Implantation refers to the penetration of the blastocyst into the endometrial wall.
After fertilization, the zygote undergoes rapid mitotic division known as cleavage. The first cleavage, completed approximately 30 hours after fertilization, produces two identical cells called blastomeres.
This cell division continues, and by day 3, there is a cluster of 16 identical blastomeres. This is known as the morula.
Blastomeres continue synchronous cell division with the morula entering the uterine cavity at around day 4.
At this point, the 32-cell stage, a fluid known as uterine milk starts to penetrate the zona pellucida to nourish the blastomeres.
The blastomeres continue to divide and as more uterine milk enters the morula, they develop a central fluid-filled cavity known as the blastocyst cavity or blastocoele.
At approximately day 5, after the development of the blastocyst cavity, the developing embryo is known as a blastocyst.
Around this time, the zona pellucida degenerates and the blastocyst 'hatches' into the uterine cavity, ready to implant into the uterine wall.
Implantation occurs approximately 6-7 days after fertilization, but can only occur if the endometrial wall is sufficiently prepared by the correct levels of hormones.
The blastocyst usually implants with its inner cell mass facing the endometrium, in the posterior portion of the fundus of the uterus.
Initially, the blastocyst attaches loosely to the endometrial wall, but this attachment becomes stronger as the blastocyst burrows into the endometrium and the endometrium becomes increasingly vascularized.
Once the blastocyst is completely embedded within the endometrium, the endometrium becomes known as the decidua.
The regions of the decidua have specific names relative to the site of blastocyst implantation: the area between the embryo and the stratum basalis is known as the decidua basalis, the area between the embryo and the uterine cavity is the decidua capsularis, and the remaining area is known as the decidua parietalis.
Implantation is completed approximately 6-12 days after fertilization has taken place. After birth, the decidua is shed and menstruation recommences.
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27 авг 2014
