The trophoblast (from

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

: 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 ...

. Trophoblasts are present four days after

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

in humans. They provide nutrients to the

embryo An embryo ( ) is the initial stage of development for 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 sp ...

and develop into a large part of the

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

. They form during the first stage of

pregnancy Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

and are the first cells to differentiate from the

fertilized Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a zygote and initiate its development into a new individual organism or off ...

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

to become extraembryonic structures that do not directly contribute to the embryo. After

blastulation Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development, the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (from ...

, 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 the o ...

of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their

totipotency Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum ...

because they can no longer form a trophoblast. They become

pluripotent Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum ...

stem cells.

Structure

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

Function

Trophoblasts are specialized cells of the

that play an important role in embryo implantation and interaction with the

decidua The decidua is the modified mucosal lining of the uterus (that is, modified endometrium) that forms every month, in preparation for pregnancy. It is shed off each month when there is no fertilized egg to support. The decidua is under the influe ...

lized maternal

uterus The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetus’ circulation via the

umbilical cord In Placentalia, placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord i ...

. This core is surrounded by two layers of trophoblasts, the cytotrophoblast and the

syncytiotrophoblast The 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 ...

. 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 The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the res ...

in the uterine spiral arteries as they remodel these vessels into wide bore conduits that are independent of maternal

vasoconstriction Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vesse ...

. 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

is a vital stage in the establishment of

. Failure of the trophoblast to invade sufficiently is important in the development of some cases of

pre-eclampsia Pre-eclampsia is a multi-system disorder specific to pregnancy, characterized by the new onset of hypertension, high blood pressure and often a significant amount of proteinuria, protein in the urine or by the new onset of high blood pressure a ...

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

placenta percreta Placenta accreta spectrum (PAS) is a medical condition that occurs when all or part of the placenta attaches abnormally to the ''myometrium'' (the muscular layer of the uterine wall) during pregnancy. This condition was first documented in medica ...

. 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 cell Embryonic stem cells (ESCs) are Cell potency#Pluripotency, pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre-Implantation (human embryo), implantation embryo. Human embryos reach the blastocyst stage 4� ...

s (ESCs) in the fact that they come from early on in the trophoblast lifetime. In the

, 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 Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of deve ...

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. Branch ...

, in a tubal pregnancy, with labeled cytotrophoblasts and syncytiotrophoblasts.

References

External links