Changes produced by pertussis antigen on the blood cells and lympho-haemapoietic tissues after early and late thymectomy or splenectomy

The effect of thymectomy and splenectomy in C3H/Bi mice on the responses of circulating leucocytes and on morphological changes of the haematopoietic tissues after injection of pertussis vaccine has been studied.

After pertussis all mice showed depletion of lymphoid cells in all the lymphoid organs as well as in bone-marrow and an increased number of leucocytes, lymphocytes, neutrophils and monocytes in the circulation. Neonatal thymectomy decreased lymphocytosis produced by pertussis. Thymectomy, at all ages studied, fostered an increase in the number of monocytes and polymorphonuclears in circulation. Splenectomy at birth or early in life provoked an increase in levels of circulating polymorphonuclears and lymphocytes in pertussis treated animals.

In neonatally thymectomized mice the depletion of lymphoid cells from lymphoid tissues after pertussis could be shown to include the thymic-independent areas. The depletion of small lymphocytes from thymus following pertussis persisted longer than depletion of small lymphocytes from spleen, marrow or lymph nodes. The longer persistence of lymphoid depletion in the thymus than in peripheral lymphoid tissues is, we believe, to be related to the central lymphoid function of thymus as a site of differentiation of lymphoid cells and to the aloofness of thymus from recirculation of fully differentiated peripheral lymphocytes.

     

Development of Natural Killer Cell Activity and Genetic Resistance to Bone Marrow Transplantation with Age: Effect of Neonatal Thymectomy

The effect of neonatal thymectomy on the development of splenic and bone marrow natural cell-mediated cytotoxicity and on genetic resistance to bone marrow transplantation was examined in mice. Natural cytotoxicity was measured by a ⁵¹Cr release assay; the ability to engraft foreign bone marrow was assayed by the spleen colony method. The natural cytolytic response of spleen cells increased progressively from youth to early adulthood, whereas that of the bone marrow declined during the same age period. Neonatal thymectomy significantly elevated the natural killer cell response of young mice only (4 weeks, spleen; 6 weeks, bone marrow). In other experiments, neonatally thymectomized and sham-operated mice were lethally irradiated at 4 or 6 weeks of age and injected with 2.5, 5.0 or 10 million rat marrow cells. Six days later spleen colonies were markedly reduced in both 4- and 6-week-old neonatally thymectomized mice with all rat marrow cell doses tested. Neonatal thymectomy did not alter the percentage of erythroid versus other colonies at either 4 or 6 weeks. In both thymectomized and sham-operated mice the number of colonies increased with increases in marrow cell dose. The data are suggestive of a production and dissemination to the spleen of cells involved in the natural cytotoxic response from the bone marrow.     

Selective depletion of lymphoid tissue by cyclophosphamide

Selective depletion of lymphocytes from the lymph follicles and cortico-medullary junction in lymph nodes and equivalent non thymus dependent areas of the spleen can be produced by cyclophosphamide (CY) (300 mg/kg) in the mouse and guinea-pig. Despite three such injections on alternate days, thymus dependent areas still contained lymphocytes. Total depletion of lymphocytes from lymph nodes and spleen was produced by combining neonatal thymectomy in the mouse or ALS treatment in the guinea-pig with CY. CY produced depletion of lymphocytes in the cortex of the thymus before the medulla. Maximal depletion occurred at 3 days and in surviving animals repopulation was evident by 7 days at the cortico-medullary junction only. Lymph follicles were found in lymph nodes of neonatally thymectomized CY treated mice following repopulation with bone marrow. These findings suggest that the lymphocytes of the lymph follicles are derived from a population of rapidly dividing cells, part of which at least can be found in the bone marrow.     

