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Implantation of the Egg

 Once fertilization has occured the zygote must travel through the oviducts in order to reach the uterus and implant itself in the stratum functionalis of the endometrium. As the zygote is traveling down the oviducts it undergoes several mitotic divisions. At this stage, the ball of cells is referred to as a morula. The morula will consist of a large amount of cells that are dividing. These cells decrease in size with each division, but the overall cell mass of the morula will not increase in diameter, as shown in picture 10 (Jones and Lopez, 2006). By staying the same diameter, the morula will be able to travel through the oviducts without getting larger and therefore risking getting stuck.

Picture 10: Morula

Around day six, the morula will enter the uterus. At this point it will be known as a blastula because it increases in diameter as the cells divide. In a late blastula, a fluid filled cavity is formed in the middle of the mass, which is known as a blastocoel. Just inside the zona pellucida will be a single layer of cells know as the trophoblast. On one side of the blastula will be a mass of cells just below the trophoblast. These cells are referred to as the inner cell mass (Jones and Lopez, 2006). Picture 11 shows a picture of a blastula.

Picture 11: Blastula

The inner cell mass will align itself with the cells of the stratum functionalis. The trophoblast will differentiate into a syncytiotrophoblast and cytotrophoblast. The syncytiotrophoblast is the section of cells against the stratum functionalis that secretes enzymes in order to allow for invagination into the endometrium. The cytotrophoblast consists of squamal cells and will develop into the extra embryonic membranes later in the pregnancy. The inner cell mass will differentiate into the hypoblast. The hypoblast is located below the inner cell mass(Jones and Lopez, 2006). Picture 12 below shows the trophoblast along with the two layers it develops into.

Picture 12: Syncytiotrophoblast and Cytotrophoblast

As the syncytiotrophoblast continues to travel further into the stratum functionalis, there will be further differentiation of the inner cell mass resulting in the epiblast. The epiblast will develop into the fetus. At this point of implantation, the amniotic cavity has appeared and is located between the epiblast and syncytiotrophoblast (Jones and Lopez, 2006). Picture 13 demonstrates a developing amniotic cavity in the implanted egg.

Picture 13: Developing Amniotic Cavity

The cytotrophoblast will have developed into a single layer of cuboidal cells. The inner cell mass has become the bi-laminar embryonic disc (Jones and Lopez, 2006). The blastocoel will become the yolk sac, which is shown in picture 14.

Picture 14: The blastocoel becomes the yolk sac

Once the blastula has invaginated itself into the stratum functionalis completely, there will be the formation of the closing plug. This closing plug will consist of the uterine tissue redeveloping the endometrial epithelium, thus completing the implantation of the egg into the uterine wall.

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