Module 11a: Fertilisation

Egg Transport

  • At ovulation, fimbriae pass over the surface of the ovary and sweep the oocyte and surrounding cumulus granulosa cells into the fallopian tube.
  • Beating cilia move the egg along the fallopian tube – takes 4 days for the egg to reach the uterus.
  • Visual reference: scanning electron micrograph showing a hamster cumulus-oocyte-complex entering the infundibulum; the outer and inner surfaces of the infundibulum are covered with cilia.

Sperm Transport and Capacitation

  • Eventual delivery to the fallopian tube involves: hundreds of millions of sperm ejaculated, but only about ~1000 reach the fallopian tube because of:
    • leakage from the vagina
    • the acidic vaginal environment
    • the length and energy requirements of the trip

Capacitation: key changes that enable fertilization

  • Step 1: Epididymal sperm are coated with proteins and cholesterol on their plasma membrane.
    • Capacitation involves replacing the wavelike beats of a sperm’s tail with a more whiplike action that propels the sperm forward in strong surges.
    • It also causes the sperm’s plasma membrane to become fragile, so that the hydrolytic enzymes in their acrosomes can be released.
  • Step 2: During ejaculation, additional seminal plasma proteins associate with the sperm membrane.
  • Step 3: Within the female tract, proteins are removed and cholesterol is depleted — sperm can now fertilise.
  • Context labels:
    • Epididymal
    • Ejaculated Seminal Plasma
    • Female Tract
    • Capacitated

Fertilization: Mechanisms and sequence

  • Structure involved: Sperm, cumulus granulosa cells, zona pellucida (ZP), ZP3 molecules, oocyte plasma membrane, oocyte sperm receptors, cortical granules.
  • Interaction sequence (as outlined in the slide progression): 1) Aided by surface hyaluronidase enzymes, a sperm cell weaves its way past cumulus granulosa cells.
    • Cortical granules are present in the oocyte (HUMAN ANATOMY AND PHYSIOLOGY, E. Marieb et al., 2016).
      2) Binding of sperm to ZP3 molecules in the zona pellucida causes a rise in Ca$^{2+}$ within the sperm, triggering the acrosomal reaction.
    • Cortical granules (present in the oocyte) are involved in subsequent steps.
      3) Acrosomal enzymes digest holes through the zona pellucida, clearing a path to the oocyte membrane.
    • Cortical granules.
      4) The sperm forms an acrosomal process, which binds to the oocyte’s sperm-binding receptors.
    • Cortical granules.
      5) The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte.
    • Cortical granules.
      6) Entry of sperm contents causes a rise in the Ca$^{2+}$ level in the oocyte’s cytoplasm, triggering the cortical reaction.
    • Cortical granules.
      7) The cortical reaction (exocytosis of cortical granules) hardens the zona pellucida and clips off sperm receptors (slow block to polyspermy).
    • Cortical granules.
  • These events culminate in the oocyte’s activation and preparation for first embryonic divisions.

Post-fertilization: Zygote formation and early embryogenesis

  • After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and the second polar body.
  • Sperm and ovum nuclei swell, forming pronuclei.
  • Pronuclei approach each other and a mitotic spindle forms between them.
  • Chromosomes of the pronuclei intermix.
  • DNA replicates in preparation for the first cleavage division.
  • Zygote is formed; subsequent embryogenesis begins.
  • Visual reference (from the diagram): Male pro-nucleus, Female pro-nucleus, mitotic spindle, centriole, polar bodies, and sperm nucleus are involved in subsequent cell divisions.

Vander’s Figure 17.29: Summary of events leading to fertilization

  • Step sequence:
    • Many sperm bind to receptors on the zona pellucida and undergo the acrosome reaction.
    • Sperm move through the zona pellucida.
    • One sperm binds to the egg plasma membrane.
    • The sperm is drawn into the egg.
    • The egg completes the second meiotic division.
    • Nuclei of sperm and egg unite.
    • Egg releases contents of secretory vesicles.
    • Enzymes enter the zona pellucida.
    • Block to polyspermy occurs as the egg enzymes are activated.
    • Zygote begins embryogenesis.

References and context

  • Fertilization and Human Development discussion appears in the Nature paper: Bianchi, E., Doe, B., Goulding, D. & Wright, G. J. Fertilization and human development, Nature.
  • Textbook reference used for the fertilization steps: HUMAN ANATOMY AND PHYSIOLOGY (E. Marieb et al.) © 2016 Pearson Education Inc.
  • Additional note: The material for this course includes content from FERTILIZATION AND PREGNANCY resources and related figures (e.g., Vander’s figure 17.29).

Key terms to remember

  • Fimbriae, infundibulum, cilia, cumulus granulosa cells, zona pellucida, ZP3, oocyte plasma membrane, oocyte sperm receptors, cortical granules, acrosomal reaction, hyaluronidase, capacitation, slow block to polyspermy, pronuclei, mitotic spindle, zygote.

Important formulas and symbols

  • Ca$^{2+}$ signaling during fertilization: rise in Ca2+Ca^{2+} within the sperm triggers the acrosomal reaction and cortical reaction in the oocyte.