THE
GENETICS OF TISSUE TRANSPLANTATION
I N MAMMALS
C. C. LITTLE
University of Maim
This paper has its justification in the fact that the subject of
the genetics of tissue transplantation is likely to become in the
not distant future of far greater general importance than it is at
present and also in the fact that much of the published work in
this, and in allied fields, has been done by medical men or institutions, and has appeared in medical journals. The result has
been that there has not been brought to the attention of experimental biologists any considerable amount of evidence as regards the type of inheritance found in the case of tissue transplantation. It is true that there have been some efforts to
describe family and individual differences in susceptibility or
non-susceptibility to tumor implants but the application of the
principles there discovered, to the cases of so-called “normal”
tissue’has not been made with sufficient clarity and critical
thoroughness.
Leo Loeb and his co-workers have done a considerable amount
of investigation of the behavior of various types of normal implants in different hosts. If, in its course, the present paper takes
or appears to take the form of a criticism directed against their
work it should be understood at the outset that none of the
details of this criticism are in any way intended to belittle or to
cast doubt upon the accuracy of the data which they have reported. When the interpretation of these data is considered,
however, there is provided an open field for conjecture and for
the advancing of any theory or theories which fit the observed
facts more completely, more accurately, or more consistently
than do those explanations which Loeb has offered.
Loeb has used as a term to define the difference found to exist
between most individuals the phrase, I ‘ individuality differen75
76
C. C. LITTLE
tial.” In this paper for economy of space, this will be abbreviated as (‘ind.” We may perhaps profitably use Loeb’s own
words to define exactly what an ind is.
i‘
. . . all the tissues of an individual have in common a
chemical characteristic through which they differ from other
individuals of the same species. This characteristic may be
designated as the individuality differential” (1). We know then
from definition that any one animal has a characteristic makeup by which it is supposed to differ from all others of the same
species as itself. This make-up is moreover described as being
chemical in nature.
There is, however, presented at the outset a very interesting
biological dilemma as to the permanency, extent, and origin of
the chemical characteristics by which any one individual differs
from all others of its species. If the difference characterizes
‘(all the tissues” its distribution is as general as the protoplasm
itself and because of its nature as uniform as the number and
form of the chromosomes. It must, moreover, be a substance
communicable to all cells in the body. It is, therefore, either
reduced to a difference which rests upon the substances inherited
at the inception of the individual, or else it must be able to circulate through the tissues with great ease. The latter fact
follows naturally, for if the difference has arisen from elements
acquired since the development of the individual began, i.e.,
during ontogeny, the environment would be continually exerting
an effect upon the underlying elements, and to obtain a consistent
and uniform characteristic for all the tissues at any one time
would, it seems, be almost impossible and would be quite so were
not ready penetration of all tissues by it entirely possible.
If however the origin of the substance or substances which
form the basis for the ind is ontogenetic, we are faced with
another real difficulty. This consists of the fact that either
we must suppose that these substances make their way into
the germ plasm, and thus provide examples of the inheritance of an acquired character; or else we must suppose that
any study of the inheritance of tbe ind will have as a source
of error the purely somatic effects which are not inherited and
GENETICS O F TISSUE TRANSPLANTATION IN MAMMALS
77
which serve to confuse the experimental conditions to a very
great extent.
In other words we must at the outset define exactly what class
or group of characteristics we are to deal with in our investigations of the ind and of its behavior. Inasmuch as there is absolutely no evidence for the inheritance of any acquired character
in the types of laboratory mammals on which the experiments
on transplantation have been performed, we can say that for our
purpose the substances which serve to diferentiate between one
individual and the others of its species are those which are represented in the germ-plasm and which therefore are subject to the
methods of analysis and of investigation which characterize all
genetic studies. Any attempt to bring into the case substances
of other nature must be at once recognized as being of secondary
importance to the main issue.
Undoubtedly age, sex, and general physical condition will
play rbles in the success or failure of implants of tissue. In
general, however, these factors themselves will be found to
trace back to genetic agents which determine to a large degree
the rate and time of maturity, susceptibility to disease, and the
rate of approach of the physiological changes which we have
come to lump under the general term senility.
There is then nothing mystical or unique about the case which
we have under consideration. There will be many times when
we shall need to keep this in mind during the analysis of Loeb’s
investigations.
For convenience we may divide the questions to be taken up
into two sub-headings, as follows: A , Critical; and B, Con. structive. Under the first heading we may group almost all, if
not all the trouble under a very significant and broad blanket,
namely ‘‘ The distinction between pedigree and genetic relationship.”
Under the second heading we shall examine three main topics
and try to show how progress can be made by bearing in mind
certairl. fundamental principles of modern genetics. These
three topics are (1) The use of tumor tissue in implantation work,
(2) The use of multiple mendelizing factors as a n explanation of
genetic results, (3) The meaning of a peculiar form of behavior
shown by F, hybrids during certain transplantation experiments.
