Blastulation
| Blastulation | |
|---|---|
Gastrula | |
| Anatomical terminology] |
Blastulation is the stage in early
A common feature of a
During blastulation, a significant amount of activity occurs within the early embryo to establish
The study of the blastula, and of cell specification has many implications in stem cell research, and assisted reproductive technology.[6] In Xenopus, blastomeres behave as pluripotent stem cells which can migrate down several pathways, depending on cell signaling.[9] By manipulating the cell signals during the blastula stage of development, various tissues can be formed. This potential can be instrumental in regenerative medicine for disease and injury cases. In vitro fertilisation involves the transfer of an embryo into a uterus for implantation.[10]
Development
The blastula stage of early embryo development begins with the appearance of the blastocoel. The origin of the blastocoel in Xenopus has been shown to be from the first
In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore exclusively from the mother.[12][13]
Midblastula transition
In many organisms including Xenopus and Drosophila, the
The mid-blastula transition is also characterized by a marked increase in
Structure
A blastula (blastocyst in mammals), is a sphere of cells surrounding a fluid-filled cavity called the blastocoel. The blastocoel contains amino acids, proteins, growth factors, sugars, ions and other components which are necessary for cellular differentiation. The blastocoel also allows blastomeres to move during the process of gastrulation.[16]
In Xenopus embryos, the blastula is composed of three different regions. The animal cap forms the roof of the blastocoel and goes on primarily to form ectodermal derivatives. The equatorial or marginal zone, which compose the walls of the blastocoel differentiate primarily into mesodermal tissue. The vegetal mass is composed of the blastocoel floor and primarily develops into endodermal tissue.[7]
In the mammalian blastocyst there are three lineages that give rise to later tissue development. The epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac.[6] In the mouse embryo, blastocoel formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient which is the result of sodium–potassium pumps that produce a high sodium gradient on the basolateral side of the trophectoderm. This movement of water is facilitated by aquaporins. A seal is created by tight junctions of the epithelial cells that line the blastocoel.[6]
Cellular adhesion
Tight junctions are very important in embryo development. In the blastula, these cadherin mediated cell interactions are essential to development of epithelium which are most important to paracellular transport, maintenance of cell polarity and the creation of a permeability seal to regulate blastocoel formation. These tight junctions arise after the polarity of epithelial cells is established which sets the foundation for further development and specification. Within the blastula, inner blastomeres are generally non-polar while epithelial cells demonstrate polarity.[16]
Mammalian embryos undergo compaction around the 8-cell stage where
Xenopus membrane polarity is established with the first cell cleavage. Amphibian EP-cadherin and XB/U cadherin perform a similar role as E-cadherin in mammals establishing blastomere polarity and solidifying cell-cell interactions which are crucial for further development.[16]
Clinical implications
Fertilization technologies
Experiments with implantation in mice show that hormonal induction, superovulation and artificial insemination successfully produce preimplantation mouse embryos. In the mice, ninety percent of the females were induced by mechanical stimulation to undergo pregnancy and implant at least one embryo.[17] These results prove to be encouraging because they provide a basis for potential implantation in other mammalian species, such as humans.
Stem cells
Blastula-stage cells can behave as pluripotent stem cells in many species. Pluripotent stem cells are the starting point to produce organ specific cells that can potentially aid in repair and prevention of injury and degeneration. Combining the expression of transcription factors and locational positioning of the blastula cells can lead to the development of induced functional organs and tissues. Pluripotent Xenopus cells, when used in an in vivo strategy, were able to form into functional retinas. By transplanting them to the eye field on the neural plate, and by inducing several mis-expressions of transcription factors, the cells were committed to the retinal lineage and could guide vision based behavior in the Xenopus.[18]
See also
References
- ^ "Blastulation". web-books.com.
- ^ "Blastula". Encyclopædia Britannica. 2013.
- ^ ]
- ISBN 978-0-7923-8336-9.
- ^ Forgács & Newman, 2005: p. 27
- ^ PMID 20364097.
- ^ PMID 9334267.
- PMID 15704150.
- PMID 12533022.
- ^ Toth, Attila. "Treatment: Addressing the Causes of Infertility in Men and Women". Macleod Laboratory. Retrieved 22 March 2013.
- PMID 5565077.
- PMID 15704150.
- ^ )
- PMID 17543857.
- PMID 16564001.
- ^ PMID 10966863.
- PMID 901897.
- PMID 19688031.
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.
- Cullen, K.E. (2009). "embryology and early animal development". Encyclopedia of life science, Volume 2. Infobase. ISBN 978-0-8160-7008-4.
- McGeady, Thomas A., ed. (2006). "Gastrulation". Veterinary embryology. Wiley-Blackwell. ISBN 978-1-4051-1147-8.