Proliferation and emigration of newly formed lymphocytes from pig spleens during an immune response

Normal young pigs were immunized intravenously with sheep red blood cells (SRBC). At various times after a second SRBC injection the spleens were connected to an extracorporeal perfusion system, and proliferating lymphoid cells in the spleens were selectively labelled with tritiated thymidine. One day later the relative and absolute numbers of spleen-derived lymphocytes were determined by autoradiography in the following organs: various parts of the spleen, mesenteric and cervical lymph nodes, thymus, bone marrow, Peyer's patches, tonsils, intestine, lung, liver and blood. From 1 to 7 days after the second SRBC injection, the spleens produced increasing numbers of lymphocytes, and labelled cells were found especially in the blood and bone marrow. The newly formed splenic lymphocytes migrated preferentially to T- but also to B-cell areas in lymph nodes, Peyer's patches and tonsils. In all organs outside the spleen nearly all labelled spleen-derived lymphocytes were small lymphocytes. However, the bone marrow contained a high proportion of labelled immature and mature plasma cells. The spleen produced large numbers of lymphocytes during the secondary immune response, many of which migrated to different organs probably as memory cells, while others were found in the bone marrow as effector cells from the immune response.     

The immunological responsiveness of germ-free mice thymectomized at birth. II. Lymphoid tissue and histopathology

The effect of neonatal thymectomy and antigenic stimulation on the lymphoid cell population has been studied in germ-free mice. Neither thymectomy nor injection of sheep erythrocytes induced any significant alteration in the blood lymphocyte levels. There was a clear-cut reduction in the cellularity of the periarteriolar lymphocyte sheaths of the spleen and of the paracortical regions of the lymph nodes in the thymectomized mice. Following stimulation with sheep erythrocytes, large pyroninophilic cells appeared in these areas in the intact germ-free controls but in only a few thymectomized mice and then in reduced numbers. Thymectomy did not influence the cellularity of the lymphoid follicles but less germinal centre and plasma cell activity occurred in response to an injection of sheep erythrocytes. Lesions suggestive of autoimmune reactivity were not found in lymphoid or nonlymphoid tissues of neonatally thymectomized germ-free mice. Lesions typical of viral infections were seen in some germ-free mice in both thymectomized and intact groups. It is concluded that the specific defect associated with the absence of the thymus is a reduction in a particular class of lymphocytes the development of which is under thymus control and the activities of which are to mediate certain defined immunological responses.     

Quantitative studies of lymphocytes and other cell populations in the bone marrow of neonatally thymectomized C3H mice

The effects of neonatal thymectomy on the development of the lymphoid, erythroid and granulocytic cell populations in mouse bone marrow have been assessed by quantitative techniques. The numbers per unit volume of bone marrow of 17 cell types were determined in neonatally thymectomized and sham thymectomized C3H mice at two, four and eight weeks of age, and compared with those of normal C3H mice. After neonatal thymectomy the numbers of small lymphocytes, large and medium-sized lymphoid cells, and erythroid cells reached normal levels at two weeks but fell progressively to 18%, 22% and 42% of normal, respectively, by eight weeks. In sham thymectomized mice these cell populations did not differ significantly from normal. Immature and mature granulocytes were elevated in numbers two weeks after either neonatal thymectomy or sham thymectomy, suggesting a transient non-specific stimulation of granulocytopoiesis. During continuous infusion of ³H-thymidine for ten days in neonatally thymectomized mice aged four weeks and eight weeks many bone marrow small lymphocytes remained unlabeled. The results demonstrate that early postnatal development of bone marrow lymphoid and erythroid cells proceeds normally in the absence of the thymus, in accord with the concept of the bone marrow as a primary site of lymphocyte production and differentiation. In addition, some slowly-renewing small lymphocytes in bone marrow appear to be thymus-independent cells.     