78
c. c.
LITTLE)
CRITICAL
(1) The Distinction between ‘Pedigree and Genetic Relationship.
If we had before us several hundred pea seedlings obtained
from a long series of self-fertiliaed pea plants, in a direct line of
descent, we should admit without any question that they formed
what is known as a pure line. Roughly speaking, that would
mean that the differences in form, or size, or vigor, which were
apparent between individual seedlings were purely somatic and
could be ignored when the question of the breeding quality of the
various seedlings was considered.’
If now we sent to various breeders of peas one or two of the
seedlings so obtained, we might after severaI years or generations of breeding receive back from the breeders a number of the
descendants of the original plants. Although these would be
to the plants originally sent, in the relationship of grandchildren with several “greats” attached, we should find that,
unless mutations had occurred, the plants which we received
back would have exactly the same breeding qualities as did
those which we sent away years before. They would then
biologically be essentially the same individuals as were the
original plants and between their inds and those of the original
plants no diference would exist.
Exactly the same conditions would be found in the case of
animals reproducing by sexual reproduction, provided a long
program of extremely close (own brother to sister matings)
inbreeding had been followed, and provided the line of descent
was kept narrow by the selection of single pair matings. If,
after a long period of this sort, the descendants of a single pair
were, by intensive breeding, multiplied to a point where several
hundreds of animals were obtained, we should have cousins,
parents, aunts, uncles, and mother-in-law all biologically the
same. All would be parts of the same germ-plasm which had
been made uniform under the deliberate and selective process of
breeding which had been employed. In such a population the
’This of course does not consider the occurrence of mutations in pmticular
individuals. The mutation factor is a necessary vrtriable, for in the present state
of our knowledge we cannot predict the time, number, or nature of mutations.
GENETICS O F TISSUE TRANSPLANTATION IN MAMMALS
79
results of transplantation of tissue from one animal to another
would not be different from those obtained when transplants
were made from one portion of an individual to another part of
the same’animal.
Loeb has used, on the basis of pedigree relationship, the terms
autotransplantation, syngenesiotransplantation and homoiotransplantation, to debignate identical, closely related, and
widely related or unrelated individuals respectively. A moment’s consideration of the matter will, however, show that the
terms are almost entirely deceptive, if the pedigree relationship
is alone considered or even if it is made the chief criterion in
judging what the result of any particular implantation of tissue
is to be.
It will be of interest to quote briefly from certain of Loeb’s
papers on the relation of au to-, syngenesio- and homoiodifferentials and on the way in which the ind is inherited. He says :
The interaction of cells and substances which possess the same “ind”
leads to the production of autosubstances which are responsible for
various conditions of tissues? After transplanting a piece of an organ
into a near relative of the donor of the tissue (syngenesiotransplant),
or into an unrelated individual of the same specie8 (homoiotransplant),
or into an individual belonging to a different species (heterotransplant), the ind is no longer adapted to its environment and acts as a
syngenesio, homoio, or heterodifferential respectively,
In these quotations we find Loeb using the terms of distinction between different types of transplants on the basis of
pedigree relationship alone. There are many errors to which a
viewpoint of this sort can very easily lead. For example, the
very occurrence, in the same species, of animals which are
entirely unrelated to the individual in question, is, in itself,
highly doubtful from a genetic point of view although quite readily
observed from a pedigree viewpoint. From the genetic aspect
the degree to which any two individuals of the same species are
related will depend upon the number of inheritable units or
genes which the two individuals possess in common. It will
be rarely if ever that we should find two animals of the same
* In this case, aa in others to follow, the term ind has been used in quotation,
although of course Loeb used the full form “individuality differential.’’ Am. Nat.,
1920, liv, 5&60.
80
C. C . LITTLE
species which do not possess a considerable number of their
genes in common. It is not desirable or accurate therefore to
focus our attention on the pedigree relationship between two
animals and to declare them to be unrelated if their ancestors
do not run together within a certain limited period of observation.
As a general thing transplants between two individuals of an
ordinary laboratory strain of animals will not be successful.
This is because a very close approximation to biological identity
is essential for the successful fulfillment of the relations oE host
and donor in any transplantation experiment. In other words,
either by chance or by a deliberate process of selective inbreeding
and choice of animals, the individuals picked for host and for
donor must be either approximately or absolutely equal in the
genetic factors which underly the physiological make-up of
their tissues. Let us see how Loeb considers the matter of the
relationship between the auto-, syngenesio-, and homoiotransplants.
He says:
After syngenesioplastic transplantation of thyroid in guinea-pigs,
the results are intermediate between those obtained after auto- and
homoiotransplantation. These findings agree with our previous
results obtained in the rat and with different organs (2).
