Blastocyst

The blastocyst stage occurs between 5 and 9 days after conception. During embryonic development, after fertilization (approximately 5–6 days in the human), the cells of the morula begin to undergo cell differentiation, and the morula changes into the blastocyst by pumping fluid to grow a lumen. In the uterus the zona pellucida surrounding the blastocyst breaks down, allowing it to implant into the uterine wall. Implantation marks the end of the germinal stage of embryogenesis, and the beginning of gestation.{{medical citation needed|date=July 2023}}

File:2904 Preembryonic Development-02.jpg from ovulation through implantation]]

The zygote undergoes several rounds of mitosis. After the 3rd cleavage division, the embryo begins the process of compaction, which, in human, is only completed when the embryo consists of 8-16 cells,{{cite journal | doi=10.1038/s41586-020-2759-x | title=Initiation of a conserved trophectoderm program in human, cow and mouse embryos | date=2020 | last1=Gerri | first1=Claudia | last2=McCarthy | first2=Afshan | last3=Alanis-Lobato | first3=Gregorio | last4=Demtschenko | first4=Andrej | last5=Bruneau | first5=Alexandre | last6=Loubersac | first6=Sophie | last7=Fogarty | first7=Norah M. E. | last8=Hampshire | first8=Daniel | last9=Elder | first9=Kay | last10=Snell | first10=Phil | last11=Christie | first11=Leila | last12=David | first12=Laurent | last13=Van De Velde | first13=Hilde | last14=Fouladi-Nashta | first14=Ali A. | last15=Niakan | first15=Kathy K. | journal=Nature | volume=587 | issue=7834 | pages=443–447 | pmid=32968278 | pmc=7116563 | bibcode=2020Natur.587..443G }}{{cite journal | url=https://doi.org/10.1038/s41586-024-07351-x | doi=10.1038/s41586-024-07351-x | title=Mechanics of human embryo compaction | date=2024 | last1=Firmin | first1=Julie | last2=Ecker | first2=Nicolas | last3=Rivet Danon | first3=Diane | last4=Özgüç | first4=Özge | last5=Barraud Lange | first5=Virginie | last6=Turlier | first6=Hervé | last7=Patrat | first7=Catherine | last8=Maître | first8=Jean-Léon | journal=Nature | volume=629 | issue=8012 | pages=646–651 | pmid=38693259 | bibcode=2024Natur.629..646F }} then becoming known as the morula. Compaction results from increased contractility of the actomyosin cortex, which pull cells together into a tighter configuration.{{cite journal | url=https://doi.org/10.1038/ncb3185 | doi=10.1038/ncb3185 | title=Pulsatile cell-autonomous contractility drives compaction in the mouse embryo | date=2015 | last1=Maître | first1=Jean-Léon | last2=Niwayama | first2=Ritsuya | last3=Turlier | first3=Hervé | last4=Nédélec | first4=François | last5=Hiiragi | first5=Takashi | journal=Nature Cell Biology | volume=17 | issue=7 | pages=849–855 | pmid=26075357 | url-access=subscription }}{{cite journal | url=https://doi.org/10.1038/s41586-024-07351-x | doi=10.1038/s41586-024-07351-x | title=Mechanics of human embryo compaction | date=2024 | last1=Firmin | first1=Julie | last2=Ecker | first2=Nicolas | last3=Rivet Danon | first3=Diane | last4=Özgüç | first4=Özge | last5=Barraud Lange | first5=Virginie | last6=Turlier | first6=Hervé | last7=Patrat | first7=Catherine | last8=Maître | first8=Jean-Léon | journal=Nature | volume=629 | issue=8012 | pages=646–651 | pmid=38693259 | bibcode=2024Natur.629..646F }} Increased contractility during compaction is observed in both mouse and human embryos,{{cite journal | url=https://doi.org/10.1038/ncb3185 | doi=10.1038/ncb3185 | title=Pulsatile cell-autonomous contractility drives compaction in the mouse embryo | date=2015 | last1=Maître | first1=Jean-Léon | last2=Niwayama | first2=Ritsuya | last3=Turlier | first3=Hervé | last4=Nédélec | first4=François | last5=Hiiragi | first5=Takashi | journal=Nature Cell Biology | volume=17 | issue=7 | pages=849–855 | pmid=26075357 | url-access=subscription }}{{cite journal | url=https://doi.org/10.1038/s41586-024-07351-x | doi=10.1038/s41586-024-07351-x | title=Mechanics of human embryo compaction | date=2024 | last1=Firmin | first1=Julie | last2=Ecker | first2=Nicolas | last3=Rivet Danon | first3=Diane | last4=Özgüç | first4=Özge | last5=Barraud Lange | first5=Virginie | last6=Turlier | first6=Hervé | last7=Patrat | first7=Catherine | last8=Maître | first8=Jean-Léon | journal=Nature | volume=629 | issue=8012 | pages=646–651 | pmid=38693259 | bibcode=2024Natur.629..646F }} but is stronger in humans, which could contribute to its fragmentation.{{cite journal | url=https://doi.org/10.15252/embj.2023114415 | doi=10.15252/embj.2023114415 | title=Cell fragmentation in mouse preimplantation embryos induced by ectopic activation of the polar body extrusion pathway | date=2023 | last1=Pelzer | first1=Diane | last2=De Plater | first2=Ludmilla | last3=Bradbury | first3=Peta | last4=Eichmuller | first4=Adrien | last5=Bourdais | first5=Anne | last6=Halet | first6=Guillaume | last7=Maître | first7=Jean-Léon | journal=The EMBO Journal | volume=42 | issue=17 | pages=e114415 | pmid=37427462 | pmc=10476277 }} 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 into trophectoderm while cells with inner position initiate their differentiation into inner cell mass fate.{{cite journal | doi=10.1038/s41586-020-2759-x | title=Initiation of a conserved trophectoderm program in human, cow and mouse embryos | date=2020 | last1=Gerri | first1=Claudia | last2=McCarthy | first2=Afshan | last3=Alanis-Lobato | first3=Gregorio | last4=Demtschenko | first4=Andrej | last5=Bruneau | first5=Alexandre | last6=Loubersac | first6=Sophie | last7=Fogarty | first7=Norah M. E. | last8=Hampshire | first8=Daniel | last9=Elder | first9=Kay | last10=Snell | first10=Phil | last11=Christie | first11=Leila | last12=David | first12=Laurent | last13=Van De Velde | first13=Hilde | last14=Fouladi-Nashta | first14=Ali A. | last15=Niakan | first15=Kathy K. | journal=Nature | volume=587 | issue=7834 | pages=443–447 | pmid=32968278 | pmc=7116563 | bibcode=2020Natur.587..443G }} The morula then develops by cavitation 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 the lumen and the inner mass of cells (the embryoblast). The inner cell mass is at the origin of embryonic stem cells.{{cite web |last1=Molnar |first1=Charles |last2=Gair |first2=Jane |title=24.6. Fertilization and Early Embryonic Development |url=https://opentextbc.ca/biology/chapter/24-6-fertilization-and-early-embryonic-development/ |access-date=19 October 2022 |language=en |date=14 May 2015}} The conceptus is then known as the blastocyst.{{cite journal | vauthors = Nissen SB, Perera M, Gonzalez JM, Morgani SM, Jensen MH, Sneppen K, Brickman JM, Trusina A | display-authors = 6 | title = Four simple rules that are sufficient to generate the mammalian blastocyst | journal = PLOS Biology | volume = 15 | issue = 7 | pages = e2000737 | date = July 2017 | pmid = 28700688 | pmc = 5507476 | doi = 10.1371/journal.pbio.2000737 | doi-access = free }}

