When a salamander limb is amputated, mature cells at the wound site (muscle, cartilage, connective tissue) dedifferentiate to form the blastema. What does 'dedifferentiation' mean in this context?
AThe cells die and are replaced by circulating stem cells from the bone marrow
BDifferentiated cells lose their specialized gene expression program, re-enter the cell cycle, and revert to a progenitor-like state capable of proliferating and re-differentiating into the cell types needed to rebuild the limb
CThe cells physically move from the limb stump to a different body location
DDedifferentiation means the cells become cancerous
Dedifferentiation is a reversal of the normal differentiation trajectory. Mature muscle fibers, for example, downregulate muscle-specific genes, fragment into mononucleated cells, re-enter the cell cycle, and become proliferative progenitors. These dedifferentiated cells form the blastema, which then re-differentiates to rebuild the missing limb structures. Lineage tracing studies show that dedifferentiated cells are lineage-restricted: muscle cells produce new muscle, cartilage cells produce new cartilage — they remember their tissue of origin. This partial dedifferentiation (enough to proliferate, not enough to become fully pluripotent) is a key feature of salamander regeneration.
Question 2 True / False
If a blastema from an amputated forelimb is transplanted to a hindlimb amputation site, it regenerates a forelimb, not a hindlimb.
TTrue
FFalse
Answer: True
The blastema retains positional identity — it 'remembers' that it came from a forelimb. This positional memory is encoded in the Hox gene expression pattern and other positional transcription factors of the blastema cells. When the blastema regenerates, it recapitulates the developmental program appropriate for its position of origin, not its new location. This demonstrates that regeneration is not a generic growth process but a re-deployment of the original developmental patterning program, with the positional coordinates carried by the cells themselves.
Question 3 Short Answer
Why do mammals have such limited regenerative capacity compared to salamanders and planarians?
Think about your answer, then reveal below.
Model answer: Several factors likely contribute: (1) Mammals respond to injury primarily with fibrosis (scarring) rather than blastema formation — the rapid inflammatory and fibrotic wound healing response seals the wound but prevents the formation of a regenerative progenitor population. (2) Mammalian cells have more restrictive chromatin modifications and cell cycle controls that make dedifferentiation difficult. (3) The immune response in mammals may be more hostile to regenerative processes than in cold-blooded vertebrates. (4) Mammals may have evolved to prioritize rapid wound closure (preventing infection) over regeneration, since the fitness cost of losing a digit is lower than the cost of a weeks-long open wound. However, mammalian regenerative capacity is not zero — the liver regenerates robustly, digit tips can regenerate in children, and the MRL mouse strain shows enhanced wound healing — suggesting that the molecular machinery for regeneration may be latently present but suppressed.
Research into why salamanders regenerate and mammals do not is actively pursuing therapeutic applications. If the molecular brakes on mammalian regeneration (fibrosis, immune response, chromatin restriction) can be identified and modulated, enhanced tissue regeneration in humans may be achievable.