TRANSPLANTATION OF CELL NUCLEI
21
by the serial transplantation of nuclei from one original somatic cell
nucleus. The germ cells from these serial-transfer embryos are grafted
to normal host embryos whose own germ cells have been removed.
The sex of these host embryos is determined by their own genetic
constitution and not by that of their implanted germ cells. They
therefore develop as males and females, which can be mated together
and so reveal mutations in the original somatic cell nucleus as long as
these do not cause lethality of germ cells. So far only very few mutations
have been found which may have arisen during the differentiation
of a somatic cell (Fischberg and Blackler, 1963); most of these are
recessive and are therefore unlikely to have any effect on differentiation.
Making the following assumptions, we can calculate the approximate
frequency with which spontaneous mutations might be present in
somatic cells. If genes mutate at the same rate in somatic cells as in
germ cells, then each gene will be mutant in about one out of every
million cells. Since only a few genes are likely to be active in any one
cell type, most of these mutations will not be lethal to the cell in question. They will however be recognizable in nuclear-transplant embryos,
since all cell types are examined. If there were 50,000 gene loci in Xenopus, then about 50,000/10
6
, or 5%, of Xenopus endoderm cells would be
mutant for one gene or another. Fischberg and Blackler (1963) have
detected a lower frequency of gene mutations than this in Xenopus
somatic cells, and there are probably very many more than 50,000 gene
loci in Xenopus. Therefore this kind of experiment has so far given no
reason for believing that somatic cells carry more mutant genes than
they would be expected to acquire as a result of spontaneous mutation.
The last type of contribution to somatic cell genetics provided by
nuclear transfer experiments consists of transplanting skin between
two adult transplant-frogs each of which is derived from the transplantation of a single intestine cell nucleus of the same tadpole. Normally
skin grafts made between genetically non-identical Xenopus are
immunologically rejected within a few weeks (Fig. 6, G, H). When skin
was exchanged between two transplant frogs derived from intestine
nuclei of the same original donor tadpole, the grafts were not rejected
(Fig. 6, E, F). This experiment shows that these two intestine cell
nuclei did not differ in respect of any of the genes responsible for graft
rejection. The number of these so-called histocompatibility loci has
been estimated to be 14-16 in two races of rat (Billingham et al., 1962).
None of these results suggest that mutations arise in somatic cells
more frequently than they would be expected to do spontaneously, and
therefore that they play any part in promoting cell differentiation.
21
by the serial transplantation of nuclei from one original somatic cell
nucleus. The germ cells from these serial-transfer embryos are grafted
to normal host embryos whose own germ cells have been removed.
The sex of these host embryos is determined by their own genetic
constitution and not by that of their implanted germ cells. They
therefore develop as males and females, which can be mated together
and so reveal mutations in the original somatic cell nucleus as long as
these do not cause lethality of germ cells. So far only very few mutations
have been found which may have arisen during the differentiation
of a somatic cell (Fischberg and Blackler, 1963); most of these are
recessive and are therefore unlikely to have any effect on differentiation.
Making the following assumptions, we can calculate the approximate
frequency with which spontaneous mutations might be present in
somatic cells. If genes mutate at the same rate in somatic cells as in
germ cells, then each gene will be mutant in about one out of every
million cells. Since only a few genes are likely to be active in any one
cell type, most of these mutations will not be lethal to the cell in question. They will however be recognizable in nuclear-transplant embryos,
since all cell types are examined. If there were 50,000 gene loci in Xenopus, then about 50,000/10
6
, or 5%, of Xenopus endoderm cells would be
mutant for one gene or another. Fischberg and Blackler (1963) have
detected a lower frequency of gene mutations than this in Xenopus
somatic cells, and there are probably very many more than 50,000 gene
loci in Xenopus. Therefore this kind of experiment has so far given no
reason for believing that somatic cells carry more mutant genes than
they would be expected to acquire as a result of spontaneous mutation.
The last type of contribution to somatic cell genetics provided by
nuclear transfer experiments consists of transplanting skin between
two adult transplant-frogs each of which is derived from the transplantation of a single intestine cell nucleus of the same tadpole. Normally
skin grafts made between genetically non-identical Xenopus are
immunologically rejected within a few weeks (Fig. 6, G, H). When skin
was exchanged between two transplant frogs derived from intestine
nuclei of the same original donor tadpole, the grafts were not rejected
(Fig. 6, E, F). This experiment shows that these two intestine cell
nuclei did not differ in respect of any of the genes responsible for graft
rejection. The number of these so-called histocompatibility loci has
been estimated to be 14-16 in two races of rat (Billingham et al., 1962).
None of these results suggest that mutations arise in somatic cells
more frequently than they would be expected to do spontaneously, and
therefore that they play any part in promoting cell differentiation.