Before cell differentiation takes place there are two transcription factors, Oct-4 and nanog that are uniformly expressed in all cells, but both of these transcription factors are turned off in the trophoblast once it has formed.{{cite book |last1=Schoenwolf |first1=Gary C. |title=Larsen's human embryology |date=2015 |location=Philadelphia, PA |isbn=9781455706846 |pages=35–37 |edition=Fifth}} The outer cells of the trophectoderm pump sodium ions into the blastocyst, which causes water to enter through osmosis. Water accumulation between cell-cell contacts breaks them open via hydraulic fracturing.{{cite journal | url=https://www.science.org/doi/full/10.1126/science.aaw7709 | doi=10.1126/science.aaw7709 | title=Hydraulic fracturing and active coarsening position the lumen of the mouse blastocyst | date=2019 | last1=Dumortier | first1=Julien G. | last2=Le Verge-Serandour | first2=Mathieu | last3=Tortorelli | first3=Anna Francesca | last4=Mielke | first4=Annette | last5=De Plater | first5=Ludmilla | last6=Turlier | first6=Hervé | last7=Maître | first7=Jean-Léon | journal=Science | volume=365 | issue=6452 | pages=465–468 | pmid=31371608 | bibcode=2019Sci...365..465D }} The fluid then collects into a single lumen in a process akin to Ostwald ripening to form the blastocoel, which determines the first axis of symmetry of the mammalian embryo.{{cite journal | url=https://www.science.org/doi/full/10.1126/science.aaw7709 | doi=10.1126/science.aaw7709 | title=Hydraulic fracturing and active coarsening position the lumen of the mouse blastocyst | date=2019 | last1=Dumortier | first1=Julien G. | last2=Le Verge-Serandour | first2=Mathieu | last3=Tortorelli | first3=Anna Francesca | last4=Mielke | first4=Annette | last5=De Plater | first5=Ludmilla | last6=Turlier | first6=Hervé | last7=Maître | first7=Jean-Léon | journal=Science | volume=365 | issue=6452 | pages=465–468 | pmid=31371608 | bibcode=2019Sci...365..465D }} 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 blastocoel, trophectoderm, and inner cell mass are hallmarks of the blastocyst.{{cite book | vauthors = Gilbert SF | title = Developmental Biology | edition = 6th | location = Sunderland (MA) | publisher = Sinauer Associates | date = 2000 | chapter = Early Mammalian Development | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK10052/ }}

