Primitive knot

Primitive knot
Details
Days	17
Latin	nodus primitivus
*Anatomical terminology* [edit on Wikidata]

The primitive knot (or primitive node) is the organizer for gastrulation in vertebrates.

Diversity

In birds, it is known as "Hensen's node", and is named after its discoverer Victor Hensen.
In amphibians, it is known as "Spemann's organizer", and is named after Hans Spemann (who, with Mangold, first identified the organizer in 1924.^[1])

Development

In chick development, the primitive knot starts as a regional knot of cells that forms on the blastodisc immediately anterior to where the outer layer of cells will begin to migrate inwards - an area known as the primitive streak, which is involved with Koller's sickle. Posterior to the node is the primitive pit, where the cells of the epiblast (the upper layer of embryonic cells) initially begin to invaginate. This invagination expands posteriorly into the primitive groove as the cells layers continue to move into the space between the embryonic cells and the yolk. This differentiates the embryo into the three germ layers - endoderm, mesoderm, and ectoderm. The primitive knot migrates posteriorly as gastrulation proceeds, eventually being absorbed into the tail bud.

Default model

The cells of the primitive node secrete many cellular signals essential for neural differentiation. After gastrulation the developing embryo is divided into ectoderm, mesoderm, and endoderm. The ectoderm gives rise to epithelial and neural tissue, with neural tissue being the default cell fate. Bone morphogenetic proteins (BMPs) suppress neural differentiation and promote epithelial growth. Therefore, the primitive node (the dorsal lip of the blastopore) secretes BMP antagonists, including noggin, chordin, and follistatin.

References

↑ Garcia-Fernàndez J, D'Aniello S, Escrivà H (2007). "Organizing chordates with an organizer". BioEssays. 29 (7): 619–24. doi:10.1002/bies.20596. PMID 17563072.

External links

Overview at nature.com
Overview at Northwestern University
Embryonic Organizers at the US National Library of Medicine Medical Subject Headings (MeSH)

Human embryogenesis in the first three weeks

Week 1

Week 2
(Bilaminar)

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	Axial mesoderm Paraxial Somite Somitomere Intermediate Lateral plate Intraembryonic coelom Splanchnopleuric mesenchyme Somatopleuric mesenchyme

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