The following statements are made regarding the amphibian early-embryonic development:

A. The Nieuwkoop centre cells  are mesodermal in origin.

B. Chordin, Noggin and Goosecoid are secreted by the Organizer.

C. The default fate of the ectoderm is to become neural tissue.

D. BMP levels are high in the presumptive dorsal mesoderm.

Which one of the following options represents the combination of all correct statements?

Correct option is C

Amphibian early embryonic development involves key molecular signals that regulate germ layer formation, neural induction, and dorsal-ventral patterning.

Analysis of Statements:

1. Statement A: Incorrect

   • The Nieuwkoop center is actually endodermal in origin, not mesodermal.
   • It is located in the dorsal-vegetal region of the early amphibian embryo and plays a crucial role in inducing mesoderm formation by signaling to overlying cells.
   • Since the statement says "mesodermal in origin", it is incorrect.

2. Statement B: Correct 

   • Chordin, Noggin, and Goosecoid are secreted by the Spemann-Mangold Organizer, which is in the dorsal mesoderm.
   • These molecules act as BMP inhibitors, promoting neural tissue formation by preventing epidermal differentiation.
   • Since this correctly describes Organizer-secreted molecules, it is correct.

3. Statement C: Correct 

   • The default fate of the ectoderm is neural tissue in the absence of BMP signaling.
   • BMP4 promotes epidermal fate, but its inhibition by Chordin, Noggin, and Follistatin allows neural induction.
   • This follows the neural default model, making this statement correct.

4. Statement D: Incorrect 

   • Bone Morphogenetic Proteins (BMPs) are high in ventral mesoderm and ectoderm, but low in the dorsal mesoderm.
   • The dorsal mesoderm expresses BMP inhibitors, ensuring low BMP levels in that region.
   • Since the statement claims BMP levels are high in the dorsal mesoderm, it is incorrect.