Diabetogenic T cells are primed both in pancreatic and gut-associated lymph nodes in NOD mice

Activation of an islet-specific immune response is an early yet essential step in autoimmune diabetes. The immune cells and antigen(s) involved in this early step and its anatomical site remain incompletely understood. To directly evaluate the site where islet-specific and diabetogenic lymphocytes are activated, we isolated lymphocytes from spleen and from pancreas-draining, gut-associated and subcutaneous lymph nodes of diabetic NOD mice and of young NOD mice, and transferred these into NOD scid/scid recipients devoid of endogenous islet-specific immune responses themselves. Although spleen lymphocytes from diabetic NOD mice induced diabetes more rapidly than lymphocytes from any other lymphoid tissue, spleen lymphocytes from young NOD donors were not superior to other lymphocytes from the same donors. At a donor-age of 6 weeks, the most-diabetogenic lymphocytes were found in pancreas-draining lymph node whereas gut-associated lymph nodes and the spleen were sources of intermediate diabetogenic activity. Lymphocytes from peripheral lymph nodes were only weakly diabetogenic at this age, and also remained the least efficient later. Surprisingly, lymphocytes isolated even from 3-week-old NOD mice had diabetogenic potential. However, such cells were almost exclusively found in gut-associated lymph nodes. This suggests that initial priming of diabetogenic cells takes place in the gut whereas pancreas-draining lymph nodes may serve as the site of amplification of the autoimmune response.     

Adult and pre-adult thymectomy of mice: contrasting effects on immune responsiveness, and on numbers of mitogen-responsive and Thy-1+ lymphocytes.

Peripheral lymphoid tissues of mice which have been thymectomized at 2 or 4 weeks of age, that is, before they achieve adult body weight, have been shown to be lacking in cells responsive to the T-cell mitogen, phytohaemagglutinin, when the animals became adult, and these mice have also been shown to have a deficient immune response against sheep erythrocytes. It is suggested these effects of pre-adult thymectomy are consequent upon removal of the prime source of T cells prior to the animal having acquired complete T-cell populations of the adult. Spleens and lymph nodes of mice thymectomized at 8 weeks of age were found to have reduced numbers of cells susceptible to the cytotoxic effects of anti-Thy-1 serum as early as 4 weeks after the operation, whereas the number of lymphocytes responsive to T-cell mitogens in these lymphoid tissues was not reduced at this time. The number of spleen-borne antibody-producing cells in a primary or secondary response was not affected by 8-week thymectomy, either when the response was tested in the operated animal, or after transfer of cells from such an animal to an irradiated recipient. The results are discussed with respect to other work on the effects of thymectomy of mice during the post-neonatal and pre-adult period.     

Numbers and phenotype of lymphocytes emigrating from sheep bone marrow after in situ labelling with fluorescein isothiocyanate

In normal young lambs the bone marrow was selectively labelled with fluorescein isothiocyanate by a temporary perfusion of one hind-leg. One day later, the incidence of bone marrow emigrants in different lymph nodes, spleen, Peyer's patches, thymus, non-perfused bone marrow and blood was determined. The emigrants were also phenotyped by the use of monoclonal antibodies and classified into monocytes or lymphocyte subsets. Large numbers of lymphocytes left the bone marrow of the perfused leg during 1 day. Considerable numbers of cells migrated to other bone marrow compartments. Varying numbers of mononuclear emigrants were found in peripheral lymphoid organs, with labelling indices ranging from 1.06% in the blood to 0.004% in the thymus. In the spleen, comparable numbers of B- and T-lymphocyte emigrants from the bone marrow were found, whereas in the blood, lymph nodes and jejunal Peyer's patches many more emigrants were T lymphocytes than B lymphocytes. In the prescapular lymph nodes, for instance, 90.4% of emigrants were T cells but only 9.6% were B cells. Based on the large numbers of lymphocytes emigrating from the bone marrow, their phenotypes and their entry into other bone marrow compartments, it it can be concluded that the bone marrow of young lambs is an integral part of the migratory route of lymphocytes.     

