Adult stem cells and asymmetric division in tissue renewal
In the classical model, an adult stem cell divides into two daughters with different fates: one stays a stem cell and the other becomes specialised.
Remember
In the classical model, when an adult stem cell divides, one daughter keeps the stem cell's place and the other becomes specialised, so the stem cell pool is preserved while lost cells are replaced.
Some adult tissues lose cells all the time. A tissue whose cells are short lived and constantly replaced is called a labile tissue, and the lining of the gut is the standard example. A stem cell is an unspecialised cell that can divide and give rise to specialised cells, and it is the source of this renewal.
In the classical model, a stem cell divides by mitosis (ordinary cell division) into two daughter cells with different fates. One daughter stays a stem cell, so the pool of stem cells keeps a steady size. This is self-renewal. The other daughter differentiates, meaning it becomes specialised, often through a progenitor stage (an immature cell already heading towards a specialised fate), into a mature cell that replaces one that has been lost. This is asymmetric division.
Stem cells are not spread evenly through the tissue. They live in small, protected places called niches, for example at the bottom of the intestinal crypts (small pits in the lining of the gut).
Asymmetric division is the classical model, not a rule for every division. Some tissues, including the intestine, also use balanced symmetric divisions, in which both daughters have the same fate, to maintain the pool: the balance is reached across the whole stem cell population rather than within each division.
Stem cells are also graded by potency, meaning how many cell types they can become. Adult tissue stem cells are unipotent (one type) or multipotent (several types). Totipotent cells, which can become any cell type, belong to the zygote and early embryonic cells.
Key facts
- Classical model
- a stem cell undergoes mitosis; one daughter remains as a stem cell while the other differentiates.
- Stem cell pool
- in the classical model, it keeps a steady size because one daughter remains a stem cell at each division.
- Where they sit
- stem cells are present in small, discrete populations, for example at the bottom of intestinal crypts.
- Labile tissue
- its cells are short lived and constantly replaced, and the lining of the gut is the standard example.
- Not a rule
- asymmetric division is the classical model, not a rule for every division, because intestinal stem cells can also use balanced symmetric divisions.
- Potency
- unipotent stem cells make one cell type, multipotent cells make several, and totipotent cells make any cell type (the zygote and early embryo).
Exam traps
- If stem cells divided only to make two differentiating daughters, mature cells would be replaced but the pool of stem cells would shrink and eventually run out, which is why in the classical model one daughter stays in reserve as a stem cell.
- If stem cells divided only to make two more stem cells, the pool would keep growing but no new mature cells would be made to replace cells lost from the lining, so balance also needs differentiating cells.
- Hypertrophy, where cells grow larger without increasing in number, adds no new cells, so it cannot match the continuous loss of cells from a rapidly renewing lining, which needs new cells made by stem cell division.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Pathology, with the answer and a full explanation after every question.
Practice these questions