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For the non-species specific developmental stage, see Blastula. For the single-celled parasite, see Blastocystis.
Blastocyst
Blastocyst just before implantation
A human blastocyst, 5 days after fertilization
Details
Carnegie stage3
Days5-9
PrecursorMorula
Gives rise toGastrula and inner cell mass
Identifiers
LatinBlastocystis
Anatomical terminology

The blastocyst is a structure formed in the early development of mammals. In humans, its formation begins 5 days after fertilization during the germinal stage of development. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of cells of the blastocyst are called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the placenta.

The human blastocyst comprises 200-300 cells following rapid cleavage throughout this stage. 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 blastocysts in in-vitro fertilization (IVF) involves culturing a fertilized egg for five days before implanting it into the uterus. It can be more a viable method of fertility treatment than traditional IVF.

Development cycle

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During human embryogenesis, the blastocyst arises from the morula in the uterus, 5 days after fertilization. The early embryo undergoes cell differentiation and structural changes to become the blastocyst. It is then prepared for implantation into the uterine wall 6 days after fertilization. Implantation marks the end of the germinal stage and the beginning of the embryonic stage of development.[1]

Early development of the embryo from ovulation through implantation in humans. The blastocyst stage occurs between 5 and 8-9 days following conception.

Blastocyst formation

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The morula, which precedes the blastocyst, is an early embryo comprised of 16 undifferentiated cells. Shortly following the morula's entry into the uterus from the Fallopian tube, the morula becomes the blastocyst through cellular differentiation and cavitation. The morula's cells differentiate into two types: an inner cell mass growing on the interior of the blastocoel and trophoblast cells growing on the exterior.[2] The animal pole refers to the side of the blastocyst where the ICM resides, while the vegetal pole is on the opposite side. Cavitation is the process by which the outer trophoblast cells secrete fluid inside the embryo. The trophoblast cells pump sodium ions into the center of the embryo, which causes water to enter through osmosis. This forms an internal fluid-filled cavity called the blastocoel. This distinguishable blastocoel cavity in addition to cellular specification are both hallmark identities of the blastocyst.[3]

Implantation

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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.[4] 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.[5]

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.

Structure

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The blastocyst is made up of blastomere cells and the blastocoel.

There are two types of blastomere cells:[6]

  • The inner cell mass, also known as the embryoblast, gives rise to the primitive endoderm and the epiblast.
    • The primitive endoderm develops into the amniotic sac which forms the fluid-filled cavity that the embryo resides in during pregnancy.[7]
    • The epiblast gives rise to the 3 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.[8]

The blastocoel fluid cavity contains amino acids, growth factors, and other necessary molecules for cellular differentiation.[10]

Cell specification

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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.[11] 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.[12] 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.[13]

Clinical implications

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Understanding of blastocyst development has important consequences in medicine and family planning, namely in pregnancy testing and in-vitro fertilization.

Pregnancy test

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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.[14]

Blastocyst in vitro fertilization

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Blastocysts have played a major role in the advancement of 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 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.[15] 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 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 cells are able to be removed without disturbing the developing blastocyst. These cells can be tested for genetic defects using immunofluorescence.[16][17]

Blastocyst in art

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Gustav Klimt's Danaë in 1907

Gustav Klimt, an artist of the early 20th Century, often depicted romantic and sexual scenes in his artwork and paintings. In 1907, Klimt undertook the task to portray the Zeus' impregnation of Danae. In the myth of Danae, daughter of the King Acrisius of Argos, it was prophesized that Acrisius would be killed by his own grandson. In order to prevent his death, the king secluded his daughter so that she would never have any children. Zeus, however tricks Danae, and impregnates her in the form of a golden shower. The son that resulted from this impregnation, Perseus, eventually does inadvertently kill Acrisius by hitting him in the head with a discus.[18] In his masterpiece, Klimt depicts Zues as the sheets of gold and string and with a black rectangle representing Zeus’ phallic structure.

Particularly intriguing in Klimt’s Danae is the shawl that enshrouds Danae. On this shawl, there are what art historians for years referred to as little golden ovaloids.[19] However, upon further investigation these ovaliods have a striking similarity to the structure of blastocysts. This depiction carries along the theme of fertility and development associated with the Greek myth. Klimt’s portrayal of Danae curled up in the fetal position also points towards human development in the womb. The themes of sexuality, fertilization, and development are all support that Klimt intentionally meant for the ovaloids to be blastocysts. However, when Klimt painted Danae in 1907, blastocysts were not common knowledge among the public. Blastocyst had not even been looked at until August Rauber and Rudolf Leuckart described them as a diagnostic stage of mammalian development in the 1880s.[20] Rauber was a leading biologist of the time and made great leaps in developmental biology by combining comparative biology and molecular pathways of early growth, development, and differentiation.[21] Klimt most likely received his knowledge of the blastocyst from frequenting salons. Emil Zuckerkandl, a respected anatomist from Vienna, frequently gave lectures at the Zuckerkandl salon concerning his research. These presentations often included slide shows of his findings. Emil’s wife Berta Zuckerkandl, was the driving force behind having respected artists come to the salon along with the more typical scientists, politicians, and philosophers. This mix of professions allowed the opportunity for Klimt to see groundbreaking biological discoveries such as the blastocyst. It also would have provided an arena for Klimt to ask questions not only to Zuckerkandl, but also to the leading embryologist in Vienna, Hans Leo Przibram.

