| Blastocyst | |
|---|---|
 Blastocyst just beforeimplantation | |
 A human blastocyst, withinner cell mass at upper right | |
| Details | |
| Carnegie stage | 3 |
| Days | 5–9 |
| Gives rise to | Gastrula |
| Identifiers | |
| Latin | blastocystis |
| MeSH | D001755 |
| TE | E2.0.1.2.0.0.12 |
| FMA | 83041 |
| Anatomical terminology | |
Theblastocyst is a structure formed in the earlyembryonic development ofmammals. It possesses aninner cell mass (ICM) also known as theembryoblast which subsequently forms theembryo, and an outer layer oftrophoblast cells called thetrophectoderm.[1][2] This layer surrounds theinner cell mass and a fluid-filled cavity orlumen known as theblastocoel.[3] In the late blastocyst, the trophectoderm is known as thetrophoblast.[2] The trophoblast gives rise to thechorion andamnion, the twofetal membranes that surround the embryo. Theplacenta derives from the embryonic chorion (the portion of the chorion that developsvilli).[4][5] The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called theblastula.
In humans, blastocyst formation begins about five days afterfertilization when a fluid-filled cavity opens up in themorula, the early embryonic stage of a ball of 16cells.The blastocyst has a diameter of about 0.1–0.2mm and comprises 100-200 cells following 7-8 rounds ofcleavage (cell division without cell growth). About seven days after fertilization,[6] the blastocyst undergoesimplantation, embedding into theendometrium of theuterine wall where it will undergo further developmental processes, includinggastrulation. Embedding of the blastocyst into the endometrium requires that ithatches from thezona pellucida, the egg coat that prevents adherence to thefallopian tube as thepre-embryo makes its way to the uterus.
The use of blastocysts inin vitro fertilization (IVF) involves culturing a fertilized egg for five days beforetransferring it into the uterus. It can be a more viable method offertility treatment than traditional IVF. The inner cell mass of blastocysts is the source ofembryonic stem cells, which are broadly applicable instem cell therapies including cell repair, replacement and regeneration.Assisted zona hatching may also be used in IVF and other fertility treatments.
The name "blastocyst" arises from theGreekβλαστόςblastós ("a sprout") andκύστιςkýstis ("bladder, capsule").
The blastocyst stage occurs between 5 and 9 days after conception. Duringembryonic development, afterfertilization (approximately 5–6 days in the human), the cells of themorula begin to undergocell differentiation, and the morula changes into the blastocyst by pumping fluid to grow alumen. In the uterus thezona pellucida surrounding the blastocyst breaks down, allowing it toimplant into the uterine wall. Implantation marks the end of thegerminal stage of embryogenesis, and the beginning ofgestation.[medical citation needed]
Thezygote undergoes several rounds ofmitosis. After the 3rdcleavage division, the embryo begins the process of compaction, which, in human, is only completed when the embryo consists of 8-16 cells,[7][8] then becoming known as themorula. Compaction results from increased contractility of the actomyosincortex, which pull cells together into a tighter configuration.[9][10] Increased contractility during compaction is observed in both mouse and human embryos,[11][12] but is stronger in humans, which could contribute to itsfragmentation.[13] Until this developmental stage, cells (blastomeres) were not specified to any particular cell lineage but, when reaching the 16-cell stage, cells at the surface of the embryo begin to differentiate intotrophectoderm while cells with inner position initiate their differentiation intoinner cell mass fate.[14] The morula then develops bycavitation to become the blastocyst, or in many other animals the blastula. Cell differentiation then further commits the morula's cells into two types:trophectoderm cells that surround thelumen and theinner mass of cells (the embryoblast). The inner cell mass is at the origin ofembryonic stem cells.[15] Theconceptus is then known as the blastocyst.[16]
Before cell differentiation takes place there are twotranscription factors,Oct-4 andnanog that are uniformly expressed in all cells, but both of these transcription factors are turned off in the trophoblast once it has formed.[17] The outer cells of thetrophectoderm pump sodium ions into the blastocyst, which causes water to enter throughosmosis. Water accumulation between cell-cell contacts breaks them open viahydraulic fracturing.[18] The fluid then collects into a single lumen in a process akin toOstwald ripening to form theblastocoel, which determines the first axis of symmetry of the mammalian embryo.[19] The side of the blastocyst where the inner cell mass forms is called the embryonic pole, and the opposite side is the abembryonic pole. Theblastocoel,trophectoderm, andinner cell mass are hallmarks of the blastocyst.[20]
Implantation is critical to the survival and development of the early human embryo. It establishes a connection between the mother and the early embryo which will continue through the remainder of the pregnancy. Implantation is made possible through structural changes in both the blastocyst and endometrial wall.[21] Thezona pellucida surrounding the blastocyst breaches, referred to ashatching. This removes the constraint on the physical size of the embryonic mass and exposes the outer cells of the blastocyst to the interior of the uterus. Furthermore,hormonal changes in the mother, specifically a peak inluteinizing hormone (LH), prepare the endometrium to receive and envelop the blastocyst. Theimmune system is also modulated to allow for the invasion of the foreign embryonic cells. Once bound to the extracellular matrix of the endometrium, trophoblast cells secrete enzymes and other factors to embed the blastocyst into the uterine wall. The enzymes released degrade the endometrial lining, whileautocrine growth factors such ashuman chorionic gonadotropin (hCG) andinsulin-like growth factor (IGF) allow the blastocyst to further invade the endometrium.[22]
Implantation in the uterine wall allows for the next step in embryogenesis,gastrulation, which includes the formation of the placenta from trophoblastic cells and differentiation of the inner cell mass into theamniotic sac andepiblast.
