How can cancer treatment damage fertility in both males and females?

So the question really is, how do chemotherapy or radiation damage fertility? And so the mechanism differs by the cancer chemotherapy—how damaging it can be to either a female ovary or a male testes. But the main way chemotherapeutics work is by targeting rapidly growing cells, and so within both the ovary and the testes, in the development of the mature gamete, either the egg or the sperm, the cells that nurse those two gametes, the somatic cells, are rapidly proliferating.

And so in fact chemotherapy and radiation target those cells preferentially. The gametes themselves, the egg or the sperm, are also particularly vulnerable to agents like chemotherapeutics, and particularly radiation to the pelvis, because they have DNA that can be easily damaged. And so those treatments can impact very directly both cell types within the gonads, the somatic cell and the gametes themselves.