You think it's just a "boxed lunch" for cells? In fact, it's a strategic "commander-in-chief"!

In the world of iPSC culture, we go through the motions day after day — changing medium, passaging cells. Those bottles of clear maintenance medium might seem to us like nothing more than "nutrient meals" for stem cells, providing some amino acids, vitamins, and sugars.

But today, let's peel back the veil. This seemingly simple liquid is, in reality, a meticulously designed "kingdom of signals" — filled with "secret codes" and "commands" that determine cell fate.

Why can iPSCs be so "willful"? Give them a little "color" (a signal), and they can differentiate into neurons, cardiomyocytes, or any cell type you desire. Conversely, if we want them to maintain their "original intention" — that is, a pluripotent state with unlimited proliferation — we must create an absolutely pure and stable environment.

The core architect of this environment is the maintenance medium. Its primary task is not to "feed" the cells, but to issue commands — continuously telling the iPSCs to stay themselves and avoid differentiation.

How does the maintenance medium issue commands? The key lies in the several core signaling pathways it precisely regulates. Let's get to know these three幕后 "commanders":

1. TGF‑β/Activin A pathway: "Who I am"

Mechanism: This pathway is critical for maintaining iPSC pluripotency (especially the transition from the naïve state to the primed state). It directly upregulates the expression of core pluripotency transcription factors (such as OCT4, SOX2, and NANOG) by activating downstream SMAD proteins. Simply put, it tells the cells every day: "Remember, you are a pluripotent stem cell! You are powerful!"

2. bFGF: "Survive and proliferate"

Mechanism: bFGF is one of the most important growth factors for maintaining iPSC self‑renewal and proliferation. It provides a sustained proliferative drive by activating pathways such as MAPK/ERK, while synergizing with other signaling pathways (e.g., TGF‑β) to collectively inhibit differentiation.

3. WNT pathway: "The master of balance"

Mechanism: The WNT pathway plays a complex and nuanced role in stem cell fate. Overly strong WNT signaling drives differentiation, whereas a moderate, stable WNT background is essential for maintaining pluripotency. iPSC maintenance medium uses specific inhibitors (e.g., IWP2) or agonists to precisely tune WNT signaling into a "sweet spot" range that supports self‑renewal without triggering differentiation.

In practice, how can you tell whether your culture system is healthy? Pay attention to these external manifestations of the cell's "signal language":

✔ Ideal colony morphology: Sharp borders, flat and compact, high nuclear‑to‑cytoplasmic ratio

✔ Stable proliferation rate: Consistent and stable population doubling time

✔ Consistent expression of pluripotency markers: OCT4, SOX2, NANOG positivity > 90%

✔ Low spontaneous differentiation rate: < 10%

When these indicators fluctuate, it often means the signaling balance has been disrupted — possibly due to compromised medium components or a need to adjust your handling practices.