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Cleavage

Zygote divides and develops to form the blastocyst* during passage through the uterine tube.

Involves multiple rounds of mitotic division without growth in size.

Key Steps:

Zygote, single cell, produced by fusion of the male and female gametes in fertilization.

Has zona pellucida, which is the acellular protective coat.

After a single round of mitosis, the zygote gives rise to two cells, aka, blastomeres.*

By the four-cell stage, the blastomeres are loosely organized, with incomplete contact between the cells.

This changes at the 8-cell stage, when compaction* reorganizes the cells and forms:

An inner cell mass, which is held together by tight junctions.
An outer cell mass, which is covered by the zona pellucida.

In the histological sample, notice the tightly packed cells and thick outer zona pellucida.

In the next stage the morula* comprises approximately 16 cells.

Sodium-potassium pumps are responsible for pulling water into the morula cell mass.
As a result, cavitation occurs

Morula gives rise to the blastocyst, with a blastocyst cavity (blastocoel*)

Blastocyst:

Zona pellucida surrounds two cell populations:

Outer cell population is now called the trophoblast. Inner cell mass, which has become concentrated to one side, is now called the embryoblast.

As we learn elsewhere, the trophoblast plays an essential role in implantation and formation of the placenta, and the embryoblast gives rise to the tissues of the embryo proper.

Cavitation establishes embryonic polarity:

Embryonic pole at the side of the embryoblast.
Abembryonic pole at the opposite side.

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Step 1

In response to ovarian hormones, the stroma has undergone decidualization, and is edematous, vascularized, and rich in mucus and glycogen.

Thus, the endometrium is ready to house and nourish an embryo.

The blastocyst emerges from the zona pellucida (aka, it "hatches"*).

It can now interact directly with the maternal environment, and does so by releasing substances that prevent its rejection.
One of these substances, human chorionic growth hormone (hCG), preserves the corpus luteum within the ovary; recall that the corpus luteum produces progesterone, which is necessary for maintenance of the endometrial lining, and, therefore, the survival of the embryo.

Thus, many early pregnancy tests rely on the presence of hCG to determine pregnancy status.

Step 2

Apposition involves alignment of the embryonic pole to the uterine wall, to which it loosely attaches.

The blastocyst rolls over its surface (some texts specify that it is the inner cell mass that "rolls" within the trophoblast).
Proper alignment relies on signaling between the epithelia and embryo: pinopods, aka, uterodomes, on the uterine wall interact with microvilli on the blastocyst and establish a loose connection.

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Cleavage & Implantation

Cleavage

Mitotic division that occurs in the uterine tube.

Implantation

Endometrial encapsulation that occurs within the uterus.

Endometrium comprises multiple layers, and, under direction from the ovarian hormones, cyclically prepares for implantation of an embryo. In the absence of implantation, the endometrial lining is shed during menstruation.

We show implantation in 4 key steps:

1. Pre-implantation events, which include "hatching" of the blastocyst from the zona pellucida. 2. Blastocyst apposition to the uterine wall. 3. Early adhesion to uterine epithelia. 4. Uterine stroma – embryo contact, which ultimately connects the maternal vascular supply to the embryo.

Cleavage & Implantation

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