http://www.ncbi.nih.gov/About/primer/genetics_cell.html NCBI] .] Embryogenesis is the process by which the
embryois formed and develops. It starts with the fertilizationof the ovum, egg, which, after fertilization, is then called a zygote. 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) and cellular differentiation, leading to development of an embryo. It occurs in both animal and plant development, but this article addresses the common features among different animals.
The egg cell (and hence the fertilized egg) is always asymmetric, having an "animal pole" (future
ectodermand 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 membrane of the egg, is made of glycoproteinsand is called vitelline membrane ( zona pellucidain mammals). Different taxashow different cellular and acellular envelopes outside of the vitelline membrane.
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
blastomeres. Depending mostly on the amount of yolkin 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
placentaor 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 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
blastularef|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
blastocystref|NCBI, characterized by an inner cell massthat 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.
gastrulationcells migrate to the interior of the blastula, consequently forming two (in diploblasticanimals) or three ( triploblastic) germ layers. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm, mesodermand endoderm. In diploblasticanimals 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 with
Invagination- forming the mouth, anus, and archenteronref|Campbell
**Delamination - the external cells divide, leaving the daughter cells in the cavityref|NCBI
*Other major changes during gastrulation:
RNA transcriptionusing embryonic genes; up to this point the RNAs used were maternal (stored in the unfertilized egg).
**Cells start major differentiation processes, losing their pluripotentiality.
In most animals, a
blastoporeis 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 deuterostomes the anus forms from the blastopore, while in protostomes it develops into the mouth.
At some point after the different germ layers are defined,
organogenesisbegins. The first stage in vertebrates is called neurulation, where the neural platefolds forming the neural tuberef|Campbell. Other common organs or structures which arise at this time include the heartand somites, but from now on embryogenesis follows no common pattern among the different taxaof the animal kingdom.
In most animals organogenesis along with
morphogenesiswill result in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.
# [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.
* [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 laevisfrom shortly after fertilizationuntil the hatching of the tadpole; acquired by MRI( [http://dx.doi.org/10.1111/j.1432-0436.2006.00114.x DOI of paper] )
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