|Blastocyst just before implantation|
|A human blastocyst, 5 days after fertilisation|
|Gives rise to||Gastrula and inner cell mass|
The blastocyst is a structure formed in the early development of mammals. In humans, its formation begins 5 days after fertilization. It is preceded by the morula. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, surrounding the inner cell mass and a fluid-filled cavity known as the blastocoele. The human blastocyst comprises 70-100 cells. This group of cells embeds itself into the endometrium of the uterine wall where it will undergo later developmental processes, including gastrulation.
The use of of blastocysts in in vitro fertilization (IVF) involves culturing a fertilized egg for five days instead of three before implanting it into the uterus. It can be more a viable treatment than traditional IVF.
The blastocyst develops from the morula, an early embryo made of 16 undifferentiated cells, through cavitation. Trophoblast cells secrete fluid inside the embryo. This forms an internal fluid-filled cavity called the blastocoel. The cells differentiate into two types, an inner cell mass (ICM) growing on the interior of the blastocele and trophoblast cells growing on the exterior. The animal pole refers to the side of the blastocyst where the ICM resides, while the vegetal pole is on the opposite side. The trophoblast cells pump sodium ions into the blastocoel, which causes water to enter through osmosis. After 24 to 48 hours, the zona pellucida breaches, referred to as hatching. This removes the constraint on the physical size of the embryonic mass and allows for implantation into the uterine wall. Once bound to the extracellular matrix of the endometrium, trophoblast cells secrete enzymes to embed the blastocyst into the uterine wall. 
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 ICM into amniotic cavity and epiblast.
The blastocyst is made up of blastomere cells and the blastocoel.
There are two types of blastomere cells:
- the inner cell mass, also known as the embryoblast, which gives rise to all later structures of the adult organism
- the trophoblast, a layer of cells forming the outer ring of the blastocyst that combine with the maternal endometrium to form the placenta. Trophoblast cells also secrete the fluids to make the blastocoel.
Multiple processes control cell lineage specification in the blastocyst to produce the trophectoderm, 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 3 different cell types, preparing the blastocyst for gastrulation.
Blastocyst in vitro fertilization
Blastocysts have played a major role in the advancement of In-vitro fertilization (IVF). 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 in 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 two or three blastocysts to be implanted, cutting down on 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 implanting through in vitro fertilization is a painless procedure in which a catheter is inserted into the vagina, guided through the service via ultrasound, and 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 cells are able to be removed without disturbing the developing blastocyst. These cells can be tested for genetic defects using immunofluorescent tags.
Blastocyst in art
In 1907 Gustav Klimt painted Danaë depicting Zeus, disguised as golden coins, impregnating Danae. On the fabric of the sheets in the painting, however there appears to be what looks like golden blastocysts. The blastocysts represent life and growth. It is hypothesized that Klimt had this very early knowledge of the blastocysts because of his association with anatomist, Emil Zuckerkandl. Zuckerlandl and Klimt frequented the same salons and Klimt was probably able to know of the microscopic cell mass because of this association.
- Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Early Mammalian Development. Available from: http://www.ncbi.nlm.nih.gov/books/NBK10052/
- Scott F. Gilbert (15 July 2013). Developmental Biology. Sinauer Associates, Incorporated. ISBN 978-1-60535-173-5.
- Gasperowicz, Malgorzata; Natale, David R.C. (2010), "Establishing Three Blastocyst Lineages—Then What?", Biology of Reproduction 84 (4): 621–630, retrieved 2013-11-13
- Yamanaka, Y.; Lanner, F.; Rossant, J. (2010), "FGF signal-dependent segregation of primitive endoderm and epiblast in the mouse blastocyst", Development 137 (5): 715–724, retrieved 2013-11-13
- Strumpf, Dan; Mao, Chai-An; Yamanaka, Yojiro; Ralston, Amy; Chawengsaksophak, Kallayanee; Beck, Felix; Rossant, Janet (2005), "Cdx2 is required for correct cell fate specification and differentiation of trophectoderm in the mouse blastocyst", Development 132: 2093–2102, retrieved 2013-11-16
- Mark Perloe, M.D.; Michael John Tucker, Ph.D (2007), Fewer Risks, New Hope: The Reality of Blastocyst Transfers, retrieved 2013-11-11
- Blastocyst transfer, retrieved 2013-11-11
- Blastocyst transfer and fertility treatment
- Risks of blastocyst transfer
- Blastocyst photos at different stages of development
- Diagram at weber.edu
- Blastocyst Differentiation Diagram