Trophoblast
Encyclopedia
Trophoblasts are cells forming the outer layer of a blastocyst
, which provide nutrients to the embryo
and develop into a large part of the placenta
. They are formed during the first stage of pregnancy
and are the first cells to differentiate
from the fertilized egg
. This layer of trophoblasts is also collectively referred to as "the trophoblast", or, after gastrulation
, the trophectoderm, as it is then contiguous with the ectoderm of the embryo.
In addition, cytotrophoblast in the tips of villi can differentiate into another type of trophoblast called the extravillous trophoblast. Extravillous trophoblast grow out from the placenta and penetrate into the decidualised uterus. This process is essential not only for physically attaching the placenta to the mother, but also for altering the vasculature in the uterus to allow it to provide an adequate blood supply to the growing fetus as pregnancy progresses. Some of these trophoblast 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 the fetus receives a steady supply of blood, and the placenta is not subjected to fluctuations in oxygen that could cause it damage.
is a vital stage in the establishment of pregnancy
:
Gestational trophoblastic disease
represents a form of proliferation.
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...
, which provide nutrients to the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
and develop into a large part of the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
. They are formed during the first stage of pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
and are the first cells to differentiate
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
from the fertilized egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
. This layer of trophoblasts is also collectively referred to as "the trophoblast", or, after gastrulation
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
, the trophectoderm, as it is then contiguous with the ectoderm of the embryo.
Function
Trophoblasts are specialised cells of the placenta that play an important role in embryo implantation and interaction with the decidualised maternal uterus. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetal circulation via the umbilical cord. This core is surrounded by two layers of trophoblast; a single layer of mononuclear cytotrophoblast that fuse together to form the overlying multinucleated syncytiotrophoblast layer that covers the entire surface of the placenta. It is this syncytiotrophoblast that is in direct contact with the maternal blood that reaches the placental surface, and thus facilitates the exchange of nutrients, wastes and gases between the maternal and fetal systems.In addition, cytotrophoblast in the tips of villi can differentiate into another type of trophoblast called the extravillous trophoblast. Extravillous trophoblast grow out from the placenta and penetrate into the decidualised uterus. This process is essential not only for physically attaching the placenta to the mother, but also for altering the vasculature in the uterus to allow it to provide an adequate blood supply to the growing fetus as pregnancy progresses. Some of these trophoblast 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 the fetus receives a steady supply of blood, and the placenta is not subjected to fluctuations in oxygen that could cause it damage.
Differentiation
The trophoblast proliferates and differentiates into 2 cell layers at approximately 6 days after fertilization for humans:| Layer | Location | Description |
| cytotrophoblast Cytotrophoblast The cytotrophoblast is the inner layer of the trophoblast, interior to the syncytiotrophoblast in an embryo.Cytotrophoblast is considered to be the trophoblastic stem cell; it differentiates into the other forms of trophoblastic tissue .-External links: - "Female Reproductive System: placental... |
inner layer | Single celled, inner layer of the trophoblast. |
| syncytiotrophoblast Syncytiotrophoblast Syncytiotrophoblast is the epithelial covering of the placenta villous tree. It is a unique tissue in that it is a multi-nucleated, terminally differentiated syncytium, extending to 13m^2... |
outer layer | Thick layer that lacks cell boundaries and grows into the endometrial stroma Stroma (animal tissue) In animal tissue, stroma refers to the connective, supportive framework of a biological cell, tissue, or organ.The stroma in animal tissue is contrasted with the parenchyma.Examples include:* Stroma of iris... . It secretes hCG Human chorionic gonadotropin Human chorionic gonadotropin or human chorionic gonadotrophin is a glycoprotein hormone produced during pregnancy that is made by the developing embryo after conception and later by the syncytiotrophoblast .. Some tumors make this hormone; measured elevated levels when the patient is not... in order to maintain progesterone Progesterone Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species... secretion and sustain a pregnancy. |
| intermediate trophoblast Intermediate trophoblast Intermediate trophoblast is a distinct subtype of trophoblastic tissue that arises from the cytotrophoblast.It is sub-categorized by location:*Implantation site intermediate trophoblast*Villous intermediate trophoblast... (IT) |
implantation site, chorion, villi (dependent on subtype) | anchor placenta (implanation site IT), unknown (chorionic & villus IT) |
Pathology
The invasion of a specific type of trophoblast (extravillous trophoblast) into the maternal uterusUterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
is a vital stage in the establishment of pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
:
- Failure of the trophoblast to invade sufficiently is important in the development of some cases of pre-eclampsiaPre-eclampsiaPre-eclampsia or preeclampsia is a medical condition in which hypertension arises in pregnancy in association with significant amounts of protein in the urine....
. - Too firm an attachment may lead to placenta accretaPlacenta accretaPlacenta accreta is a severe obstetric complication involving an abnormally deep attachment of the placenta, through the endometrium and into the myometrium...
.
Gestational trophoblastic disease
Gestational trophoblastic disease
Gestational trophoblastic disease is a term used for a group of pregnancy-related tumours. These tumours are rare, and they appear when cells in the womb start to grow out of control. The cells that form gestational trophoblastic tumours are called trophoblasts and come from tissue that grows to...
represents a form of proliferation.
