Human development days 4-7

Definitions & Key takeaways

After blastocyst formation at around day four to five, the blastocyst starts to implant into the uterus at around day six or seven. During blastocyst implantation, the trophoblast binds to the endometrial wall with L-selectin and integrin molecules. The blastocyst implants into the decidua basalis, which happens in the secretory phase of the menstrual cycle, also known as the implantation window.

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During fertilization, the sperm and egg fuse to form a new diploid cell called a zygote. The zygote’s cells divide, forming a new, multicellular cluster called a blastocyst, which travels to the uterus where it implants itself on surface of the uterine wall.
A few hours after sperm and egg have fused into the zygote it undergoes a process called cleavage, where it divides into a pair of 2 new cells called blastomeres.
The blastomeres keep splitting so it turns into a loose clump of four cells, then eight cells, and finally into a more structured “mulberry”-shaped 16-cell cluster called a morula, with inner and outer cell masses.
The morula’s cells are held in place by the zona pellucida. The morula gradually develops an outer cell mass of trophoblast cells and an interior cell cluster with a fluid-filled cavity at the core called the blastocoel.
As soon as the blastocoel forms, the morula is no more: it’s now a water balloon-shaped structure called a blastocyst. The cells making up the inner cell wall of the blastocyst are collectively called the embryoblast - because they will go on to eventually become the fetus.
The embryoblast cells cluster together at one end of the blastocyst in an area called the embryonic pole. Meanwhile, the trophoblast flattens out, forming a barrier called the epithelial wall.
Fully-formed, the blastocyst “hatches” from the zona pellucida around the end of day 4, and is ready to attach itself to the wall of the uterus.
Trophoblast cells help it implant into the wall of the uterus by putting L-selectin molecules on their surface which are a proteins bound to carbohydrates that bind like a lock and key to carbohydrate receptors on the uterine wall.
Once there, the trophoblast proliferates into the wall, called syncytiotrophoblasts. During implantation, the mucosal tissues of the uterus are in a secretory phase, which is where the corpus luteum releasing large quantities of progesterone, a sex hormone that floods the body and reaches the glands and arteries of the uterus.
The corpus luteum is located in the ovary, and develops in the sport where the unfertilized egg was initially released. Responding to this flood of progesterone, the uterine arteries become large and coiled, engorging the endometrium layer - the tissues lining the uterus - with blood.
All right, as a quick recap…Just a few hours after its formation at around 1-2 days, the zygote undergoes a series of mitotic divisions, taking us through days 2-4 and developing into a blastocyst at around day 5.
This new cluster of cells attaches to the outer wall of the endometrium, where it starts to implant and reach into the endometrial wall.
Meanwhile, the endometrial tissues react to a surge in progesterone released from the corpus luteum. They become engorged with blood and fluids, swelling up in preparation for the blastocyst’s arrival, and this takes us to about day 7.