Spemann organizer
Spemann's organizer , also known as the Spemann-Mangold organizer , is the oldest and best-known example of an embryonic signal center in embryology . It was discovered by Hans Spemann (1869–1941) and his doctoral student Hilde Mangold (1898–1924). For his achievements in the field of embryology, Spemann received the Nobel Prize in Physiology or Medicine in 1935 . During the development of amphibians , for example newts and frogs, the formation of the embryo is controlled by a specific group of cells. During ontogenesis , this cell group “organizes” the development of all other cells and was therefore called the organizer by Spemann.
yellow - yolk-poor part
red - gray crescent
green - yolk-rich part
The Spemann organizer is located in the so-called "gray crescent" of the fertilized egg cell. Even before the first division ( furrowing ), the zygote of an amphibian can be divided into three sections. The dark colored yolk-rich part and the light yolk-poor part are separated from each other by the gray crescent moon (red in the illustration). This is caused by the rotation of the cell cortex as a result of fertilization, whereby, among other things, components of the Wnt signal path are transported and collect in an area opposite the sperm entry point , the gray crescent. During the subsequent cell divisions , the cells that have emerged from the area of the gray crescent receive significantly higher concentrations of Wnt signal components.
On the other hand, on the yolk-rich, vegetal side of the embryo, special mRNA molecules (maternal) are provided, such as vg1 , which codes for a component of the TGF-β signaling pathway, and vegt -mRNA, which codes for a transcription factor . In the zone where the present mRNAs overlap with the Wnt signal components located in the gray crescent, the Nieuwkoop center is initially formed, which in turn is formed by the secretion of ligands of the Nodal signaling pathway (another branch of the TGF-β signaling pathway) Organizer induced further above it.
By secreting inhibitors of the Wnt and BMP signaling pathways (another branch of the TGF-β signaling pathway), the organizer then takes on a central role in the pattern formation of the embryo, both with regard to the belly / back differentiation (ventro-dorsal axis) as well as the head / tail distinction (anterio-posterior axis).
In the former process, chordin , noggin and follistatin play a special role as they are inhibitors of the BMP signaling pathway. While BMP acts on cells from the ventral side, Chordin influences the cells from the dorsal side. This creates an activity gradient that controls the differentiation of other structures. For example, on the ventral side, where BMP activity is highest, the epidermis is formed from the ectoderm , while on the opposite BMP-inactive side, the ectoderm becomes neuroectoderm , from which nerve tissue later emerges (see also neurulation ).
The same applies to the mesoderm , from which blood islands emerge on the ventral side and the chorda dorsalis on the dorsal side . This axis structure of the chordates is formed directly by the organizer, which itself is of mesodermal origin, is displaced by gastrulation movements and lengthened to the notochord. At the same time, the organizer secretes inhibitors of the Wnt signaling pathway, such as proteins of the Dickkopf / Dkk and sFRP families, which in turn cause the formation of anterior structures such as the head at the front, while at the other end the increased activity of the Wnt signals for the formation of posterior ones Structures like the tail leading. The fact that overexpression of Dickkopf homologues leads to an anteriorized embryo with a sharply shortened tail and a significantly enlarged head is what gives these proteins their name. Wnt and BMP thus act as shapers, morphogens , which, depending on their concentration or activity, are able to induce the formation of different cell types.
The organizing cell group is therefore of crucial importance for the formation of the longitudinal axis of the embryo. The invagination of the blastopore during gastrulation is associated with a specialization of the tissues in its vicinity. The role played by the cluster of cells located on the blastopore lip has earned him the name of organizer. The role of the organizer was proven by Hans Spemann's doctoral student Hilde Mangold through a microsurgical experiment on newt embryos: If the organizer is transplanted to another location in another embryo at the beginning of gastrulation, the target embryo begins gastrulation at two locations. In this way, as a result of two organizing regions, a double set of body structures is created on a single embryo. Depending on the location of the implantation, this leads to more or less mature Siamese twins . Manipulating the localization of Wnt signal components in the gray crescent can also lead to different forms of axis formation. If the Wnt signal path z. B. activated outside the crescent structure creates another organizer and another body axis.
Signal proteins also play a central role in the subsequent shape-forming processes, for example face formation. By manipulating the underlying mechanisms, deviations can be brought about, for example a doubling of the face. By mutating a gene that is necessary for the synthesis of the sonic hedgehog protein , similar malformations can occur naturally.
Organization centers comparable to the Spemann-Mangold organizer can also be found in the other vertebrates, such as the dorsal shield in zebrafish or the primitive knot of amniotes , also called Hensenscher knot in birds .
literature
- Viktor Hamburger : The Heritage of Experimental Embryology: Hans Spemann and the Organizer. Oxford University Press , 1988, ISBN 0-19-505110-6 (English).
- Werner A. Müller, Monika Hassel: Developmental and reproductive biology of humans and animals. Springer textbook. 4th edition. Springer Science + Business Media , 2005, ISBN 3-540-24057-8 , Chapter 12.4: Induction of heads and torsos in the vertebrate embryo with transplanted organizational tissue, p. 331f ( preview ; as of May 25, 2009).
- Peter E. Fässler, Klaus Sander : Hilde Mangold (1898–1924) and Spemann's organizer: achievement and tragedy. In: Roux's Arch. Dev. Biol. 205, 1996, pp. 323-332.
- Klaus Sander, Peter E. Fässler: Introducing the Spemann-Mangold organizer: experiments and insights that generated a key concept in developmental biology . In: Int. J. Dev. Biol. 45, 2001, pp. 1-11.