A germ cell is a reproductive cell, or any cell that gives rise to one. In humans, the most familiar germ cells are sperm and eggs. What makes them unique is their role as the only cells in the body capable of passing genetic information to the next generation. Every other cell, from skin to muscle to brain tissue, is a somatic cell, and its DNA dies with you.
Germ Cells vs. Somatic Cells
Your body contains trillions of somatic cells and a much smaller population of germ cells, but the distinction between them is fundamental. Somatic cells are diploid, meaning they carry two complete sets of chromosomes, 46 in total, one set from each parent. If a somatic cell picks up a DNA mutation during your lifetime, that change can affect you personally (potentially leading to cancer, for instance) but it cannot be inherited by your children.
Germ cells, by contrast, are the bridge between generations. A mutation in a germ cell, called a germline mutation, gets built into the egg or sperm and can be passed to every cell in a future child’s body. This is why genetic conditions like cystic fibrosis or sickle cell disease run in families. The parents may carry the mutation in their reproductive cells without being affected themselves, then pass it along at fertilization.
How Germ Cells Make Sperm and Eggs
Germ cells start out diploid, just like somatic cells. To become functional sperm or eggs, they undergo meiosis, a specialized type of cell division that cuts the chromosome count in half. The process involves one round of DNA copying followed by two rounds of division, producing cells with 23 chromosomes instead of 46. These haploid cells are called gametes.
In males, meiosis turns one germ cell into four sperm cells. In females, the same process yields one mature egg and three tiny leftover cells called polar bodies, which don’t function as eggs. During the first stage of meiosis, something important happens: matching chromosomes from your mother and father pair up and physically swap segments of DNA. This genetic shuffling, called recombination, is why siblings who share the same parents can look and behave so differently. Each sperm or egg ends up with a unique combination of genetic material.
When a sperm and egg fuse at fertilization, their 23 chromosomes combine to restore the full set of 46 in a new organism.
Where Germ Cells Come From
Germ cells don’t originate in the ovaries or testes. They form much earlier in embryonic development as a small cluster of cells called primordial germ cells, or PGCs, which arise far from the future reproductive organs. These cells then make a remarkable journey through the developing embryo to reach the gonads.
In humans and mice, PGCs are initially carried along passively as the embryo folds and reshapes during early development. They end up embedded in the tissue of the developing gut, then actively migrate out of the gut wall and travel through surrounding tissue to reach two structures called the gonadal ridges, which will become the ovaries or testes. The whole migration happens during the first several weeks of embryonic life. Once PGCs settle into the gonads, they multiply and eventually begin the process of becoming mature sperm or eggs, a process that continues from puberty onward in males, while females are born with their lifetime supply of immature egg cells already in place.
Potency and Stem Cell Research
In their normal role, germ cells are considered unipotent: they can only produce one cell type, either sperm or eggs. But researchers have found that under the right conditions, cells derived from primordial germ cells or from certain adult germ line cells can behave like pluripotent stem cells, meaning they regain the ability to develop into many different tissue types. This makes germ cells an area of significant interest in regenerative medicine and stem cell biology, since they retain a hidden flexibility that most other specialized cells in the body have lost.
Germ Cell Tumors
When germ cells grow abnormally, they can form tumors called germ cell tumors. These most commonly occur in the testes or ovaries, but because primordial germ cells migrate through the body during embryonic development, some can settle in the wrong location. This is why germ cell tumors occasionally appear in unexpected places: the brain (particularly in a structure called the pineal gland), the chest (in the mediastinum, the area between the lungs), and the back wall of the abdomen (the retroperitoneum).
Germ cell tumors fall into two broad categories. Seminomas (called germinomas when they occur in the brain) tend to grow more slowly and respond well to treatment. Non-seminomatous tumors include several subtypes, such as embryonal carcinoma and yolk sac tumors, which can behave more aggressively. The distinction matters because it shapes how the tumor is treated and what the outlook looks like. Testicular germ cell tumors are among the most treatable cancers, particularly when caught early.
Germ Cell Loss and Fertility
Because germ cells are the sole source of sperm and eggs, losing them directly impacts fertility. In males, aging gradually depletes the stem cells that replenish sperm. Research in mice has shown that in aged testes, roughly 20% of the sperm-producing tubes develop abnormalities like germ cell depletion and failure to release mature sperm. Even in the tubes that still function, the number of germ cells drops because the stem cells divide less frequently.
Interestingly, the germ cells themselves don’t appear to show typical aging markers. Instead, the support cells surrounding them deteriorate, and their numbers decline. This means the environment around germ cells may matter as much as the germ cells themselves. Beyond aging, germ cell loss can also be triggered by chemical exposures, poor nutrition, certain medical treatments like chemotherapy, and various diseases. In females, where the supply of egg precursor cells is fixed at birth, any significant loss is permanent and brings menopause closer.