Klimt thus successfully used cutting edge biological knowledge to represent the successful impregnation of Danae. This tactic was also very important because Klimt’s paintings had been under scrutiny, because his recent paintings were too explicit. This move towards using a new discovery allowed him thus to get his point across subtly so that later generations would be able to behold his masterpiece.

See also

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References

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  1. ^ Sherk, Stephanie Dionne (2006), "Prenatal Development", Gale Encyclopedia of Children's Health, retrieved 2013-12-07
  2. ^ Clinic, Mayo (2012), "Fetal development: The first trimester", Mayo Foundation for Medical Education, retrieved 2013-12-07
  3. ^ Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Early Mammalian Development. Available from: http://www.ncbi.nlm.nih.gov/books/NBK10052/
  4. ^ Shang, Shuang (2013), "Physiological and molecular determinants of embryo implantation", Molecular Aspects of Medicine, 34 (5): 939–980, doi:10.1016/j.mam.2012.12.011, PMC 4278353, PMID 23290997
  5. ^ Srisuparp S, Strakova Z, Fazleabas AT (2001). "The role of chorionic gonadotropin (CG) in blastocyst implantation". Arch Med Res. 32 (6): 627–34. doi:10.1016/s0188-4409(01)00330-7. PMID 11750740.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ Scott F. Gilbert (15 July 2013). Developmental Biology. Sinauer Associates, Incorporated. ISBN 978-1-60535-173-5.
  7. ^ Schoenwolf, Gary C., and William J. Larsen. Larsen's Human Embryology. 4th ed. Philadelphia: Churchill Livingstone/Elsevier, 2009. Print.
  8. ^ James JL, Stone PR, Chamley LW (2005). "Cytotrophoblast differentiation in the first trimester of pregnancy: evidence for separate progenitors of extravillous trophoblasts and syncytiotrophoblast". Reproduction. 130 (1): 95–103. doi:10.1530/rep.1.00723. PMID 15985635.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ^ Vićovac L, Aplin JD (1996). "Epithelial-mesenchymal transition during trophoblast differentiation". Acta Anat (Basel). 156 (3): 202–16. doi:10.1159/000147847. PMID 9124037.
  10. ^ Gasperowicz, Malgorzata; Natale, David R.C. (2010), "Establishing Three Blastocyst Lineages—Then What?", Biology of Reproduction, 84 (4): 621–630, doi:10.1095/biolreprod.110.085209, PMID 21123814, S2CID 25300129, retrieved 2013-11-13
  11. ^ 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, doi:10.1242/dev.043471, PMID 20147376, S2CID 28481311, retrieved 2013-11-13
  12. ^ 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 (9): 2093–2102, doi:10.1242/dev.01801, PMID 15788452, S2CID 188191, retrieved 2013-11-16
  13. ^ Damsky CH, Librach C, Lim KH, Fitzgerald ML, McMaster MT, Janatpour M; et al. (1994). "Integrin switching regulates normal trophoblast invasion". Development. 120 (12): 3657–66. doi:10.1242/dev.120.12.3657. PMID 7529679. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  14. ^ "Human Chorionic Gonadotropin (hCG)", WebMD, 2010, retrieved 2013-12-07
  15. ^ Fong CY, Bongso A, Ng SC, Anandakumar C, Trounson A, Ratnam S (1997). "Ongoing normal pregnancy after transfer of zona-free blastocysts: implications for embryo transfer in the human". Hum Reprod. 12 (3): 557–60. doi:10.1093/humrep/12.3.557. PMID 9130759.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ Mark Perloe, M.D.; Michael John Tucker, Ph.D (2007), Fewer Risks, New Hope: The Reality of Blastocyst Transfers, retrieved 2013-11-11
  17. ^ Blastocyst transfer, retrieved 2013-11-11
  18. ^ http://edweb.sdsu.edu/people/bdodge/scaffold/gg/perseus.html, retrieved 2013-12-07 {{citation}}: Missing or empty |title= (help)
  19. ^ Maxmen, Amy (2010), "Gustav Klimt's mysterious embryos", New Scientist, retrieved 2013-12-07
  20. ^ Gilbert, Scott F.; Brauckmann, Sabine (2011), "http://www.mitpressjournals.org/doi/abs/10.1162/LEON_a_00166", Leonardo, 44 (3): 221–227, doi:10.1162/LEON_a_00166, S2CID 15216777, retrieved 2013-12-07 {{citation}}: External link in |title= (help)
  21. ^ Brauckmann, Sabine (2006). "August Rauber (1841-1917): From the primitive streak to Cellularmechanik". The International Journal of Developmental Biology. 50 (5): 439–49. doi:10.1387/ijdb.052127sb. PMID 16586344.

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


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Category:Embryology

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