Teratocarcinogenesis and spontaneous parthenogenesis in mice.

Teratomas are rare in most strains of mice. Testicular teratomas are common in some sublines of inbred strain 129. Ovarian teratomas are common in inbred strain LT. Testicular teratomas are derived from primordial germ cells and can be experimentally produced by grafting 121/2-day genital ridges to the testes of adults. They develop into testes and for some strains most have teratomas. Ovarian teratomas are derived from parthenogenetically activated ovarian oocytes that have completed the first meiotic division. Teratomas of either sex can be experimentally produced by grafting early embryos to various sites in adults. Embryo-derived teratomas originate directly from undifferentiated embryonal cells. Occasionally teratomas are malignant (teratocarcinomas) and can be maintained as transplantable tumors. Some form embryoid bodies that resemble normal early embryos. When the stem cells of some transplantable teratocarcinomas are injected into blastocysts and transferred to the uteri of pseudopregnant females, they participate in normal development and contribute to the formation of all major tissues including functional sperm and eggs. Spontaneous parthenogenesis is common in strain LT oocytes after ovulation. The eggs cleave, form blastocysts which implant in the uterus, but after the egg cylinder stage they become disorganized and are aborted. Eight-cell embryos from the pigmented LT strain were aggregated with embryos of albino strain 129 and transferred to the uteri of pseudopregnant females. They participated in development and contributed to the formation of normal chimeric tissues. Offspring from eggs derived from parthenogenetic embryonal cells were produced, demonstrating that parthenogenetic embryonic cells are totipotent. It is still a mystery why parthenogenetic embryos will not survive in utero.