View source for Embryo

[[Image:Wrinkledfrog_embryos.jpg|thumb|260px|Embryos (and one [[tadpole]]) of the wrinkled frog (''Rana rugosa'')]]
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An '''embryo''' (from [[Greek language|Greek]]: ''{{Polytonic|ἔμβρυον}}'', plural ''{{Polytonic|ἔμβρυα}}'', lit. "that which grows," from en- "in" + bryein "to swell, be full") is a [[multicellular organism|multicellular]] [[ploidy|diploid]] [[eukaryote]] in its earliest stage of [[development]], from the time of first [[cell division]] until [[birth]], [[Egg (biology)|hatching]], or [[germination]]. In humans, it is called an embryo from the moment of [[fertilisation]] until the end of the 8th week, whereafter it is instead called a [[fetus]].

==Development==
{{Main|Embryogenesis}}
The development of the embryo is called embryogenesis. In organisms that [[sexual reproduction|reproduce sexually]], once a [[spermatid|sperm]] fertilizes an [[ovum|egg cell]], the result is a [[cell (biology)|cell]] called the [[zygote]] that has all the [[DNA]] of two parents. The resulting embryo derives 50 percent of its genetic makeup from each parent. In [[plant]]s, [[animal]]s, and some [[protist]]s, the zygote will begin to divide by [[mitosis]] to produce a multicellular organism. The result of this process is an embryo. 

In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of [[blastula]], [[gastrula]], and [[organogenesis]]. The blastula stage typically features a fluid-filled cavity, the [[blastocoele|blastocoel]], surrounded by a sphere or sheet of cells, also called [[blastomere]]s.

[[Image:6 weeks pregnant.png|left|thumb|Human embryo at six weeks gestational age<ref>[http://www.3dpregnancy.com/static/pregnancy-week-6.html 3D Pregnancy] (Image from gestational age of 6 weeks). Retrieved [[2007-08-28]].  A rotatable 3D version of this photo is available [http://www.3dpregnancy.com/rotatable/6-weeks-pregnant.html here], and a drawing is available [http://www.3dpregnancy.com/pictures/pregnancy-week-6.html here].</ref>]] During [[gastrulation]] the cells of the blastula undergo coordinated processes of cell division, invasion, and/or migration to form two ([[diploblasty|diploblastic]]) or three ([[triploblasty|triploblastic]]) tissue layers. In triploblastic organisms, the three [[germ layer]]s are called [[endoderm]], [[ectoderm]] and [[mesoderm]]. However, the position and arrangement of the germ layers are highly species-specific, depending on the type of embryo produced. In [[vertebrate]]s, a special population of embryonic cells called the [[neural crest]] has been proposed as a "fourth germ layer", and is thought to have been an important novelty in the evolution of head structures.

During organogenesis, molecular and cellular interactions between germ layers, combined with the cells' developmental potential or [[competence (biology)|competence]] to respond, prompt the further differentiation of organ-specific cell types.  For example, in [[neurogenesis]], a subpopulation of ectoderm cells is set aside to become the [[brain]], [[spinal cord]] and [[peripheral nervous system|peripheral nerves]]. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including [[angiogenesis]] (formation of new blood vessels from pre-existing ones), [[chondrogenesis]] (cartilage), [[myogenesis]] (muscle), [[osteogenesis]] (bone), and many others.

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "[[recapitulation theory|ontogeny recapitulates phylogeny]]."<ref>Gould, Stephen.  [http://www.sjgarchive.org/library/ontogeny.html Ontogeny and Philogeny], page 206 (1977): "recapitulation was not 'disproved'; it could not be, for too many well-established cases fit its expectations."</ref> For example, the backbone is a common structure among all vertebrates such as [[fish]], [[reptile]]s and [[mammal]]s, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The [[cerebrum]] in humans, which is the most sophisticated part of the [[human brain|brain]], develops last. This rule is not absolute, but it is recognized as being partly applicable to development of the human embryo.

