Characterization of dimethylnitrosamine-induced focal and nodular lesions in the livers of newborn mice

Newborn Swiss-Webster mice were given an intraperitoneal injection of 25 micrograms of dimethylnitrosamine. At weaning they began receiving 0.05% phenobarbital in the drinking water to promote the lesions for the term of the study. Preneoplastic foci and hyperplastic nodules were identified histologically by two markers, resistance to exogenous iron accumulation and an increase in gamma-glutamyltranspeptidase activity. AT 8, 12, and 16 weeks of age, livers of affected male mice exhibited 12, 18, and 12 iron-resistant foci/cm2 and 13, 9, and 9 gamma-glutamyltranspeptidase-positive foci/cm2, respectively (average for median right and right anterior sublobes). Iron-resistant nodules were first observed at 8 weeks; however, gamma-glutamyltranspeptidase-positive nodules were not noted until 12 weeks. In animals that received dimethylnitrosamine but were not placed on phenobarbital, there was an average of 5 foci/cm2 (iron-resistant or gamma-glutamyltranspeptidase-positive) at 12 weeks while no nodules were noted. This model could provide a practical short-term in vivo tool for the detection of early sequential cellular alterations produced by initiators, inhibitors, and promoters of carcinogenesis.     

Abnormalities in liver iron accumulation during N-2-fluorenylacetamide hepatocarcinogenesis that are dependent or independent of continued carcinogen action

The characteristics of liver iron accumulation were studied during N-2-fluorenylacetamide (FAA)-induced hepatocarcinogenesis in rats. After injection of iron-dextran in control rats, hepatocytes accumulated stainable iron evenly throughout hepatic lobules. During the feeding of FAA, iron accumulation was reduced in the midzonal and centrilobular regions. After FAA removal, hepatocytes in these regions again accumulated high amounts of iron. Hepatocellular altered foci induced by FAA displayed rather uniform (> 94%) iron-exclusion during FAA feeding. After FAA removal, however, iron-exclusion was lost in a fraction of the foci, while others (40-64%) remained resistant to iron accumulation. A large majority of liver neoplasms (> 93%) displayed resistance to cellular iron accumulation both during FAA feeding and after removal of FAA. Thus, iron-exclusion by liver neoplasms is carcinogen-independent and irreversible, in contrast with that of normal hepatocytes which is completely carcinogen-dependent and reversible. Altered foci appear to represent two populations: one is characterized by reversible iron-exclusion whereas the other, like neoplasms, possesses permanent iron-exclusion.     

Isolation and enrichment of cells resistant to iron accumulation from carcinogen-induced rat liver altered foci

Enrichment of cells from rat liver iron-excluding foci induced by 2-acetylaminofluorene was performed using density gradients. Rats were continuously fed a diet containing the carcinogen for 8 to 13 weeks to induce altered foci that were iron excluding and positive for γ-glutamyl transpeptidase activity. At weekly intervals, and after loading with iron by subcutaneous injection, the livers were perfused with collagenase and dissociated. Fractionation of dissociated control and treated liver cells on Ficoll gradients yielded three cellular fractions. In preparations from carcinogen-exposed rats the percentage of hepatocytes excluding iron and positive for γ-glutamyl transpeptidase was always higher in the top (least dense) fraction than in the other lower fractions or the original population of dissociated hepatocytes. Moreover, the total number of hepatocytes in the top fraction increased progressively with the duration of feeding. These observations indicate that iron-excluding hepatocytes of enzyme-altered foci are less dense than other hepatocytes and can be enriched by gradient centrifugation. The progressive increase in the number of these altered liver cells with continued carcinogen exposure is consistent with a role for these cells in the development of liver tumors.     

Iron resistance of hepatic lesions and nephroblastoma in rainbow trout (Salmo gairdneri) exposed to MNNG

Histochemical markers are important for the early detection of chemically initiated neoplasia in experimental animal studies. The marker, iron resistance, was evaluated in the Shasta strain of rainbow trout (Salmo gairdneri). Twenty-one-day-old trout embryos were exposed to 100 ppm aqueous N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) for 30 min in a static water bath. Fish were fed a semipurified diet, and sampled monthly from the 4th to the 9th month. Two days before sampling, fish were iron-loaded with a single ip dose of 0.30 mg iron dextran/100 g body weight. Livers and kidneys were conventionally processed to paraffin sections for iron, or hematoxylin and eosin (H&E) staining. Normal hepatocytes accumulated iron in pericanalicular locations, but in hepatocytes from carcinogen-altered foci and tumors, iron staining was clearly reduced or absent. Normal renal tubule cells exhibited slight to moderate iron staining, while those from nephroblastoma were iron resistant. These results establish iron resistance as a property of preneoplastic and neoplastic trout hepatocytes and nephroblastoma cells for the first time. Iron resistance may offer a practical histochemical marker in experimental fish models of hepatocellular carcinoma and nephroblastoma.     

