Editing Blastocoel

{{Short description|Fluid-filled or yolk-filled cavity that forms in the blastula}}
{{Infobox embryology
| Name          = Blastocoel
| Latin         =
| Image         =Blastocyst_-_2.png
| Caption       = Mammalian blastocoel
| Image2        = Diagram of Blastocyst stage.png
| Caption2      = <br>Schematic diagram showing the [[blastocyst]], with its [[inner cell mass|embryoblast (inner cell mass)]] and its [[trophoblast]] layer, alongside the surface of the [[endometrium]].
| System        =
| CarnegieStage = 3
| Days          = 5
| Precursor     = [[Morula]]
| GivesRiseTo   = [[Gastrula]], <br>[[primitive yolk sac]]
}}
The '''blastocoel''' ({{IPAc-en|ˈ|b|l|æ|s|t|ə|ˌ|s|iː|l}}), also spelled '''blastocoele''' and '''blastocele''', and also called '''cleavage cavity''', or '''segmentation cavity'''<ref name="MW2">{{cite web |title=Definition of BLASTOCOEL |url=https://www.merriam-webster.com/dictionary/blastocoel |website=Merriam-Webster |language=en}}</ref> is a fluid-filled or yolk-filled cavity that forms in the [[blastula]] during very early [[embryonic development]]. At this stage in mammals the blastula is called the [[blastocyst]], which consists of an outer [[epithelium]], the [[trophectoderm]], enveloping the [[inner cell mass]] and the blastocoel .

It develops following [[cleavage (embryo)|cleavage]] of  the [[zygote]] after [[fertilization]].<ref name="CIBA">{{cite book |last1=Biggers |first1=JD |last2=Borland |first2=RM |last3=Powers |first3=RD |chapter=Transport Mechanisms in the Preimplantation Mammalian Embryo |title=Ciba Foundation Symposium 52 - the Freezing of Mammalian Embryos |journal=Ciba Foundation Symposium |series=Novartis Foundation Symposia |date=1977 |volume=52 |issue=52 |pages=129–53 |doi=10.1002/9780470720332.ch7 |pmid=145938|isbn=9780470720332 }}</ref><ref name="Kalt">{{cite journal |last1=Kalt |first1=Marvin R. |title=The relationship between cleavage and blastocoel formation in Xenopus laevis. I. Light microscopic observations |journal=Journal of Embryology and Experimental Morphology |volume=26 |issue=1 |pages=37–49 |year=1971 |pmid=5565077 |url=http://dev.biologists.org/content/26/1/51.short }}</ref> It is the first fluid-filled cavity or [[lumen (anatomy)|lumen]] formed as the embryo enlarges,<ref name="Heasman">{{cite journal |last1=Heasman |first1=Janet |last2=Crawford |first2=Aaron |last3=Goldstone |first3=Kim |last4=Garner-Hamrick |first4=Peggy |last5=Gumbiner |first5=Barry |last6=McCrea |first6=Pierre |last7=Kintner |first7=Chris |last8=Noro |first8=Chikako Yoshida |last9=Wylie |first9=Chris |title=Overexpression of cadherins and underexpression of β-catenin inhibit dorsal mesoderm induction in early Xenopus embryos |journal=Cell |volume=79 |issue=5 |pages=791–803 |year=1994 |pmid=7528101 |doi=10.1016/0092-8674(94)90069-8 |s2cid=33403560 }}</ref> and is the essential precursor for the [[Cellular differentiation|differentiated]] [[gastrulation|gastrula]].<ref name="Gilbert">{{cite book |last1=Gilbert |first1=Scott F. |title=Developmental biology |date=2010 |publisher=Sinauer Associates |location=Sunderland, Mass. |isbn=978-0-87893-384-6 |edition=9th}}</ref>{{page needed|date=April 2016}} In the ''[[Xenopus]]'' a very small cavity has been described in the two-cell stage of development.<ref name="Wolpert1">{{cite book |last1=Wolpert |first1=Lewis |title=Principles of development |date=2015 |location=Oxford, United Kingdom |isbn=9780199678143 |page=375 |edition=Fifth}}</ref>

