Human fibroblasts and induced pluripotent stem cells (iPSCs) derived from them are genetically identical, yet they express dramatically different sets of genes. The transformation from a specialized skin cell into a stem cell requires switching on hundreds of dormant genes, silencing others, and fundamentally rewiring how the cell grows, divides, and generates energy. These differences span pluripotency networks, cell cycle control, metabolic pathways, and even small regulatory RNA molecules.
Pluripotency Genes Turn On in iPSCs
The most striking difference is the activation of pluripotency genes. Fibroblasts are terminally differentiated cells. They do not express the core transcription factors that define stem cells: Oct4, Sox2, and Nanog. These genes are effectively locked down in fibroblasts through chemical modifications to the DNA and its surrounding proteins.
iPSCs, by contrast, express all three at high levels, along with surface markers like TRA-1-60 and TRA-1-81 and additional pluripotency-associated genes such as LIN28 and ZFP42. This suite of active genes is what gives iPSCs the ability to become any cell type in the body. The process of creating iPSCs requires artificially forcing fibroblasts to express four transcription factors (Oct4, Sox2, Klf4, and c-Myc, collectively known as the Yamanaka factors), and among these, high expression of Oct4 relative to the others is especially important for efficient reprogramming.
Interestingly, Klf4 is naturally expressed in fibroblasts, where it serves a different role related to maintaining fibroblast identity. During reprogramming, the context of Klf4’s activity changes as the broader gene network around it shifts.
Cell Cycle Control Is Fundamentally Rewired
Fibroblasts divide a limited number of times before entering a permanent growth arrest called senescence. This process is governed by tumor suppressor pathways, particularly those involving p16 and p21, two proteins that act as brakes on cell division. As fibroblasts age in culture, levels of p16 and p21 rise, eventually halting proliferation entirely.
iPSCs handle cell division very differently. During reprogramming, the gene locus encoding p16 (and a related protein, p14) acquires chemical marks that silence it completely. The p53-p21 pathway, another major growth checkpoint, is also transiently inactivated during the conversion. This allows iPSCs to self-renew indefinitely, a hallmark of stem cells. The tradeoff is reduced genomic stability: removing these safeguards means iPSCs are more tolerant of DNA damage that would normally stop a fibroblast from dividing.
When iPSCs are later differentiated back into fibroblasts in the lab, those “re-made” fibroblasts show higher baseline levels of p21 and p53 than the original fibroblasts they came from. They also appear more sensitive to stress and harder to immortalize, requiring the suppression of p53 before they can be grown indefinitely. This suggests that the reprogramming process leaves a lasting imprint on how these pathways are regulated, even after the cell returns to a fibroblast-like state.
Metabolic Gene Programs Shift
Fibroblasts generate most of their energy through oxidative phosphorylation, a process that takes place in the mitochondria and relies on oxygen to efficiently convert nutrients into cellular fuel. The genes that support this process, encoding components of the electron transport chain and mitochondrial enzymes, are highly active in fibroblasts.
iPSCs rely far more on glycolysis, a less efficient but faster method of energy production that breaks down glucose in the cell’s main compartment rather than in the mitochondria. This metabolic switch is not incidental. It mirrors the metabolism of embryonic stem cells and early embryos, which also favor glycolysis. The shift involves upregulating genes for glycolytic enzymes and glucose transporters while dialing down mitochondrial energy production genes. Cells that fail to make this metabolic transition during reprogramming generally do not become fully pluripotent.
MicroRNA Profiles Are Nearly Opposite
Beyond protein-coding genes, fibroblasts and iPSCs express very different profiles of microRNAs, small RNA molecules that regulate gene expression by silencing specific messenger RNAs. One of the clearest distinctions involves miR-302a-3p, a microRNA strongly associated with stem cell identity. All iPSC lines express miR-302a-3p at significantly higher levels than fibroblasts. This microRNA helps maintain pluripotency by suppressing genes that would push the cell toward differentiation.
The reverse pattern holds for miR-145-5p, a microRNA linked to differentiation. Fibroblasts express it at high levels, while iPSCs express it at very low levels. The statistical difference is stark in both directions, with p-values below 0.001 across multiple iPSC lines tested. These microRNA switches reinforce the broader gene expression changes: miR-302a-3p helps keep pluripotency genes active, while the silencing of miR-145-5p removes a force that would otherwise suppress them.
Epigenetic Memory Lingers From the Original Cell
Despite all these changes, iPSCs do not perfectly erase every trace of the fibroblast they came from. Early-passage iPSCs retain what researchers call “epigenetic memory,” residual chemical marks on the DNA that reflect the gene expression profile of the donor cell. These marks bias the iPSCs toward differentiating back into the cell type they originated from rather than becoming other cell types equally well.
This has been demonstrated by comparing iPSCs made from different source tissues. When iPSCs were generated from blood cell precursors and muscle precursors taken from the same animal, each set of iPSCs retained higher expression of markers specific to its tissue of origin. For fibroblast-derived iPSCs, this means genes associated with connective tissue identity may remain slightly more accessible than genes for, say, blood or nerve cells.
Epigenetic memory tends to fade with extended passaging (growing cells through many rounds of division), as the iPSC state gradually overwrites the old marks. But in the early stages after reprogramming, the fibroblast signature is measurably present. This has practical implications for researchers choosing iPSCs for specific experiments: if the goal is to generate a particular cell type, starting from a source tissue related to that lineage can improve efficiency.
How These Differences Fit Together
The gene expression differences between fibroblasts and iPSCs are not random. They form an interconnected network. Pluripotency transcription factors like Oct4 and Sox2 directly activate genes for self-renewal and glycolytic metabolism while repressing differentiation-associated genes. MicroRNAs like miR-302a reinforce this by silencing transcripts that conflict with the stem cell program. The shutdown of p16 and the p53-p21 axis removes the proliferation limits that define fibroblast biology. And the metabolic shift to glycolysis supports the rapid cell division that pluripotent cells require.
In essence, nearly every major gene regulatory layer, from transcription factors to microRNAs to epigenetic marks to metabolic enzymes, is reconfigured during the conversion from fibroblast to iPSC. The two cell types share the same genome but read it in profoundly different ways.