TRANSPLANTATION OF CELL NUCLEI
27
tadpoles. The genes concerned in ribosomal UNA synthesis are therefore functionally active in intestine cells but are inactive or repressed
during cleavage. Brown and Gurdon have found that when intestinal
epithelium cell nuclei of Xenopus are transplanted to enucleated eggs,
the previously active ribosomal RNA genes of the donor nuclei at once
become inactive. They then recommence activity again when the resulting transplant-embryos start to gastrulate.
These cytological and chemical changes are examples of natural
changes in nuclear function induced by the cytoplasm. We know that
these changes are harmless to the transplanted nuclei, since such nuclei
support normal transplant-embryo development. These experiments
show that nuclear activity is controlled by the type of cytoplasm in
which the nuclei lie. Since transplanted nuclei swell up so much immediately after transfer, the chromosomes of the transplanted nucleus
are quickly exposed to the new cytoplasm. We can therefore conclude
that the repression and activation of genes is immediately controlled
by the environment in which they lie. Further study of this kind of
natural cytoplasmically-induced change in the state of gene activity
could be very revealing.
2. Transfer of Insect Nuclei to Free Cytoplasm
Kröger (1960) has provided cytological evidence of reversible gene
activity by transplanting larval salivary gland nuclei of Drosophila
busckii into a drop of exudate (cytoplasm and nuclei) squeezed out of a
few early embryos of Drosophila melanogaster. The operation was carried
out under a drop of oil. A nucleus was dissected out from a salivary
gland and pushed into the drop of embryo exudate which had been
diluted with distilled water. After incubation for 3 h the preparation
was fixed and stained with aceto-orcein, and changes in the puff pattern
of the polytene chromosomes (see Section IV, C) were recorded. As a
control a whole salivary gland was incubated in the embryo exudate
for the same period of time.
FIG. 6. A-D. Cytological changes in transplanted nuclei. Original photographs.
A, Section of intestinal epithelium of feeding Xenopus tadpoles showing the striated
border (sb) and a nucleus (n).
B, Swollen intestine nucleus 40 min after transfer to an enucleated egg.
C, Cells and nuclei of a late blastula developed from a fertilized egg.
D, Blastula nucleus 40 min after transfer.
E-H. Skin grafts in nuclear-transplant frogs. Original photographs.
E and F, Adult frogs derived from two transplanted intestine nuclei of the same
original donor tadpole. Skin grafts (circled in white) have been permanently accepted.
G and H, Unrelated frogs from fertilized eggs which have rejected skin grafts within
the usual period of 2-4 weeks at 20° C. Skin grafts by method of Simnett (1964).
B
A . M . 4
27
tadpoles. The genes concerned in ribosomal UNA synthesis are therefore functionally active in intestine cells but are inactive or repressed
during cleavage. Brown and Gurdon have found that when intestinal
epithelium cell nuclei of Xenopus are transplanted to enucleated eggs,
the previously active ribosomal RNA genes of the donor nuclei at once
become inactive. They then recommence activity again when the resulting transplant-embryos start to gastrulate.
These cytological and chemical changes are examples of natural
changes in nuclear function induced by the cytoplasm. We know that
these changes are harmless to the transplanted nuclei, since such nuclei
support normal transplant-embryo development. These experiments
show that nuclear activity is controlled by the type of cytoplasm in
which the nuclei lie. Since transplanted nuclei swell up so much immediately after transfer, the chromosomes of the transplanted nucleus
are quickly exposed to the new cytoplasm. We can therefore conclude
that the repression and activation of genes is immediately controlled
by the environment in which they lie. Further study of this kind of
natural cytoplasmically-induced change in the state of gene activity
could be very revealing.
2. Transfer of Insect Nuclei to Free Cytoplasm
Kröger (1960) has provided cytological evidence of reversible gene
activity by transplanting larval salivary gland nuclei of Drosophila
busckii into a drop of exudate (cytoplasm and nuclei) squeezed out of a
few early embryos of Drosophila melanogaster. The operation was carried
out under a drop of oil. A nucleus was dissected out from a salivary
gland and pushed into the drop of embryo exudate which had been
diluted with distilled water. After incubation for 3 h the preparation
was fixed and stained with aceto-orcein, and changes in the puff pattern
of the polytene chromosomes (see Section IV, C) were recorded. As a
control a whole salivary gland was incubated in the embryo exudate
for the same period of time.
FIG. 6. A-D. Cytological changes in transplanted nuclei. Original photographs.
A, Section of intestinal epithelium of feeding Xenopus tadpoles showing the striated
border (sb) and a nucleus (n).
B, Swollen intestine nucleus 40 min after transfer to an enucleated egg.
C, Cells and nuclei of a late blastula developed from a fertilized egg.
D, Blastula nucleus 40 min after transfer.
E-H. Skin grafts in nuclear-transplant frogs. Original photographs.
E and F, Adult frogs derived from two transplanted intestine nuclei of the same
original donor tadpole. Skin grafts (circled in white) have been permanently accepted.
G and H, Unrelated frogs from fertilized eggs which have rejected skin grafts within
the usual period of 2-4 weeks at 20° C. Skin grafts by method of Simnett (1964).
B
A . M . 4