04. Fertilisation and Contraception

• requirements for fertilisation:

  • sperm must be matured and capacitated.
  • egg must be arrested at metaphase II.
  • must be in the same place at the same time.
  • must be synchronised with receptive endometrium
• ejaculation is approximately 3ml in humans
• STIs are found in seminal plasma
• bulbourethral gland secretes pre-ejaculate during intercourse

  • important for lubrication and neutralisation
• spermatozoa leave the testis immotile and unable to recognise or bind an egg

  • need to undergo maturation in the male tract.
  • need to undergo capacitation in the female tract
• capacitation is the sperm being treated with uterine washings. This gives them the ability to undergo the acrosome reaction

  • has flagella beat hyperactivated
• ovulation = cumulus-oocyte complex is picked up by ciliated fimbriae on the end of the uterine tube
• 200 million sperm are deposited in the upper vagina but only around 1000 enter each uterine tube
• cervical mucous is the least viscous during days 9-16 of the menstrual cycle, increasing the chances of sperm making it through the cervix when there is highest chance of egg being present
• fertilisation happens in the ampulla region of the uterine tube
• sperm remain capable of fertilisation for about 5 days within the female tract
• oocyte is only viable for 24 hours after ovulation
• sperm interaction with the egg:

  1. penetration of cumulus
  2. zona binding
  3. acrosome reaction
  4. zona penetration
  5. fuse with plasma membrane
• zona pellucida = extracellular protein matrix surrounding the egg

  • persists post fertilisation
  • acrosome reaction releases proteolytic contents, exposing new membrane for oocyte fusion
• once the sperm has penetrated the zona pellucida, the equatorial segment of sperm head fuses with the oocyte plasma membrane

  • sperm nucleus encased by a vesicle composed of internalised oocyte membrane
  • large increase in free Ca2+ concentration sweeps across the egg
• IZUMO = sperm membrane receptor in fusion, only detectable after acrosome reaction
• JUNO and MAIA are receptors for IZUMO
• JUNO binds to IZUMO forming a complex which undergoes a conformational change creating a binding pocket for MAIA
• there are Ca2+ oscillations every 3-15 minutes after the sperm binds to the egg

  • triggered by phospholipase C
• MPF = M-phase promoting factor

  • made of cdk1 and cyclin B
  • causes block that stops the egg transitioning to anaphase
  • stabilised by CSF
• raised Ca2+ levels suppress CSF activity and destroy cyclin B

  • activates anaphase promoting complex
• cohesin ring is present around sister chromatids, opposing the pulling force of microtubules
• securin = inhibits separase activity

  • separase = enzyme that cleaves cohesin
  • separase inhibited until securing un=biquitinated by APC/C
  • leads to resumption of the cell cyclin in the oocyte
• block to polyspermy:

  caused by increase in Ca2+

  • fast block = membrane depolarises after fusion (occurs in marine species)
  • cortical granules below the plasma membrane are released causing cortical reaction, takes around an hour
• cortical reaction is a mixture of enzymes that diffuse into the zona pellucida following exocytosis

  • induces zona reaction

  zona reaction = alteration in zona pellucida structure catalysed by proteases so sperm can no longer bind
• the JUNO protein is shed from the oocyte membrane due to the cortical reaction, preventing sperm fusing
• sperm provide the sex of the baby and the centriole that forms the spindle for first cell division

  oocyte provides the cytoplasm, the organelles and the mitochondria
• zygotic stage:

  • de-condensation of sperm DNA and protamine/histone exchange
  • male and female pronuclei replicate DNA
  • pronuclei migrate towards each other
  • guided by sperm aster (microtubules that radiate from the centrosome
• syngamy = 18-24 hours after fusion

  • pronuclear membranes break down
  • chromatin intermixes
  • nuclear envelope reforms around the zygote nucleus
  • cleavage begins marking the beginning of embryogenesis
• the zygote cleaves to form two blastomeres

  • genetic testing possible up to implantation (8 cells)

  morula = near the end of the uterine tube

  • there is no cytoplasmic synthesis so the blastomere size decreases with each division
• blastocyte forms on late day 4/5

  • distinct inner cell mass and single layered trophoblast layer
  • embryonic and abembryonic pole
  • embryonic genome is activated

  implantation occurs after the blastocyte has expanded out of a hole in the zona pellucida

  • late day 6 onwards
• endometrial structure:

  • uterine lining = endometrium
  • basal layer = attached to myometrium, remains intact during menstruation
  • functional layer = undergoes proliferation then shedding (menstruation)
• follicular phase = proliferative phase

  • after menstruation the endometrium is very thin and consists of only the basal layer
  • in the first 14 days of the menstrual cycle the endometrial cells proliferate
• luteal phase = secretory phase

  • after ovulation the ovaries produce progesterone
  • progesterone stimulates synthesis of secretory material by the glands, providing nutrition for the blastocyst
• hormonal contraception:

  • mimic hormone levels during luteal phase
  • feedback inhibition on HPG axis
  • constant exposure to progesterone suppresses ovulation in most women
  • also causes thickening of cervical mucus
  • oestrogen exerts additional negative feedback and induces PR expression which increases the progesterone effect
• emergency contraception:

  • morning after pill = higher levels of hormones than normal contraceptive pill, progesterone only
  • prevents ovulation
• barrier methods of contraception:

  • prevent sperm and eggs meeting
  • spermicides
  • male condom
  • female condom
  • diaphragm and cap
• IUDs:

  • cause foreign body response in the uterus, producing lots of leukocytes and prostaglandins
  • uterus becomes hostile to sperm and embryos
  • can also cause heavy periods