Cell populations

Cell fate: quiescence phase and division capacity

After coming into existence as a result of mitosis, a cell decides its further fate. A huge portion of the cell pool undergoes differentiation, acquiring narrow specialization. In the process, cells leave the active cell cycle and enter the reproductive quiescence stage, designated as $G_0$.

Depending on how firmly a cell is "stuck" in this quiescence, four groups are distinguished:

1. Non-dividing (terminally differentiated). These structures permanently lose the ability to return to the $G_1$ phase and resume proliferation. Classic examples are nerve cells (neurons), heart muscle cells (cardiomyocytes), as well as retinal photoreceptors (rods and cones).
2. Rarely dividing (conditionally renewing). They are capable of remaining in the quiescence stage for a very long time (months), but can re-enter the cell cycle if necessary. A prominent representative is the liver cell (hepatocyte).
3. Rapidly proliferating (renewing). They practically do not linger in quiescence, completing a full cycle every 12–36 hours. These include the basal layer of the epidermis, the epithelial lining of the small intestine, and all hematopoietic elements.
4. Ultrashort cycle. Found in the embryonic period during rapid cleavage of eggs in lower animals (amphibians, echinoderms). The entire cycle takes about 30 minutes.

Classification of populations by renewal rate

Based on how a tissue maintains its population size and whether it is capable of division and renewal, all cell populations of the body are divided into three global types.

1. Stable populations

These are tissues whose elements have completely lost the ability to divide. The number of cells in such systems is strictly stabilized at the very early stages of their differentiation. Throughout life, the number of such cells can only decrease due to natural cell death. Replenishing losses through the division of remaining cells is impossible.

• Typical representatives: cardiomyocytes, neurons.

Growing and renewing populations

2. Growing populations

They are characterized by the ability not only for renewal but also for active growth, which leads to an increase in total tissue mass.

Growth is achieved in two ways: by increasing the actual number of cells and through the process of polyploidization (a multiple increase in the chromosome set). Cells here are long-lived and perform highly specialized functions. However, their key feature is that they retain the ability to respond to stimulating signals. If tissue is damaged, cells exit quiescence and enter the cycle, restoring the population size.

• Where found: liver, kidneys, thyroid and pancreas.

3. Renewing populations

Here, a continuous conveyor-belt process takes place. A strict balance is maintained in the tissue: the natural loss of old differentiated cells as a result of cell death is completely counterbalanced by the birth of new ones.

The main source of such continuous renewal is cambial cells. These are poorly differentiated elements that actively divide, after which their progeny undergo differentiation and replace the dead cells.

• Where found: skin epidermis, blood and bone marrow cells, intestinal epithelium.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of cell transition from the G1 phase to the reproductive quiescence stage (G0).
• The role of apoptosis and necrosis in maintaining cellular balance.
• Cell polyploidization: causes and biological significance.
• Properties and types of cambial (stem) cells in various tissues.
• Differences between the embryonic cell cycle and the somatic cell cycle.

Other topics in the "Cell Division" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.