4.5. Spemann and Mangold's Organizer, Part 2

Now, we have learned that Spemann and Mangold's Organizer, a section of dorsal mesoderm tissue, induces neural issue formation. What molecule was the organizer secreting to induce this?

To answer this, researchers first excised tissue from the animal pole of a blastula embryo and of a gastrula embryo (we call it the animal cap), isolated each, and tracked how each animal cap developed. Here's what happened:

The isolated animal cap of the blastula (pregastrula) became the epidermis, and isolated animal cap of the gastrula became neural tissue!

So, what converts the gastrula animal cap into neural tissue?

Researchers again turned to the blastula animal cap to see if they could induce gastrula-animal-cap-like behavior from the blastula cells (i.e., turn blastula animal cap into neural tissue). This was a smart way to test out what secreted molecules would (or wouldn't) induce neural formation.

Researchers found something interesting. If they isolate the blastula and continue letting it develop (as they did in the previous experiment), the animal cap becomes the epidermis. However, if the animal cap cells are dispersed in the petri dish (i.e., sort of spread out), the animal cap becomes neural tissue!

Why does the simple action of animal cap cell dispersal suddenly induce neural cells?

Researchers identified a molecule called BMP (short for bone morphogenic proteins) secreted by the isolated developing animal cap. Turns out, BMP is secreted by default between the animal cap cells, which induces the epidermis. So, it's actually the LACK of BMP that induces neural cells. When cells are dispersed, the BMP signal dilutes and dissipates. This ridding of BMP gives rise to neural cells.

Here are the critical points: In the presence of BMP, ectoderm becomes epidermis; to get neural tissue, the dorsal side of the embryo must remove BMP from ectodermal tissue.

Of course, while scientists can manually disperse cells in a dish, cells cannot be dispersed during development. And because the mesoderm signals the ectoderm to become neural tissue (See Spemann and Mangold's Organizer, Part I), these mesoderm signals must be ones that remove BMP from ectoderm cells.

Indeed, during gastrulation, the mesoderm secretes two molecules, noggin and chordin, that blocks BMP from binding to ectodermal cells, therefore preventing ectodermal cells near the mesoderm from becoming epidermis tissue.

This LACK of molecular signaling (BMP signaling) required for ectodermal tissue to become neural tissue is called the neural default model.

So let's put it together--here is what's happening in the embryo:

1. there’s BMP all over the embryo

2. in the organizer forming region, noggin and chordin are secreted and travel into ectoderm, mesoderm, and endoderm

3. noggin and chordin diffuse away from Spemann’s organizer (dorsal mesoderm)

4. noggin and chordin binds and inhibits BMP in the dorsal ectoderm, allowing those cells to become neural plate

As always, I hope you learned something new (even if it is a factoid) and are finding developmental neurobiology interesting (even if just a little bit more). Hope to see you back.

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