- Embryogenesis
[
http://www.ncbi.nih.gov/About/primer/genetics_cell.html NCBI] .] Embryogenesis is the process by which theembryo is formed and develops. It starts with thefertilization of theovum , egg, which, after fertilization, is then called azygote . The zygote undergoes rapid mitotic divisions, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as cleavage) andcellular differentiation , leading to development of an embryo. It occurs in both animal and plant development, but this article addresses the common features among differentanimal s.The zygote
The egg cell (and hence the fertilized egg) is always asymmetric, having an "animal pole" (future
ectoderm andmesoderm ), two of three primitive tissue types, and a "vegetal pole" (futureendoderm ), it is also covered with different protective envelopes. The first envelope, the one which is in contact with the membrane of the egg, is made ofglycoproteins and is called vitelline membrane (zona pellucida inmammals ). Differenttaxa show different cellular and acellular envelopes outside of the vitelline membrane.Cleavage
The zygote undergoes rapid 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 ofyolk in the egg, the 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 ormilk . 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 zygoteref|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:
Blastula and Gastrula
After the cleavage has produced over 100 cells, 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 aninner 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 (indiploblastic animals) or three (triploblastic )germ layer s. The embryo during this process is called agastrula . The germ layers are referred to as theectoderm ,mesoderm andendoderm . Indiploblastic 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 cellsref|NCBI
**Ingression - cells move withpseudopods ref|NCBI
**Invagination - forming themouth ,anus , andarchenteron ref|Campbell
**Delamination - the external cells divide, leaving the daughter cells in the cavityref|NCBI
**Polar proliferation*Other major changes during gastrulation:
**HeavyRNA transcription using embryonic genes; up to this point theRNA s used were maternal (stored in the unfertilized egg).
**Cells start major differentiation processes, losing their 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. Indeuterostome s the anus forms from the blastopore, while inprotostome s it develops into the mouth.Organogenesis
At some point after the different germ layers are defined,
organogenesis begins. The first stage invertebrate s is calledneurulation , where theneural plate folds forming theneural tube ref|Campbell. Other common organs or structures which arise at this time include theheart andsomite s, but from now on embryogenesis follows no common pattern among the differenttaxa of theanimal kingdom .In most animals organogenesis along with
morphogenesis will result in alarva . The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.ee also
*
Mammalian embryogenesis
*Drosophila embryogenesis
*Plant embryogenesis
*Cdx2 gene
*Homeobox genesReferences
# [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.
# Campbell, Neil A.; Reece, Jane B.; Biology Benjamin Cummings, Pearson Education Inc 2002.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 frogXenopus laevis from shortly afterfertilization until the hatching of the tadpole; acquired byMRI ( [http://dx.doi.org/10.1111/j.1432-0436.2006.00114.x DOI of paper] )
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