==Embryos of plants and animals==
*'''Plants''': In [[botany]], a [[seed plant]] ''embryo'' is part of a [[seed]], consisting of precursor tissues for the leaves, stem (see '''[[hypocotyl]]'''), and root (see '''[[radicle]]'''), as well as one or more '''[[cotyledon]]s'''. Once the embryo begins to [[germination|germinate]] &mdash; grow out from the seed &mdash; it is called a '''seedling'''. Plants that do not produce seeds, but do produce an embryo, include the [[bryophyte]]s and [[fern]]s. In these plants, the embryo is a young plant that grows attached to a parental '''[[gametophyte]]'''.
*'''Animals''': The embryo of a [[placental mammal]] is defined as the organism between the first [[cell division|division]] of the '''''[[zygote]]''''' (a fertilized [[ovum]]) until it becomes a '''[[fetus]]'''. In humans, the embryo is defined as the product of conception from implantation in the [[uterus]] through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until [[childbirth|birth]] or hatching. In humans, this is from the eighth week of [[gestation]].

==The human embryo==
===Growth===
[[Image:Tubal Pregnancy with embryo.jpg|thumb|180px|250px|A 10mm embryo from an [[ectopic pregnancy]], still in the oviduct. This embryo is about five weeks old (or from the 7th week of pregnancy).]] 
{{seealso|Human embryogenesis|Prenatal development}}
'''Week 1-3''' 5-7 days after fertilization, the blastula attaches to the wall of the uterus ([[endometrium]]). When it comes into contact with the endometrium it performs [[implantation]]. Implantation connections between the mother and the embryo will begin to form, including the [[umbilical cord]]. The embryo's growth centers around an axis, which will become the spine and spinal cord.     The brain, spinal cord, heart, and gastrointestinal tract begin to form.<ref>NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm</ref>

'''Week 4-5''' Chemicals produced by the embryo stop the woman's [[menstrual cycle]]. Neurogenesis is underway, showing brain activity at about the 6th week.  The heart will begin to beat around the same time. Limb buds appear where the arms and legs will grow later. Organogenesis begins. The head represents about one half of the embryo's axial length, and more than half of the embryo's [[mass]]. The brain develops into five areas, along with vertebra and bones beginning to form. The heart starts to beat and blood starts to flow.<ref>NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm</ref>

'''Week 6-8''' Myogenesis and neurogenesis have progressed to where the embryo is capable of motion, and the eyes begin to form. Organogenesis and growth continue. Hair has started to form along with all essential organs. Facial features are beginning to develop. At the end of the 8th week, the embryonic stage is over, and the [[fetus|fetal]] stage begins.<ref>NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm</ref>

===Status===

The status of the human embryo is debated by some [[Bioethics|bioethicists]]. Some Christian Ethicists believe that an embryo does, in fact, possess [[Person|personhood]].  Gilbert Meileander, Christian ethics professor at the private Lutheran university [[Valparaiso University]] for example, identifies conception as the point at which a new individual human being comes into existence, since "when sperm and ovum join to form the zygote, the individual's genotype is established."<ref>Gilbert Meilander, ''Bioethics: A Primer for Christians'' (2nd ed.; Grand Rapids: Eerdmans, 2005), p. 29.</ref> The [[NIH]] defines the embryonic stage as the beginning of developed human form <ref>NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm</ref>

==Footnotes==
{{reflist|2}}

==See also==
*[[Embryogenesis]]
*[[Embryology]]
*[[In vitro fertilization]]
*[[Plant embryogenesis]]
*[[Embryo space colonization]]
*[[Pregnancy]]
*[[Embryo adoption]]
*[[Sexual]]

{{start box}}
{{succession box|title=[[Stages of human development]] |before=[[Zygote]]|after=[[Fetus]]|years=Embryo}}
{{end box}}
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==External links==
{{commonscat|Embryology}}
*[http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm Chart of human fetal development]
*[http://raven.zoology.washington.edu/embryos/ A Comparative Embryology Gallery]
*[http://www.ivanbelchev.com/myembryo Development of the human embryo]
*[http://www.visembryo.com/baby/index.html The Visible Embryo from fertilization to birth]
*[http://www.hfea.gov.uk The Human Fertilisation and Embryology Authority (HFEA)]
*'''''The Developing Human (Keith L.Moore'')''' http://www.youtube.com/watch?v=Rb0uZefwQnc

{{Embryology}}

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[[Category:Embryology]]
[[Category:Developmental biology]]
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