Primitive knot
Gray's	subject #6 47
Days	17

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

Contents 1 Diversity 2 Development 3 References 4 External links

[edit] 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])

[edit] 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. Differential secretion of factors by the node also causes development of the right-left axis in the embryo.

[edit] References

1. ^ 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. 

[edit] External links

• Overview at nature.com
• Overview at Northwestern University
• MeSH Embryonic+Organizers

v • d • e Developmental biology > Human embryogenesis (development of embryo) and development of fetus First three weeks Week 1 Fertilization · Egg activation · Zygote · Cleavage · Morula · Blastula (Blastomere) · Blastocyst · Inner cell mass Week 2 (Bilaminar) Hypoblast · Epiblast Week 3 (Trilaminar) Germ layers Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) · Gastrula/Gastrulation · Regional specification Ectoderm Surface ectoderm · Neuroectoderm · Somatopleure · Neurulation · Neural crest Endoderm Splanchnopleure Mesoderm Chorda- · Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) · Intermediate · Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure) Extraembryonic/ uterus Trophoblast (Cytotrophoblast, Syncytiotrophoblast) Blastocoele · Yolk sack/exocoelomic cavity · Heuser's membrane · Extraembryonic coelom · Vitelline duct Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) · Allantois Placenta · Decidua (Decidual cells) · Chorionic villi/Intervillous space · Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion) Histogenesis Programmed cell death (Apoptosis) · Stem cells · Germ line development Organogenesis Limb development: Limb bud · Apical ectodermal ridge/AER other structures: Eye development · Cutaneous structure development · Heart development · Development of the urinary and reproductive organs (Some dates are approximate—see Carnegie stages and a timeline.)