A blending or combination result is here hinted at. The
violence of the reactions involved in the elimination of the transplant are the measures by which the results spoken of as characterizing auto- or homoiotransplants are judged. The histology of the reactions has been carefully worked out by Loeb,
and the fact has been clearly brought out that the elimination
of the implant is a function of the degree of similarity or difference between the donor and host. This fact cannot however
be thought of as involving anything except a clear idea which all
who have been carrying on transplantation work of any kind
have long known and utilized in their work.
It will, moreover, be apparent that to focus our attention upon
the degree of reaction to be observed in deciding the fate of
various implants is to run the danger of confusion between the
GENETICB
OF TISSUE TRANSPLANTATION
IN MAMMALS
81
outward manifestation of a number of hereditary units working
as a complex and the nature of those units themselves. The
study of the genetics of tissue transplantation will not progress
if we spend our time studying, in material whose genetic nature
is little or not at all known, the varying degrees of violence of
lymphocytic or other protective reactions set up by the organism
against implants of foreign tissue. That ’some degree of trouble
has already resulted from this method is to be judged from the
following:
To a certain extent syngenesiotoxins still take the place of autosubstances characteristic of the interaction between the own tissues
and body fluids. Some syngenesiotoxins, however, can do so only to
a very slight extent; homoiotoxins are still less able t o take the place
of auto-substances and least of all heterotoxins (3).
There is a slight amount of unconscious irony in this statement, for while the degree to which syngenesiotoxins replace
auto-substances is spoken of as “certain” there cannot well be
anything much more uncertain, if the explanation here used is
followed. Some of this uncertainty is to be found in another
quotation which should be taken in connection with the statement already quoted to the effect that the results of syngenesiotransplantation are intermediate between those of auto- and
homoio-transplants.
All degrees of variation between the two extremes of results resembling those in autotransplantation . , . and of homoiotransplantation . . . are obtained after transplantation . . . into near relatives.
We do not find a half-way condition. The different members of a
family may behave very differently (4).
Although this statement appears t o contradict that made by
the same worker on another occasion, it is probably much more
nearly correct. In fact it agrees with the results which have
been frequently described by other authors, for example, in
ovarian transplantation in guinea-pigs by Castle and Phillips
( 5 ) and in mouse tumors by the writer and Tyzzer (6) some
years before Loeb’s paper was published.
It may be pointed out, in passing, that the results described
by Loeb are certainly to be expected if the animals used were
not descended from a particularly inbred race. I n other words
82
c, c.
LITTL?
the animals which go to make up an ordinary laboratory race of
mammals are by no means alike genetically, They vary, in
different cases, from those which are apparently practically
identical, to those which bear so little genetic resemblance to
one another that if their pedigree did not provide the mechanical
road by which they might, in some way, be related one would
never guess their common origin either by physiological or by
genetic behavior.
One more quotation may be given along these same lines:
Occasionally pieces behave after homoiotransplantation in a way
which is characteristic of syngenesiotransplantation. At present we
must admit the possibility that in such cases the donor and host had
after all been related to each other; 80 that in reality we had to deal,
not with a homoiotransplantation, but with a more distant syngenesiotransplantation (7).
Here we have an excellent example of the degree t o which the
pedigree relationship idea has grasped the imagination of Loeb.
These exceptional cases showingsome degree of similarity without
any directly traceable line of pedigree descent from a common
ancestor are disturbing. They are considered ‘ I for the present, ”
and although it is genetically entirely possible to get results from
this sort of case that are identical with autotranspIantation, yet
because of pedigree they are treated as being “more distant
syngenesio,” nearer than that they cannot go.
There is another group of facts which the hypothesis of pedigree “relationship” meets with little success and from which
it retires somewhat discomfited; these are the experiments involving transplantation from parents to children, from child to
parent, and between sibs. That these are considered by Loeb
as belonging to the group of syngenesiotransplantations, regardless of the degree of similarity or difference possessed by
the a’ncestors,is shown by the following quotation:
Tissues transplanted from parents to children, or from sisters and
brothers to sisters and brothers, or from children to parents, behave in
a manner intermediate between tissues after homoio- and autotransplantation (8).
Following this statement we may consider the results of the
various types of transfers as described by Loeb:
GENETICS OF TISSUE TRANSPLANTATION IN MAMMALS
83
We found transplantation from brother to brother to give the beat
results but even here the mixing of the inds called forth the develop
ment of syngenesiotoxins which usually were relatively mild, but in
certain cases would be more severe,
and again,
Transplantation from mother to child on the whole, resembled that of
transplantation from brother to brother, but it seemed to be somewhat
less favorable . . . . Transplantation from child to mother led to the
production of toxic effects which were almost as marked as those produced by the homoiotoxins (9).
