Fertilization: Biology and Medicine

Female Reproductive System

• Uterus and Ovaries
  • Ovaries are female gonads.
  • Ovaries produce the female egg.
• Menstrual Cycle
  • Monthly cycle where women ovulate and release an egg.
  • Process of releasing an egg is called ovulation.

Structure of the Egg

• Secondary Oocyte
  • Surrounded by the zona pellucida (ZP), a thick protective membrane.
  • ZP is filled with sperm recognition proteins called ZP3.
  • Corona radiata is a layer of cells surrounding the ZP.

Fertilization Process

• Time Frame
  • Egg must be fertilized within 24 hours, otherwise a period occurs.
• Sperm Capacitation
  • Sperm undergoes capacitation in the vagina.
  • Capacitated sperm initiates the acrosome reaction after docking with ZP3 proteins.
  • Leads to fertilization of the egg.

Post-Fertilization Events

• Formation of Zygote
  • Secondary oocyte undergoes meiosis II, forming a polar body and an ovum.
  • Incorporates sperm genes to form a zygote on day 1 of development.

Early Development Stages

• Cleavage
  • Occurs from days 2-3.
  • Zygote divides into two cells, then four, then eight, etc.
  • Cytoplasm amount remains constant.
• Morula Formation
  • On day 4, cleavage results in a 16-cell ball called a morula.
• Blastocyst Formation
  • On day 5, the morula cleaves into a 32-cell blastocyst.
  • Zona pellucida surrounds the blastocyst.

Implantation Preparation

• Zona Pellucida Shedding
  • On day 7, the ZP is shed from the blastocyst.
  • Occurs just prior to implantation.
• Blastocyst Structure
  • Hollow cell mass sphere.
  • Lump of cells on one side binds to the endometrium (uterus lining).