Frequencies of background cytoplasmic Ig-containing cells in various lymphoid organs of athymic and euthymic mice as a function of age and immune status

The distribution of background Ig-secreting cells, measured as cells containing cytoplasmic immunoglobulin (C-Ig cells), over spleen, bone marrow, lymph nodes and Peyer's patches was studied in congenitally athymic (nude) mice and heterozygous euthymic mice as a function of age and immune status (germ-free (GF) vs specific pathogen-free (SPF]. In young athymic as well as in young euthymic mice, the spleen was found to contain the great part of all C-Ig cells, irrespective of whether the mice were GF or SPF. The number of C-Ig cells in the spleen was found to be rather constant over the life span, while the number of C-Ig cells in the bone marrow of all groups of mice greatly increased with age. This indicates that the relative shift of C-Ig cells to the bone marrow is neither dependent on the presence of the thymus, nor on the microbiological status of the mice. However, at young and intermediate age the microbiological status of the mice did affect the total number of C-Ig cells per mouse. This was mainly due to the effect upon the bone marrow, mesenteric lymph nodes and Peyer's patches. At these ages the background Ig synthesis in these organs appeared to be mainly dependent on external antigenic stimulation, in contrast to the spleen, where the Ig synthesis appeared to be mainly due to endogenous stimulation. The Ig (sub)class distribution of the C-Ig cells was different for all different organs tested. Hardly or no difference in percentage distribution was found between the GF nude and GF heterozygous mice. Most C-Ig cells in spleen, bone marrow and lymph nodes of the GF mice were of the IgM isotype. C-IgG and C-IgA cells occurred in substantial percentages only in bone marrow and lymph nodes. In the lymph nodes of GF nude mice a remarkably high percentage of C-IgA cells was found.     

Nomarski differential interference contrast studies of murine lymphocytes

Nomarski differential interference contrast microscopy (DIC) of lymphocyte surface morphology was combined with immunofluorescence studies of T and B cell markers on the thymus, lymph nodes, spleen, peripheral blood lymphocytes and thoracic duct lymph of female CBA mice. DIC identified smooth cells and several categories of villous cells; more extreme forms were present in lymph. Most B cells seemed to belong to the smooth group and most peripheral T cells to the villous group. Thymus cells were almost entirely smooth, but treatment with cortisone increased the proportion of villous cells to 50%. The surface morphology of lymphocytes was highly labile preventing direct identification or separation of T and B cells. In vivo removal of T cells by adult thymectomy, lethal irradiation and bone marrow reconstitution caused the villous cells to decrease. During recovery from irradiation, T lymphocytes tended to parallel villous cells, B lymphocytes smooth cells, but there were differences between the spleen and lymph nodes. Mice deprived of T1 cells by adult thymectomy showed a modest decrease of smooth cells in the spleen and blood; mice depleted of T2 cells by anti-lymphocyte serum, or which were naturally deficient in T2 cells, were markedly lacking in villous cells. Thoracic duct lymph, which is rich in T2 cells, had a high proportion of extremely villous lymphocytes. Exposure to lymph induced extreme villous features in lymph node cells, and it was found that the thoracic duct lymph was markedly hypertonic to serum, although varying in osmolarity throughout the day. It is suggested that the villous shape of T2 cells is a circulatory adaptation, necessitated by the peculiar character of the lymphatic system in mice.     

Histological Development of the Immune Tissues of a Marsupial,the Red‐Tailed Phascogale (Phascogale calura)

Red‐tailed phascogale (Phascogale calura) pouch young at birth were relatively underdeveloped in comparison with their eutherian counterparts, and the lymphoid tissues of the immune system were found to be histologically immature. The phascogale thymus rapidly developed in the first few days of pouch life and was quickly populated with lymphocytes. By the end of pouch life, involution of the thymus was underway. The bone marrow started to develop in the early stage of pouch life, although adipocytes and megakaryocytes were not observed until slightly later. The liver was hematopoietic from birth and reached histological maturity toward the end of pouch life. The lymph nodes were difficult to detect macroscopically because of their small size, but were easily identified microscopically later in pouch life, particularly in the mesentery, and these lymph nodes exhibited germinal centers by the end of pouch life. The early spleen was predominately mesenchymal, but exhibited some erythropoiesis. Follicles with well‐developed germinal centers were not observed until the latest stage of pouch life. Although intraepithelial lymphocytes were detected in the intestines early in pouch life, the discrete lymphoid aggregates and Peyer's patches characteristic of the gut‐associated lymphoid tissue (GALT) were not detected until later in pouch life. This is the first report of histological development in phascogale pouch young, as well as the first report of the thymus, bone marrow, and lymph nodes in this dasyurid species at any age. Anat Rec, 299:207–219, 2016. © 2015 Wiley Periodicals, Inc.     

