How does asymmetric cell division work in stem cells?

Answer

Asymmetric cell division in stem cells is necessary to ensure that when a stem cell divides, one daughter cell remains stem cell while the other daughter cell can differentiate into a cell type with a specific function. If the cell division were symmetrical, then both daughter cells would remain a stem cell, and then no new cells with a certain function could be formed, or both daughter cells would differentiate and then there would be no stem cell left.

The mechanism or mechanisms responsible for it have not yet been fully elucidated. The general principle is that a stem cell is polarized, and this is the result of both signals inside the cell and signals that the cell receives from outside. In the latter case, one speaks of the niche.

Signals from the outside: The cell can receive signals from surrounding cells of the niche or the extracellular matrix on one side, and not on the other side. These are receptor-ligand interactions that control the orientation of the spindle, i.e. the axis along which the cell will divide.

Signals from within: Certain proteins or RNAs are distributed heterogeneously in the cell, so more at one pole than the other. When the cell divides along a certain axis, one daughter cell will contain more of these proteins, which serve as a signal for gene expression, than the other. And this then leads to two different daughter cells (asymmetry). The centrosomes in the cell also play an important role. The centrosome, which controls the formation of the spindle, divides itself semi-conservatively: one daughter cell will keep the old centrosome (= the mother centrosome), and the other will receive a newly formed centrosome (= daughter centrosome). The cell with the parent centrosome remains a stem cell while the other can differentiate. Certain signaling proteins or RNAs are thought to stick to those centrosomes and play a role in the process.

For more details, I refer to an interesting review article in the Journal of Cell Biology of September 19, 2018 (DOI: 10.1083/jcb.201807037).