Embryogenesis
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2008-07-16T18:51:41Z
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{{otheruses4|embryogenesis of all organisms|human embryogenesis|Human embryogenesis}}
[[Image:Cell differentiation.gif|right|thumb|400px|Embryonic cells differentiate into a variety of different cell types. Image from [http://www.ncbi.nih.gov/About/primer/genetics_cell.html NCBI].]]'''Embryogenesis''' is the process by which the [[embryo]] is formed and develops. It starts with the [[fertilization]] of the [[ovum]], egg, which, after fertilization, is then called a [[zygote]]. The zygote undergoes rapid [[mitosis|mitotic]] [[cell division|division]]s, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as [[cleavage (embryo)|cleavage]]) and [[cellular differentiation]], leading to development of an embryo. It occurs in both animal and [[plant embryogenesis|plant development]], but this article addresses the common features among different [[animal]]s.
==The zygote==
The egg cell (and hence the fertilized egg) is always asymmetric, having an "'''animal pole'''" (future [[ectoderm]] and [[mesoderm]]), two of three primitive tissue types, and a "'''vegetal pole'''" (future [[endoderm]]), it is also covered with different protective envelopes. The first envelope, the one which is in contact with the [[cell membrane|membrane]] of the egg, is made of [[glycoproteins]] and is called '''vitelline membrane''' ('''[[zona pellucida]]''' in [[mammals]]). Different [[taxa]] show different cellular and acellular envelopes outside of the vitelline membrane.
==Cleavage==
{{main|Cleavage (embryo)}}
[[Image:Gray9.png|thumb|300px|Cleavage during embryogenesis]]
The zygote undergoes rapid [[Cleavage (embryo)|cell divisions]] with no significant growth, producing a cluster of cells that is the same size as the original zygote. The different cells derived from cleavage, up to the blastula stage, are called '''[[blastomere]]s'''. Depending mostly on the amount of [[yolk]] in the egg, the [[cleavage (embryo)|cleavage]] can be '''holoblastic''' (total) or '''meroblastic''' (partial){{ref|Campbell}}.
Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via [[placenta]] or [[milk]]. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk; i.e. birds and reptiles. Because cleavage is impeded in the vegetal pole, there is a very uneven distribution and size of cells, being more and bigger at the animal pole of the zygote{{ref|Campbell}}.
In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:
{| align="center" class="toccolours" border="1" style="border:1px solid gray; border-collapse:collapse;"
|+'''Cleavage patterns followed by holoblastic and meroblastic eggs'''
|-
!Holoblastic
!Meroblastic
|- valign="top"
|
*Radial ([[sea urchin]], [[amphioxus]])
*Bilateral ([[tunicate]]s, [[amphibian]]s)
*Spiral ([[annelid]]s, [[mollusk]]s)
*Rotational ([[mammal]]s)
|
*Discoidal ([[fish]], [[bird]]s, [[reptile]]s)
*Superficial ([[insect]]s)
|- valign="top"
|}
==Blastula and Gastrula==
{{main|Gastrulation}}
After the cleavage has produced over 100 [[Cell (biology)|cell]]s, the embryo is called a '''[[blastula]]'''{{ref|Campbell}}. The blastula is usually a spherical layer of cells (the '''blastoderm''') surrounding a fluid-filled or yolk-filled cavity (the '''blastocoel''').
Mammals at this stage form a structure called the '''[[blastocyst]]'''{{ref|NCBI}}, characterized by an [[inner cell mass]] that is not present in the blastula. The blastocyst must not be confused with the blastula; even though they are similar in structure their cells have different fates.
During '''[[gastrulation]]''' cells migrate to the interior of the blastula, consequently forming two (in [[diploblastic]] animals) or three ([[triploblastic]]) [[germ layer]]s. The embryo during this process is called a '''[[gastrula]]'''. The germ layers are referred to as the [[ectoderm]], [[mesoderm]] and [[endoderm]]. In [[diploblastic]] animals only the ectoderm and the endoderm are present {{ref|Campbell}}.
*Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
**[[Epiboly]] - expansion of one cell sheet over other cells{{ref|NCBI}}
**Ingression - cells move with [[pseudopods]]{{ref|NCBI}}
**[[Invagination]] - forming the [[mouth]], [[anus]], and [[archenteron]]{{ref|Campbell}}
**Delamination - the external cells divide, leaving the daughter cells in the cavity{{ref|NCBI}}
**Polar proliferation
*Other major changes during gastrulation:
**Heavy [[RNA transcription]] using embryonic genes; up to this point the [[RNA]]s used were maternal (stored in the unfertilized egg).
**Cells start major [[cellular differentiation|differentiation]] processes, losing their [[pluripotent|pluripotentiality]].
In most animals, a [[blastopore]] is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In [[deuterostome]]s the anus forms from the blastopore, while in [[protostome]]s it develops into the mouth.
==Organogenesis==
At some point after the different germ layers are defined, '''[[organogenesis]]''' begins. The first stage in [[vertebrate]]s is called '''[[neurulation]]''', where the [[neural plate]] folds forming the [[neural tube]]{{ref|Campbell}}. Other common organs or structures which arise at this time include the [[heart]] and [[somite]]s, but from now on embryogenesis follows no common pattern among the different [[taxa]] of the [[animal kingdom]].
In most animals organogenesis along with '''[[morphogenesis]]''' will result in a [[larva]]. The hatching of the larva, which must then undergo [[metamorphosis (biology)|metamorphosis]], marks the end of embryonic development.
==See also==
*[[Mammalian embryogenesis]]
*[[Drosophila embryogenesis]]
*[[Plant embryogenesis]]
*[[Cdx2|Cdx2 gene]]
*[[Homeobox|Homeobox genes]]
==References==
#{{note|Campbell}} [http://www.ncbi.nih.gov/About/primer/genetics_cell.html What is a cell?] 2004. A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI.
#{{note|Campbell}} Campbell, Neil A.; Reece, Jane B.; '''Biology''' Benjamin Cummings, Pearson Education Inc 2002.
{{embryology}}
{{Developmental biology}}
==External links==
*[http://www.ivanbelchev.com/myembryo Development of the embryo] (retrieved November 20, 2007)
*[http://yourembryo.blogspot.com/ Human Embryo] (retrieved November 20, 2007)
*[http://www.blackwell-synergy.com/doi/suppl/10.1111/j.1432-0436.2006.00114.x/suppl_file/Movie+S3.mov Video] of embryogenesis of the frog [[Xenopus laevis]] from shortly after [[fertilization]] until the hatching of the tadpole; acquired by [[MRI]] ([http://dx.doi.org/10.1111/j.1432-0436.2006.00114.x DOI of paper])
[[Category:Embryology]]
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