HMEC stands for Human Mammary Epithelial Cells. These cells line the ducts and lobules of the mammary gland and are the cells from which the majority of human breast cancers arise. Immortalization of HMECs refers to the process by which these cells acquire the ability to proliferate indefinitely.

In normal physiological conditions, HMECs, like most human cells, have a finite lifespan. This is due to a phenomenon known as replicative senescence, where cells cease to divide after a certain number of divisions. This is largely due to the progressive shortening of telomeres, the protective caps at the ends of chromosomes, during each round of cell division.

However, in certain conditions, such as in many cancers, cells can become “immortal” – they acquire the ability to divide indefinitely. This often involves the activation of telomerase, an enzyme that adds back the telomeric sequences and prevents telomere shortening, thereby bypassing replicative senescence.

In the context of breast cancer, some HMECs may acquire mutations or undergo other changes that lead to their immortalization. This is typically an early event in the process of carcinogenesis and is often followed by additional changes that promote uncontrolled growth, resistance to cell death, angiogenesis, and other hallmarks of cancer.

It’s important to note that immortalization alone does not make a cell cancerous; it’s only one piece of the puzzle. However, by allowing cells to escape from the normal constraints on their lifespan, immortalization can set the stage for the accumulation of additional changes that drive the transformation of normal cells into cancer cells.