Gastrulation

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Introduction

Human embryo primitive streak
Hans Spemann (1869 - 1941)

Gastrulation means the formation of gut (Greek, gastrula = belly), but has now a more broad sense to to describe the formation of the trilaminar embryo. The epiblast layer, consisting of totipotential cells, derives all 3 embryo layers: endoderm, mesoderm and ectoderm. The primitive streak is the visible feature which represents the site of cell migration to form the additional layers.

Historically, gastrulation was one of the earliest observable morphological event occurring in the frog embryo. Currently, the molecular and physical mechanisms that regulate patterning and migration during this key event are being investigated in several different animal models. This region was also called the "Spemann's organiser" after Hans Spemann (1869 - 1941) who identified the region in amphibia. The same region in birds it is known as "Hensen's node" named for Victor Hensen (1835 – 1924) and is also known generally as the primitive node or knot. In humans, it is proposed that similar mechanisms regulate gastrulation to those found in other vertebrates.


  • primitive node - region in the middle of the early embryonic disc epiblast from which the primitive streak extends caudally (tail)
    • nodal cilia establish the embryo left/right axis
    • axial process extends from the nodal epiblast
  • primitive streak - region of cell migration from the epiblast layer forming sequentially the two germ cell layers (endoderm and mesoderm)

Epithelial to Mesenchymal Transition

Gastrulation cartoon

Epithelial cells (organised cellular layer) which loose their organisation and migrate/proliferate as a mesenchymal cells (disorganised cellular layers) are said to have undergone an Epithelial Mesenchymal Transition (EMT). Mesenchymal cells have an embryonic connective tissue-like cellular arrangement, that have undergone this process may at a later time and under specific signaling conditions undergo the opposite process, mesenchyme to epithelia. In development, this process can be repeated several times during tissue differentiation.

This process occurs at the primitive streak where epiblast cells undergo an epithelial to mesenchymal transition in order to delaminate and migrate.


Links: Lecture - Week 3 | Week 3 | stage 7 | stage 8 | Endoderm | Mesoderm | Ectoderm | Epithelial Mesenchymal Transition | Notochord
Historic Embryology: Gastrulation (1910) | Frog Gastrulation (1951)

Some Recent Findings

  • Deficiency in crumbs homolog 2 (Crb2) affects gastrulation and results in embryonic lethality in mice[1] "The Crumbs family of transmembrane proteins has an important role in the differentiation of the apical membrane domain in various cell types, regulating such processes as epithelial cell polarization. The mammalian Crumbs protein family is composed of three members. Here, we inactivated the mouse Crb2 gene with gene-targeting techniques and found that the protein is crucial for early embryonic development with severe abnormalities appearing in Crb2-deficient embryos at late-gastrulation. Our findings indicate that the primary defect in the mutant embryos is disturbed polarity of the epiblast cells at the primitive streak, which affects epithelial to mesenchymal transition (EMT) during gastrulation, resulting in impaired mesoderm and endoderm formation, and embryonic lethality by embryonic day 12.5. These findings therefore indicate a novel role for the Crumbs family of proteins."
  • Zebrafish eve1 regulates the lateral and ventral fates of mesodermal progenitor cells at the onset of gastrulation[2] "Our data show that Eve1 functions together with Ved, Vent and Vox in a transcriptional network to prevent the spread of anti-Bmp gene activity from the dorsal side, leading to the establishment of the Bmp gradient activity along the dorsoventral axis to induce distinct transcriptional outputs in mesodermal progenitor cells (MPCs) to maintain the lateral and ventral MPC fates during gastrulation."

Gastrulation Movies

Animation showing the secondary spread of mesoderm following the migration of endoderm through the primitive streak.
Mesoderm 001 icon.jpg
 ‎‎Week 3 Mesoderm
Page | Play
Nodal-cilia-001-icon.jpg
 ‎‎Nodal Cilia
Page | Play
Quail HH stage 2 fibronectin movement.jpg
 ‎‎Gastrulation ECM
Page | Play

Human Gastrulation

The site of gastrulation, the primitive streak is visible during week 3 on the epiblast dorsal surface of the embryonic disc.

Stage 7

Stage7-sem4.jpg Stage7-bf4.jpg

Stage7 primitive streak labelled.jpg


Links: Carnegie stage 7

Stage 8

Stage8 bf10.jpg

Primitive pit

Stage8 bf11.jpg

Primitive groove and primitive streak

Gastrulation Concepts

Chicken-gastrulation2.jpg


Chicken-gastrulation3.jpg


Links: Chicken Development


Germ Layer Markers

Stem cell researchers have used the following markers to identify early differentiation of cells in the three germ layer in embryoid bodies.[3]


Links: Induced Stem Cells | Alpha-Fetoprotein

Frog Gastrulation

Rugh 071.jpg Frog Gastrulation[4]

(A) The blastula stage, prior to any gastrulation movement.

(B) Movement of the blastula cells preliminary to gastrulation.

(C) Blastoporal view of successive phases of gastrulation; (solid line) lip of blastopore, {dotted line) germ ring, to be subsequently incorporated into the blastoporal lips.

(D) Lateral view of sagittal section during late gastrulation showing the origin of the mesial notochord, and the lateral mesoderm from the proliferated chorda-mesoderm cells at the dorsal lip.

(E) Composite drawing to illustrate the germ layer relations in the later gastrula of the frog. The medullary plate (ectoderm) is not indicated; {alternate dots and dashes) notochord, {heavy stippling) notochord, {sparse stippling) mesoderm, (cellular markings) ectoderm.

Additional Images

References

  1. <pubmed>22072575</pubmed>
  2. <pubmed>20950598</pubmed>
  3. <pubmed>20369364</pubmed>
  4. Rugh, R. The Frog Its Reproduction and Development (1951)


Reviews

<pubmed></pubmed> <pubmed>20566281</pubmed> <pubmed>20298258</pubmed> <pubmed>19945376</pubmed> <pubmed>19761865</pubmed> <pubmed>19253398</pubmed> <pubmed>18721878</pubmed> <pubmed>16344860</pubmed> <pubmed>11170350</pubmed> <pubmed>10369659</pubmed>


Articles

<pubmed></pubmed> <pubmed></pubmed> <pubmed>23204916</pubmed> <pubmed>21761476</pubmed> <pubmed>20485500</pubmed>

Books

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Cite this page: Hill, M.A. (2020, October 28) Embryology Gastrulation. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Gastrulation

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© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G