Primary cell lines are cultures derived directly from living tissues, typically taken from animals or humans. These cells maintain many of the characteristics of the original tissue, allowing researchers to study biological processes in a more natural context. Primary cell lines are often used in research for drug testing, disease modeling, and understanding cellular functions. Unlike immortalized cell lines, which can divide indefinitely, primary cell lines have a limited lifespan and will eventually stop growing. This limitation makes them valuable for specific experiments but also requires careful management to ensure they are used effectively within their viable period.