Blastocyst

Blastocyst
Gastrula
Identifiers
Latin	blastocystis
MeSH	D001755
TE	E2.0.1.2.0.0.12
FMA	83041
Anatomical terminology [edit on Wikidata ]

The blastocyst is a structure formed in the early

The name "blastocyst" arises from the Greek βλαστός blastós ("a sprout") and κύστις kýstis ("bladder, capsule").

In non-mammalian animals, this is a structure consisting of an undifferentiated ball of cells and is called a

.

In humans, blastocyst formation begins about five days after

. The blastocyst has a diameter of about 0.1–0.2 makes its way to the uterus.

The use of blastocysts in

may also be used in IVF, and other fertility treatments.

Development cycle

The blastocyst stage occurs between 5 and 9 days after conception. During

]

Blastocyst formation

human embryo from ovulation through implantation

The

Before cell differentiation takes place there are two transcription factors, Oct-4 and nanog that are uniformly expressed on all of the cells, but both of these transcription factors are turned off in the trophoblast once it has formed.^[9] The side of the blastocyst where the inner cell mass forms is called the embryonic pole, and the opposite side is the abembryonic pole. The outer layer of trophoblast cells, resulting from compaction, pumps sodium ions into the blastocyst, which causes water to enter through osmosis and form the internal fluid-filled blastocyst cavity (blastocoel). The blastocoel, trophoblast cells, and inner cell mass are hallmarks of the blastocyst.^[10]

Implantation

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 the amniotic sac and epiblast.

Structure

There are two types of blastomere cells:^[13]

• The inner cell mass, also known as the
  primitive endoderm
  and the embryo proper (epiblast).
  • The primitive endoderm develops into the amniotic sac which forms the fluid-filled cavity that the embryo resides in during pregnancy.^[14]
  • The epiblast gives rise to the three
    germ layers of the developing embryo during gastrulation (endoderm, mesoderm, and ectoderm
    ).
• 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.[15]
  • After implantation, cytotrophoblast is the inner layer of the trophoblast, composed of stem cells which give rise to cells comprising the chorionic villi, placenta, and syncytiotrophoblast.
  • After implantation,
    proteolytic enzymes to break down the endometrial extracellular matrix to allow for implantation of the blastocyst in the uterine wall.^[16]

The blastocoel fluid cavity contains

Cell specification

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 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 as

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 to extraembryonic membranes and cell 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.^[20] 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.^[21]

In the mouse, primordial germ cells are specified from epiblast cells, a process that is accompanied by extensive genome-wide epigenetic reprogramming.^[22] Reprogramming involves global DNA demethylation and chromatin reorganization resulting in cellular totipotency.^[22] The process of genome-wide demethylation involves the DNA base excision repair pathway.^[23]

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 triggers further specification into the three different cell types, preparing the blastocyst for gastrulation.^[24]

Clinical implications

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Pregnancy tests

The level of human chorionic gonadotropin (hCG) secreted by the blastocyst during implantation is the factor measured in a pregnancy 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 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 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.^[25] 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 the cervix 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 few

Embryo transfer process

In an

womb

.

See also

• Developmental biology

References

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

External links