Cells involved in the immune response: XXII. The demonstration of thymus-specific antigens in the rabbit

Horse anti-rabbit thymus cell serum (HARTS) was obtained by immunizing a horse with rabbit thymocytes intravenously at weekly intervals for 3 weeks. The horse was bled 2 weeks later and the antiserum was analysed for its cytotoxic activity with respect to the lymphocytes of the various lymphoid organs. It was demonstrated that the cytotoxic activity of the antiserum was several orders of magnitude greater for thymus cells than for cells of the other organs tested. Only thymus and lymph node cells were capable of absorbing the thymocytotoxic activity of the antiserum; however, ten to fifteen times as many lymph node cells as thymus cells were required to neutralize the thymocytotoxic activity of the serum. Absorption of the antiserum with the cells of the other lymphoid organs (spleen, bone marrow, appendix, sacculus rotundus, Peyer's patches and circulating leucocytes) resulted in a slight but significant decrease in the thymocytotoxic activity. At no time was the thymocytotoxic activity completely absorbed with cells of these organs. The cytotoxic activity of the antiserum with respect to the cells of the different lymphoid organs other than the thymus could be abolished following absorption of the antiserum with the cells of any of the lymphoid organs. On the basis of our data, it is concluded that (a) the thymocytes possess two groups of antigens—one thymocyte specific and one common to all rabbit lymphocytes and (b) only the lymph nodes of all the lymphoid organs other than the thymus possess significant numbers of thymus-derived or T-cells. However, the proportion of these cells in the lymph node does not exceed 7–10 per cent, a figure much lower than that found in the lymph nodes of the mouse. Less than 1 per cent of the circulating lymphocytes in the rabbit are T-cells.     

Frequencies of background cytoplasmic Ig-containing cells in various lymphoid organs of athymic asplenic (lasat), athymic,asplenic and normal BALB/c mice

The frequency of immunoglobulin-containing plasma blasts and plasma cells (C-Ig cells) was determined by means of immunofluorescence in various lymphoid organs of hereditarily athymic asplenic (lasat), athymic (nude), asplenic (all on a BALB/c background) and normal BALB/c mice. Two age groups were tested, namely 8 and 14 weeks.The total number of C-Ig cells in spleen, bone marrow, lymph nodes and Peyer''s patches together was the largest in the normal BALB/c mice, the smallest in the lasat mice, and intermediate in the athymic mice and the asplenic mice. The mice of all four groups had more C-Ig cells at the age of 14 weeks than at the age of 8 weeks. In 8-week-old BALB/c mice and in athymic nude mice most C-Ig cells were located in the spleen. In lasat mice and in thymus-bearing asplenic mice the bone marrow was the major site of localization of C-Ig cells. In all four groups of mice the number of C-Ig cells increased considerably with age, especially in the bone marrow. In athymic mice, whether asplenic or not, C-Ig cell numbers in the Peyer''s patches were deficient.The percentage distribution of C-IgM, C-IgG1, C-IgG2, C-IgG3 and C-IgA cells was different for different lymphoid organs, and was dependent on both the spleen and the thymus. In normal BALB/c mice C-IgM cells were the most frequent in the spleen, whereas the other classes of C-Ig cells were the most frequent in the other lymphoid organs. In absolute numbers, C-IgM cells were the most numerous in the two groups of eusplenic mice, and C-IgG and C-IgA cells in the two groups of euthymic mice.     