COMPARATIVE STUDY OF ABNORMALITY IN GLYCOGEN STORING CAPACITY AND OTHER HISTOCHEMICAL PHENOTYPIC CHANGES IN CARCINOGEN-INDUCED HEPATOCELLULAR PRENEOPLASTIC LESIONS IN RATS

A sequential comparison was made between abnormal glycogen storage and other histochemical phenotypic changes in hepatocellular precancerous lesions (altered foci and neoplastic nodules) during various stages in the process of development of cancer in rat liver. N-2-fluorenylacetamide was fed to male rats for 8 weeks and groups of rats were killed at the end of carcinogen feeding and at 12 and 24 weeks on control diet. Foci rich in glycogen storage accounted for a majority of all foci over the course of experiment, while foci devoid of glycogen storage, which were absent at the end of carcinogen feeding, gradually increased in number during maintenance. Glycogen-deficient lesions that might appear to arise from glycogen-rich lesions displayed hyper-basophilia demonstrated by toluidine blue reaction, but often lacked gamma-glutamyl transpeptidase activity. Resistance to iron accumulation was consistently shown in all precursor lesions for hepatocellular carcinoma in the siderotic liver regardless of abundance or absence of cellular glycogen. It was suggested that properties such as loss of glycogen storing capacity, hyperbaso-philia, and some cellular atypicality resembling those of carcinoma cells might be essential elements for malignant progression.     

Influence of type 2 diabetes on the inflammatory response in rats

OBJECTIVES AND DESIGN: To verify whether the inflammatory responses in animals with type 2 diabetes are altered to an extent similar to that in type 1 diabetes. MATERIALS: Male newborn (2 days old) Wistar rats were made diabetic by streptozotocin (160 mg/kg, i.p.) and used 8-10 weeks later (10 rats/group). METHODS: The inflammatory responses were evaluated using paw edema (induced by local injection of carrageenan or dextran), pleurisy (by pleural injection of carrageenan), increases in vascular permeability (induced by intradermal injection of histamine, serotonin and bradykinin) and leukocyte counts in peripheral blood and pleural exudate. RESULTS: Diabetic animals showed reduced inflammatory responses to carrageenan but not to dextran. The increase in vascular permeability induced by serotonin and bradykinin was reduced whereas that to histamine was not altered in diabetic compared to control rats. Although the pleural exudate was reduced, leukocyte counts were similar in diabetic and control rats. Insulin (2 IU, 4 h before), though effective in reducing blood sugar levels, did not restore the altered responses in diabetic rats. In contrast to that in rats with type 1 diabetes, in rats with type 2 diabetes, removal of the adrenal glands restored the reduced inflammatory responses. CONCLUSIONS: Insulin resistance in type 2 diabetic rats led to reduced inflammatory responses, which were partially corrected by adrenalectomy.     

Carcinogen-induced alterations in rat liver DNA adduct formation determined by computerized fluorescent image analysis

These studies employed continuous feeding of a carcinogenic level of N-2-acetylaminofluorene to male rats for 28 days. Under these conditions normal hepatocytes are known to be inhibited from proliferation, whereas xenobiotic-resistant putative preneoplastic hepatocytes with altered liver enzyme phenotypic expression appear to have a growth advantage. A novel technique using computerized fluorescent image analysis of triple-stained frozen liver sections was developed and used to visualize three different molecular markers in individual hepatic cells. Proliferating liver cells were identified by anti-5-bromodeoxyuridine immunostaining in livers of rats injected with 5-bromodeoxyuridine 1 hour before sacrifice. Anti-cytokeratin immunostaining was used to identify bile ducts and putative oval cells. Characterization of DNA adduct formation was achieved with an antiserum specific for N-(deoxyguanosine-8-yl)-2-aminofluorene, the major DNA adduct of 2-acetylaminofluorene. The image analysis demonstrated low but distinct DNA adduct concentrations in putative oval cells identified by anti-cytokeratin staining and in scattered, replicating liver cells recognized by anti-5-bromodeoxyuridine. Adducts were not detected in replicating foci consisting of 3 to 11 nuclei. It is possible that proliferating liver cells that have low N-2-acetylaminofluorene-DNA adduct levels may clonally expand to become foci protected from further adduct accumulation and preneoplastic liver lesions. Thus, the computerized fluorescent image analysis demonstrated here may provide a novel procedure for identification of carcinogen-induced liver cell alterations.     

