The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the

blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...

. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

and develop into a large part of the placenta. They form during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg to become extraembryonic structures that do not directly contribute to the embryo. After

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...

, the trophoblast is contiguous with the

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their totipotency and are no longer totipotent stem cells because they cannot form a trophoblast. They are now pluripotent stem cells.

Structure

2907 Embroyonic Disc, Amniotic Cavity, Yolk Sac-02

The trophoblast proliferates and differentiates into two cell layers at approximately six days after fertilization for humans.

Function

Trophoblasts are specialized cells of the placenta that play an important role in embryo implantation and interaction with the decidualized maternal uterus. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetus’ circulation via the umbilical cord. This core is surrounded by two layers of trophoblasts, the

cytotrophoblast "Cytotrophoblast" is the name given to both the inner layer of the trophoblast (also called layer of Langhans) or the cells that live there. It is interior to the syncytiotrophoblast and external to the wall of the blastocyst in a developing embryo ...

and the

syncytiotrophoblast Syncytiotrophoblast (from the Greek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is the epithelial covering of the highly vascular embryonic placental villi, which invades the wall of the uterus to establish nut ...

. The cytotrophoblast is a layer of mono-nucleated cells that resides underneath the syncytiotrophoblast. The syncytiotrophoblast is composed of fused cytotrophoblasts which then form a layer that covers the placental surface. 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 the extravillous 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 the endothelial cells in the uterine spiral arteries as they remodel these vessels into wide bore conduits that are independent of maternal vasoconstriction. 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.

Clinical significance

The invasion of a specific type of trophoblast (extravillous trophoblast) into the maternal uterus is a vital stage in the establishment of pregnancy. Failure of the trophoblast to invade sufficiently is important in the development of some cases of

pre-eclampsia Pre-eclampsia is a disorder of pregnancy characterized by the onset of high blood pressure and often a significant amount of protein in the urine. When it arises, the condition begins after 20 weeks of pregnancy. In severe cases of the disease ...

. Invasion of the trophoblast too deeply may cause conditions such as placenta accreta,

placenta increta Placenta accreta occurs when all or part of the placenta attaches abnormally to the ''myometrium'' (the muscular layer of the uterine wall). Three grades of abnormal placental attachment are defined according to the depth of attachment and invasio ...

, or placenta percreta. Gestational trophoblastic disease is a pregnancy-associated concept, forming from the villous and extravillous trophoblast cells in the placenta. Choriocarcinoma are trophoblastic tumors that form in the uterus from villous cells. Trophoblast stem cells (TSCs) are cells that can regenerate and they are similar to embryonic stem cells (ESCs) in the fact that they come from early on in the trophoblast lifetime. In the placenta, these stem cells are able to differentiate into any trophoblast cell because they are pluripotent.

Additional images

File:Gray10.png, Blastodermic vesicle of ''Vespertilio murinus''. File:Gray11.png, Section through embryonic disk of ''Vespertilio murinus''. File:Gray35.png, Transverse section of a chorionic villus. File:Gray39.png, Scheme of placental circulation. File:HumanEmbryogenesis.svg, The initial stages of human embryogenesis File:Histopathology of tubal pregnancy.jpg, Histopathology of a

chorionic villus Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood. They are an essential element in pregnancy from a histomorphologic perspective, and are, by definition, a product of conception. Branche ...

, in a

tubal pregnancy Tubal ( he, תֻבָל, ''Ṯuḇāl'', ), in Genesis 10 (the "Table of Nations"), was the name of a son of Japheth, son of Noah. He is known to be the father of the Caucasian Iberians (ancestors of the Georgians) according to primary sources ...

, with labeled cytotrophoblasts and syncytiotrophoblasts.

References

External links