These results are entirely in line with the experience of those
who have worked with mixed stocks in the implantation of tumor
tissue as well as of normal tissues. They are not in the least
in agreement with results described by the writer and Johnson in 1922 (10). Here we used mice of known genetic make-up
and after consideration of the genetic constitution devised a
series of transplantations of spleen which were calculated to
test the validity of any system of classification of relationship
which was based on pedigree alone. In certain strains where
we expected it, transplants from child to parent, from parent to
child and between sibs were equally successful and all were indistinguishable from autotransplants. The relationship and
results described by Loeb broke down entirely and, as we expected, failed to materialize. The experiments referred to will
be taken up again later in more detail.
That Loeb himself is in doubt as to the meaning of his results
will be apparent from the two following quotations.
The difference between results obtained after transplantation from
parents to children and . . . from an animal to his sisters and brothers,
is so small that it may be entirely accidental. Tissues transplanted
from children to mother . . . show an intermediate behavior. . . .
This result may also be accidental and due to the relatively small
number of mother rats used in this series (ll),
and further
Sex does not influence the four variables in the case of thyroid transplantation in the guinea-pig. Whether the inferiority in results obtained after transplantation from child to mother which we found
previously, is due to the action of a constitutional or of an extraneous
factor remains still to be determined (12).
84
C. C. LITTLE
The indefiniteness of the above statements will without doubt
be apparent. It is largely the product of the use of stocks of
unknown genetic make-up. It really seems that with the exception of the histological findings already referred to, Loeb’s work
has merely extended to a number of new organs and tissues the
methods and type of work done by many experimenters with
the implantation of tumor tissue. It is also clear that no adequate genetic analysis of his material can be made, because its
genetic constitution is unknown and ignored. No controls are
therefore available and no experiments to test genetic similarity
or differences can be planned. The sole method at his disposal
is to contrast brother with brother, sister with uncle, aunt with
cousin until a family party at Thanksgiving bears to a member
of the family newly acquired by marriage, a simpler relationship
than does the biology and genetics of Loeb’s various experiments
to the would-be analyzer. It is especially unfortunate that the
genetics of the transplantation of tissues will be the first acquaintance which many well-meaning medical men will have
with that field of biology. Even if their contact with genetics
is first made through the work of over-ardent eugenic field
workers it may, through irritation, survive the shock more
readily than if it becomes completely submerged in a maze of
experiments and facts collected on the basis of pedigree relationship.
CONSTRUCTIVE BUGQEBTIONB
As has already been intimated these suggestions will be
grouped under three main topics. The first of these is the relation of experimental work done in the field of tumor transplantation to that done with the so-called normal tissues.
Many histologists have shown that in spite of its unique
physiological behavior, tumor tissue retains to a considerable
degree the characteristics of the tissues which gave rise to the
neoplasm. Thus, for example, sarcomas show in their histological structure the unmistakable marks of connective tissue.
By contrast tumors of the liver show the characteristics of liver
tissue and so on for the various sites in which neoplasms can be
formed. Further than this the implantation of bits of tumor
GENETICS OF TISSUE TRANSPLANTATION IN MAMMALS
85
into new localities on the individual that gave rise to it are in
their relation to its tissues in the nature of an autoplastic implant just as the implants of any other of its tissues would be.
It is quite true that tumor tissue has an amazing power of
growth, but that fact in itself presupposes no incompatibility
between the results obtained after implantation of tumor tissue
and those following implantation of normal tissue. The power
of growth which tumors possess may possibly make them, on
the whole, more apt to grow than are normal tismes, but there
is every reason to think that this increased power of growth is
in the nature of a constant and would be felt as such in the
various types of implantation ranging from auto- to heterotransplants. If such is the case, and if tumors possess, as they
apparently do, the same genetic constitution as the animal
which gives rise to them, their ind should be equal to that
possessed by the normal tissues of the animal on which they
originated.
Tumor tissue, however, has in its power of rapid growth a
factor of great value in the genetic study of tissue transplantation. This factor is found in the ability on the part of a tumor to
increase to such a mass that it may conveniently be used for the
inoculation of hundreds of individuals. The advantage of this
should be obvious. It gives us the opportunity of testing a very
large number of animals in their reaction to a constant or nearly
constant piece of tissue. It removes in so far as it is possible to do
so the variable introduced by the use of tissue derived from a
large number of individuals. It thus simplifies the experimental conditions to a very great degree and makes possible
methods of analysis which are entirely impossible in ordinary
laboratory races of animals if many animals are used as the host
and as the donors of tissue.
It may be of interest to state that Loeb recognizes, not only
the similarity in reactions following autotransplants of tumor
and normal tissue, but also in the cases of homoio- and of heterotransplants in the two types of tissue. Certain specific quotations will aid in bringing out his viewpoint in this matter.
