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
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
in humans. They provide nutrients to the
embryo 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
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proc ...
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 a ...
to become extraembryonic structures that do not directly contribute to the embryo. After
gastrulation, the trophoblast is contiguous with the
ectoderm of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their
totipotency Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
and are no longer
totipotent stem cells because they cannot form a trophoblast. They are now
pluripotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
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
placenta that play an important role in embryo
implantation and interaction with the
decidualized maternal
uterus
The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
. 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 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 is physiologi ...
. 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 embr ...
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 nu ...
. 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 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 rest of the vessel ...
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 vess ...
. 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
The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
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 invas ...
, or
placenta percreta.
Gestational trophoblastic disease is a pregnancy-associated concept, forming from the villous and extravillous trophoblast cells in the placenta.
Choriocarcinoma
Choriocarcinoma is a malignant, trophoblastic cancer, usually of the placenta. It is characterized by early hematogenous spread to the lungs. It belongs to the malignant end of the spectrum in gestational trophoblastic disease (GTD). It is also c ...
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 pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre- implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist ...
s (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
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry)
In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...
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 de ...
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
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.
See also
*
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 nu ...
*
Hydatidiform mole
A molar pregnancy also known as a hydatidiform mole, is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus. A molar pregnancy is a type of gestational trophoblastic disease that used to be known as a ''hy ...
References
External links
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{{Authority control
Animal developmental biology