== In mammals==
After fertilization, the [[zygote]] undergoes several rounds of [[Cleavage (embryo)|cleavage]] divisions forming daughter cells known as [[blastomere]]s. At the 8- or 16-cell stage, the embryo undergoes [[Cleavage (embryo)#Compaction|compaction]] and forms the [[morula]]. Eventually, the [[morula]] is a solid ball of cells that has a small group of internal cells surrounded by a larger group of external cells. Then blastomeres undergo [[cellular differentiation]] with internal cells adopting the [[inner cell mass]] fate and the external layer becoming [[trophectoderm]]. The inner cell mass will go on to become the actual embryo. The external, surrounding cells develop into [[trophoblast]] cells, which only contribute to extra-embryonic tissues. At this stage there is no [[lumen (anatomy)|lumen]] within the embryo. In a process called [[Cavitation (embryology)|cavitation]], [[trophectoderm]] cells transport fluid into the embryo to create a blastocoel, the fluid-filled [[lumen (anatomy)|lumen]]. The membranes of the [[trophectoderm]] cells contain sodium (Na<sup>+</sup>) pumps, Na<sup>+</sup>/K<sup>+</sup>- ATPase and Na<sup>+</sup>/H<sup>+</sup> exchangers, that pump sodium into the embryo. The oviduct cells stimulate these trophoblast sodium pumps as the fertilized egg travels down the fallopian tube towards the uterus.<ref name="Wiley">{{cite journal |last1=Wiley |first1=Lynn M. |title=Cavitation in the mouse preimplantation embryo: and the origin of nascent blastocoele fluid |journal=Developmental Biology |volume=105 |issue=2 |pages=330–42 |year=1984 |pmid=6090240 |doi=10.1016/0012-1606(84)90290-2 }}</ref> The accumulation of sodium pulls in water through [[osmosis]].<ref name="CIBA"/> The accumulation of water breaks open cell-cell contacts via [[hydraulic fracturing]].<ref name="science.org">{{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 }}</ref> To form a single lumen, the fluid from multiple water pockets collects into a single entity in process akin to [[Ostwald ripening]].<ref name="science.org"/> The blastocoel further expands and the inner cell mass becomes positioned on one side of the trophoblast cells forming a mammalian blastula, called a [[blastocyst]]. The axis formed by the [[inner cell mass]] and the blastocoel is the first axis of symmetry of mammalian embryo and determines its attachment point to the uterus.

== In amphibians ==
An amphibian embryo in the 128- cell stage is considered a blastula as the blastocoel in the embryo becomes apparent during this stage. The fluid-filled cavity forms in the [[Polarity in embryogenesis|animal hemisphere]] of the frog. However, the early formation of the blastocoel has been traced back to the very first [[cleavage furrow]]. It was demonstrated in the frog embryo that the first cleavage furrow widens in the animal hemisphere creating a small intercellular cavity that is sealed off via tight junctions.<ref name="Kalt"/> As cleavage continues, the cavity expands to become the developed blastocoel. The blastocoel is a crucial component of amphibian embryo development. It permits cell migration during gastrulation and prevents the cells beneath the blastocoel from interacting prematurely with the cells above the blastocoel. For instance, the blastocoel prevents the vegetal cells destined to become [[endoderm]] from coming in contact with those cells in the [[ectoderm]] fated to give rise to the skin and nerves.<ref name="Nieuwkoop">{{cite journal |last1=Nieuwkoop |first1=PD |title=The organization center of the amphibian embryo: its origin, spatial organization, and morphogenetic action |journal=Advances in Morphogenesis |volume=10 |pages=1–39 |year=1973 |doi=10.1016/b978-0-12-028610-2.50005-8 |pmid=4581327 |isbn=9780120286102 }}</ref>

=== Damage to blastocoel ===
The blastocoel can be damaged and abolished if the adhesion between blastomeres, provided by cell adhesion molecules like EP-cadherin, is destroyed as mRNA by [[oligonucleotide]]s. If the mRNA is destroyed, then there’s no EP-cadherin, little to no blastomere adhesion and the blastocoel is non-existent.<ref name="Heasman"/> During the next stage of embryonic development, amphibian [[gastrulation]], the blastocoel is displaced by the formation of the [[archenteron]], during mid-gastrulation. At the end of gastrulation, the blastocoel has been obliterated.<ref name="Purcell">{{cite journal |last1=Purcell |first1=SM |last2=Keller |first2=R |title=A different type of amphibian mesoderm morphogenesis in Ceratophrys ornata. |journal=Development |date=January 1993 |volume=117 |issue=1 |pages=307–17 |doi=10.1242/dev.117.1.307 |pmid=8223254}}</ref>

== In sea urchins ==
At the 120- cell stage, the sea urchin embryo is considered a blastula because of its developed blastocoel, which every embryonic cell surrounds and touches. Every cell is in contact with the proteinaceous fluid of the blastocoel on the inside and touches the hyaline layer on the outside. The loosely connected blastomeres are now tightly connected because of [[tight junction]]s that create a seamless epithelium that completely encircles the blastocoel.<ref name="Galileo">{{cite journal |last1=Galileo |first1=Deni S. |last2=Morrill |first2=John B. |title=Patterns of cells and extracellular material of the sea urchinLytechinus variegatus (Echinodermata; Echinoidea) embryo, from hatched blastula to late gastrula |journal=Journal of Morphology |volume=185 |issue=3 |year=1985 |pages=387–402 |doi=10.1002/jmor.1051850310 |pmid=29991195 |s2cid=51615081 }}</ref> Even as the blastomeres continue to divide, the blastula remains one-cell thick and thins out as the embryo expands outward. This is accomplished in part due to the influx of water that expands the blastocoel and pushes the cells surrounding it outwards. At this point, the cells have become specified and are ciliated on the opposite side of the blastocoel. The vegetal plate and animal hemisphere develop and secrete a hatching enzyme that digests the fertilization envelope and allows the embryo to now become a free-swimming hatched blastula.<ref name="Cherr">{{cite journal |last1=Cherr |first1=GN |last2=Summers |first2=RG |last3=Baldwin |first3=JD |last4=Morrill |first4=JB |title=Preservation and visualization of the sea urchin embryo blastocoelic extracellular matrix. |journal=Microscopy Research and Technique |date=15 June 1992 |volume=22 |issue=1 |pages=11–22 |doi=10.1002/jemt.1070220104 |pmid=1617206|s2cid=32044141 }}</ref>

