Development of a Blastocyst and Implantation in the Endometrium
- After fertilization, the zygote undergoes a series of transformations that culminate in the formation of a blastocyst, which is then implanted into the endometrium of the uterus.
- Think of the zygote as a seed planted in fertile soil.
- Just as a seed needs nutrients and support to grow, the developing embryo requires the nurturing environment of the uterus to thrive.
How the Zygote Becomes a Blastocyst
- Cleavage Divisions
- The zygote undergoes mitotic divisions called cleavage.
- These divisions increase the number of cells without increasing the overall size of the embryo.
- The embryo progresses from a single cell to a solid ball of cells.
- Blastocyst Formation
- By day 5–6, the embryo becomes a blastocyst—a hollow structure with two distinct parts:
- Trophoblast: The outer layer of cells, which will form the placenta.
- Inner Cell Mass (ICM): A cluster of cells inside the blastocyst that will develop into the embryo.
- By day 5–6, the embryo becomes a blastocyst—a hollow structure with two distinct parts:
- The blastocyst is surrounded by the zona pellucida, a protective glycoprotein layer.
The zona pellucida prevents the blastocyst from implanting prematurely in the oviduct, which could lead to an ectopic pregnancy.
Implantation: How the Blastocyst Anchors in the Uterus
- Hatching
- Around day 6–7, the blastocyst breaks free from the zona pellucida, a process called hatching.
- This step allows the blastocyst to directly interact with the uterine lining.
- Attachment to the Endometrium
- Trophoblast cells secrete enzymes that digest part of the endometrial lining, enabling attachment.
- The trophoblast differentiates into two layers:
- Cytotrophoblast: The inner layer that remains cellular.
