Syncytiotrophoblast (ST)

Definition - What does Syncytiotrophoblast (ST) mean?

A syncytiotrophoblast (ST) is the outermost layer of the trophectoderm (the cells surrounding the embryo at the blastocyst stage of development). The syncytiotrophoblast forms shortly prior to implantation and actively invades the uterine wall. After the blastocyst has fully embedded in the uterus, the syncytiotrophoblast goes on to develop the outer layer of the fetal portion of the placenta. If a woman's immune system identifies the implanting embryo as a foreign object it may result in pregnancy complications or even miscarriage.

FertilitySmarts explains Syncytiotrophoblast (ST)

Shortly before coming into contact with the endometrium, the trophectoderm (rim of cells encompassing the embryo) differentiates into two different layers of cells. The inner layer is called the cytotrophoblast (CT) and the outer layer is called the syncytiotrophoblast (ST).

The syncytiotrophoblast produces enzymes that break down the endometrial lining of the uterus. It embeds deep within the endometrium, coming into contact with the maternal blood supply. Once the blastocyst has firmly implanted in the endometrium, the syncytiotrophoblast grows quickly and entirely surrounds the embryo. It continues to erode uterine tissue and blood vessels. By around day 13 the early circulatory system connecting the uterus and placenta develops.

The syncytiotrophoblast secretes hormones and growth factors that promote fetal and placental development and growth. The hormones it produces include human chorionic gonadotropin (hCG), estrogen, progesterone, and human placental growth hormone. Progesterone is also secreted by the corpus luteum. By the end of the first trimester of pregnancy, the placenta is able to produce enough progesterone on its own and the corpus luteum is no longer needed.

Typically, a woman's immune cells ignore the invasion of the embryo into the uterine lining. However, it's possible that the woman's body can incorrectly identify the embryo as a foreign object and initiate an immune response. This could lead to complications such as preeclampsia, preterm labor, or miscarriage.

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