TRANSPLANTATION OF CELL NUCLEI
5
by some of the cell cytoplasm. The whole broken cell is then injected
into the centre of the enucleated recipient egg.
As a result of refinements in technique over several years, a high
percentage of eggs receiving transplanted blastula nuclei will develop
normally, especially in Rana pipiens and Xenopus laevis (Table I). As
is also shown in Table I, a number of other amphibian species have been
used for nuclear transplantation experiments. The main problems in
using Amphibia for these experiments seem to be the tendency for the
egg cytoplasm to leak out of the punctured membranes, and the difficulty of finding a means of artificial activation ; however these cannot be
all, as is shown by the unrewarded efforts of Lehman (1955) to transplant nuclei in newts.
Amphibia constitute very convenient material for nuclear transfer
experiments partly because their eggs can withstand a remarkable
amount of experimental punishment and partly because of the very
large size of their cells compared to those of most other animals. They
are particularly suitable for transplanting nuclei from different kinds
of somatic cells, but cannot be used with the present methods for the
transfer of nuclei to any other kind of cytoplasm than that of eggs.
2. Insects
A method of transplanting nuclei in the honeybee has been worked
out by DuPraw (1960), who has very kindly allowed the author to
quote much of his unpublished work in the following account. In the
honeybee the sausage-shaped eggs are 1-5-2-0 mm long and, as is
common in the insects, free nuclei are distributed throughout the cytoplasm during cleavage. The eggs remain syncytial until the blastoderm
stage. By means of a fine glass pipette, some of the contents of a preblastoderm embryo are withdrawn, and the cytoplasm with several to
many nuclei is injected into a fertilized (worker) or unfertilized (drone)
egg through the animal pole (Fig. 2). Immediately after injection a
cotton fibre is used to constrict off about 15-20% of the animal end of
the egg which contains the egg or zygote nucleus. This treatment not
only enucleates the egg but also prevents desiccation through the injection pore which otherwise occurs; it results in the development of
dwarf but in other respects apparently normal larvae. It is interesting
that the honeybee egg is extremely sensitive to temperature, and both
the pipette and eggs must be kept at exactly 35-5° C.
DuPraw has obtained vigorous young larvae from the injection of
several preblastoderm nuclei and cytoplasm into fertilized eggs. Indirect
evidence from time-lapse photography indicates that the zygote nucleus
is reliably excluded by the constriction. However, nuclear-transplant
embryos do not survive until the adult stage, and therefore it has not
5
by some of the cell cytoplasm. The whole broken cell is then injected
into the centre of the enucleated recipient egg.
As a result of refinements in technique over several years, a high
percentage of eggs receiving transplanted blastula nuclei will develop
normally, especially in Rana pipiens and Xenopus laevis (Table I). As
is also shown in Table I, a number of other amphibian species have been
used for nuclear transplantation experiments. The main problems in
using Amphibia for these experiments seem to be the tendency for the
egg cytoplasm to leak out of the punctured membranes, and the difficulty of finding a means of artificial activation ; however these cannot be
all, as is shown by the unrewarded efforts of Lehman (1955) to transplant nuclei in newts.
Amphibia constitute very convenient material for nuclear transfer
experiments partly because their eggs can withstand a remarkable
amount of experimental punishment and partly because of the very
large size of their cells compared to those of most other animals. They
are particularly suitable for transplanting nuclei from different kinds
of somatic cells, but cannot be used with the present methods for the
transfer of nuclei to any other kind of cytoplasm than that of eggs.
2. Insects
A method of transplanting nuclei in the honeybee has been worked
out by DuPraw (1960), who has very kindly allowed the author to
quote much of his unpublished work in the following account. In the
honeybee the sausage-shaped eggs are 1-5-2-0 mm long and, as is
common in the insects, free nuclei are distributed throughout the cytoplasm during cleavage. The eggs remain syncytial until the blastoderm
stage. By means of a fine glass pipette, some of the contents of a preblastoderm embryo are withdrawn, and the cytoplasm with several to
many nuclei is injected into a fertilized (worker) or unfertilized (drone)
egg through the animal pole (Fig. 2). Immediately after injection a
cotton fibre is used to constrict off about 15-20% of the animal end of
the egg which contains the egg or zygote nucleus. This treatment not
only enucleates the egg but also prevents desiccation through the injection pore which otherwise occurs; it results in the development of
dwarf but in other respects apparently normal larvae. It is interesting
that the honeybee egg is extremely sensitive to temperature, and both
the pipette and eggs must be kept at exactly 35-5° C.
DuPraw has obtained vigorous young larvae from the injection of
several preblastoderm nuclei and cytoplasm into fertilized eggs. Indirect
evidence from time-lapse photography indicates that the zygote nucleus
is reliably excluded by the constriction. However, nuclear-transplant
embryos do not survive until the adult stage, and therefore it has not