Comparative study of abnormality in glycogen storing capacity and other histochemical phenotypic changes in carcinogen-induced hepatocellular preneoplastic lesions in rats

A sequential comparison was made between abnormal glycogen storage and other histochemical phenotypic changes in hepatocellular precancerous lesions (altered foci and neoplastic nodules) during various stages in the process of development of cancer in rat liver. N-2-fluorenylacetamide was fed to male rats for 8 weeks and groups of rats were killed at the end of carcinogen feeding and at 12 and 24 weeks on control diet. Foci rich in glycogen storage accounted for a majority of all foci over the course of experiment, while foci devoid of glycogen storage, which were absent at the end of carcinogen feeding, gradually increased in number during maintenance. Glycogen-deficient lesions that might appear to arise from glycogen-rich lesions displayed hyperbasophilia demonstrated by toluidine blue reaction, but often lacked gamma-glutamyl transpeptidase activity. Resistance to iron accumulation was consistently shown in all precursor lesions for hepatocellular carcinoma in the siderotic liver regardless of abundance or absence of cellular glycogen. It was suggested that properties such as loss of glycogen storing capacity, hyperbasophilia, and some cellular atypicality resembling those of carcinoma cells might be essential elements for malignant progression.     

Histogenesis of dieldrin and DDT-induced hepatocellular carcinoma in Balb/c mice

Male, Balb/c mice were fed diets containing dieldrin (10 ppm) and DDT (100-175 ppm) for 75 weeks. Control and treated mice were serially killed and their livers analyzed by histological and histochemical procedures after 2, 4, 8, 16, 36, 52 and 75 weeks of exposure. Mice administered both chlorinated hydrocarbons initially responded with centrolobular hepatocytomegaly. The cells were characterized by decreased glucose-6-phosphatase and succinate dehydrogenase activity. At later periods 52 through 75 weeks, foci of phenotypically-altered hepatocytes were noted. The cells of these lesions were basophilic or clear-staining in hematoxylin and eosin-stained sections and displayed increased gamma glutamyl transpeptidase activity. In mice preloaded with iron dextran, cells of foci were negative for iron when the surrounding parenchyma was siderotic. Hepatocellular adenomas (HA) and carcinomas (HPC) were composed of cells with increased gamma glutamyl transpeptidase and glucose-6-phosphate dehydrogenase and decreased glucose-6-phosphatase and succinate dehydrogenase activity. In iron loaded mice, the cells of HA and HPC did not stain for iron in otherwise siderotic surroundings. Both hepatocellular foci and adenomas may be potential precursors of mouse hepatocellular carcinomas.     

The resistance of spontaneous mouse hepatocellular neoplasms to iron accumulation during rapid iron loading by parenteral administration and their transplantability.

Spontaneous mouse liver nodules were found to be resistant to iron accumulation induced either by dietary overload or by a rapid protocol of subcutaneous injection of iron dextran. Transplants of 11 liver nodules into the mammary fat pad gave rise to neoplastic growth. Transplants of the less differentiated nodules grew more frequently and rapidly than better differentiated nodules and produced pulmonary metastases. Therefore, these mouse liver nodules are considered to be neoplasms, and their resistance to iron accumulation suggests that this marker will be useful in studying their histogenesis.     

Chemopreventive effects of vanadium toward 1,2-dimethylhydrazine-induced genotoxicity and preneoplastic lesions in rat colon

