Renner Pietzak*

ABSTRACT

Stem cells are a special population of cells with the extraordinary ability to transform into a wide range of cell types that make up the human body. During early development and growth, these cells can mature into specialized structures, while in adult tissues they act as a built-in repair system, dividing continuously as needed to replace worn-out or damaged cells. Each time a stem cell divides, the daughter cells retain the ability either to remain stem cells or to differentiate into cells with distinct biological roles—such as neurons, muscle fibers, or blood cells. Two defining features set stem cells apart from other cells. First, they are undifferentiated, meaning they lack the specialized structures needed for specific bodily functions. Second, they possess the capacity for long-term self-renewal through continuous cell division. In organs like the bone marrow and digestive system, stem cells divide frequently to keep tissues healthy. In contrast, organs such as the pancreas or heart contain stem cells that divide only under specific or stressful conditions. Historically, research focused on two main categories of stem cells: embryonic stem cells and adult (somatic) stem cells. Mouse embryonic stem cells were first isolated in 1981, paving the way for scientists to derive human embryonic stem cells in 1998 using embryos generated through in vitro fertilization. Another scientific milestone occurred in 2006 when researchers discovered how to “reprogram” adult cells back into a stem-cell-like state. These reprogrammed cells—known as induced pluripotent stem cells (iPSCs)—behave similarly to embryonic stem cells. Stem cells are crucial to organisms for several reasons. In early embryonic development, specifically in the blastocyst stage (3 to 5 days post-fertilization), the inner cell mass forms every tissue of the body, from vital organs to reproductive cells. Even in adults, small pools of stem cells exist in organs such as the brain, muscle, and bone marrow, continuously replacing cells lost to injury, aging, or disease.