Which of the following cells is immortal?

The correct option is Germ cells.

To understand why germ cells are considered immortal, let us break down the concepts of cellular immortality and biological aging step-by-step:

1. Cellular Senescence and the Hayflick Limit
Most cells in a multicellular organism are somatic cells (such as glomerular cells in the kidney or cells of the pituitary gland). Somatic cells have a limited lifespan and can only divide a finite number of times before they enter a state of permanent growth arrest called senescence. This limit is known as the Hayflick Limit, and it is primarily governed by the shortening of telomeres (the protective caps at the ends of chromosomes) during each round of cell division.

2. The Role of Telomerase
When a cell divides, the DNA replication machinery cannot fully copy the very ends of linear chromosomes. In somatic cells, which lack significant telomerase activity, telomeres get progressively shorter with each division. Eventually, they become critically short, signaling the cell to stop dividing or undergo apoptosis (programmed cell death). This makes somatic cells biologically "mortal."

3. Why Germ Cells are "Immortal"
Germ cells (the progenitors of gametes like sperm and eggs) express high levels of the enzyme telomerase. Telomerase actively rebuilds and maintains telomere length. Because germ cells can continuously restore their telomeres, they do not undergo replication-induced senescence. They pass genetic information from one generation to the next without aging-related degradation of their chromosome ends. Thus, the lineage of germ cells is biologically immortal, ensuring the continuity of life across generations.