Scrotum - Sac that holds and protects the testes at slightly lower than body temperature
• Sperm


  • Cells highly adapted for their function, with a flagellum (tail) that allows the sperm to swim to meet the egg
  • Sperm (and egg cells) are also adapted to their function in being haploid
• The female reproductive system

  1. Ovary - Produces ova (eggs)

  2. Cervix - The opening of the uterus, widens during the process of birth

  3. Vagina - The penis places sperm here during sexual intercourse
• Fertilisation and pregnancy


  1. If a sperm and an ovum meet and fuse (join) in an oviduct, fertilisation will result

  2. The zygote divides by mitosis and grows into a ball of cells, referred to as an embryo, that develops further as it travels down the oviduct into the uterus

  3. In the uterus, the embryo sinks into the thick uterine lining and becomes attached (in a process called implantation)

  4. At the point where the embryo begins to develop in the uterus lining, the placenta and umbilical cord form

  5. A protective membrane, the amnion, develops around the embryo. It contains a fluid, the amniotic fluid, within which the growing embryo develops

  6. During pregnancy, useful materials, including oxygen and dissolved nutrients, pass from the mother to the foetus through the placenta and umbilical cord. Waste excretory materials including carbon dioxide and urea pass from the foetus back to the mother
• Placenta


  • The structure that links the uterus wall to the foetus via the umbilical cord. It is here that the exchange of materials takes place between the mother and the foetus
• The surface area between the uterine wall and the placenta is further increased by small villi (extensions) in the placenta that extend into the uterus wall
• Prostate gland


  Adds fluid to nourish the sperm
• Oviduct


  The part of the female reproductive system in which fertilisation occurs
• Amniotic fluid


  Cushions the delicate developing embryo
• You should be aware of the roles of mitosis and meiosis in reproduction and the development of the human embryo
• Meiosis and mitosis in fertilisation and embryo development

  1. Meiosis produces haploid gametes/gametes with half the number of chromosomes of other cells; it ensures that the diploid number is restored at fertilisation

  2. Mitosis maintains chromosome number/diploid number during the growth of the embryo
• Testosterone


  Produced by the testes in males, leads to the development of secondary sexual characteristics
• Oestrogen


  Produced by the ovaries in females, leads to the development of secondary sexual characteristics
• The menstrual cycle


  1. Ovulation - The ovum is released on day 14 (approximately)

  2. Menstruation - The breakdown and removal of the blood-rich uterine lining at the end of each cycle

  3. Oestrogen - Stimulates ovulation and starts the build-up and repair of the uterine wall after menstruation

  4. Progesterone - Continues the build-up of the uterine lining after ovulation
• Menstruation occurs when the levels of both oestrogen and progesterone fall at the end of the cycle
• The menstrual cycle stops if pregnancy occurs and only restarts after the baby is born
• Testosterone


  The male hormone that leads to the development of secondary sexual characteristics
• Oestrogen


  The hormone that stimulates ovulation during the menstrual cycle
• Fertility problems


  • Females unable to produce eggs or blockages preventing movement of eggs down the oviducts, and males having low sperm counts or impotence
• Fertility treatment


  1. Giving females fertility drugs to increase egg production

  2. Collecting eggs from ovaries and adding to sperm in a test tube (in vitro fertilisation, or IVF)

  3. Placing embryos back into the uterus
• Fertility treatments can raise ethical issues, as IVF treatment can be used to screen for abnormalities or for particular characteristics, e.g. selecting the sex of the embryo
• When replacing the embryos back into the uterus, it is important to strike a balance between increasing the chances of success and avoiding the potential for multiple births. For this reason, only two embryos are often placed back into the uterus following IVF
• Before replacing the embryos back into the uterus, the mother has to be given hormones to ensure that the uterine lining is at a stage of development where implantation can occur
• Methods of contraception


  • Mechanical - Male condom, Female condom
  • Chemical - Contraceptive pill, Implants
  • Surgical - Vasectomy, Female sterilisation
• Condoms


  Act as a barrier to prevent the sperm entering the woman, also protect against sexually transmitted infections
• Contraceptive pill


  Taken regularly by the woman and prevents the ovaries from releasing ova by changing hormone concentrations
• Vasectomy


  Cutting of sperm tubes, preventing sperm from entering the penis
• Condoms can be unreliable if not used properly
• Male sterilisation (vasectomy) prevents pregnancy by cutting the sperm tubes, preventing sperm from entering the penis