Organ distribution and fate of newly formed splenic lymphocytes in the pig

The production of lymphoid cells in the pig spleen was studied autoradiographically after selective labeling of the spleen using an extracorporeal perfusion circuit. Tritiated thymidine was added as a DNA precursor. One to 4 days after local labeling of the spleen the relative and absolute number of spleenderived lymphocytes were determined in the following organs: mesenteric, cervical and inguinal lymph nodes, thymus, bone marrow, Peyer's patches, tonsils, three different parts of the gut, lung, liver, and blood. The labeled lymphocytes which migrated to these organs were all small lymphocytes, except for some large cells in the lamina propria. In the bone marrow, however, a considerable number of the spleen-derived immigrants were transformed into plasma cells. The total number of labeled lymphocytes decreased dramatically from Day 1 to Day 4 after labeling, indicating a high percentage of short-lived cells. Within the spleen, plasma cells had the highest labeling index of about 30% at Day 1 but this dropped to only 1.5% on Day 3. The organ distribution of the splenic emigrants changed from Day 1 to Day 4 with a relative increase in lymphocytes found in lymph nodes and a decrease in the lung and the intestinal wall. The newly formed splenic lymphocytes migrated to T- and B-cell areas in lymph nodes, Peyer's patches, and tonsils. In the intestinal wall labeled lymphocytes were found in the lamina propria and also as intraepithelial lymphocytes. There was no obvious redistribution between organ compartments with time after labeling of the spleen. The spleen produces large numbers of lymphocytes, which show typical organ distribution and homing to areas in lymphoid and nonlymphoid organs.     

Kinetics of recovery of serum Ig levels and of cytoplasmic Ig positive cells in various lymphoid organs of nude mice after thymus transplantation.

The long-term effects of thymus transplantation in nude mice were studied with regard to the number of cytoplasmic immunoglobulin positive plasmablasts and plasma cells (C-Ig cells) in various lymphoid organ and their immunoglobulin (Ig) class distribution profile. These data were correlated with the serum Ig levels of the same mice. Four weeks after thymus transplantation, the number of C-Ig cells in the spleen of nude mice had increased two- to three-fold over that found in normal nude mice and normal heterozygous littermates of the same age. This overshoot subsided at 8 weeks after thymus transplantation. The increase of the C-Ig cell number in the other lymphoid organs tested (bone marrow, mesenteric lymph nodes and Peyer''s patches) started later than in spleen, and did not show a clear overshoot. Almost complete recovery of the C-Ig cell pattern to that of normal littermates was found 32 weeks post-transplantation. Analysis of the Ig class distribution of the C-Ig cells showed that the increase of the C-Ig cell numbers after thymus transplantation in nude mice was almost exclusively confined to IgG1, IgG2 and IgA. The increase of C-IgG1 and C-IgG2 cells in spleen and bone marrow correlated with a simultaneous increase of the serum IgG1 and IgG2 levels, suggesting that these organs are the major source of serum IgG in young adult mice.     

T and B lymphocytes in New Zealand Black mice: an analysis of the theta, TL and MBLA markers

The lymphocytes of thymus, blood, spleen, lymph nodes and bone marrow were studied in NZB mice between the ages of 1 and 14 months, and compared with lymphocytes of A and CBA strain mice of the same age. By standard cytotoxicity techniques, the proportion of cells possessing the θ, TL and MBLA markers was found to be similar in NZB in mice and controls. In 14-month old NZBs, whose spleen was largely replaced by reticulum cell sarcoma, and in younger recipients of the passaged tumour, there was a reduction in the percentage of θ and MBLA-positive cells in the spleen. In a few mice at 4 and 9 months, small numbers of MBLA-positive cells were present in the thymus and there was a corresponding decrease in θ-positive cells. TL-positive cells were not present outside the thymus, and θ-positive cells were not present in the bone marrow in unusual numbers. NZB peripheral lymphocytes appeared to have the same surface concentration of θ as those of A or CBA mice, as judged by anti-θ titration curves. The reticulum cell sarcoma was shown to be θ-negative and MBLA-negative, while an NZB thymoma was θ-positive, TL-positive and MBLA-negative. It was concluded that the peripheral lymphoid organs contain a large population of T lymphocytes of abnormal character.     

