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At the time of implantation, the endometrium, the innermost layer of the uterine wall is in the progestational or secretory phase of the menstrual cycle. Under the influence of progesterone which is produced by the corpus luteum, the endometrium becomes very thickened, and the uterine arteries and glands become coiled. The glands begin to secrete a glycogen rich secretion to nourish the blastocyst as it invades into the endometrium

  • Day 09
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    The cells of the trophoblast differentiate into two layers: an inner cytotrophoblast which retains distinct cellular boundaries and an outer syncytiotrophoblast which is a syncytium.
    The syncytiotrophoblast displaces endometrial cells at the point of implantation. Apoptosis of the endometrial cells facilitates the process. Endometrial stromal cells near the implantation site assume a polyhedral shape and accumulate lipids and glycogen to become decidual cells. The syncytiotrophoblast ingests these cells as a source of nutrition. A mucus plug seals the defect in the endometrium where the blastocyst invaded.
    Spaces begin to develop in the synciotrophoblast. These are called
    trophoblastic lacunae.
  • Day 12
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    As the blastocyst invades the endometrium, spaces begin to develop in the syncytiotrophoblast. These spaces are called lacunae.
    Maternal blood vessels (sinusoids) are also eroded by the
    syncytiotrophoblast and maternal blood begins to fill the lacunae. Since the syncytiotrophoblast is part of the chorion, the human placenta is considered to be of the hemochorial type in which maternal and fetal blood come into close association without mixing together.

  • Day 13
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    At the embryonic pole of the blastocyst, the cytotrophoblast begins to form finger-like projections that will eventually from the core of the primary villi of the placenta. Note that on the abevbryonic pole, the lacunae are small and cytotrophoblast remains as a single layer of cells.