Ultrastructural analysis of the early development of teratocarcinomas.

Genital ridges of 12-day embryos of strain 129 mice were transplanted into the testes of adult mice of the same strain. Of the transplants that differentiated into fetal testes, 80% contained intratubular teratocarcinomas by the 7th day after transplantation. These teratocarcinomas were studied ultrastructurally, daily from the 7th-14th day following transplantation, to gain information about the histogenesis of the tumors and the mode of development of multipotent cells. The embryonal carcinoma cells, which are the stem cells of teratocarcinomas, so closely resembled primordial germ cells that the former appeared to be derived from the latter. The cytoplasm of these multipotential cells was characterized by large numbers of dispersed ribosomes and polysomes with few membranous organelles other than mitochondria. Among the earliest signs of differentiation was the development of rough-surfaced endoplasmic reticulum and Golgi complexes, followed by modification of the plasma membrane.