{{Main|Implantation (embryology)}}

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.{{cite journal | vauthors = Zhang S, Lin H, Kong S, Wang S, Wang H, Wang H, Armant DR | title = Physiological and molecular determinants of embryo implantation | journal = Molecular Aspects of Medicine | volume = 34 | issue = 5 | pages = 939–80 | date = October 2013 | pmid = 23290997 | pmc = 4278353 | doi = 10.1016/j.mam.2012.12.011 }} 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), prepare the endometrium to receive and envelop the blastocyst. The immune 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, while autocrine growth factors such as human chorionic gonadotropin (hCG) and insulin-like growth factor (IGF) allow the blastocyst to further invade the endometrium.{{cite journal | vauthors = Srisuparp S, Strakova Z, Fazleabas AT | title = The role of chorionic gonadotropin (CG) in blastocyst implantation | journal = Archives of Medical Research | volume = 32 | issue = 6 | pages = 627–34 | year = 2001 | pmid = 11750740 | doi = 10.1016/S0188-4409(01)00330-7 }}

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.

There are two types of blastomere cells:{{cite book| vauthors = Gilbert SF |title=Developmental Biology |url=https://books.google.com/books?id=-e_bmgEACAAJ |date=15 July 2013 |publisher=Sinauer Associates, Incorporated |isbn=978-1-60535-173-5}}{{page needed|date=December 2013}}

• The inner cell mass, also known as the embryoblast, gives rise to 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.{{cite book | vauthors = Schoenwolf GC, Larsen WJ | title = Larsen's Human Embryology | edition = 4th | location = Philadelphia | publisher = Churchill Livingstone/Elsevier | date = 2009 }} {{page needed|date=December 2013}}
• 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.{{cite journal | vauthors = James JL, Stone PR, Chamley LW | title = Cytotrophoblast differentiation in the first trimester of pregnancy: evidence for separate progenitors of extravillous trophoblasts and syncytiotrophoblast | journal = Reproduction | volume = 130 | issue = 1 | pages = 95–103 | date = July 2005 | pmid = 15985635 | doi = 10.1530/rep.1.00723 | doi-access = free }}
• 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, 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.{{cite journal | vauthors = Vićovac L, Aplin JD | title = Epithelial-mesenchymal transition during trophoblast differentiation | journal = Acta Anatomica | volume = 156 | issue = 3 | pages = 202–16 | year = 1996 | pmid = 9124037 | doi = 10.1159/000147847 }}

The blastocoel fluid cavity contains amino acids, growth factors, and other molecules necessary for cellular differentiation.{{cite journal | vauthors = Gasperowicz M, Natale DR | title = Establishing three blastocyst lineages--then what? | journal = Biology of Reproduction | volume = 84 | issue = 4 | pages = 621–30 | date = April 2011 | pmid = 21123814 | doi = 10.1095/biolreprod.110.085209 | doi-access = free }}

