12. The figure above represents a late zebrafish gastrula. The following concepts may be proposed during further development of the embryo.
    A. The concentration of FGF decreases from the yolk towards the epidermis, along with the increase of BMP activity from the dorsal to the ventral axis.
    B. Increase in FGF activity in the epidermis with concomitant decrease in BMP activity towards the ventral axis.
    C. Neural induction in zebrafish is independent of the organizer and depends on activation of BMP signaling.
    D. In comparison, both Xenopus and chick embryos require activation of FGF for neural induction to occur in addition to BMP inhibition.
    Which of the above statements are true?
    (1) A and C (2) B and C
    (3) A and D (4) C and D

The true statements regarding the development of the late zebrafish gastrula, based on morphogen gradients and neural induction research, are:

A. The concentration of FGF decreases from the yolk towards the epidermis, along with the increase of BMP activity from the dorsal to the ventral axis.

D. In comparison, both Xenopus and chick embryos require activation of FGF for neural induction to occur in addition to BMP inhibition.

Multiple studies show that in zebrafish, FGF (Fibroblast Growth Factor) signaling is high near the yolk and decreases toward the outer epidermis, while BMP (Bone Morphogenetic Protein) activity increases from the dorsal side (where neural tissues form) toward the ventral side (epidermis). BMP inhibition and FGF activation are critical for neural induction in zebrafish, Xenopus, and chick embryos, unlike option C, which is incorrect, since neural induction in zebrafish depends on BMP inhibition just as it does in other vertebrates. Option B does not represent the scientific consensus about FGF or BMP gradients.​

Therefore, the correct answer is:

(3) A and D

Introduction

Morphogen gradients play a decisive role in establishing body axes and specifying tissue fates during early embryogenesis. In zebrafish, the orchestration of gradients such as FGF and BMP is critical for patterning along the dorsal-ventral and anterior-posterior axes and for neural induction in the late gastrula.

FGF and BMP Gradients Shape the Gastrula

• FGF (Fibroblast Growth Factor) is concentrated near the yolk and diminishes toward the epidermal regions.

• BMP (Bone Morphogenetic Protein) activity is lowest on the dorsal side and increases ventrally, directing cell fates from neural to epidermal.

• This orchestration ensures neural tissues form dorsally and epidermal tissues ventrally, with intermediate gradients defining other structures.​

Neural Induction Requires FGF and BMP Inhibition

• Neural induction in zebrafish is not independent of the organizer or BMP inhibition.

• Both FGF activation and BMP inhibition are necessary for proper neural tissue specification in vertebrates, including Xenopus and chick embryos.

• Chordin, FGF signaling, and mesodermal factors work together, with FGF also repressing BMP transcripts directly in early development.​

• This model is consistent across multiple vertebrate species and highlights conserved developmental pathways.

Why Other Statements Are Incorrect

• Neural induction in zebrafish is not independent of organizer or BMP inhibition (option C is false).

• FGF does not increase in the epidermis; its activity decreases away from the yolk, the opposite of statement B.

Conclusion

Zebrafish development is governed by gradients of FGF and BMP, with neural induction relying on both FGF activation and BMP inhibition—an evolutionary conserved mechanism shared among vertebrates such as Xenopus and chicks.

Final Answer:
(3) A and D