Fertilization & Implantation

  • the genetic material from a sperm cell (spermatozoon) and secondary oocyte merge into a single nucleus
  • fertilization normally occurs in the Fallopian tube 12-24 hours after ovulation
  • ejaculated sperm can remain viable for about 48 hours and a secondary oocyte is viable for about 24 hours after ovulation
  • peristaltic contractions and the action from the cilia transport the oocyte through the tube
  • sperm swim to the oocyte by whip-like movements from its tail
  • sperm produce an enzyme (acrosin) that stimulates sperm motility and migration
  • uterine contractions stimulated by the prostaglandins in semen aids sperm movement
  • sperm undergo functional changes (capacitation) in the female reproductive tract: the membrane around the acrosome becomes fragile so that several destructive enzymes are released
  • the enzymes help penetrate the ring of cells (corona radiate) that surround the oocyte
  • one sperm penetrates and enters a secondary oocyte, this is called syngamy
  • syngamy causes depolarization, which triggers the release of calcium ions into the cell
  • calcium ions stimulate the release of granules, that in turn, promote change changes in the zona pelllucida to block entry of the other sperm
  • oocyte completes equatorial division (meiosis ll)
  • the nucleus from the sperms head and the nucleus from the ovum fuse to produce segmentation nucleus
  • the segmentation nucleus contains 23 chromosomes from the male pronucleus and 23 chromosomes from the female pronucleus
  • the fertilized ovum is now called a zygote
  • rapid mitotic cell divisions of the zygote are called cleavage
  • cleavage is completed 30 hours after fertilization
  • successive cleavages produce a solid sphere of cells called morula
  • the morula is the same size as the original zygote
  • at 4 1/2 - 5 days, the dense cluster of cells has developed into a hollow ball of cells and enters the uterine cavity, it is now called a blastocyst

Implantation

  • blastocyst remains free within the uterine cavity for a short period of time
  • blastocyst enlarges and receives nourishment from glycogen rich secretions from the uterine glands
  • 6 days after fertilization the blastocyst attaches to the endometrium
  • usually implants on the posterior wall of the fundus
  • develops two layers in the region of contact between the blastocyst and endometrium, synctiotrophoblast and cytotrophobast
  • syncytiotrophoblast secretes enzymes that enables the blastocyst to penetrate the uterine lining, the enzymes digest and liquefy the endometrial cells
  • the trophoblast secretes hCG that rescues the corpus luteum from degeneration and sustains its secrestions of progesterone and estrogens, thus menstruation does not begin