Embryology
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'''Embryology''' is the study of the development of an [[embryo]]. An embryo is defined as any organism in a stage before birth or hatching, or in plants, before [[germination]] occurs.
[[Image:Embryo, 8 cells.jpg|thumb|240px|right| Morula, 8 cell stage]]
Embryology refers to the development of the fertilized egg cell ([[zygote]]) and its differentiation into tissues and organs. After cleavage, the dividing cells, or [[morula]], becomes a hollow ball, or [[blastula]], which develops a hole or pore at one end.
[[Image:Blastulation.png|thumb|240px|right|'''1''' - morula, '''2''' - blastula]]
In animals, the [[blastula]] develops in one of two ways that divides the whole animal kingdom into two halves. If in the [[blastula]] the first pore ([[blastopore]]) becomes the mouth of the animal, it is a [[protostome]]; if the first pore becomes the anus then it is a [[deuterostome]]. The [[protostomes]] include most [[invertebrate]] animals, such as insects, worms and molluscs, while the [[deuterostomes]] includes more advanced animals including the [[vertebrates]]. In due course, the [[blastula]] changes into a more differentiated structure called the [[gastrula]].
[[Image:Gastrulation.png|thumb|240px|'''1''' - blastula, '''2''' - gastrula with blastopore; '''orange''' - ectoderm, '''red''' - endoderm.]]
The [[gastrula]] with its [[blastopore]] soon develops three distinct layers of cells (the [[germ layers]]) from which all the bodily organs and tissues then develop:
* The innermost layer, or [[endoderm]], gives rise to the digestive organs, lungs and bladder.
* The middle layer, or [[mesoderm]], gives rise to the muscles, skeleton and blood system.
* The outer layer of cells, or [[ectoderm]], gives rise to the nervous system and skin.
In humans, the term embryo refers to the ball of dividing cells from the moment the [[zygote]] implants itself in the [[uterus]] wall until the end of the eighth week after conception. Beyond the eighth week, the developing human is then called a fetus. Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have adapted into the environment of pregnancy. These similarities among species are called [[analogous structures]], which are structures that have the same or similar function and mechanism, but evolved separately.
[[Image:6 weeks pregnant.png|right|thumb|Human embryo at six weeks gestational age]]
==History==
As recently as the 18th century, the prevailing notion in human embryology was [[preformation]]: the idea that the egg or sperm contains an embryo--a preformed, miniature infant, or "homunculus"--that simply becomes larger during development. The competing explanation of embryonic development was [[epigenesis]], originally proposed 2,000 years earlier by Aristotle. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As [[microscopy]] improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.<ref>Campbell et al. (p. 987)</ref>
[[Image:Haeckel drawings.jpg|150px|right|Ernst Haeckel embryo drawings]]
Modern embryological pioneers include [[Gavin de Beer]], [[Charles Darwin]], [[Ernst Haeckel]], [[J.B.S. Haldane]], and [[Joseph Needham]], while much early embryology came from the work of [[Aristotle]] and the great [[Italy|Italian]] anatomists: [[Aldrovandi]], [[Aranzio]], [[Leonardo da Vinci]], [[Marcello Malpighi]], [[Gabriele Falloppia]], [[Girolamo Cardano]], [[Emilio Parisano]], [[Fortunio Liceti]], [[Stefano Lorenzini]], [[Spallanzani]], [[Enrico Sertoli]], [[Mauro Rusconi]], etc.<ref>http://www.ijdb.ehu.es/fullaccess/fulltext.0009/ft515.pdf Massimo De Felici, Gregorio Siracus, ''The rise of embryology in Italy: from the Renaissance to the early 20th Century,'' Int. J. Dev. Biol. 44: 515-521 (2000)</ref> Other important contributors include [[William Harvey]], [[Kaspar Friedrich Wolff]], [[Pander]], [[Karl Ernst von Baer]], and [[August Weismann]].
[[Image:10dayMouseEmb.jpg|thumb|150px|right|Histological film 10 day mouse embryo]]
After the 1950s, with the [[DNA]] helical structure being unravelled and the increasing knowledge in the field of [[molecular biology]], [[developmental biology]] emerged as a field of study which attempts to correlate the genes with morphological change; and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.
==Vertebrate and invertebrate embryology==
Many principles of embryology apply to both invertebrate animals as well as to vertebrates.<ref> Parker, Sybil. "Invertebrate Embryology," ''[http://books.google.com/books?vid=ISBN0079115047&id=CMC32Rmo9tYC&q=%22invertebrate+embryology%22+and+%22mcgraw-hill%22&dq=%22invertebrate+embryology%22+and+%22mcgraw-hill%22&pgis=1 McGraw-Hill Encyclopedia of Science & Technology]'' (McGraw-Hill 1997).</ref> Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a [[larva]] before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to it parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form many insects develop through at least one larval stage.
[[Image:Beetle larvae filtered sw.jpg|thumb|150px|right| Beetle larvae]]
==Modern embryology research==
Currently, embryology has become an important research area for studying the [[genetic]] control of the development process (e.g. [[morphogens]]), its link to [[cell signalling]], its importance for the study of certain diseases and [[mutation]]s and in links to [[stem cell]] research.
==See also==
*[[Ontogeny]]
*[[Embryogenesis]]
*[[Recapitulation theory]]
*[[Prenatal development]]
*[[Protostomes]]
*[[Deuterostomes]]
*[[Germ layers]]
*[[Epigenesis (biology)]]
*[[Developmental biology]]
*[[Cell signalling]]
*[[Hedgehog signaling pathway]]
*[[Morphogens]]
*[[Embryo drawing]]
==References==
* [http://embryology.med.unsw.edu.au/ UNSW Embryology] Large resource of information and media
* [http://www2.merriam-webster.com/cgi-bin/mwmednlm?book=Medical&va=embryo] Definition of embryo according to Webster
==Further reading==
* Scott F. Gilbert. ''Developmental Biology''. Sinauer, 2003. ISBN 0-87893-258-5.
* Lewis Wolpert. ''Principles of Development''. Oxford University Press, 2006. ISBN 0-19-927536-X.
==External links==
{{commonscat|Embryology}}
* [http://www.rsrevision.com/Alevel/ethics/embryology/index.htm Embryo Research] UK philosophy and ethics website discussing the ethics of embryology
* [http://www.religioustolerance.org/res_emb.htm Human embryo research] Canadian website covering the ethics of human embryo research
* [http://www.indiana.edu/~anat550/embryo_main/index.html Indiana University's Human Embryology Animations]
* [http://www.cambridgenetwork.co.uk/views/biolines What is a human admixed embryo?]
{{Embryology}}
{{Development of digestive system}}
{{Development of urinary and reproductive systems}}
{{Development of nervous system}}
{{Development of circulatory system}}
{{Development of respiratory system}}
{{Embryology of head and neck}}
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