In the present study, we have evaluated the antitumor effects of vanadium by monitoring DNA damage and chromosomal aberrations (CAs) during the early preneoplastic stage of 1,2-dimethylhydrazine (1,2-DMH)-induced colon cancer in male rats. Treatment with 20 mg/kg 1,2-DMH for 6 weeks resulted in the formation of aberrant crypt foci (ACF), a putative preneoplastic lesion associated with colon cancer development, while cotreatment with ammonium monovanadate (0.5 ppm in the drinking water) reduced ACF formation by 50% (P < 0.001). The 6-week treatment with 1,2-DMH also resulted in significantly higher levels of DNA damage in rat colon as measured by the Comet assay (higher mean values for length-to-width ratios (L:W) of DNA mass (P < 0.01) and mean frequencies of cells with comets (P < 0.001)). The vanadium cotreatment reduced DNA damage in colon cells by 32% (P < 0.02 and P < 0.001 for L:W and tailed cells, respectively). 1,2-DMH treatment also produced a 10-fold increase in the frequency of CAs in rat colon (P < 0.001), while cotreatment with vanadium resulted in a reduction in CAs after 2, 4, and 6 weeks of 1,2-DMH exposure (P < 0.01). Analysis of antioxidant defense enzyme activity in colonic mucosa indicated that glutathione reductase and catalase activities were increased in 1,2-DMH-treated rats; cotreatment with vanadium reduced these activities when compared to the carcinogen control (P < 0.001 and P < 0.02). These results demonstrate that the early protective effect of vanadium in chemically induced rat colon carcinogenesis may be mediated by a reduction of carcinogen-induced DNA damage.     

The way we teach general pathology in Oslo, Norway

Hepatocellular carcinoma was induced in rats by administering aflatoxin B₁ (AFB₁) for 6 weeks. Malignant tumours were preceded by foci and nodules of altered hepatocytes of three histological types, composed of basophilic, eosinophilic, and vacuolated cells. In addition, there were areas of altered hepatocytes that were considered as hyperplastic. Lectins were used as histochemical markers to compare the expression of membrane glycoproteins in hepatocellular carcinomas and hepatic nodules with non-nodular or control hepatocytes. There were marked changes in the lectin-binding patterns of the hepatocellular carcinoma cells and the eosinophilic nodules. The lectin-binding patterns of basophilic nodules, vacuolated nodules, and hyperplastic areas were similar to non-nodular or untreated hepatocytes. The similarity in the lectin-binding changes of the eosinophilic nodules and hepatocellular carcinomas suggests that the eosinophilic nodules may be an early stage in the development of carcinoma.     

Effects of the peroxisome proliferator di(2-ethylhexyl)phthalate on enzymes in rat liver and on carcinogen-induced liver altered foci in comparison to the promoter phenobarbital

The livers of rats given either the peroxisome proliferating hepatocarcinogen di(2-ethylhexyl)phthalate (DEHP) following initiation by 2-acetylaminofluorene (AAF) or the neoplasm promoter phenobarbital (PB) were studied for changes in 8 histochemical properties. Male F344 rats were fed 200 ppm AAF for 7 weeks to induce hepatocellular altered foci, and were then fed diets containing either no chemical, 12,000 ppm DEHP or 500 ppm PB for 24 weeks. In hepatocytes, DEHP increased alkaline phosphatase activity throughout the lobule, but reduced gamma-glutamyltransferase (GGT) activity in periportal hepatocytes. PB, in contrast, increased GGT activity in periportal hepatocytes. In foci that were induced by AAF, DEHP reduced the histochemical activity of GGT and did not increase the number, mean volume or volume % of foci detected by deficiencies in iron storage, glucose-6-phosphatase, adenosine triphosphatase or fibronectin. PB enhanced the expression of all 8 phenotypic abnormalities in foci such that either more profiles were detected or the area of foci was increased.     

P-Nitrobenzoic acid alpha2u nephropathy in 13-week studies is not associated with renal carcinogenesis in 2-year feed studies.

The objective of this study was to characterize the renal toxicity and carcinogenicity of p-nitrobenzoic acid in F344 rats. Dose levels in 13-week and 2-year studies ranged from 630-10,000 ppm and 1,250-5,000 ppm, respectively. At 13 weeks, renal lesions included minimal to mild hyaline droplet accumulation in male rats and karyomegaly in male and female rats. At 2 years, renal lesions included proximal tubule epithelial cell hyperplasia in male rats and oncocytic hyperplasia in high-dose male and female rats, and a decreased severity of nephropathy in males and females. The hvaline droplets in renal tubular epithelial cells of male rats at 13 weeks were morphologically similar to those described in alpha2u-globulin nephropathy. Using immunohistochemical methods, alpha2u-globulin accumulation was associated with the hyaline droplets. In addition, at 13 weeks, cell proliferation as detected by PCNA immunohistochemistry was significantly increased in males exposed to 5,000 and 10,000 ppm when compared to controls. Cytotoxicity associated with alpha2U-globulin nephropathy such as single-cell necrosis of the P2 segment epithelium or accumulation of granular casts in the outer medulla did not occur in the 13-week study. In addition, chronic treatment related nephrotoxic lesions attributed to accumulation of alpha2u-globulin such as linear foci of mineralization within the renal papilla, hyperplasia of the renal pelvis urothelium and kidney tumors were not observed. Although there was histologic evidence of alpha2u-globulin accumulation in male rats at 13 weeks, the minimal severity of nephropathy suggests that the degree of cytotoxicity was below the threshold, which would contribute to the development of renal tumors at 2 years.     