This
. . . comparison between phenomena observed in tumor and
c. c. LITTLE
86
tissue growth may suggest that a study of tissue growth may not only
assist in the interpretation of tumor growth, but that conversely, the
analysis of tumor growth may help to lay the foundation for a physiology of tissue growth. . . . This comparison leads to the conclusion
that . . . perhaps all of the phenomena observed in tumor growth find
a parallel in the growth of normal tissues under special conditions. . . .
How far these differentials act alone or in combination with constituents of the body fluids of the host . . . etc. . . . is as yet not fully
determined. But it is certain that in all these reactions no essential
difference exists between tumors and normal tissues (13).
As an experiment providing an example of the similarity of
reaction in the two case8 it may be of interest to give the results
of transplants of spleen, of carcinoma and of sarcoma all derived
from supposedly uniform animals. The donors in this case are
animals belonging to a race of Japanese waltzing mice. These
animals had been subjected to a process of rigorous inbreeding
of the type calculated to produce genetic uniformity. The results obtained cover a period of several years during which time
pedigree relationships had passed well beyond a point where they
could in any way serve either as a basis for prediction or as an
aid to analysis. A tabulation of the three groups of experimental work has been made below.
Pedigree Relationehip
1
Raoe or Generation
Between sibs. . . . . . . . . . . . . . . . . . . Jap. Waltzing
Parents to children. . . . . . . . . . . . . . Jap. Waltzing
No recent relationship. .......... Jap. Waltzing
Parents to children. . . . . . . . . . . . . .FI Jap. "on waltz.
*1Children to parents. . . . . . . . . . . .Fl Jap. Non waltz.
No recent relationship. ..........F1Jap. Non walts.
I
c$gasaroorna
All+
All
All
All
All
+
+
+
All +
+
+
+
+
All
All
All
All
None
made
All
+
Spleen
+
+
+-
All
All
All -IAll
All
All
+
From the above table it will be clear that the pedigree relationship has no effect upon the results when animals of the
closely inbred Japanese waltzing race are used.
1 The type of mating marked with an asterisk, is one in which the tissue used was
in one case spleen and in the other carcinoma derived from FI generation hybrid
animals. It is included merely because it shows how definite the result ia when
animals of known genetic constitution are used as the donors and hosts, and how on
the basis of such results it is possible to show the complete correspondence between
the behavior of tumor tissue and normal tissue in the testa made.
GENETICS OF TISBUE TRANSPLANTATION IN MAMMALS
87
We may, therefore, conclude that, for the purpose of analyzing
the phenomena of the inheritance of the factors which underly
susceptibility and non-susceptibility to transplants of tissue,
experiments with tumor and with normal tissue are of equal
value, and further that in the absence of evidence to the contrary, they may be said to be essentially similar in their behavior
under the experimental conditions of transplantation.
The Multiple Factor Hypothesis
The second matter of interest, under the head of constructive
suggestions, is the use of the multiple factor hypothesis in the
genetic analysis of the results of the study of tissue transplantation. Loeb has suggested that multiple factors might be involved in the case of certain transplantable tumors with which
he and Fleisher (14) worked some years ago. I n commenting
on the use of the multiple factor in this connection the writer
and Tyzzer in 1916 pointed out that the hypothesis of multiple
factors which we then advanced was markedly different from
that used by Loeb and Fleisher. What they apparently had in
mind was that the race was the unit of genetic analysis. When
a number of animals picked from a race which had been carried
on as distinct for some time in the laboratory were inoculated
with a given tumor, a certain proportion of them grew the tumor,
while the others failed to do so. The proportion which grew
the tumor was reduced to a per cent basis and the value so obtained was considered as the characteristic of that race. Other
races would of course have other per cents, and the difference
between the two percents shown by any two races was supposed,
by them, to be relatively constant, and to depend upon multiple
factors. Something of the same use of the multiple factor
hypothesis is being made by Loeb in the case of his studies.on
the inheritance of the inds.
A considerable series of experiments reported by Tyzzer, by
Strong (15), and by the writer (l6), (17), (18), running over
several years, and involving several races of mice of known
genetic constitution, show clearly that, while multiple mendelizing factors are undoubtedly involved, their behavior is quite
distinct from that suggested by Loeb and his co-workers.
88
C. C. LITTLE
That Loeb’s use of the multiple factor hypothesis in his studies
of the inheritance of the ind is very indefinite, can, I think, be
shown by a few quotations. Thus he says:
The rapidity with which the transplants attract lymphocytes in
various kinds of transplantations is graded, and these gradations
corrtispond to the gradations in the relationships between cell proteids
and constituents of the body fluids in donor and host (10).
In the case of the tissues we have to assume the existence of inds
which are composed of multiple chemical groups; therefore Mendelian
heredity would be that of multiple factors.
