20
J. B. GURDON
nuclei from differentiating cells promote abnormal development more
often than do nuclei from early embryonic cells. In some cases this
abnormal development is associated with stable changes in the mitotic
products of the transplanted nuclei. There is not the same agreement
regarding the question of whether these nuclear changes occur during
normal cell differentiation or whether they arise as a result of transplantation. The main evidence for denying the occurrence during normal
development of stable changes restricting the developmental capacity
of nuclei comes from the nearly normal tadpoles obtained from transplanted adenocarcinoma nuclei of Rana and the normal adult frogs
obtained from transplanted intestine nuclei of Xenopus. In addition,
Briggs et al. (1960) have shown that the more severely abnormal transplant-embryos, such as arrested blastulae and gastrulae, usually contain obvious chromosomal abnormalities which were not present in the
donor nuclei, and which may differ from one nucleus to another in the
same transplant-embryo. They must therefore take place following
transplantation. I t is possible that less severe nuclear changes not
visible cytologically might arise for the same reason. These would allow
development to proceed further before arrest, and might account for
abnormal transplant-embryo development and for transplant-frog
sterility. The significance of these abnormalities remains an unsolved
question, but until nuclear changes in Rana are shown to be cell-type
specific, it seems safer to attribute them to consequences of transplantation rather than to suppose that they arise during normal cell differentiation.
2. Nuclear Transplantation and Somatic Cell Genetics
Genetic methods have been able to reveal very little about the
genetics of somatic cells in animals since somatic cells cannot be made
to differentiate into gametes once they have already differentiated as
some other kind of cell. Though it is theoretically unlikely, it is possible
that specific gene mutations might regularly take place in the nuclei of
somatic cells during normal cell differentiation. Nuclear transfer experiments have provided three kinds of information relevant to this
question.
A functional test for the presence of genes in somatic cell nuclei is
provided by nuclear transplantation. The experiments outlined above
have shown that intestine nuclei of Xenopus have not undergone any
dominant mutations altering the activity of genes needed in the differentiation of any cell types except perhaps gametes.
Fischberg and Blackler (1963) have approached this question by
combining the methods of nuclear transplantation and germ-cell grafts.
The method consists of obtaining several genetically identical embryos
J. B. GURDON
nuclei from differentiating cells promote abnormal development more
often than do nuclei from early embryonic cells. In some cases this
abnormal development is associated with stable changes in the mitotic
products of the transplanted nuclei. There is not the same agreement
regarding the question of whether these nuclear changes occur during
normal cell differentiation or whether they arise as a result of transplantation. The main evidence for denying the occurrence during normal
development of stable changes restricting the developmental capacity
of nuclei comes from the nearly normal tadpoles obtained from transplanted adenocarcinoma nuclei of Rana and the normal adult frogs
obtained from transplanted intestine nuclei of Xenopus. In addition,
Briggs et al. (1960) have shown that the more severely abnormal transplant-embryos, such as arrested blastulae and gastrulae, usually contain obvious chromosomal abnormalities which were not present in the
donor nuclei, and which may differ from one nucleus to another in the
same transplant-embryo. They must therefore take place following
transplantation. I t is possible that less severe nuclear changes not
visible cytologically might arise for the same reason. These would allow
development to proceed further before arrest, and might account for
abnormal transplant-embryo development and for transplant-frog
sterility. The significance of these abnormalities remains an unsolved
question, but until nuclear changes in Rana are shown to be cell-type
specific, it seems safer to attribute them to consequences of transplantation rather than to suppose that they arise during normal cell differentiation.
2. Nuclear Transplantation and Somatic Cell Genetics
Genetic methods have been able to reveal very little about the
genetics of somatic cells in animals since somatic cells cannot be made
to differentiate into gametes once they have already differentiated as
some other kind of cell. Though it is theoretically unlikely, it is possible
that specific gene mutations might regularly take place in the nuclei of
somatic cells during normal cell differentiation. Nuclear transfer experiments have provided three kinds of information relevant to this
question.
A functional test for the presence of genes in somatic cell nuclei is
provided by nuclear transplantation. The experiments outlined above
have shown that intestine nuclei of Xenopus have not undergone any
dominant mutations altering the activity of genes needed in the differentiation of any cell types except perhaps gametes.
Fischberg and Blackler (1963) have approached this question by
combining the methods of nuclear transplantation and germ-cell grafts.
The method consists of obtaining several genetically identical embryos