Scientists have found that the parallels between the gestation of an embryo and the growth of a cancerous tumor are pretty precise as the two share the same genes.
In a piece for The New York Times titled "A Tumor, the Embryo's Evil Twin," George Johnson describes the similarities between the two while also referencing Susan Sontag's Illness Is Metaphor. In her book about her struggle with cancer, Sontag described her tumor as a "demonic pregnancy" and a "fetus with its own will." Johnson says Sontag probably didn't realize how right she was.
One example of how embryos and tumors share traits involves something called "sonic hedgehog," which drives home how geeky-y Sega developers really were:
Consider the gene SHH. The name is short for sonic hedgehog. (Hedgehog genes were discovered in fruit flies and when mutated they cause the larvae to be covered with a profusion of bristles.) In a human embryo, sonic hedgehog is involved with establishing the bilateral symmetry of the brain, skeleton and other organs. Later in life it can run amok, interacting with genes like SMO (for smoothened — another fruit fly derivation) to bring on a human brain cancer called medulloblastoma and askin cancer called basal cell carcinoma.
The comparisons go on, not just in the way that embryos and tumors grow, but in the way our bodies react to them:
In the early days of pregnancy, the primitive embryo — this rapidly dividing glob of cells — eats out a spot in the uterine lining using corrosive enzymes called proteases. Then it holds tight for the duration with the help of proteins like integrin, a kind of biological glue. Both types of molecules are also used by a cancer as it digs in and adheres to its berth.
With this knowledge, those nightmare stories about tumors with hair and teeth seem to make a lot more sense—but are no less terrifying.