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 cell fate determination is carried out in part by fibroblast growth factor (FGF) signaling which generates a MAP kinase pathway to alter cellular genomes.{{cite journal | vauthors = Yamanaka Y, Lanner F, Rossant J | title = FGF signal-dependent segregation of primitive endoderm and epiblast in the mouse blastocyst | journal = Development | volume = 137 | issue = 5 | pages = 715–24 | date = March 2010 | pmid = 20147376 | doi = 10.1242/dev.043471 | s2cid = 28481311 | doi-access = }} Further segregation of blastomeres into the trophectoderm 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 trophoblast.{{cite journal | vauthors = Strumpf D, Mao CA, Yamanaka Y, Ralston A, Chawengsaksophak K, Beck F, Rossant J | title = Cdx2 is required for correct cell fate specification and differentiation of trophectoderm in the mouse blastocyst | journal = Development | volume = 132 | issue = 9 | pages = 2093–102 | date = May 2005 | pmid = 15788452 | doi = 10.1242/dev.01801 | doi-access = free | hdl = 2381/1521 | hdl-access = free }} 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.{{cite journal | vauthors = Menchero S, Sainz de Aja J, Manzanares M | title = Our First Choice: Cellular and Genetic Underpinnings of Trophectoderm Identity and Differentiation in the Mammalian Embryo | journal = Current Topics in Developmental Biology | volume = 128 | pages = 59–80 | date = 2018 | pmid = 29477171 | doi = 10.1016/bs.ctdb.2017.10.009 | publisher = Elsevier | hdl = 20.500.12105/10490 | isbn = 978-0-12-804252-6 | hdl-access = free }} 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.{{cite journal | vauthors = Menchero S, Rollan I, Lopez-Izquierdo A, Andreu MJ, Sainz de Aja J, Kang M, Adan J, Benedito R, Rayon T, Hadjantonakis AK, Manzanares M | display-authors = 6 | title = Transitions in cell potency during early mouse development are driven by Notch | journal = eLife | volume = 8 | pages = e42930 | date = April 2019 | pmid = 30958266 | pmc = 6486152 | doi = 10.7554/eLife.42930 | doi-access = free }}

In the mouse, primordial germ cells are specified from epiblast cells, a process that is accompanied by extensive genome-wide epigenetic reprogramming.{{cite journal | vauthors = Hackett JA, Sengupta R, Zylicz JJ, Murakami K, Lee C, Down TA, Surani MA | title = Germline DNA demethylation dynamics and imprint erasure through 5-hydroxymethylcytosine | journal = Science | volume = 339 | issue = 6118 | pages = 448–52 | date = January 2013 | pmid = 23223451 | pmc = 3847602 | doi = 10.1126/science.1229277 | bibcode = 2013Sci...339..448H }} Reprogramming involves global DNA demethylation and chromatin reorganization resulting in cellular totipotency. The process of genome-wide demethylation involves the DNA base excision repair pathway.{{cite journal | vauthors = Hajkova P, Jeffries SJ, Lee C, Miller N, Jackson SP, Surani MA | title = Genome-wide reprogramming in the mouse germ line entails the base excision repair pathway | journal = Science | volume = 329 | issue = 5987 | pages = 78–82 | date = July 2010 | pmid = 20595612 | pmc = 3863715 | doi = 10.1126/science.1187945 | bibcode = 2010Sci...329...78H }}

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.{{cite journal | vauthors = Damsky CH, Librach C, Lim KH, Fitzgerald ML, McMaster MT, Janatpour M, Zhou Y, Logan SK, Fisher SJ | display-authors = 6 | title = Integrin switching regulates normal trophoblast invasion | journal = Development | volume = 120 | issue = 12 | pages = 3657–66 | date = December 1994 | doi = 10.1242/dev.120.12.3657 | pmid = 7529679 | url = http://dev.biologists.org/cgi/pmidlookup?view=long&pmid=7529679 | access-date = 2013-12-10 | archive-date = 2020-05-27 | archive-url = https://web.archive.org/web/20200527184814/https://dev.biologists.org/content/120/12/3657.long | url-status = dead | url-access = subscription }}

{{more citations needed section|date=December 2013}}

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 (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.{{cite journal | vauthors = Fong CY, Bongso A, Ng SC, Anandakumar C, Trounson A, Ratnam S | title = Ongoing normal pregnancy after transfer of zona-free blastocysts: implications for embryo transfer in the human | journal = Human Reproduction | volume = 12 | issue = 3 | pages = 557–60 | date = March 1997 | pmid = 9130759 | doi = 10.1093/humrep/12.3.557 | doi-access = free }} 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 trophectoderm cells can be removed without disturbing the developing blastocyst. These cells can be tested for chromosome aneuploidy using preimplantation genetic screening (PGS), or specific conditions such as cystic fibrosis, often known as preimplantation genetic diagnosis (PGD).{{cite journal | vauthors = Wang J, Sauer MV | title = In vitro fertilization (IVF): a review of 3 decades of clinical innovation and technological advancement | journal = Therapeutics and Clinical Risk Management | volume = 2 | issue = 4 | pages = 355–64 | date = December 2006 | pmid = 18360648 | pmc = 1936357 | doi = 10.2147/tcrm.2006.2.4.355 | doi-access = free }}

In an embryo transfer procedure following an initial ultrasound, a speculum is used to open the walls of the vagina, and using a catheter an embryo is passed through the tube for placement into the womb.

• Developmental biology

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Category:Embryology