Introduction to Neural Tube and Cell Fate Patterning

The neural tube is the embryonic precursor of the central nervous system, formed from the neural plate in vertebrate embryos. Proper formation and patterning of the neural tube depend on the precise specification of cells into neural tissue (neural plate) or epidermis (skin). This specification is regulated by gradients of signaling molecules, including BMPs and Sonic hedgehog (Shh), which control the dorsal-ventral patterning and differentiation within the ectoderm.

BMP Signaling and Its Role in Cell Fate

BMPs, a subgroup of the TGF-β superfamily, are crucial morphogens that direct ectodermal cells to adopt different fates based on concentration gradients:

• High BMP levels: Induce the differentiation of ectodermal cells into epidermis (skin cells). The epidermis forms the outermost protective layer of the embryo.

• Low BMP levels: Allow ectodermal cells to become neural plate cells that will eventually differentiate into the central nervous system structures.

• Intermediate BMP levels: Are critical for the formation of intermediate cell populations such as neural crest cells, which migrate to form diverse tissues including peripheral nerves and pigment cells.

BMP signaling is tightly regulated by antagonists secreted by the neural plate and surrounding tissues, ensuring correct spatial patterning.

Sonic Hedgehog and Neural Tube Patterning

While BMP controls dorsal fates (epidermis and neural crest), Sonic hedgehog (Shh) signaling emanating from the notochord and floor plate patterns ventral neural tube structures such as motor neurons. Shh complements BMP gradients, together orchestrating the complex dorsal-ventral axis of the neural tube.

Mechanism Summary

• Ectodermal cells exposed to high BMP become epidermis.

• Cells with low BMP become neural plate and contribute to the brain and spinal cord.

• Intermediate BMP induces neural crest cells at the border between neural plate and epidermis.

• Sonic hedgehog provides ventralizing signals, antagonizing BMP’s dorsalizing influence.

Conclusion

The true and fundamental statement regarding BMP in neural tube development is that high levels of BMP specify epidermis. This graded BMP activity, combined with Shh signaling, generates a well-organized and patterned neural tube necessary for subsequent vertebrate nervous system development.

Final Answer:
(1) High levels of BMP specify the cells to become epidermis.