Blastulation: Difference between revisions

{{Use dmy dates|date=JuneApril 20132023}}

| Name = BlastulaBlastulation

| Caption = Blastulation: from '''1'''. - [[morula,]] to '''2''' -. blastula.

[[File:Blastula (PSF).jpg|thumb|A. [[BlastocoelMorula]] and B. cross section of a blastula displaying the [[blastocoel]] and [[blastoderm]] of early animal [[embryonic development]]]]

'''Blastulation''' is the stage in early [[animal]] [[embryonic development]] that produces the '''blastula'''.<ref name="web-books">{{citeIn webmammalian |title=Blastulationdevelopment, |url=https://www.web-books.com/MoBio/Development/04Mblastulation.htmlthe |website=wwwblastula develops into the [[blastocyst]] with a differentiated [[inner cell mass]] and an outer [[trophectoderm]].web-books.com}}</ref> The blastula (from Greek ''[[wikt:βλαστός#Greek|βλαστός]]'' ({{transl|grc|blastos}} meaning ''sprout'')) is a hollow sphere of [[cell (biology)|cell]]s (known as [[blastomere]]s) surrounding an inner fluid-filled cavity (called the [[blastocoel]]).<ref name="web-books">{{cite web |title=Blastulation |url=https://www.web-books.com/MoBio/Development/04Mblastulation.html |website=web-books.com}}</ref><ref>{{cite encyclopedia |year= 2013 |encyclopedia=Encyclopædia Britannica|title=Blastula|url=http://www.britannica.com/EBchecked/topic/69108/blastula}}</ref> Embryonic development begins with a [[sperm]] [[fertilizing]] an [[egg cell]] to become a [[zygote]], which undergoes many [[cleavage (embryo)|cleavages]] to develop into a ball of cells called a ''[[morula]]''. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the [[gastrula]] in which the [[germ layer]]s of the embryo form.<ref name=Gilbert>{{cite book|last=Gilbert|first=Scott|title=Developmental Biology 9th Ed + Devbio Labortatory Vade Mecum3.|year=2010|publisher=Sinauer Associates Inc.|isbn=978-0-87893-558-1|pages=243–247, 161|url=http://ebooks.sinauer.com/gilbert9e/text.php}}{{Dead link|date=November 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

A common feature of a [[vertebrate]] blastula is that it consists of a layer of blastomeres, known as the [[blastoderm]], which surrounds the blastocoel.<ref>{{cite book|author=Lombardi, Julian|chapter=Embryogenesis|title=Comparative vertebrate reproduction|publisher=Springer|year=1998|isbn=978-0-7923-8336-9|page=226|chapter-url=https://books.google.com/books?id=cqQX9RMPAegC&pg=PA226}}</ref><ref>Forgács & Newman, 2005: [https://books.google.com/books?id=rUyVWQhk7CkC&pg=PA27 p. 27]</ref> In [[mammal]]s, the blastula is referred to as a [[blastocyst]]. The blastocyst contains an [[embryoblast]] (or inner cell mass) that will eventually give rise to the definitive structures of the [[fetus]], and a [[trophoblast]] which goes on to form the extra-embryonic tissues.<ref name=Gilbert /><ref name=Cockburn>{{cite journal|last=Cockburn|first=Katie|author2=Rossant, Janet|title=Making the blastocyst: lessons from the mouse|journal=Journal of Clinical Investigation|date=1 April 2010|volume=120|issue=4|pages=995–1003|doi=10.1172/JCI41229|pmid=20364097|pmc=2846056}}</ref>

During blastulation, a significant amount of activity occurs within the early embryo to establish [[cell polarity]], [[cell fate determination|cell specification]], [[regional specification|axis formation]], and to regulate [[gene expression]].<ref name=Heasman /> In many animals, such as ''[[Drosophila]]'' and ''[[Xenopus]]'', the [[midblastula|mid blastula transition]] (MBT) is a crucial step in development during which the [[maternal effect|maternal mRNA]] is degraded and control over development is passed to the embryo.<ref name="Tadros 2005">{{cite journal|last=Tadros|first=Wael|author2=Lipshitz, Howard D.|title=Setting the stage for development: mRNA translation and stability during oocyte maturation and egg activation in ''Drosophila'' |journal=Developmental Dynamics|date=1 March 2004|volume=232|issue=3|pages=593–608|doi=10.1002/dvdy.20297|pmid=15704150|doi-access=free}}</ref> Many of the interactions between blastomeres are dependent on [[cadherin]] expression, particularly [[CDH1 (gene)|E-cadherin]] in mammals and EP-cadherin in [[amphibian]]s.<ref name=Heasman>{{cite journal|last=Heasman|first=J|title=Patterning the ''Xenopus'' blastula |journal=Development|date=November 1997|volume=124|issue=21|pages=4179–91|doi=10.1242/dev.124.21.4179|pmid=9334267|url=http://dev.biologists.org/content/124/21/4179.long}}</ref>

