Michael Galton

ASYNCHRONOUS REPLICATION OF THE MOUSE SEX CHROMOSOMES.

Abstract The Y-chromosome in male cells and one of the X-chromosomes in female cells were identified in C57BL/10Jax strain mouse embryo tissue culture on the basis of their late replication, utilizing a technique involving the uptake of tritiated thymidine, and autoradiography.

DNA Replication Patterns of the Sex Chromosomes in Somatic Cells of the Syrian Hamster

The pattern of terminal DNA synthesis of the sex chromosomes of the Syrian hamster was studied by means of tritiated-thymidine incorporation and autoradiography. In the female , an e

Chromosomes of Testicular Teratomas

Eight testicular teratomas arising in proven males were examined for nuclear sex chromatin. Nuclear DNA content was measured in seven of the tumors and attempts were made to study the karyotype in seven and the chromosome late-replication pattern in five. Sex-chromatin-positive stromal, endothelial and epithelial cells possessing near-diploid or near-triploid DNA values were present in five of the eight tumors. Aneuploid DNA values of chromatin-negative regions corresponded with the aneuploid karyotype found in six tumors. The modal chromosome numbers ranged from 53 to 111 Not only was there marked karyotypic instability within each tumor but there was no overall similarity between different tumors. The terminal sequence of chromosome replication, successfully studied in two cases, one of which possessed sex-chromatin-positive cells, failed to reveal a late-replicating X-chromosome. Unfortunately, it was impossible to prove that the chromatin-positive teratoma did not contain a second X-chromosome: the failure to demonstrate an unequivocal XX sex chromosome constitution might be connected with special cultural requirements of the sex-chromatin-positive cells. However, other varieties of embryonic and undifferentiated cell tumors in males have been reported to display nuclear sex chromatin, and even certain normal testicular elements are sex-chromatin-positive. It is concluded that the existence of sex-chromatin-positive teratomas in males does not necessarily imply production from haploid gametes. The present observations do not rule out gametal origin. However, they provide more support for the idea that teratomas arise from diploid cells, possibly misplaced blastomere or primordial germ cells.

FACTORS INVOLVED IN THE REJECTION OF SKIN TRANSPLANTED ACROSS A WEAK HISTOCOM-PATIBILITY BARRIER: GENE DOSAGE, SEX OF RECIPIENT, AND NATURE OF EXPRESSION OF HISTOCOMPATIBILITY GENES

The proportion of permanently surviving skin grafts from mice of the C57BL/10J strain and from its congenic resistant line, B10.LP, on their F2 hybrid progeny is consistent with a P1 histocompatibility difference at two or possibly more loci in the Vth linkage group near the agouti locus. Male F2 recipients were incapable of rejecting P1 skin grafts foreign at certain single alleles, whereas female F2 progeny were reactive to grafts foreign at one of these alleles. The increased survival time of grafts to B10.LP hosts from (C57BL/10J x BIO.LP) F1 hybrid mice, compared with C57BL/10J donors, is tentatively attributed to a gene dosage effect. Heterozygous grafts were slightly less vulnerable than homozygous grafts to pre-existing immunity, which perhaps indicates the existence of fewer antigen receptor sites on the heterozygous grafts. Female members of both the P1 and F2 genera-lions consistently displayed a more vigorous pattern of allograft rejection than males.

Sex chromosomes of the chinchilla: Allocycly and duplication sequence in somatic cells and behavior in meiosis

SummaryThe X-chromosome of the chinchilla is the largest member of the complement. Since its area equals approximately 9% of that of the haploid autosome set, it may represent a “duplicate-type X-chromosome”, in contrast to the “original-type” in which the ratio X:autosomes ranges from 5 to 6.5%. Unlike other known species with the “duplicate-type X-chromosome”, the chinchilla possesses a very small Y-chromosome.It was inferred from the allocyclic behavior and asynchronous replication pattern of the sex chromosomes in somatic cells that one X-chromosome in the female and the male X-chromosome manifest a single genetically active region: the presumed active segment is inserted in the longer arm adjacent to the centromere. The remainder of these X-chromosomes, the entire second X-chromosome in the female and the Y-chromosome displayed positive heteropycnosis in mitotic prophase and completed DNA synthesis prior to mitosis later than any other elements: this cytologic behavior was held to reflect genetic inertness.The existence of genetic inactivation of much of the “duplicate-type” X-chromosome mass of the chinchilla supports the hypothesis that a constant optimal ratio between the functional portion of the X-chromosomes and the autosomes is maintained in mammals despite a wide range of relative sizes of the X-chromosomes.In female meiosis there is a free exchange of chiasmata between the X-chromosomes at first meiotic metaphase. In the male, however, the association between the large X- and small Y-chromosomes during meiosis is invariably end-to-end, denoting the absence of synapsis. This contrasts with the occurrence of side-by-side pairing of the XY-bivalent in other mammals possessing the “duplicate-type X-chromosome” but in which the Y-chromosome is correspondingly large.

