Q.96. Neurogen (Ngn) a newly discovered protein in chicken. is produced by the notochord and the floor
plate (FP). Ngn induces cells of the neural tube (NT) to become neurons. It is known that from
ventral to dorsal direction cells at different levels give rise to distinct types of neuronal cells. Which
of the following observations will cast a doubt in the claim that Ngn is a morphogen?
(A) Ngn is a cytosolic protein
(B) Artificial mis-expression of Ngn at identical level through out NT does not affect the neuronal
cell types formed in the NT
(C) Ngn is an integral membrane protein
(D) All of the above
(B) Artificial mis-expression of Ngn at identical level through out NT does not affect the neuronal cell types formed in the NT is the observation that casts doubt on Ngn being a morphogen.
What is a Morphogen?
A morphogen is a signaling molecule that forms a concentration gradient across a tissue and induces distinct cell fates at different threshold concentrations. Neurogenin (Ngn), produced by the notochord and floor plate, patterns the neural tube (NT) from ventral to dorsal, generating specific neuron types. For Ngn to qualify as a morphogen, uniform expression should mimic the gradient’s effects by producing varied cell types based on local concentrations.
Why Option (B) Casts Doubt
Artificial uniform mis-expression of Ngn throughout the NT eliminates any gradient. If Ngn is a morphogen, cells should still differentiate into distinct neuron types based on absolute levels. No change in cell types indicates Ngn cannot specify positional identity in a concentration-dependent manner, violating the morphogen definition.
Analysis of All Options
| Option | Description | Supports Morphogen Claim? | Explanation |
|---|---|---|---|
| (A) Ngn is a cytosolic protein | Ngn stays inside producing cells | No, but doesn’t disprove strongly | Morphogens typically diffuse (secreted), but cytosolic proteins can act non-cell-autonomously via unconventional mechanisms. Primary issue is lack of gradient evidence . |
| (B) Artificial mis-expression… no effect | Uniform Ngn levels don’t alter neuron types | No—casts strongest doubt | Fails French flag test; uniform concentration should pattern if concentration-dependent . |
| (C) Ngn is an integral membrane protein | Ngn embedded in cell membrane | No | Membrane proteins rarely diffuse to form gradients, preventing long-range morphogen action . |
| (D) All of the above | Combines A, B, C | Partially | Includes (B), the definitive disproof, but (A) and (C) are weaker . |