The study of the blastula, and of cell specification has many implications in [[embryonicStem stem cellscell#Research|stem cell]] research]], and [[assisted reproductive technology]].<ref name=Cockburn /> In ''Xenopus'', blastomeres behave as [[Cell potency|pluripotent]] stem cells which can migrate down several pathways, depending on [[cell signaling]].<ref name=Gurdon>{{cite journal|last=Gourdon|first=John B.|author2=Standley, Henrietta J.|title=Uncommitted ''Xenopus'' blastula cells can be directed to uniform muscle gene expression by gradient interpretation and a community effect|journal=The International Journal of Developmental Biology (Cambridge, UK)|volume=46|issue=8|pages=993–8|pmid=12533022|url=http://www.ijdb.ehu.es/web/paper.php?doi=12533022|date=December 2002}}</ref> By manipulating the cell signals during the blastula stage of development, various [[Tissue (biology)|tissue]]s can be formed. This potential can be instrumental in [[regenerative medicine]] for disease and injury cases. [[In vitro fertilisation]] involves implantationthe [[embryo transfer|transfer of aan blastulaembryo]] into a mother's [[uterus]] for [[Implantation (embryology)|implantation]].<ref name=Toth>{{cite web|last=Toth|first=Attila|title=Treatment: Addressing the Causes of Infertility in Men and Women|url=http://www.fertilitysolution.com/Infertility/Treatment.html|publisher=Macleod Laboratory|access-date=22 March 2013}}</ref> Blastula cell implantation could serve to eliminate [[infertility]].

The blastula stage of early embryo development begins with the appearance of the blastocoel. The origin of the blastocoel in ''Xenopus'' has been shown to be from the first [[cleavage furrow]], which is widened and sealed with [[tight junctions]] to [[Cavitation (embryology)|create a cavity]].<ref name="Kalt">{{cite journal|last=Kalt|first=MR|title=The relationship between cleavage and blastocoel formation in ''[[Xenopus laevis]]''. I. Light microscopic observations.|journal=Journal of Embryology and Experimental Morphology|date=August 1971|volume=26|issue=1|pages=37–49|pmid=5565077}}</ref>

In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore exclusively from the mother.<ref>{{cite journal|last=Tadros|first=W|author2=Lipshitz, HD|title=Setting the stage for development: mRNA translation and stability during oocyte maturation and egg activation in ''Drosophila'' |journal=Developmental Dynamics|date=March 2005|volume=232|issue=3|pages=593–608|pmid=15704150|doi=10.1002/dvdy.20297|doi-access=free}}</ref><ref name=Etkin>{{cite journalbook|last=Etkin|first=LD|titlechapter=Regulation of the midMid-blastulaBlastula transitionTransition in amphibians.Amphibians |journaltitle=The Molecular Biology of Cell Determination and Cell Differentiation|veditors= Browder LW |series= Developmental Biology |place=New York|orig-date=1985|year=1988|volume=5|pages=209–25|pmid=3077975|doi=10.1007/978-1-4615-6817-9_7|isbn=978-1-4615-6819-3 }}</ref>

The mid-blastula transition is also characterized by a marked increase in [[transcription (genetics)|transcription]] of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point, either by proteins such as [[Smaug_Smaug (protein)|SMAUG]] in ''Drosophila''<ref>{{cite journal|last=Tadros|first=W|author2=Westwood, JT |author3=Lipshitz, HD |title=The mother-to-child transition.|journal=Developmental Cell|date=June 2007|volume=12|issue=6|pages=847–9|pmid=17543857|doi=10.1016/j.devcel.2007.05.009|doi-access=free}}</ref> or by [[microRNA]].<ref>{{cite journal|last=Weigel|first=D|author2=Izaurralde, E|title=A tiny helper lightens the maternal load.|journal=Cell|date=24 March 2006|volume=124|issue=6|pages=1117–8|pmid=16564001|doi=10.1016/j.cell.2006.03.005|doi-access=free}}</ref> These two processes shift the control of the embryo from the maternal mRNA to the nuclei.