DNA Replication Patterns of the Sex Chromosomes of the Pigeon (Columba livia domestica)

The late replication pattern of the pigeon chromosomes was studied by means of the technique of terminal uptake of tritiated thymidine and autoradiography. The W-chromosome in the female completed rep

Antibody formation during pregnancy.

Antibody responses to sheep red blood cells, bacterial endotoxin, and bovine γ-globulin were compared in pregnant and nonpregnant mice, hamsters, and guinea pigs. The antibody response was not significantly altered by pregnancy and, thus, generalized suppression of the immune response is not an explanation for the privileged position of the fetus as a well tolerated homograft in the mother.

Entry of lymph node cells into the normal thymus.

SUMMARY Labeled lymphoid cells obtained from the lymph nodes draining a skin graft were administered to syngeneic mice of the opposite sex. Labeling of proliferating cells in the lymph nodes was accomplished by the repeated injection of tritiated thymidine into the base of the graft. Labeled cells, presumably donor lymph node cells, were present in small number evenly distributed in autoradiographs of sections of the thymus of both neonatal and adult recipients. At the time of observation, 1, 2, or 3 days after donor cell inoculation, donor cells were not observed in mitosis. The exemption of the thymus from direct involvement in the immune response cannot be accounted for by the exclusion of lymph node cells and therefore it is more likely to depend on factors operating within the thymus itself.

THE RELATION OF THYMIC CHIMERISM TO ACTIVELY ACQUIRED TOLERANCE

Equation of the cellular events underlying actively acquired tolerance produced in newborn mice (Billingham ei d., 1956) with those responsible for immunologic unresponsiveness to homograft antigens in adults (Brent & Gowland, 1963) has necessitated critical reevaluation of the mechanisms of tolerance. Several theories assume a plurality of response to antigen based on the degree of exposure and the developmental status of the immunologically competent cells: nonreactive stem cells are supposed to pass through a transitional phase, during which they are specifically suppressed or eliminated by contact with antigen (Medawar, 1961; Mitchison, 1961; Burnet, 1962), whereas mature cells are appropriately sensitized; paradoxically, antigen excess may eliminate competent cells (Michie & Howard, 1962; Brent & Gowland, 1962 & 1963). Two theories of tolerance have been derived from Consideration of these attributes of immunologic reactivity. The “heterogeneous” or “stem cell” theory calls for the elimination of both immature and mature cells by the tolerance-conferring antigen (Brent & Gowland, 1962 & 1963). In a modification of the “homogeneous” theory, it is stated that only mature cells are susceptible to the action of antigen, which sensitizes in low dosage but renders tolerant in high dosage (Michie & Howard, 1962). An important consideration in attempting to discriminate between these two conflicting concepts is the participation of the thymus in the maintenance of tolerance. The revised clonal selection theory of immunity (Burnet, 1962) assigns a primordial role to this organ; the thymus offers a suitable environment for the “first level” differentiation of immunologically competent cells and the coincident elimination of potentially self-reactive clones. The avoidance of self-reactivity is attributed to the suppressor effect of native antigen during the maturation process. The exclusion of foreign antigen, necessary to ensure comprehensive immunologic capacity, may be effected by a blood-thymus barrier (Marshall & White, 1961; Clark, 1963; Weiss, 1963). Homograft tolerance is often associated with thymic chimerism, although the proportion of donor cells is quite variable. In irradiated adult mice restored with foreign cells, virtually the entire dividing cell population in the thymus is ot donor origin (Ford & Micklem, 1963). Similarly, a high proportion of donor cells may be present in the thymus when tolerance is induced in the neonatal period by an inoculum capable of mounting a graft-versus-host reaction (Trentin & Session, 1963), and this is presumably a reflection of the increased territory available to the donor cells following the graft-versus-host destruction of host lymphoid tissues (Brent & Gowland, 1963). When adult F1 hybrid cells are used to confer tolerance in neonatal parental strain recipients, a combination which precludes graft-versus-host reactivity, the proportion of antigenically recognizable donor cells (in the spleen) is greatly reduced (one to five per cent; Brent & Gowland, 1963). I t has been inferred that split tolerance involving the Y antigen is independent of persistence of antigen (Billingham & Silvers, 1 9 6 0 ~ ) . However, reappraisal of this conclusion is necessary, since both antigenic moieties of the

Chromosomes of “Mongoloid” Hamsters.

Summary The chromosomes were studied in 8 adult Syrian hamsters which had developed “mongolism” following an intracerebral inoculation of virus in the neonatal period. No significant chromosome abnormalities were detected. It is concluded that viral-induced “mongolism” in the hamster is an example of selective postnatal developmental arrest and that the particular manifestations of the condition can be accounted for on the basis of cell depletion due to the lethal effect of virus on dividing cells.