blastula

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The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres and rearrangement of blastomeres around this cavity in such a way as to form the type of definitive blastula characteristic of each species. The blastocoele originates as an intercellular space which sometimes arises as early as the four- or eight-cell stage. Thus blastulation is initiated during early cleavage stages, and formation of the definitive blastula is thought to terminate cleavage and to initiate gastrulation. Initially the diameter of the blastula is no greater than that of the activated egg; subsequently it increases. See also Gastrulation.

The blastula is usually a hollow sphere. Its wall may vary from one to several cells in thickness. In eggs which contain considerable amounts of yolk the blastocoele may be eccentric in position, that is, shifted toward the animal pole. The animal portion of its wall is always completely divided into relatively small cells, whereas the vegetative portion tends to be composed of relatively large cells and may be incompletely cellulated in certain species. The blastocoele contains a gelatinous or jellylike fluid, which originates in part as a secretion by the blastomeres and in part by passage of water through the blastomeres or intercellular material, or both, into the blastocoele.

The wall of the blastula is a mosaic of cellular areas, each of which will normally produce a certain structure during subsequent development. In other words, each area of cells in the wall of the blastula has a certain prospective fate which will be realized in normal development.

The stage of an embryo that consists of just over a hundred cells — a stage reached about one week after fertilization. At this stage the cells are just at the very beginning of cellular differentiation and are said to be totipotent (See totipotency).

Conversion of the morula to the blastula by development of a blastocele.

• Reproduction and Development - blastula: hollow ball of cells formed from developing egg in early stage of noneutherian embryo

Blastula
Blastulation: 1 - morula, 2 - blastula.
Days	4
Precursor	Morula
Gives rise to	Gastrula

The blastula (from Greek βλαστός (blastos), meaning "sprout") is a hollow sphere of cells formed during an early stage of embryonic development in animals ^[1]. The blastula is created when the zygote undergoes the cell division process known as cleavage.^[2] The blastula is preceded by the morula and is followed by the gastrula in the developmental sequence.^{[citation needed]}

A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds an interior central cavity known as the blastocoel.^{[3] [4]} In mammals, blastulation leads to the formation of the blastocyst, which must not be confused with the blastula. The blastocyst contains an embryoblast, which is homologous to the blastula. However, it also includes the trophoblast, which goes on to form the extraembryonic tissues.^{[citation needed]}

Contents 1 See also 2 References 2.1 Notes 2.2 Bibliography 3 Further reading

See also

• Cellular differentiation
• Gastrulation
• Polarity in embryogenesis

References

Notes

1. ^ http://www.britannica.com/EBchecked/topic/69108/blastula
2. ^ Forgács & Newman, 2005: p. 24
3. ^ Lombardi, Julian (1998). "Embryogenesis". Comparative vertebrate reproduction. Springer. p. 226. ISBN 978-0-7923-8336-9. http://books.google.com/books?id=cqQX9RMPAegC&pg=PA226. 
4. ^ Forgács & Newman, 2005: p. 27

Bibliography

• Forgács, G. & Newman, Stuart A. (2005). "Cleavage and blastula formation". Biological physics of the developing embryo. Cambridge University Press. ISBN 978-0-521-78337-8. http://books.google.com/books?id=rUyVWQhk7CkC&pg=PA24. 

Further reading

• Cullen, K.E. (2009). "embryology and early animal development". Encyclopedia of life science, Volume 2. Infobase. ISBN 978-0-8160-7008-4. http://books.google.com/books?id=iM_O62qBSQYC&pg=PA283. 
• McGeady, Thomas A., ed. (2006). "Gastrulation". Veterinary embryology. Wiley-Blackwell. ISBN 978-1-4051-1147-8. http://books.google.com/books?id=n4C0TUeR7mUC&pg=PA34. 

v t e Developmental biology > Human embryogenesis (development of embryo) and development of fetus (TE E2.0) First three weeks Week 1 Fertilization · Oocyte activation · Zygote · Cleavage · Morula · Blastula (Blastomere) · Blastocyst · Inner cell mass Week 2 (Bilaminar) Hypoblast · Epiblast Week 3 (Trilaminar) Germ layers Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) · Gastrula/Gastrulation · Regional specification · Embryonic disc Ectoderm Surface ectoderm · Neuroectoderm · Somatopleuric mesenchyme · Neurulation · Neural crest Endoderm Splanchnopleuric mesenchyme Mesoderm Chorda- · Paraxial (Somite/Somitomere) · Intermediate · Lateral plate (Intraembryonic coelom, Splanchnopleuric mesenchyme/Somatopleuric mesenchyme) M: OBS phys/devp/memb mthr/fetu/infc, epon proc, drug(2A/G2C)

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