| Trophoblast | |
|---|---|
 Blastocyst with aninner cell mass and trophoblast. | |
| Details | |
| Days | 6 |
| Gives rise to | Caul |
| Identifiers | |
| Latin | trophoblastus; massa cellularis externa |
| MeSH | D014327 |
| TE | E6.0.1.1.2.0.2 |
| FMA | 83029 |
| Anatomical terminology | |
Thetrophoblast (fromGreektrephein: to feed; andblastos: germinator) is the outer layer of cells of theblastocyst. Trophoblasts are present four days afterfertilization in humans.[1] They provide nutrients to theembryo and develop into a large part of theplacenta.[2][3] They form during the first stage ofpregnancy and are the first cells todifferentiate from thefertilizedegg to become extraembryonic structures that do not directly contribute to the embryo. Afterblastulation, the trophoblast is contiguous with theectoderm of the embryo and is referred to as the trophectoderm.[4] After the first differentiation, the cells in the human embryo lose theirtotipotency because they can no longer form a trophoblast. They becomepluripotent stem cells.
The trophoblast proliferates and differentiates into two cell layers at approximately six days after fertilization for humans.
| Layer | Location | Description |
|---|---|---|
| Cytotrophoblast | The inner layer | A single-celled inner layer of the trophoblast. |
| Syncytiotrophoblast | The outer layer | A thick layer that lacks cell boundaries and grows into theendometrialstroma. It secreteshCG in order to maintainprogesterone secretion and sustain a pregnancy. |
| Intermediate trophoblast (IT) | The implantation site,chorion, villi (dependent on subtype) | An anchorplacenta (implantation site IT). |
Trophoblasts are specialized cells of theplacenta that play an important role in embryoimplantation and interaction with thedecidualized maternaluterus.[5] The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetus’ circulation via theumbilical cord. This core is surrounded by two layers of trophoblasts, thecytotrophoblast and thesyncytiotrophoblast. The cytotrophoblast is a layer of mono-nucleated cells that resides underneath the syncytiotrophoblast.[6] The syncytiotrophoblast is composed of fused cytotrophoblasts which then form a layer that covers the placental surface.[6] The syncytiotrophoblast is in direct contact with the maternal blood that reaches the placental surface. It then facilitates the exchange of nutrients, wastes and gases between the maternal and fetal systems.
In addition, cytotrophoblasts in the tips of villi can differentiate into another type of trophoblast called theextravillous trophoblast. Extravillous trophoblasts grow out from the placenta and penetrate into the decidualized uterus. This process is essential not only for physically attaching the placenta to the mother, but also for altering the vasculature in the uterus. This alteration allows an adequate blood supply to the growing fetus as pregnancy progresses. Some of these trophoblasts even replace theendothelial cells in the uterinespiral arteries as they remodel these vessels into wide bore conduits that are independent of maternalvasoconstriction. This ensures that the fetus receives a steady supply of blood, and the placenta is not subjected to fluctuations in oxygen that could cause it damage.[7]
The invasion of a specific type of trophoblast (extravillous trophoblast) into the maternaluterus is a vital stage in the establishment ofpregnancy. Failure of the trophoblast to invade sufficiently is important in the development of some cases ofpre-eclampsia. Invasion of the trophoblast too deeply may cause conditions such asplacenta accreta,placenta increta, orplacenta percreta.
Gestational trophoblastic disease is a pregnancy-associated concept, forming from the villous and extravillous trophoblast cells in the placenta.[8]
Choriocarcinoma are trophoblastic tumors that form in the uterus from villous cells.[8]
Trophoblast stem cells (TSCs) are cells that can regenerate and they are similar toembryonic stem cells (ESCs) in the fact that they come from early on in the trophoblast lifetime.[9] In theplacenta, these stem cells are able to differentiate into any trophoblast cell because they are pluripotent.[9]
