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Gastrulation

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Gastrulation

Gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. This diagram is color-coded: ectoderm, blue; endoderm, green; blastocoel (the yolk sack), yellow; and archenteron (the gut), purple.

Gastrulation is a phase early in the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.[1][2]

Gastrulation takes place after epithelia), or as a mesh of isolated cells, such as mesenchyme.[2][5]

The molecular mechanism and timing of gastrulation is different in different organisms. However, some common features of gastrulation across topological structure of the embryo, from a simply connected surface (sphere-like), to a non-simply connected surface (torus-like); (2) the differentiation of cells into one of three types (endodermal, mesodermal, and ectodermal); and (3) the digestive function of a large number of endodermal cells.[6]

Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life."

The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel, in his 1872 work "Biology of Calcareous Sponges".[7]

Although gastrulation patterns exhibit enormous variation throughout the animal kingdom, they are unified by the five basic types of cell movements that occur during gastrulation: 1) invagination 2) involution 3) ingression 4) delamination 5) epiboly.[8]

Contents

  • In amniotes 1
    • Overview 1.1
    • Loss of symmetry 1.2
    • Formation of the primitive streak 1.3
    • Epithelial to mesenchymal transition and ingression 1.4
  • See also 2
  • References 3
    • Notes 3.1
    • Bibliography 3.2
  • Further reading 4
  • External links 5

In amniotes

Overview

Blastopore
Dorlands
/Elsevier
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Anatomical terminology

Gastrulation involves the creation of the blastopore, an opening into the archenteron. Note that the blastopore is not an opening into the blastocoel, the space within the blastula, but represents a new inpocketing that pushes the existing surfaces of the blastula together. In amniotes, gastrulation occurs in the following sequence: (1) the embryo becomes asymmetric; (2) the primitive streak forms; (3) cells from the epiblast at the primitive streak undergo an epithelial to mesenchymal transition and ingress at the primitive streak to form the germ layers.[4]

The distinction between protostomes and deuterostomes is based on the direction in which the mouth (stoma) develops in relation to the blastopore. Protostome derives from the Greek word protostoma meaning "first mouth"(πρώτος + στόμα) whereas Deuterostome's etymology is "second mouth" from the words second and mouth (δεύτερος + στόμα).

The major distinctions between deuterostomes and protostomes are found in embryonic development:

  • Mouth/anus
    • In protostome development, the first opening in development, the blastopore, becomes the animal's mouth.
    • In deuterostome development, the blastopore becomes the animal's anus.
  • Cleavage
    • Protostomes have what is known as spiral cleavage which is determinate, this meaning that the fate of the cells is determined as they are formed.
    • Deuterostomes have what is known as radial cleavage that is indeterminate.

Loss of symmetry

In preparation for gastrulation, the embryo must become asymmetric along both the BMP, FGF, nodal, and Wnt. Visceral endoderm surrounds the epiblast. The distal visceral endoderm (DVE) migrates to the anterior portion of the embryo, forming the “anterior visceral endoderm” (AVE). This breaks anterior-posterior symmetry and is regulated by nodal signaling.[4]

Epithelial to Mesenchmyal Cell Transition – loss of cell adhesion leads to constriction and extrusion of newly mesenchymal cell.

Formation of the primitive streak

The primitive streak is formed at the beginning of gastrulation and is found at the junction between the extraembryonic tissue and the epiblast on the posterior side of the embryo and the site of ingression.[9] Formation of the primitive streak is reliant upon nodal signaling[4] in the Koller's sickle within the cells contributing to the primitive streak and BMP4 signaling from the extraembryonic tissue.[9][10] Furthermore, Cer1 and Lefty1 restrict the primitive streak to the appropriate location by antagonizing nodal signaling.[11] The region defined as the primitive streak continues to grow towards the distal tip.[4]

During the early stages of development, the primitive streak is the structure that will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the first embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation.[12] The primitive streak extends through this midline and creates the antero-posterior body axis,[13] becoming the first symmetry-breaking event in the embryo, and marks the beginning of gastrulation.[14] This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position,[13][15] where they will differentiate into the three germ layers.[12] The localization of the cell adhesion and signaling molecule beta-catenin is critical to the proper formation of the organizer region that is responsible for initiating gastrulation.

