Primitive knot
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
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. 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 germ layers - endoderm, mesoderm, and ectoderm. The primitive knot migrates posteriorly as gastrulation proceeds, eventually being absorbed into the tail bud.
The cells of the primitive knot secrete many cellular signals essential for gastrulation - including Fibroblast Growth Factors, Sonic hedgehog, and retinoic acid. Cilia beating within the knot cause differential buildup of secreted factors in the nodal cavity, leading to development of the right-left axis in the embryo.
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