It is not improbable that even in the case of tissues, the number of
these groups is limited and that all the individuals of the same species
have a choice only between a relatively small number of groups. Other
chemical groups would be characteristic of species and in this case also
the number of groups which constitute a species differential may be
limited (20).
He further adds that
. . . the ind is not inherited according to the rules of alternating
Mendelian heredity of simple monohybrid characters, but that all
degrees of blending are observed. We may conceive of all gradations
[in inds] . . . as corresponding to gradually increasihg quantitative
differences in the same substance present in the majority of the tissues
of the same individual. The inheritance of these inds is distinct from
the inheritance of other characters of organs and tissues. Both sets
of characters may follow different rules of heredity (21).
There seems to be little reason for the statement that the type
of inheritance involved in the case of the ind is, in any way,
fundamentally different from the type found in the case of any
morphological character that involves more than one unit. As
the writer and Bagg have pointed out in a paper now in press,
the rule among morphological characters of mammals seems to
be complexity of Mendelian units rather than the simple one
factor case which first made itself obvious to investigators of
color inheritance. The behavior of the genetic factors in the
case of tumor implants is orderly, and to 8 high degree predictable. A series of cases increasing in simplicity have been investigated. Thus the writer and Tyzzer (1916) reported a case
involving apparently, from 12-14 mendelian units; Tyzzer and
the writer in 1916 also gave a case of from 4-6 units in a sarcoma
of the Japanese waltzing mouse. The writer and Strong, in a
paper now in press, mentioned in the Harvey Lectures of 1921,
GENETICS OF TISBUE TRANSPLAfiTATION IN MAMMALS
89
show cases of two and three factors respectively, and Miss B. W.
Johnson in the writer’s laboratory is at present investigating
what seems to be a second three-factor case of entirely different
origin. It is clear that the principle of multiple mendelizing
factors as the underlying type of the inheritance of tissue implants is proved by these experiments.
From the ratios in which the susceptible and non-susceptible
young appear in the Fz generation, it is not only possible to show
the number of factors involved, but also to show that the factors
are functional when present in ti single representation, i.e.,
heterozygous, and that they must be present simultaneously
and act as a complex.
We may summarize and describe the way in which multiple
factors influence the successful or unsuccessful treatment of
tissue implants somewhat as follows. The tissues of any individual are the product of a complex of genetic factors interacting
with the cell substance of the egg and of its descendant cells.
In a very closely inbred race, the different individuals have the
same, or nearly the same complex of genes, and there is therefore na way in which any difference can be detected. In an
ordinary laboratory race, the various individuals are probably
characterized by different complexes many of which, however,
approach each other closely, and some of which either coincide,
or come so near it that implants between the individuals with
such complexes are as successful as are autotransplants. The
best place to study the inheritance of the factors which underlie
tissues, whether normal or tumor, is in crosses between two
closely inbred races, within each of which there is sufficient
homogeneity to allow successful transplantation of the tissue
in question between different individuals in approximately 100
per cent of the cases attempted. I n such crosses, the Fz and
back-cross generations give a chance to measure in terms of
Mendelian units, the degree of difference between the two complexes which characterize the two parent races. By inoculating
different tissues into the same or genetically similar animals, it
is possible to discover whether differences in genetic factors are
to be found in various organs, or whether all organs of a single
7
90
C. C. LITTLE
individual have the same genetic make-up. Eventually, after
more extensive experiments have been made, it should be possible
to identify by physiological means or by linkage, some of the
genetic factors which are particularly involved in the formation
of tissues.
Genetic Characteristic of F, Hybrids
The third general matter to be considered is the way in which
relationship of the two genetic complexes which characterize two
such inbred races, is shown by the first generation hybrids between them. Loeb has found that the chief characteristic of the
first generation hybrid between two individuals is that it tends
to show an ind which is intermediate between the two parents.
This is undoubtedly because he is dealing with animals so mixed
in genetic constitution that the complex resulting from a cross between any two of them will have some of the genes which were
active in the parents and some which they may have possessed
but did nbt show because they were recessive in action, or for
some other reason were inhibited from expressing themselves.
If, however, one works with genetically homogeneous (homozygous) material, the gametes formed by the parent type will
each of them contribute to the formation of the F1 hybrid aZ2
of the genes that characterize its complex (in, however, a single
representation instead of a double one found in the zygote).
The first hybrid generation would then be formed of animals
which were made up of the complexes of both parent races. It
is of prime importance to know whether or not the two complexes lose their respective identity or whether they remain
together, each retaining for use under particular circumstances
their own genetic characteristics. That the latter is the condition which actually obtains is indicated by the behavior of
first generation hybrids between animals of two distinct races of
mice, each of which has been inbred to a point of genetic uniformity.
