99. Blastocyst formation.

cleavage

Cleavage is a series of rapid mitotic divisions that begin approximately 30 hours after fertilization and continue for about 3-4 days. The purpose of cleavage is to increase the number of cells, known as blastomeres, without increasing the overall size of the embryo.

cleavage process

• Start: The cleavage process begins at the 2-cell stage.

• End: It continues through several stages until it reaches the morula stage (a solid ball of 16-32 cells).

characteristics of cleavage

• Total: The cleavage divisions affect the entire embryo.

• Unequal: Cleavage results in the formation of blastomeres of different sizes, known as microblastomeres (smaller cells) and macromeres (larger cells).

• Asynchronous: The division is not simultaneous; it starts with the 2-cell stage, progresses to the 4-cell, 8-cell, and then to the 16-cell stage.

main events during cleavage

• Increase in Cell Number: Through mitotic divisions, the number of cells increases rapidly from 2 to 4, then 8, and eventually to 16 or more. This is the morula stage.

• Decrease in Cell Size: With each division, the size of individual cells (blastomeres) decreases, as the cells divide without an increase in the overall size of the embryo.

• Compaction: At the 8-cell stage, the blastomeres undergo a process called compaction, where they maximize their contact with each other, creating a more compact, cohesive cell mass. This process is mediated by the adhesion molecule E-cadherin.

• Morula Formation: About 3 days after fertilization, a 16-cell morula is formed.

blastocyte formation

• After the morula stage, the embryo enters the uterine cavity, and fluid begins to penetrate the inner cell mass.

• The inner cell mass is now called the embryoblast, which will eventually give rise to the embryo proper.

• The outer cell mass becomes the trophoblast, which will form the epithelial wall of the blastocyst and contribute to the formation of the placenta.

• The zona pellucida (the protective glycoprotein coat surrounding the oocyte and early embryo) begins to disintegrate, allowing the blastocyst to increase in size and prepare for implantation.

Key Events During Blastocyst Formation:

• The blastocyst cavity forms as fluid fills the space between the inner cell mass (embryoblast) and the outer cell mass (trophoblast).

• The blastocyst is now a hollow ball of cells with a fluid-filled cavity.

• Approximately 4-5 days after fertilization, the blastocyst is ready to begin the process of implantation into the uterine mucosa (the endometrium).

implantation

Implantation is the process by which the blastocyst attaches to and begins to penetrate the uterine lining, which is crucial for establishing a connection between the embryo and the maternal blood supply for nutrients and gas exchange.