14
J. B. GURDON
to extend the application of nuclear transplantation two approaches
seem hopeful. One is to develop a medium in which isolated nuclei can
be kept alive for short periods. Encouraging results have been obtained
with Amoeba and especially Acetabularia. The other approach is the use
of naturally multinucleate cells. In cleaving insect eggs and in Neurospora hyphae several additional nuclei can be injected. It is not always
necessary to enucleate these cells since in Neurospora the injected nuclei
can give rise to their own kind of spores without being affected by host
nuclei present in the same cell.
III. Results of Nuclear Transfer Experiments
A. Stable Changes in Living Nuclei
1. Nuclear-transplant Embryos and Frogs
The technique of nuclear transplantation in Amphibia was originally
developed to find out whether the nuclei of differentiating cells undergo
stable changes during development. Although nuclei have now been
successfully transplanted in several Amphibia, it is at present the work
on Rana pipiens and Xenopus laevis that has contributed the most
important results relevant to this question. Comparable experiments in
these two species have yielded results which are in some respects divergent. A detailed description and criticism of these experiments has
been published by the author (1963). In this section we will summarize
only the main conclusions from this work and the evidence for them.
Development of nuclear transplant-embryos
When a nucleus is transplanted to an unfertilized and enucleated egg,
abnormal cleavage or even a total lack of cleavage sometimes results.
This does not happen very often with blastula nuclei, but with nuclei
from differentiating cells of tadpoles it happens in the majority of cases
(Table II). The probable cause of deficient and abnormal cleavage has
been identified by a cytological analysis of eggs fixed soon after receiving
transplanted nuclei (Gurdon, 1962c). The cytological appearance of
these eggs was compared with the development of other eggs to which
the same kind of nuclei were transplanted. A total lack of cleavage in
injected eggs is associated with such technical faults as the failure to
break the donor cell wall, or the inadvertant removal of the donor
nucleus from the egg with the retraction of the pipette. Injected eggs
which show only irregular cleavage furrows, or which are normally
cleaved in one part but irregularly cleaved elsewhere, have chromosomes
clumped or irregularly distributed on a multipolar spindle at their first
mitosis. This is possibly caused by taking a donor nucleus which
J. B. GURDON
to extend the application of nuclear transplantation two approaches
seem hopeful. One is to develop a medium in which isolated nuclei can
be kept alive for short periods. Encouraging results have been obtained
with Amoeba and especially Acetabularia. The other approach is the use
of naturally multinucleate cells. In cleaving insect eggs and in Neurospora hyphae several additional nuclei can be injected. It is not always
necessary to enucleate these cells since in Neurospora the injected nuclei
can give rise to their own kind of spores without being affected by host
nuclei present in the same cell.
III. Results of Nuclear Transfer Experiments
A. Stable Changes in Living Nuclei
1. Nuclear-transplant Embryos and Frogs
The technique of nuclear transplantation in Amphibia was originally
developed to find out whether the nuclei of differentiating cells undergo
stable changes during development. Although nuclei have now been
successfully transplanted in several Amphibia, it is at present the work
on Rana pipiens and Xenopus laevis that has contributed the most
important results relevant to this question. Comparable experiments in
these two species have yielded results which are in some respects divergent. A detailed description and criticism of these experiments has
been published by the author (1963). In this section we will summarize
only the main conclusions from this work and the evidence for them.
Development of nuclear transplant-embryos
When a nucleus is transplanted to an unfertilized and enucleated egg,
abnormal cleavage or even a total lack of cleavage sometimes results.
This does not happen very often with blastula nuclei, but with nuclei
from differentiating cells of tadpoles it happens in the majority of cases
(Table II). The probable cause of deficient and abnormal cleavage has
been identified by a cytological analysis of eggs fixed soon after receiving
transplanted nuclei (Gurdon, 1962c). The cytological appearance of
these eggs was compared with the development of other eggs to which
the same kind of nuclei were transplanted. A total lack of cleavage in
injected eggs is associated with such technical faults as the failure to
break the donor cell wall, or the inadvertant removal of the donor
nucleus from the egg with the retraction of the pipette. Injected eggs
which show only irregular cleavage furrows, or which are normally
cleaved in one part but irregularly cleaved elsewhere, have chromosomes
clumped or irregularly distributed on a multipolar spindle at their first
mitosis. This is possibly caused by taking a donor nucleus which