生物素多聚葡聚糖胺追踪显示皮质脊髓束的走行和分布

背景：生物素多聚葡聚糖胺的作用机制及代谢过程目前还不十分清楚,可能和与生物素连接的其他复合物相似。 目的：选择最佳的生物素多聚葡聚糖胺追踪(示踪)实验方法,使生物素多聚葡聚糖胺能充分显示皮质脊髓束的走行和分布。 方法：将15只SD雌性大鼠用于生物素多聚葡聚糖胺条件实验,分为双侧大脑皮质注射10%生物素多聚葡聚糖胺0.5 μL/位点存活2周组、单侧(右侧)大脑皮质注射10%生物素多聚葡聚糖胺1 μL/位点存活3周组及双侧大脑皮质注射10%生物素多聚葡聚糖胺0.5 μL/位点存活3周组,每组5只。 结果与结论：在3组进行生物素多聚葡聚糖胺追踪实验的大鼠中,双侧大脑皮质注射10%生物素多聚葡聚糖胺0.5 μL/位点存活3周组的追踪效果最好。提示在双侧大脑皮质上选择多点注射,以每个位点注射0.5 μL的10%生物素多聚葡聚糖胺,注射后大鼠存活3周为最佳的生物素多聚葡聚糖胺追踪实验方案。     

Tissue response in the rat and the mouse to degradable dextran hydrogels

Two types of hydroxyethyl-methacrylated dextran (dex-HEMA) hydrogels differing in crosslink density were compared for local tissue responses and degradation characteristics in mice and rats. Implants (1 mm thick, rat: 10 mm diameter, mouse: 6 mm diameter) varying in degree of HEMA substitution (DS5 and DS13, meaning 5 or 13 HEMA groups per 100 glucose units of dextran) were subcutaneously implanted and tissue responses were evaluated at week 2, 6, and 13 after implantation. In the rat after 2 weeks a slight fibrous capsule was formed composed of macrophages and fibroblasts sometimes accompanied by a minimal infiltrate. Small fragments, surrounded by macrophages and giant cells indicated hydrogel degradation. After 13 weeks DS5 implants were resorbed while parts of the DS13 implants were still present. In the mouse a moderate to strong capsule formation was present at 2 weeks accompanied by inflammatory cells (macrophages and polymorphonuclear granulocytes) and debris. Draining lymph node activation was observed. Skin ulceration was present irrespective of the type of implant. Clear differences in the tissue responses between the rat and mouse were noted, as well as between implants of different degree of substitution. Mice showed a more pronounced early inflammatory response compared with rats, whereas the degradation was more complete in rats than in mice. The differences in histology between the hydrogels disappeared over time at 13 weeks after implantation and similar responses were noted for both types of hydrogels. Both in mice and rats the DS5 hydrogels showed a faster degradation rate than the DS13 hydrogels.     

高铁对大鼠大脑皮层二价金属转运蛋白1表达的影响

目的 探讨在脑铁代谢中发挥重要生理作用的二价金属转运蛋白1(DMT1)的表达及其调控机制.方法 大鼠(n=6)侧脑室注射右旋糖酐铁3d和7d后,采用铁组织化学法检测脑内铁含量的变化,免疫组织化学技术检测大脑皮层中DMT1的两种亚型,即DMT1(+IRE)和DMT1(-IRE)蛋白表达的变化.结果 铁组织化学染色结果显示,大鼠侧脑室注射右旋糖酐铁500μg/(只·d)7d后,大脑皮层中二价铁和三价铁均显著增高.同时,免疫组织化学结果表明,与对照组相比,脑内达高铁状态时大脑皮层DMT1(+IRE)蛋白表达显著升高,而DMT1(-IRE)蛋白表达无显著变化.结论 在大鼠大脑皮层中,DMT1(+IRE)蛋白对铁水平的升高更为敏感,其表达与脑铁水平(尤其是二价铁)呈正相关.高铁对脑内不同区域内不同亚型DMT1表达的影响存在特异性.     