There are two types of blastomere cells:[23]
The blastocoel fluid cavity containsamino acids,growth factors, and other molecules necessary forcellular differentiation.[27]
Multiple processes control cell lineage specification in the blastocyst to produce the trophoblast, epiblast, and primitive endoderm. These processes include gene expression, cell signaling, cell-cell contact and positional relationships, andepigenetics.
Once the inner cell mass has been established within the blastocyst, it prepares for further specification into the epiblast and primitive endoderm. This process of specification known ascell fate determination is carried out in part byfibroblast growth factor (FGF) signaling which generates aMAP kinase pathway to alter cellular genomes.[28] Further segregation of blastomeres into the trophectoderm and inner cell mass are regulated by thehomeodomain protein,Cdx2. This transcription factor represses the expression ofOct4 andNanog transcription factors in thetrophoblast.[29] These genomic alterations allow for the progressive specification of both epiblast and primitive endoderm lineages at the end of the blastocyst phase of development preceding gastrulation. Much of the research conducted on these early embryonic stages is on mouse embryos and specific factors may differ between mammals.
During implantation, the trophoblast gives rise toextraembryonic membranes andcell types that will eventually form most of the fetal placenta, the specialized organ through which the embryo obtains maternal nourishment necessary for subsequent exponential growth.[30] The specification of the trophoblast is controlled by the combination of morphological cues arising from cell polarity with differential activity of signaling pathways such as Hippo and Notch, and the restriction to outer cells of lineage specifiers such as CDX2.[31]
In the mouse, primordialgerm cells are specified fromepiblast cells, a process that is accompanied by extensivegenome-wideepigenetic reprogramming.[32] Reprogramming involves globalDNA demethylation andchromatin reorganization resulting in cellulartotipotency.[32] The process ofgenome-wide demethylation involves the DNAbase excision repair pathway.[33]
Trophoblasts expressintegrin on their cell surfaces which allow for adhesion to the extracellular matrix of the uterine wall. This interaction allows for implantation and triggers furtherspecification into the three different cell types, preparing the blastocyst for gastrulation.[34]
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The level ofhuman chorionic gonadotropin (hCG) secreted by the blastocyst during implantation is the factor measured in apregnancy test. hCG can be measured in both blood and urine to determine whether a woman is pregnant. More hCG is secreted in a multiple pregnancy. Blood tests of hCG can also be used to check for abnormal pregnancies.
In vitro fertilization (IVF) is an alternative to traditionalin vivo fertilization for fertilizing an egg with sperm and implanting that embryo into a female's womb (uterus). For many years the embryo was inserted into the uterus two to three days after fertilization. However at this stage of development it is very difficult to predict which embryos will develop best, and several embryos were typically implanted. Several implanted embryos increased the likelihood of a developing fetus but also led to the development of multiple fetuses. This was a major problem and drawback for using embryos in IVF.
The use of blastocysts for human IVF has proved successful. A blastocyst is implanted five to six days after the eggs have been fertilized.[35] After five or six days it is much easier to determine which embryos will result in healthy live births. Knowing which embryos will succeed allows just one blastocyst to be implanted, cutting down dramatically on the health risk and expense of multiple births. Now that the nutrient requirements for embryonic and blastocyst development have been determined, it is much easier to give embryos the correct nutrients to sustain them into the blastocyst phase.
Embryo transfer following in vitro fertilization is a procedure in which a catheter is inserted into the vagina, guided through thecervix via ultrasound, and into the uterine cavity where the blastocysts are inserted into the womb.
Blastocysts also offer an advantage because they can be used to genetically test the cells to check for genetic problems. There are enough cells in a blastocyst that a fewtrophectoderm cells can be removed without disturbing the developing blastocyst. These cells can be tested forchromosomeaneuploidy usingpreimplantation genetic screening (PGS), or specific conditions such ascystic fibrosis, often known aspreimplantation genetic diagnosis (PGD).[36]
In anembryo transfer procedure following an initialultrasound, a speculum is used to open the walls of thevagina, and using acatheter an embryo is passed through the tube for placement into thewomb.
This article incorporates text in thepublic domain from the 20th edition ofGray's Anatomy(1918)
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