Discovery by Embryologist Leroy Stevens: Pluripotent Embryonic Stem Cells

A teratoma tumor

In 1958, one of the earliest steps towards the discovery of iPS cells occurred when embryologist Leroy Stevens discovered a large tumor on the testes of a mouse. The tumor was composed of many different mouse parts from all three germ layers, including muscle, skin, teeth, bone, and hair. After transplanting bits of the tumor into healthy mice, he observed that some of the bits developed into varied cell and tissue types, while groups of undifferentiated cells also formed. Stevens publishes his results, asserting that the cells from inside the tumor were pluripotent, a sort of cancerous embryonic cell (“Stem Cells”).