Blastocyst just before implantation
A human blastocyst, with inner cell mass at upper right
|Gives rise to||Gastrula and Inner cell mass|
The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoele. The trophoblast gives rise to the placenta. The name "blastocyst" arises from the Greek βλαστός blastos ("a sprout") and κύστις kystis ("bladder, capsule").
In humans, blastocyst formation begins about 5 days after fertilization, when a fluid-filled cavity opens up in the morula, a ball consisting of a few dozen cells. The blastocyst has a diameter of about 0.1-0.2 mm and comprises 200-300 cells following rapid cleavage (cell division). After about 1 day (5–6 days post-fertilization), which is the time usually required to reach the uterus, the blastocyst begins to embed itself into the endometrium of the uterine wall where it will undergo later developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, which prevents it from adhering to the oviduct as it makes its way to the uterus.The blastocyst is completely embedded in the endometrium only 11–12 days after fertilization.
The use of blastocysts in in-vitro fertilization (IVF) involves culturing a fertilized egg for five days before implanting it into the uterus. It can be a more viable method of fertility treatment than traditional IVF. The inner cell mass of blastocysts is also a source of embryonic stem cells.
During human embryogenesis, the blastomeres of the morula continue mitosis and compaction to form the blastula - a hollow sphere of cells surrounding a fluid filled cavity. Approximately 5–6 days after fertilization the blastomeres of the blastula begin to undergo cell differentiation and its structure changes to become the blastocyst. In the uterus the zona pellucida of the blastocyst breaks down allowing it to implant into the uterine wall approximately 6 days after fertilization. Implantation marks the end of the germinal stage of embryogenesis.
The zygote develops by mitosis, and when it has developed into 16 cells becomes known as the morula. The morula then develops by cavitation to become the blastula. Cellular differentiation then develops the blastula's cells into two types: trophoblast cells that surround the blastocoel and an inner mass of cells (the embryoblast). This is then known as the blastocyst. The side of the blastocyst where the inner cellular mass (ICM) forms is referred to as the animal pole, while the vegetal pole refers to the side opposite this. The outer layer of trophoblast cells resulting from compaction pump sodium ions into blastocyst which cause water to enter through osmosis and form the internal fluid-filled blastocyst cavity (blastocoel). This cavity in addition to cellular specification are both hallmark identities of the blastocyst.
Implantation is critical to the survival and development of the early 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. The zona pellucida surrounding the blastocyst breaches, referred to as hatching. 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 in luteinizing hormone (LH) prepares the endometrium to receive the blastocyst and envelope it. 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, while autocrine growth factors such as human chorionic gonadotropin (hCG) and insulin-like growth factor (IGF) allow the blastocyst to further invade the endometrium.
Implantation in the uterine wall allows for the next step in embryogenesis, gastrulation, which includes formation of the placenta from trophoblastic cells and differentiation of the ICM into the amniotic sac and epiblast.
The blastocyst is made up of cells from the inner cell mass and the blastocoel.
There are two types of blastomere cells:
- The inner cell mass, also known as the embryoblast, gives rise to the primitive endoderm and the epiblast.
- The trophoblast is a layer of cells forming the outer ring of the blastocyst that combines with the maternal endometrium to form the placenta. Trophoblast cells also secrete factors to make the blastocoel.
- Cytotrophoblast is the inner layer of the trophoblast, composed of stem cells which give rise to cells comprising the chorionic villi, placenta, and syncytiotrophoblast.
- Syncytiotrophoblast is the outermost layer of the trophoblast. These cells secrete proteolytic enzymes to break down the endometrial extracellular matrix to allow for implantation of the blastocyst in the uterine wall.
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, and epigenetics.
Once the ICM has been established within the blastocyst, this cell mass prepares for further specification into the epiblast and primitive endoderm. This process of specification is determined in part by fibroblast growth factor (FGF) signaling which generates a MAP kinase pathway to alter cellular genomes. Further segregation of blastomeres into the trophoblast and inner cell mass are regulated by the homeodomain protein, Cdx2. This transcription factor represses the expression of Oct4 and Nanog transcription factors in the trophectoderm. 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.
Trophoblasts express integrin on their cell surfaces which allow for adhesion to the extracellular matrix of the uterine wall. This interaction allows for implantation and also triggers further specification into the three different cell types, preparing the blastocyst for gastrulation.
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Levels of human chorionic gonadotropin secreted by the blastocyst during implantation is the factor measured in a pregnancy test. HCG can be measured in both the blood and urine to determine if 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
In vitro fertilization is an alternative to traditional in vivo fertilization for fertilizing an egg with sperm and implanting that embryo into a female’s womb. For many years the embryo was inserted into the fallopian tube 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 helped to guarantee that there would be a developing fetus but it also led to the development of multiple fetuses. This was a major problem and drawback for using embryos to IVF.
A recent breakthrough for in vitro fertilization is the use of blastocysts. A blastocyst would be implanted five to six days after the eggs had been fertilized. 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 sources for embryonic and blastocyst development has been determined, it is much easier to give embryos the correct nutrients in order to sustain them into the blastocyst phase. Blastocyst implantation through in vitro fertilization is a painless procedure in which a catheter is inserted into the vagina, guided through the cervix via ultrasound, into the uterus 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 genomic problems. There are enough cells in a blastocyst that a few trophectoderm cells are able to be removed without disturbing the developing blastocyst. These cells can be tested for chromosome aneuploidy using preimplantation genetic screening (PGS).
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