What Are Polyploid Cells and Why Are They Important?

Polyploid cells contain more than two complete sets of chromosomes, a state distinct from the diploid (two sets) condition of most human cells. While commonly associated with the plant kingdom, where it often contributes to desirable agricultural traits, polyploidy also occurs in specific animal cells, including those in the human body. Its presence has implications for normal biological functions and disease states.

How Polyploid Cells Are Formed

One primary mechanism is endoreduplication, a modified cell cycle where a cell replicates its DNA but forgoes the subsequent division into two daughter cells. This process involves cycles of DNA synthesis (S phase) and growth (G phase) without entering mitosis, leading to a single cell with multiple copies of its genome.

Another route to polyploidy is through cell fusion, where two or more separate cells merge into a single, larger cell. This creates a multinucleated cell that contains the combined chromosomal content of the original cells. This process is observed in the formation of certain tissues, such as muscles and bone-resorbing osteoclasts.

Errors during cell division, either mitosis or meiosis, can also generate polyploid cells. If the final step of cell division, known as cytokinesis, fails, a cell that has already duplicated its chromosomes will not split, resulting in a single cell with double the number of chromosomes. Similarly, failures in chromosome separation during the production of reproductive cells can lead to diploid gametes, which, upon fertilization, create a polyploid organism.

Polyploid Cells in the Natural World

Polyploidy is a widespread phenomenon, particularly within the plant kingdom, where it is a major driver of evolution and diversity. It is estimated that 30–80% of living plant species are polyploid. Many staple agricultural crops, such as wheat, cotton, potatoes, and strawberries, are polyploid, a condition often associated with larger fruits and flowers, as well as increased vigor. For example, bread wheat is hexaploid, and many cultivated strawberries are octoploid.

In the animal kingdom, polyploidy is less common at the whole-organism level but does occur in certain species of fish, salamanders, and insects. For instance, some species of salmon and trout are naturally tetraploid (four sets of chromosomes). In these animals, polyploidy can contribute to unique developmental pathways.

Within humans and other mammals, specific tissues are composed of polyploid cells that carry out specialized functions. Some examples include:

• Hepatocytes in the liver, which are integral to detoxification and regeneration.
• Cardiomyocytes (heart muscle cells) that support the heart’s high metabolic workload.
• Megakaryocytes in the bone marrow, which are large cells that fragment to produce platelets for blood clotting.
• Placental cells that support fetal development during pregnancy.

The Impact of Polyploid Cells on Human Health

While certain polyploid cells are a normal part of healthy tissue, their appearance can also be linked to disease, most notably cancer. Many tumors contain a significant population of polyploid cancer cells, which can arise from errors in cell division common in cancer. This state can contribute to genomic instability, as the division of polyploid cells often leads to aneuploidy—an incorrect number of individual chromosomes—which can fuel tumor evolution and adaptation.

Polyploid cells in tumors are often associated with resistance to chemotherapy and radiation. The extra sets of genes may provide a buffer against cellular damage induced by these treatments, allowing some cancer cells to survive and repopulate the tumor. Researchers have found that polyploidy enables cells to tolerate higher levels of this damage before dying.

Beyond cancer, an increase in polyploid cells can be a response to cellular stress or part of the aging process. In organs with limited regenerative capacity, such as the heart, polyploidization can be a way to maintain tissue function when diploid cells are lost or damaged. However, this can also contribute to age-related declines in organ function. For example, the accumulation of polyploid cells in the cornea of the eye is associated with age-related vision disorders.

Studying and Utilizing Polyploid Cells

Scientists study polyploid cells to understand fundamental aspects of biology, including cell cycle regulation, genome stability, and cell size control. By examining how and why cells become polyploid, researchers gain insights into both normal development and the progression of diseases like cancer.

In agriculture, polyploidy has been intentionally harnessed for decades to improve crops. Scientists can induce polyploidy using chemicals like colchicine, which disrupts cell division and leads to chromosome doubling. This technique has been used to create new plant varieties with enhanced traits, such as seedless watermelons, larger flowers, and increased resistance to environmental stressors. Ongoing research in this area aims to create hardier and more productive crops.

The characteristics of polyploid cells are also being explored in biotechnology and medicine. Because polyploidy is a feature of many aggressive cancers, it represents a potential target for new therapies designed to eliminate these resilient cells. Furthermore, understanding the role of polyploidy in tissue regeneration could inform new strategies in regenerative medicine.