Pre-Implantation Development Notes

Pre-Implantation Development

This lecture discusses the events occurring from fertilization to blastocyst implantation in the uterus, covering the first week of development.

Definition of Pre-Implantation

• Pre-implantation refers to all events from sperm fertilization of an egg to the implantation of the fertilized egg into the uterine wall.
• Essentially, it encompasses the first week of a new individual's development.

Key Events in Pre-Implantation

1. Fertilization:
   • The fusion of sperm and egg.
   • Refer to previous video for details.
2. Cleavage:
   • The zygote (fertilized egg) undergoes rapid cell division via mitosis.
   • One cell divides into two, then four, eight, and so on.
3. Blastocyst Formation:
   • The structure formed after cleavage.
   • It travels to the uterus and implants into the uterine wall.

Fertilization Reminder

• Upon sperm entry, the oocyte completes meiosis II, releasing the second polar body.
• The sperm and oocyte nuclei (pro-nuclei) fuse to form a single diploid nucleus.
• The fertilized egg with this single nucleus is called a zygote.
• The zygote then undergoes cleavage.

Cleavage Process

• The zygote divides into two cells approximately 24 hours after fertilization.
• After another 24 hours, the two cells divide into four.
• By day three, there's an eight-cell structure.
• Cleavage is this rapid mitotic division that transforms the single-celled zygote into a cluster of cells.

Morula Stage

• At day four a solid ball of cells is formed, referred to as a morula.

Location within the Female Reproductive Tract

• Fertilization ideally takes place in the ampulla of the oviduct.
• The cleavage process (two-cell, four-cell, and eight-cell stages) occurs in the ampulla and isthmus of the oviduct.
• The morula stage appears around day four when the structure enters the uterus.

Role of the Zona Pellucida

• The zona pellucida is a thick, jelly-like coat surrounding the oocyte and early cell structures.
• It protects the oocyte and ensures only highly capable sperm fertilize it.
• It holds the cells together during cleavage, preventing them from separating.
• It prevents implantation in the oviduct, thus preventing ectopic pregnancies.

Blastocyst Formation and Implantation

• By day five, the zona pellucida dissolves, allowing implantation to occur.
• The structure transforms into a blastocyst, characterized by a fluid-filled cavity.
• Implantation occurs around day six on the side of the blastocyst where the inner cell mass is located.

Blastocyst Structure

• Unlike the morula, the blastocyst isn't a solid ball of cells; it has a cavity.
• The cells differentiate into two types:
  • Trophoblast cells: Outer cells that eventually contribute to the placenta.
  • Inner cell mass (Embryoblast): A cluster of cells on one side that will develop into the embryo.

Cell Differentiation and Function

• The inner cell mass (embryoblast) becomes the embryo.
• The trophoblast cells become the embryonic part of the placenta.
• Implantation begins as the blastocyst burrows into the endometrium.