A tissue stem cell divides asymmetrically, producing one daughter that remains a stem cell and one that differentiates. Why is this asymmetric division essential for tissue homeostasis?
AIt ensures the stem cell pool size remains constant while continuously producing differentiated cells to replace those lost to turnover
BIt is not essential — symmetric division works equally well for all tissues
CAsymmetric division is faster than symmetric division
DIt prevents the stem cell from ever dividing again
Tissues like the gut epithelium, blood, and skin continuously lose differentiated cells (shed, apoptotic, senescent) that must be replaced. If stem cells divided symmetrically into two stem cells, the pool would expand uncontrollably (tumor-like). If they divided into two differentiated cells, the stem cell pool would be depleted. Asymmetric division maintains the stem cell pool (one daughter retains stem cell identity) while producing replacement differentiated cells (the other daughter enters the differentiation pathway). In practice, tissues use a mix of asymmetric and population-asymmetric (stochastic symmetric) divisions, but the net result must balance self-renewal and differentiation.
Question 2 True / False
The stem cell niche is simply a physical location with no active signaling role — stem cells maintain their identity cell-autonomously.
TTrue
FFalse
Answer: False
The niche is an active signaling microenvironment that maintains stem cell identity through direct cell-cell contact (Notch signaling, adherens junctions), secreted factors (Wnt ligands, BMP antagonists, growth factors), and extracellular matrix components. When stem cells are removed from their niche (by transplantation or dissociation), they typically differentiate — demonstrating that stem cell identity is not purely cell-autonomous but depends on continuous niche signals. The intestinal crypt niche, for example, provides Wnt ligands from Paneth cells and BMP antagonists from the surrounding mesenchyme, both of which are required to maintain intestinal stem cell self-renewal.
Question 3 Short Answer
How does the concept of a cancer stem cell relate to normal stem cell biology?
Think about your answer, then reveal below.
Model answer: The cancer stem cell hypothesis proposes that tumors contain a subpopulation of cells with stem-like properties: self-renewal (sustaining tumor growth indefinitely) and the ability to differentiate (producing the heterogeneous cell types found in the tumor). If correct, this means that therapy must eliminate the cancer stem cells specifically; killing bulk tumor cells without targeting the self-renewing subpopulation allows tumor regrowth. Cancer stem cells may arise from normal stem cells that acquire oncogenic mutations, or from differentiated cells that reactivate self-renewal pathways. The shared molecular machinery (Wnt, Notch, Hedgehog signaling) between normal stem cells and cancer stem cells makes targeting one without harming the other a major therapeutic challenge.
Evidence for cancer stem cells is strong in some cancers (acute myeloid leukemia, glioblastoma) and debated in others. The concept has stimulated important research into the molecular mechanisms of self-renewal and has shifted therapeutic focus from killing all rapidly dividing cells to targeting the self-renewal capacity of the stem cell compartment.