Altered liver acini induced in diabetic rats by portal vein islet isografts resemble preneoplastic hepatic foci in their enzymic pattern.

As demonstrated previously, liver acini draining the blood from intraportally transplanted pancreatic islets in streptozotocin-diabetic rats are altered in various respects. The hepatocytes in these acini store glycogen and/or fat, and they show an increase in proliferation as well as in apoptotic activity. Thus, they are phenotypically similar to carcinogen-induced preneoplastic liver foci (glycogen-storing foci and sometimes also mixed cell foci). By means of catalytic enzyme histochemistry or immunohistochemistry, we investigated the activity of key enzymes of alternative pathways of carbohydrate metabolism and some additional marker enzymes (well known from studies on preneoplastic hepatic foci) in the altered liver acini surrounding the islet isografts. In addition, the expression of glucose transporter proteins 1 and 2 (GLUT-1 and GLUT-2) were investigated immunohistochemically. The activities of hexokinase, pyruvate kinase, glyceraldehyde-3-phosphate dehydrogenase, and glucose-6-phosphate dehydrogenase were increased, whereas the activities of glycogen phosphorylase, adenylate cyclase, glucose-6-phosphatase, and membrane-bound adenosine triphosphatase were decreased in the altered liver acini. The expression of GLUT-2 was also decreased. GLUT-1 and glutathione S-transferase placental form were not expressed, and the activities of glycogen synthase and gamma-glutamyl-transferase remained unchanged. All changes of the enzyme activities were in line with the well known effects of insulin and resembled alterations characteristic of preneoplastic liver foci observed in different models of hepatocarcinogenesis. It remains to be clarified in long-term experiments whether or not these foci represent preneoplastic lesions and may proceed to neoplasia.     

Distribution of ferritin in the rat hippocampus after kainate-induced neuronal injury

A gradual increase in iron occurs in the lesioned hippocampus after neuronal injury induced by the excitotoxin kainate, and the present study was carried out to investigate whether this increase in iron might be associated with changes in expression of the iron binding protein, ferritin. An increase in ferritin immunoreactivity was observed in glial cells of the hippocampus, as early as three days after intracerebroventricular injections of kainate. The number of ferritin positive cells peaked four weeks after the kainate injection, and decreased eight and twelve weeks after injection. They were found to be mostly microglia and oligodendrocytes by double immunofluorescence labeling with glial markers. A number of ferritin-labeled endothelial cells were also observed via electron microscopy. The decline in ferritin immunoreactivity four weeks after the injection of kainate is accompanied by an increase in the number of ferric and ferrous iron positive cells in the lesioned tissue. A substantial non-overlap between ferritin and iron-containing cells was observed. In particular, spherical ferric or ferrous iron-laden cells in the degenerating hippocampus were unlabeled for ferritin for long time periods after the kainate injection. An increase in iron, together with a reduced expression of iron binding proteins such as ferritin at long time intervals after kainate lesions, could result in a relative decrease in ferritin-induced ferroxidase activity and the presence of some of the iron in the ferrous form. It is postulated that this may contribute to chronic neuronal injury, following acute kainate-induced neurodegeneration.     

The response of the newborn rat to intrapleural dextran: Vascular and cellular aspects

The development of hind paw oedema, in response to dextran injected into the pleural cavity of rats, has been shown to differ with the age of rat studied. It was found that newborn rats were less responsive than adults, with respect to oedema formation induced by dextran. Attempts to suppress the dextran-induced foot oedema using mepyramine and cyproheptadine, were successful only in rats from the adult age group.A semiquantitative analysis was made of the inflammatory cellular exudate produced by the intrapleural injection of dextran into rats of different ages. In contrast to the reaction of adult rats, it was found that the accumulation of polymorphs in the pleura of newborn animals was delayed. The polymorph was the dominant cell type found in exudates induced either by intrapleural saline or dextran, in rats from the 6-hour age group. This contrasted strikingly with comparable exudates induced in adult rats, whose major cell type was the mononuclear cell (after saline), and the polymorph, followed by the mononuclear cell (after dextran).Fewer cells were observed in the reaction of 6-hour-old rats, compared with 7-day-old animals following intrapleural injection of the same volume of a 6-percent solution of dextran.