Epithelial to mesenchymal transition and ingression

In order for the cells to move from the epithelium of the epiblast through the primitive streak to form a new layer, the cells must undergo an epithelial to mesenchymal transition (EMT) to lose their epithelial characteristics, such as cell-cell adhesion. FGF signaling is necessary for proper EMT. FGFR1 is needed for the up regulation of Snail1, which down regulates E-cadherin, causing a loss of cell adhesion. Following the EMT, the cells ingress through the primitive streak and spread out to form a new layer of cells or join existing layers. FGF8 is implicated in the process of this dispersal from the primitive streak.[11]

See also

References

Notes

  1. ^ Mundlos 2009: p. 422
  2. ^ a b McGeady, 2004: p. 34
  3. ^ Hall, 1998: pp. 132-134
  4. ^ a b c d e Arnold & Robinson, 2009
  5. ^ Hall, 1998: p. 177
  6. ^ Harrison 2011: p. 206
  7. ^ Ereskovsky 2010: p. 236
  8. ^ Gilbert 2010: p. 164.
  9. ^ a b Tam & Behringer, 1997
  10. ^ Catala, 2005: p. 1535
  11. ^ a b Tam, P.P. & Loebel, D.A (2007). "Gene function in mouse embryogenesis: get set for gastrulation". Nat Rev Genet 8 (5): 368–81.  
  12. ^ a b Mikawa T, Poh AM, Kelly KA, Ishii Y, Reese DE. (2004). "Induction and patterning of the primitive streak, an organizing center of gastrulation in the amniote.". Dev Dyn 229 (3): 422–32.  
  13. ^ a b Downs KM. (2009). "The enigmatic primitive streak: prevailing notions and challenges concerning the body axis of mammals.". Bioessays 31 (8): 892–902.  
  14. ^ Chuai M, Zeng W, Yang X, Boychenko V, Glazier JA, Weijer CJ. (2006). "Cell movement during chick primitive streak formation.". Dev Biol. 296(1)) (1): 137–49.  
  15. ^ Chuai M, Weijer CJ. (2008). "The mechanisms underlying primitive streak formation in the chick embryo.". Curr Top Dev Biol. 81: 135–56.  
  16. ^ See

Bibliography

  • Arnold, Sebastian J.;  
  • Catala, Martin (2005). "Embryology of the Spine and Spinal Cord". In Tortori-Donati, Paolo et al.. Pediatric Neuroradiology: Brain. Springer.  
  • Ereskovsky, Alexander V. (2010). The Comparative Embryology of Sponges. Springer.  
  • Gilbert, Scott F. (2010). Developmental Biology (Ninth ed.). Sinauer Associates.  
  •  
  •  
  • McGeady, Thomas A., ed. (2006). "Gastrulation". Veterinary embryology. Wiley-Blackwell.  
  • Mundlos, Stefan (2009). "Gene action: developmental genetics". In Speicher, Michael et al.. Vogel and Motulsky's Human Genetics: Problems and Approaches (4th ed.). Springer.  
  • Tam, Patrick P.L. & Behringer, Richard R. (1997). "Mouse gastrulation: the formation of a mammalian body plan".  

Further reading

  • Baron, Margaret H. (2001). "Embryonic Induction of Mammalian Hematopoiesis and Vasculogenesis". In Zon, Leonard I. Hematopoiesis: a developmental approach. Oxford University Press.  
  • Cullen, K.E. (2009). "embryology and early animal development". Encyclopedia of life science, Volume 2. Infobase.  
  • Forgács, G. & Newman, Stuart A. (2005). "Cleavage and blastula formation". Biological physics of the developing embryo. Cambridge University Press.  
  • Forgács, G. & Newman, Stuart A. (2005). "Epithelial morphogenesis: gastrulation and neurulation". Biological physics of the developing embryo. Cambridge University Press.  
  • Hart, Nathan H. & Fluck, Richard A. (1995). "Epiboly and Gastrulation". In Capco, David. Cytoskeletal mechanisms during animal development. Academic Press.  
  • Knust, Elizabeth (1999). "Gastrulation movements". In Birchmeier, Walter & Birchmeier, Carmen. Epithelial Morphogenesis in Development and Disease. CRC Press. pp. 152–153.  
  • Kunz, Yvette W. (2004). "Gastrulation". Developmental biology of Teleost fishes. Springer.  
  • Nation, James L., ed. (2009). "Gastrulation". Insect physiology and biochemistry. CRC Press.  
  • Ross, Lawrence M. & Lamperti, Edward D., eds. (2006). "Human Ontogeny: Gastrulation, Neurulation, and Somite Formation". Atlas of anatomy: general anatomy and musculoskeletal system. Thieme.  
  • Sanes, Dan H. et al. (2006). "Early embryology of metazoans". Development of the nervous system (2nd ed.). Academic Press. pp. 1–2.  
  • Stanger, Ben Z. & Melton, Douglas A. (2004). "Development of Endodermal Derivatives in the Lungs, Liver, Pancreas, and Gut". In Epstein, Charles J. et al. Inborn errors of development: the molecular basis of clinical disorders of morphogenesis. Oxford University Press.  

External links

  • Gastrulation animations
  • Gastrulation illustrations and movies from Gastrulation: From Cells To Embryo edited by Claudio Stern
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