When tests of this sort are made the results are very striking
and consistent. In, for example, a cross between Japanese
waltzing mice and non-waltzers of a closely inbred race the hybrids which result in the first generation will grow the tissue of
GENETICS O F TISSUE TRANSPLANTATION I N MAMMALS
91
either parent race. In other words the genetic factors which
make up the Jap-waltzing complex are present in only a single
representation but they function nevertheless, and are determining agents in allowing the Japanese mouse tissue to grow.
In the very same hybrid animals there is, however, a single
representation of the factors which characterize the nonwaltzing complex. When a bit of tissue from an animal of this
non-waltzing race is inoculated into such a hybrid the animal
reacts favorably to it on the basis of the non-waltzing complex
which it possesses. The first generation hybrids are, in these
cases, therefore really dual in nature. When one set of circumstances require, they can produce the reaction necessary to
support growth of one type of tissue, and when different conditions obtain they can reverse their behavior and grow tissue
of very different genetic properties.
There is a further development of this line of reasoning which
gives us some interesting insight into the reason why implants
from child to parent are not as successful as are those from
parent to child. One possible reason for this phenomenon has
been hinted at by Loeb as follows:
In the case of transplantation from child to mother . . . the graft
would lack one half the chromosomes and therefore the corresponding
chemical groups present in the cells of the graft. The result should
approach that of homoiotransplantation, which we indeed find to be
the case (22).
The idea which seems to underlie Loeb’s explanation is
probably correct as far as it concerns an inherited chemical
difference which determines the fate of the graft, and so far as it
mentions the chromosomes as the basis for genetic difference.
On the other hand, in the description of the graft in its relation
to the host, in the transplants from child to parent there is
either a typographical error or else a badly mixed conception
of what the basis of genetic difference really is. It is not so much
that the graft lacks any chromosomes or anything else, as it is
that the graft consists of tissue derived from a dual origin, bearing the genetic factors which the parent other than that used as
the host, has introduced. Since, now, these factors will, unless
the two parents have an almost identical genetic constitution,
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be of very different nature from those possessed by the other
parent, it is clear that the hybrid resulting from a cross between
two genetically unlike parents will have in its combination of
genes, at least several that the parent used as a host will lack.
If now the genes underlying tissue formation are operative in
determining the biological and physiological properties of the
tissues derived from their interaction, it will follow that the
tissue of the offspring of two such parents will differ from either
parent and therefore is recognized by them as foreign and is
eliminated after transplantation just as is any tissue which
differs in its genetic constitution from the animal used as the
host.
The dual capacity in which the hybrid between any two genetic complexes acts, is very interesting. As above stated, it
shows that the two genetic complexes which went in to the
make-up of the hybrid can be separated when it comes to recognizing the elements which the hybrid has in common with either
of the parent types. On the other hand, the hybrid itself
builds up its tissues out of the interaction of the two complexes
so that the resulting tissue is a combination of the two as well as
the sum of their genetic compositions. These facts bring out the
point that the host is the agent which discriminates in favor of,
or against the implanted tissue. If the host recognizes in a
physiological way the implanted bit of tissue as foreign to itself,
it will eliminate the implant. If on the other hand the implant
has no foreign or antagonistic elements the host does not eliminate it but leaves the decision as to its future to the local blood
supply, and to the inherent power of metabolism and growth
which the implanted tissue possesses.
That the implant does not need to be identical genetically
with the host provided it has no factors recognized by the host as
foreign, is shown by two main lines of experimental evidence.
The first of these is given by the work of Murphy and others in
growing rat and mouse tumors in early chick embryos. Here
the embryo is unable to recognize any tissue as foreign until
such a time as it has developed enough of its own physiological
characteristics to enable it to express its individuality. When
GENETICS O F TISSUE TRANSPLANTATION IN MAMMALS
93
this point is reached, however, it proceeds to eliminate the
implant of tumor tissue promptly and effectively. The second
line of evidence is found in the growth of the embryo of mammals as an implant in the uterus. To be sure, the mother has a
well developed system of protective mechanisms, but it seems
likely that most of the structures developed for the care of the
embryo, have, in evolution, a morphogenetic or mechanical
function rather than any attempt to keep the physiological
nature of the embryo apart from that of the mother. Logically
there are two ways in which the mother could tolerate the
growth of the mammalian embryo. The first of these is by
having a physiological make-up identical with it, the second
is under a condition in which the embryo has no definite physiological characteristics which are individual enough to be recognized as foreign by the mother, until well along in its ontogeny.
That it is, in all probability, the latter of these two explanations that applies, is indicated by the fact that embryonic tissue
has been shown to have distinctly fewer individually characteristic differences than has the tissue of the same animal when adult.
