D3.1.16 Development of a blastocyst and implantation in the endometrium (HL) Notes

Development of a Blastocyst and Implantation in the Endometrium

1. 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.

How the Zygote Becomes a Blastocyst

1. Cleavage Divisions
   1. The zygote undergoes mitotic divisions called cleavage.
   2. These divisions increase the number of cells without increasing the overall size of the embryo.
   3. The embryo progresses from a single cell to a solid ball of cells.
2. Blastocyst Formation
   1. By day 5–6, the embryo becomes a blastocyst—a hollow structure with two distinct parts:
      1. Trophoblast: The outer layer of cells, which will form the placenta.
      2. Inner Cell Mass (ICM): A cluster of cells inside the blastocyst that will develop into the embryo.
3. The blastocyst is surrounded by the zona pellucida, a protective glycoprotein layer.

Implantation: How the Blastocyst Anchors in the Uterus

1. Hatching
   1. Around day 6–7, the blastocyst breaks free from the zona pellucida, a process called hatching.
   2. This step allows the blastocyst to directly interact with the uterine lining.
2. Attachment to the Endometrium
   1. Trophoblast cells secrete enzymes that digest part of the endometrial lining, enabling attachment.
   2. The trophoblast differentiates into two layers:
      1. Cytotrophoblast: The inner layer that remains cellular.
      2. Syncytiotrophoblast: The outer layer that invades the endometrium and facilitates implantation.
3. Invasion and Nutrient Exchange
   1. The syncytiotrophoblast forms finger-like projections that penetrate the endometrium and establish contact with maternal blood vessels.
   2. These projections create the foundation for the placenta, which will support the embryo throughout pregnancy.

Hormonal Support: Why hCG Is Critical for Early Pregnancy

1. hCG signals the corpus luteum to produce progesterone, maintaining the endometrium and preventing menstruation.
2. Pregnancy tests detect hCG levels as an early indicator of successful implantation.

Why Is Implantation Critical?

1. Nutrient Supply: The blastocyst’s nutrient reserves are limited, so implantation ensures a continuous supply of nutrients and oxygen from the mother.
2. Placenta Formation: Implantation initiates the development of the placenta, which sustains the embryo throughout pregnancy.
3. Hormonal Communication: The secretion of hCG and other hormones ensures that the maternal body supports the pregnancy.

Challenges and Implications

1. Ectopic Pregnancy
   1. If the blastocyst implants outside the uterus (e.g., in the oviduct), it results in an ectopic pregnancy.
   2. This condition is non-viable and can be life-threatening if untreated.
2. Implantation Failure
   1. Implantation may fail due to issues with the blastocyst (e.g., chromosomal abnormalities) or the endometrium (e.g., insufficient thickness or hormonal support).