By: N. Shreya Mohan (MSIWM042)

The Spemann-Mangold organizer are a consortium of cells that are required for the commencement of the neural tissue during the development of an amphibian embryo. Hilde Mangold, the then doctorate student along her mentor Hans Spemann, first published this work in 1924. This discovery gave so much scope to the developmental biology field, that this became one of the few doctoral theses to have won the Nobel prize in 1935. They showed that, of all the tissues in the early gastrula stage, only one has its fate determined. The overview of this organizer proved that destiny of the cells can be altered and influenced by factors from other cell clusters.

This self-differentiating tissue is the dorsal lip (the dorsal bordering region of the blastopore, which acts as the centre of differentiation) of the blastopore, the tissue derived from the grey crescent cytoplasm. When this dorsal lip tissue was transplanted into the so-called belly skin region of another gastrula, it not only continued to be blastopore tip, but also started the process of gastrulation and embryogenesis in the nearby tissues. Later on, two conjoined twins were formed instead of one. 

In this experiment, Spemann and Mangold used two different pigmented embryos from two newt species- that is, the darkly pigmented Triturus taeniatus and the non-pigmented Triturus cristatus. Two different species were taken for this experiment for transplantation by Spemann and Mangold as it would be easier to identify which one was the host and donor tissues respectively. At first, the dorsal lip of an early T. taeniatus gastrula was removed and then implanted into the area of an early T. cristalus gastrula was supposed to turn into ventral epidermis. As predicted, the dorsal lip tissue invaginated (cleaved) showing the qualities of self-determination and disappeared under the vegetal cells. The egg is divided into two regions- the animal pole (top part of the egg) and the vegetal pole (bottom part of the cell). Usually, the genetic material and proteins are unevenly distributed among these two poles.

The donor tissue (the pigmented species) of newt then continued to self-differentiate and divide into a structure called chordamesoderm (notochord) and other mesodermal structures respectively which usually comes from the original dorsal lip. 

Now, these newly made donor- derived mesodermal cells move forward for further participation in differentiation. As they are in movement, the host cells participate in the formation of new embryo. It creates organs that normally never would have formed before. In the secondary embryo, a somite could be seen containing both pigmented (that is, the donor) and the unpigmented (which is the host) tissues. What was more shocking was, the dorsal lip was able to interact strongly with the host tissue to form a fully formed neural tube derived solely from the host’s ectoderm. Back then, Spemann referred to the dorsal lip cells and their derivatives as the organizer. The reason being was because-

  • They could induce the host’s ventral tissue to change their fates to form a neural tube and a dorsal mesodermal tissue (most commonly the somite).
  • They could systematically organize host and donor tissues into a secondary embryo with clear anterior-posterior and dorsal-ventral regions.

Because, there are numerous inductions during the embryonic developments, this key induction wherein the progeny of dorsal lip cells induces the dorsal axis and the neural tube is traditionally called the Primary embryonic organizer.

REFERENCES-

https://www.khanacademy.org/science/biology/developmental-biology/signaling-and-transcription-factors-in-development/a/frog-development-examples

https://en.wikipedia.org/wiki/Spemann-Mangold_organizer

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