In vitro spleen leucocyte migration and phytohaemagglutinin-stimulated lymphocyte transformation of immunosuppressed mice cells. II. Cells from mice treated by azathioprine and/or immune stimulation or thymectomy

An in vitro study of the spleen leucocyte migration ability and the phytohaemag-glutinin (PHA) stimulated peripheral lymphocyte response was made on cells derived from mice treated by azathioprine, immune stimulation, or thymectomy and azathioprine. These responses were compared with those shown by normal mice and found to be considerably depressed. Thymectomy combined with azathioprine produced the greatest change which persisted at 40 days. These changes suggested that azathioprine in common with thymectomy, antilymphocyte globulin and radiotherapy produced some of its immunosuppression by depressing T-cell function.     

Delayed development of immunological responsiveness in neonatally thymectomized mice: a time-course study

Haemolysin responses to first injection of sheep erythrocytes in neonatally thymectomized, neonatally sham-thymectomized and intact Swiss albino mice were tested when the mice were 10 days, 4 weeks, 6–7 weeks and 6 months of age. The serum haemolysin activity was assessed at a number of times after injection of antigen (time-course study). Neonatal thymectomy of Swiss mice was followed by a decreased and delayed haemolysin response. These abnormalities in antibody response following neonatal thymectomy became less obvious when the age at which the mice were injected was increased, indicating that delayed development of immunological responsiveness had occurred in neonatally thymectomized Swiss mice.     

Formation of lymph follicles and germinal centers in the somatic and mesenteric lymph nodes of growing mice during ontogenesis

We investigated the age-dependent changes that occur in the numbers of lymph follicles and germinal centers in various lymph nodes in BALB/C and ICR mice aged between four days and 16 to 18 weeks. Young adult BALB/C mice have a relatively small body size, compared to ICR mice at the same stage, where there is a relatively large body size. In BALB/C mice somatic (popliteal, brachial, axillary, inguinal, submandibular and deep cervical) and mesenteric lymph nodes were examined. In ICR mice only the somatic (popliteal, brachial and axillary) lymph nodes were examined. In both BALB/C and ICR mice, the primary follicles were apparent in most somatic nodes by the 6th postnatal day. Up to 28 days of age, the number of follicles per node increased, reaching different levels in nodes from different locations. Thereafter, in most of the somatic nodes in BALB/C mice the number of follicles increased only slightly, although there was a substantial increase in ICR mice, reaching a peak or a plateau at 8 or 12 weeks of age. In the mesenteric (ileocecal) nodes in BALB/C mice, the primary follicles first appeared at 10 to 12 days, then there was a linear increase until a plateau level was reached at 8 weeks of age. Germinal centers appeared in the mesenteric nodes at 28 days and increased rapidly in number thereafter. In most somatic nodes germinal centers were scarcely observable until 8 weeks of age. Based on our observations we have three suggestions. Firstly, in BALB/C mice there were two different patterns of age-dependent changes in the numbers of lymph follicles in the somatic and the mesenteric nodes during ontogenesis. These different patterns are probably due to variations in the magnitude of the exogenous antigen stimulatory effect. Secondly, it seems likely that the variations in the numbers of lymph follicles that are produced in somatic nodes at different locations during the first 28 days after birth relate to the dimensions of the body regions that are drained by that particular somatic node at that stage of development. Thirdly, in the relatively small BALB/C mice, the ontogenetic production of lymph follicles in a somatic node is mostly completed during the first four weeks of life, whereas in the relatively larger ICR mice, this process may continue until the young adult stage of 8 weeks.