A blastula ([[blastocyst]] in [[mammal]]s), is a sphere of cells surrounding a fluid-filled cavity called the [[blastocoel]]. The blastocoel is a fluid filled cavity which contains [[amino acid]]s, [[protein]]s, [[growth factor]]s, sugars, ions and other components which are necessary for [[cellular differentiation]]. The blastocoel also allows blastomeres to move during the process of [[gastrulation]].<ref name=Fleming>{{cite journal|last=Fleming|first=Tom P.|author2=Papenbrock, Tom |author3=Fesenko, Irina |author4=Hausen, Peter |author5= Sheth, Bhavwanti |title=Assembly of tight junctions during early vertebrate development|journal=Seminars in Cell & Developmental Biology|date=1 August 2000|volume=11|issue=4|pages=291–299|doi=10.1006/scdb.2000.0179|pmid=10966863}}</ref>

In the mammalian blastocyst (term for mammalian blastula) there are three lineages that give rise to later tissue development. The [[epiblast]] gives rise to the fetus itself while the trophoblast develops into part of the [[placenta]] and the primitive endoderm becomes the [[yolk sac]].<ref name=Cockburn /> In the mouse embryo, blastocoel formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient which is the result of [[sodium–potassium pump]]s that produce a high sodium gradient on the basolateral side of the trophectoderm. This movement of water is facilitated by [[aquaporin]]s. A seal is created by tight junctions of the [[epithelium|epithelial cells]] that line the blastocoel.<ref name=Cockburn />

Mammalian embryos undergo compaction around the 8-cell stage where [[CDH1 (gene)|E-cadherins]] as well as [[alpha -catenin|alpha]] and [[beta -catenin|beta]] [[catenin]]s are expressed. This process makes a ball of embryonic cells which are capable of interacting, rather than a group of diffuse and undifferentiated cells. E-cadherin adhesion defines the [[epithelial polarity|apico-basal axis]] in the developing embryo and turns the embryo from an indistinct ball of cells to a more polarized [[phenotype]] which sets the stage for further development into a fully formed blastocyst.<ref name=Fleming />

Experiments with implantation in mice show that [[Ovulation induction|hormonal induction]], [[Controlled ovarian hyperstimulation|superovulation]] and [[artificial insemination]] successfully produce preimplantation micemouse embryos. In the mice, ninety percent of the females were induced by mechanical stimulation to undergo pregnancy and implant at least one embryo.<ref name= Watson>{{cite journal|last=Watson|first=J.G.|title=Collection and Transfer of Preimplantation Mouse Embryos|journal=Biology of Reproduction|volume=17|issue=3|pmid=901897 |date=October 1977|pages=453–8|doi=10.1095/biolreprod17.3.453|doi-access=free}}<!--|accessdate= 1 April 2013--></ref> These results prove to be encouraging because they provide a basis for potential implantation in other mammalian species, such as humans.

Blastula-stage cells can behave as pluripotent stem cells in many species. Pluripotent stem cells are the starting point to produce organ specific cells that can potentially aid in repair and prevention of injury and degeneration. Combining the expression of [[transcription factor]]s and locational positioning of the blastula cells can lead to the development of induced functional organs and tissues. Pluripotent ''Xenopus'' cells, when used in an in vivo strategy, were able to form into functional [[retina]]s. By transplanting them to the eye field on the [[neural plate]], and by inducing several mis-expressions of transcription factors, the cells were committed to the retinal lineage and could guide vision based behavior in the ''Xenopus''.<ref name=Viczian>{{cite journal|last=Viczian|first=Andrea S.|author2=Solessio, Eduardo C. |author3=Lyou, Yung |author4= Zuber, Michael E. |title=Generation of Functional Eyes from Pluripotent Cells|journal=PLoSPLOS Biology|volume=7|issue=8|pmid=19688031 |doi=10.1371/journal.pbio.1000174|date=August 2009|pages=e1000174 |pmc=2716519 |doi-access=free }}</ref>

==AlsoSee seealso==

{{reflistReflist|30em}}

* {{cite book|author=Forgács, G.|author2=Newman, Stuart A.|name-list-style=amp|chapter=Cleavage and blastula formation|title=Biological physics of the developing embryo|publisher=Cambridge University Press|year=2005|isbn=978-0-521-78337-8|chapter-url=https://books.google.com/books?id=rUyVWQhk7CkC&pg=PA24}}

* {{cite book|author=Cullen, K.E.|chapter=embryology and early animal development|title=Encyclopedia of life science, Volume 2|publisher=Infobase|year=2009|isbn=978-0-8160-7008-4|chapter-url=https://books.google.com/books?id=iM_O62qBSQYC&pg=PA283}}

* {{cite book|editor=McGeady, Thomas A.|chapter=Gastrulation|title=Veterinary embryology|publisher=Wiley-Blackwell|year=2006|isbn=978-1-4051-1147-8|chapter-url=https://books.google.com/books?id=n4C0TUeR7mUC&pg=PA34}}