Week 1 of Embryonic Development

What is the current stage of development of the fertilized egg?

What is the function of the zona pellucida?

What is the term for the process of the embryo breaking out of the zona pellucida?

What is the term for the valleys that form in the endometrial lining in preparation for implantation?

What is the term for the contact between the outer cell layers of the embryo and the endometrial lining?

What is the function of the trophoblasts in the process of implantation?

What is the term for the process by which the embryo becomes firmly embedded in the endometrium?

What occurs as a result of the trophoblasts invading the endometrial tissue?

What is happening to the uterine endometrial cells as the embryo implants?

What forms from the fusion of trophoblasts?

What are the finger-like projections of syncytiotrophoblasts called?

What is the function of the membrane of trophoblasts between the fetal and uterine blood vessels?

What is the name of the structure that forms from the growth of the villi and uterine blood vessels?

What happens to the cytotrophoblasts as the villi grow?

What is the purpose of the placenta?

What is the name of the blood vessels that develop within the villi?

The blastocyst has been fertilized for about a week and has been bouncing around in the fallopian tube before entering the endometrial cavity within the uterus.
The blastocyst is surrounded by a thick layer of glycoproteins called the zona pellucida, which starts to disintegrate, allowing the outer cells to become exposed.

The endometrium is anticipating the blastocyst's arrival and starts proliferating, forming valleys (crypts) for the blastocyst to rest in.
The blastocyst's outer cells (trophoblasts) come into contact with the endometrium, a process called apposition.
The trophoblasts multiply and invade into the endometrial tissue, causing adhesion and getting the blastocyst stuck.

The endometrial cells continue to divide and grow, eventually embedding the blastocyst entirely within the endometrium.
The endometrial blood vessels form pools of blood, which start to coalesce and eventually form a placenta-like structure.
The trophoblasts continue to divide and fuse, forming multi-nucleated cells called syncytiotrophoblasts, which grow into the endometrium.
The original trophoblasts that haven't fused are called cytotrophoblasts.

The syncytiotrophoblasts form finger-like projections (villi) that go into the endometrium, increasing the surface area for nutrient transfer.
The villi are lined with cytotrophoblasts, which help to form fetal blood vessels.
The fetal blood vessels are in close contact with the uterine blood vessels, allowing for nutrient and waste transfer.
Over time, the structure continues to grow, eventually forming the placenta, which lines the entire inside of the uterine cavity.