Fertilization Process Overview

Fertilization

• Fertilization is the merging of a sperm cell from a male and an ovum (egg) from a female, creating a zygote.
• This process is essential for sexual reproduction and initiates embryonic development.

Sperm Transport

• Sperm are deposited in the female reproductive tract after ejaculation.
• They travel through the cervix, uterus, and fallopian tubes.
• Capacitation is a crucial process for sperm, enabling them to fertilize an egg.
• It involves biochemical changes in the sperm's plasma membrane.

Interaction with the Egg

• The egg is surrounded by layers: the corona radiata (granulosa cells) and the zona pellucida (glycoprotein layer).
• Sperm must penetrate these layers to reach the egg's plasma membrane.
• Acrosome reaction: The acrosome releases enzymes that digest the zona pellucida, allowing sperm to reach the egg.
• Sperm binding: Sperm binds to specific receptors on the egg's surface after penetrating the zona pellucida.

Fusion of Sperm and Egg

• Membrane fusion: Upon binding, the plasma membranes of the sperm and egg fuse.
• This allows the sperm's nucleus to enter the egg.
• Cortical reaction: After sperm entry, the egg releases enzymes modifying the zona pellucida, preventing polyspermy (multiple sperm entering).

Formation of the Zygote

• After fusion, the sperm and egg nuclei combine.
• This results in a diploid zygote with a complete set of chromosomes.
• The zygote travels down the fallopian tube towards the uterus.

Cleavage

• Cleavage: Rapid mitotic divisions after fertilization forming a multicellular structure (blastula).
• Blastomeres are the smaller cells produced by these divisions.
• Cleavage stages: Two-cell, four-cell, eight-cell, and 16-cell (Morula).
• Radial Cleavage: Cells arranged symmetrically (typical in deuterostomes).
• Spiral Cleavage: Cells arranged at an angle (typical in protostomes).

Blastocyst Formation

• Fluid accumulation in the morula forms a blastocoel (cavity).
• The morula transforms into a blastocyst.
• Blastocyst components:
  • Inner Cell Mass (ICM): Develops into the embryo.
  • Trophoblast: Forms the placenta and aids in implantation.

Implantation

• The blastocyst reaches the uterus around 5-6 days post-fertilization.
• It implants in the uterine wall (endometrium).
• Trophoblast cells invade the endometrium.
• The blastocyst establishes connections for nutrient exchange.