7) Fertilization Lecture Notes

Fertilization

  • Results in a zygote, requiring sperm and egg nuclei fusion.

Sperm Structure

  • Head: Haploid nucleus & acrosome (enzymes for egg penetration).
  • Middle Piece: Mitochondria (ATP production).
  • Tail: Flagellum for movement.

Egg Structure

  • Contains plasma membrane, vitelline envelope, and jelly coat.

Sperm Transport

  • Peristaltic actions of uterus, cervix, and oviduct aided by oxytocin and PGF2α\alpha.
  • Semen deposit sites vary among species (vaginal, intracervical, transcervical).

Egg Transport

  • Ciliated epithelial cells pick up ovum.
  • Estrogen retains egg (favors sperm motility);
  • Progesterone hastens movement (inhibits sperm motility).

Fertilization Process

  • Acrosome enzymes digest jelly coat; acrosome extends a filament to attach to vitelline envelope.
  • Sperm and egg nuclear membranes fuse; egg plasma membrane changes to prevent other sperm entrance and vitelline envelope becomes fertilization envelope.
  • Egg moves into fallopian tube for fertilization and then to uterus for implantation.

Accomplishing Fertilization

  • Oocyte viable for 12-24 hours; sperm viable for 24-72 hours.
  • Coitus must occur no more than three days before or 24 hours after ovulation.

Sperm Capacitation

  • Sperm undergoes capacitation in female reproductive tract for fertilization capacity.
  • Involves cellular changes facilitated by Ca++ influx, inhibited by secondary sex gland secretions, and dilution by uterus.

Acrosome Reaction

  • Involves fusion of acrosomal vesicle with sperm cell membrane and release of enzymes.
  • Initiated by contact with egg jelly, causing exocytosis and proteolytic enzyme release.

Sperm-Egg Interaction

  • Bindin proteins mediate species-specific recognition; species specificity occurs at sperm attraction, activation, and adhesion

Sperm Recognition & Penetration in Mammals

  • Zona Pellucida glycoproteins (ZP 1, 2, and 3) bind sperm, triggering the acrosomal reaction.
  • Acrosomal reaction releases acrosin, hyaluronidase, and other enzymes for penetration.
  • Sperm penetrates zona pellucida with acrosin, sperm plasma membrane fuses with oocyte plasma membrane initiating the cortical reaction.

Acrosomal Reaction and Sperm Penetration

  • Sperm binds to zona pellucida, releasing enzymes to digest it.
  • Beta protein binds to oocyte receptors; alpha protein inserts into membrane.
  • Mammals: sperm contacts egg on the side of its head.

Blocks to Polyspermy

  • Fast block: Membrane depolarization prevents sperm fusion.
  • Slow block: Zonal inhibiting proteins (ZIPs) destroy sperm receptors and detach bound sperm.

Fast Block to Polyspermy

  • Influx of sodium ions causes surface depolarization, blocking further sperm binding.

Slow Block to Polyspermy

  • Sperm head fuses, releasing intracellular calcium.
  • Cortical granules fuse with egg cell membrane, releasing proteinases and glycosidases, hydrolyzing ZP3, and making the zona "refractory".

Cortical Granule Exocytosis

  • Releases enzymes and mucopolysaccharides, forming fertilization membrane and osmotic gradient.

Cell Division and Early Embryonic Development

  • Zygote undergoes cleavage (cell division without growth).

Cleavage

  • Increases the number of cells in the developing embryo.

8-Cell Stage

  • Deuterostomes (radial cleavage): 8 cells sit directly on top of each other.
  • Protostomes (spiral cleavage): cells spiral.

Morula

  • Solid ball of 16-32 cells.

Blastula

  • Hollow ball of cells with fluid-filled space (blastocoel).

Gastrula

  • Blastula folds inward, forming two layers (ectoderm and endoderm).

Gastrula Layers

  • Ectoderm: Forms nervous system and outer skin.
  • Mesoderm: Forms muscle, bone, and cardiovascular system.
  • Endoderm: Forms digestive system organs, respiratory lining, and bladder.

Fertilization Process

  • Membrane contact, sperm entry, prevention of polyspermy, egg activation, meiotic completion, and pronuclei fusion.

Fertilization in Sea Urchins

  • Acrosomal reaction: Sperm membrane changes upon contact, Ca++ influx, acrosomal enzyme release, and formation of acrosomal process with bindins.

Fertilization in Sea Urchins cont.

  • Acrosomal process contacts vitelline envelope, bindins attach to ZP3 receptors.
  • Sperm penetrates, initiates blocks to polyspermy.

Blocks to Polyspermy (Sea Urchins)

  • Fast block: Na+ influx causes positive membrane potential, preventing sperm binding and initiating slow block.
  • Slow block: Ca++ release causes cortical granules to burst, releasing enzymes that break bonds and hydrate, forming fertilization membrane.

Mammalian Fertilization

  • Sperm must meet egg in oviduct. Uses hyaluronidase & flagellum. Sperm binds to ZP3 of the zona pellucida using acrosin. Sperm-egg membranes contact, block to polyspermy begins, & egg is metabolically activated.

Mammalian Fertilization (cont.)

  • Second meiotic block lifted, sperm nuclear membrane breaks down, DNA replicates, and chromosomes align.

Establishment of Polarity

  • Craniocaudal, dorsoventral, and mediolateral axes.
  • Primary polarity by yolk distribution (animal vs. vegetal pole).
  • Secondary polarity by gray crescent formation.