Cultured cells refer to cells that are grown and maintained under controlled conditions outside of their original organism or natural environment. Cell culture is a crucial technique in biological research, allowing scientists to study the behavior, function, and response of cells under different conditions, as well as develop therapies and perform drug testing.

There are several types of cultured cells, including:

1. Primary cells: These cells are isolated directly from animal or human tissues and then cultured. Primary cells maintain many characteristics of the original tissue, making them useful for studying physiological processes. However, they have a limited lifespan and can undergo a limited number of cell divisions before they stop growing, a phenomenon known as replicative senescence.
2. Cell lines: Cell lines are immortalized cells that have been genetically modified or derived from tumors, allowing them to proliferate indefinitely in culture. These cells are often used in research because they are easier to maintain and manipulate than primary cells. However, they may exhibit altered characteristics compared to their original tissue due to genetic changes, which can limit their relevance in some studies.
3. Stem cells: Stem cells are undifferentiated cells that can differentiate into various specialized cell types. They can be cultured and manipulated to produce specific cell types for research or therapeutic purposes. There are different types of stem cells, including embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and adult stem cells (such as mesenchymal stem cells or hematopoietic stem cells).

Cultured cells are typically grown in a controlled environment within an incubator, which maintains optimal temperature, humidity, and gas composition (usually 5% CO2) to support cell growth. Cells are grown in a nutrient-rich culture medium that supplies essential components such as amino acids, vitamins, minerals, and growth factors. The medium may also contain animal serum (e.g., fetal bovine serum) to provide additional nutrients and support cell growth.

Cell culture requires careful maintenance to avoid contamination and ensure optimal growth conditions. Cells need to be monitored regularly and passaged (transferred to a fresh culture vessel with new medium) once they reach a certain density to prevent overcrowding and maintain healthy growth. Additionally, sterile techniques must be employed when handling cultured cells to prevent contamination by bacteria, fungi, or other microorganisms.