Thus, adult mice quickly recognize as foreign and eliminate
implants of rat tumor while newly born mice will support the
growth of the same tumor for eight or ten days before they reach
a point where they are able to recognize it as foreign and to
start against it a protective reaction. Also, as has been clearly
shown, mice which are not yet sexually mature do not discriminate against and eliminate transplants of tumor from genetically unrelated mice as rapidly as do the same animals when
sexual maturity has been attained. The internal secretions of
the gonads have without any question a distinct effect in giving
an opportunity for the various genetic characteristics to express
themselves. Before these secretions are active, the animal has
not completely developed the physio'ogical characteristics which
are potential to it. After they become active, however, the
full individuality of the animal expresses itself. If the above
description is the one which accurately explains the facts, we
should expect that in a race in which some of the animals were
going to grow the implants and some were not, that the sex
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C. C. LITTLE
which matures the more rapidly should, if a series of animals of
different ages were inoculated, show a higher percentage of
successful implantations. This would be especially true in
observations taken at a time when that particular sex had
matured and the other sex had not yet reached the full development of a sexual maturity. That such is the case has been
shown by the writer (23) in a series of age groups of mice inoculated with a tumor (sarcoma, J. W. B.) of the Japanese waltzing
mouse.
SUMMARY AND CONCLUSIONS
It is not easy to include in a summary the points which are
developed by a communication of the present type. In general,
however, we may say that the whole theory of the hereditary
behavior of the individuality differential as advanced by Loeb,
rests on the assumption that the pedigree relationship of animals
is per se an index of their genetic relationship, and that this
assumption is unsupported by a whole host of evidence which
underlies the modern concepts of genetics. This fact makes the
use of such terms as syngenesiotranspIantation of no real scientific value. The terms “auto” and “homoio,” long used to
describe a certain type of transplant, need revision to conform
with the advances that have been made in our knowledge of
the genetic constitution of the individual.
The essential similarity of normal and tumor tissue for purposes of transplantation work, from the viewpoint of genetics,
is clear, as are certain advantages which tumor tissue possesses.
The multiple factor hypothesis of mendelizing units can be
successfully applied to all known cases of the genetics of tissue
transplantation. When it is so applied, it is in a markedly
different form from that advanced by Loeb and Fleisher and by
other investigators. We must suppose that a complex of interacting factors is involved. All the factors which go to make up
the necessary complex must be present simultaneoudy. This is
a very different principle from that underlying the inheritance
of a character which depends upon a single gene, the expression
of which is modified by the presence of a number of subsidiary
genes. It also differs from the conception of multiple allelo-
GENETICS OF TISSUE TRANSPLANTATION I N MAMMALS
95
morphs and from the type of inheritance that depends on
duplicate genes.
The peculiar nature of the first generation hybrid between
different races is also a matter of interest. When material of
known genetic composition is used the intermediate results
described by Loeb are not found. The first generation hybrids
arising from a cross between two homogeneous races which
differ from one another, have the ability to grow the tissue of
both parent races.
REFERENCES
(1)
(2)
(3)
(4)
LOEB:American Naturalist, 1920, liv, 55.
LOEB: J. M. Res., 1918, xxxix, 56.
LOEB:American Naturalist, 1920, liv, 45.
LOEB: J. M. Res., 1918, xxxviii, 418.
( 5 ) CASTLE AND PHILLIPS: Carnegie Inst. Of Wash., Publ. NO. 144, pp. 26, 1911.
(6) LITTLEAND TYZZER:
J. M. Res., 1916, xxxiii, 393.
(7) LOEB:J. M. Res., 1918, xxxix, 211, 212.
(8) LOEB: J. M. Res., 1918, xxxviii, 418.
(9) LOEB: American Naturalist, 1920, liv, 55.
(10) LITTLEAND JOHNSON: Proc. SOC.Exper. Biol. & Med., 1922, xix, 163.
(11) LOEB:J. M. Res., 1918, xxxviii, 418.
(12) LOEB:Proc. SOC.Exper. Biol. & Med., 1921, xviii, 153.
(13) LOEB:J. Cancer Res., 1917, ii, 147 and 150.
(14) LOEBAND FLEISHER: Centralbl. f. Bakteriol., 1912, lxvii, 135.
(15) STRONG:J. Exper. Zool., 1922, xxxvi, 67.
(16) LITTLE: Science, 1920, li, 467.
(17) LITTLE:Abt’s Pediatrics, 1923, i, 171.
(18) LITTLE: The Harvey Lectures, 1921-22, Series xvii.
(19) LOEB: J. M. Res., 1918, xxxix, 39-58.
(20) LOEB:American Naturalist, 1920, liv, 55-60.
(21) LoEn: J. M. Res., 1918, xxxviii, 419.
(22) LoEn: American Naturalist, 1920, liv, 55.
(23) LITTLE: J. Exper. Zool., 1920, xxxi, 307.