=== Development of primary mesenchyme ===
Important to the sea urchin blastula is the ingression of the [[primary mesenchyme]]. After the blastula hatches from the fertilization envelope, the [[Vegetal pole|vegetal]] side of the blastula begins to flatten and thicken as a small cluster of these cells develop long, thin processes called [[filopodia]]. These cells then dissociate and ingress into the blastocoel and are called the primary mesenchyme. The cells move randomly along the inside of the blastocoel, until they become localized in the ventrolateral region of the blastocoel.<ref name="Cherr"/><ref name="Galileo"/>

== In birds ==
Similar to mammals, fertilization of the avian ovum occurs in the oviduct. From there the [[blastodisc]], a small cluster of cells in the animal pole of the egg, then undergoes [[Cleavage (embryo)|discoidal]] [[meroblastic]] cleavage. The blastoderm develops into the epiblast and hypoblast and it is between these layers that the blastocoel will form. The shape and formation of the avian blastodisc differs from amphibian, fish, and echinoderm blastulas, but the overall spatial relationship of the blastocoel remains the same.<ref name="Gilbert"/>{{page needed|date=April 2016}}

=== Formation of primitive streak ===
The avian blastocoel is important during the development of the primitive streak. The ingression of the endodermal precursor cells form the epiblast into the blastocoel and the migration of lateral cells of the posterior epiblast towards the center form the early [[primitive streak]]. As these cells converge inward, a depression forms called the [[primitive groove]] and functions as an opening through which cells travel into the blastocoel. As cells migrate into the blastocoel, they undergo an [[Epithelial–mesenchymal transition|epithelial-to-mesenchymal transformation]].<ref name="Gilbert"/>{{page needed|date=April 2016}}

== In zebrafish ==
Unlike amphibian, echinoderm, mammalian, and avian embryos, [[zebrafish]] do not have a defined blastocoel. Rather, they have small, irregular extracellular spaces that are formed between the cells of the [[blastodisc]] sitting atop the yolk.<ref name="Kimmel">{{cite journal |last1=Kimmel |first1=Charles B. |last2=Ballard |first2=William W. |last3=Kimmel |first3=Seth R. |last4=Ullmann |first4=Bonnie |last5=Schilling |first5=Thomas F. |title=Stages of embryonic development of the zebrafish |journal=Developmental Dynamics |volume=203 |issue=3 |pages=253–310 |year=1995 |pmid=8589427 |doi=10.1002/aja.1002030302 |s2cid=19327966 |doi-access= }}</ref>

==References==
{{reflist}}

==Further reading==
*Dorlands Staff (2004). "blastocoel [distionary entry]". ''Dorland's Illustrated Medical Dictionary'' (online). Amsterdam, NDE: Elsevier-Saunders. Retrieved 30 January 2016. "blastocoel...[blaso- + -coele] the fluid-filled cavity of the mass of cells (blastula) produced by cleavage of fertilized ovum. Sometimes spelled...[c]alled...'Also' blastocoelic ...pertaining to the blastocoele."; [https://web.archive.org/web/20160624025353/http://www.dorlands.com/wsearch.jsp Dorlands.com]
*{{cite book |last1=Gilbert |first1=Scott F |year=2000 |chapter=Early Mammalian Development |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10052/ |title=Developmental Biology |edition=6th |location=Sunderland |publisher=Sinauer Associates |isbn=0-87893-243-7 |url=https://archive.org/details/developmentalbio00gilb }}
*{{cite book |last1=Gilbert |first1=Scott F |year=2000 |chapter=Early Amphibian Development |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10113/ |title=Developmental Biology |edition=6th |location=Sunderland |publisher=Sinauer Associates |isbn=0-87893-243-7 |url=https://archive.org/details/developmentalbio00gilb }}
*{{cite book |last1=Gilbert |first1=Scott F |year=2000 |chapter=The Early Development of Sea Urchins |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK9987/ |title=Developmental Biology |edition=6th |location=Sunderland |publisher=Sinauer Associates |isbn=0-87893-243-7 |url=https://archive.org/details/developmentalbio00gilb }}

{{Embryology}}
{{Extraembryonic and fetal membranes}}

[[Category:Animal developmental biology]]