Adenomatous hyperplasia of the liver resembling focal nodular hyperplasia in patients with chronic liver disease

Two nodules of hepatic adenomatous hyperplasia (AH) resembling focal nodular hyperplasia were found in two patients with cirrhosis or chronic active hepatitis. Imaging techniques suggested that the nodules were hepatocellular carcinoma. Pathological examination showed that the nodules (approximately 1.0 cm in diameter) were clearly demarcated from the surrounding liver tissue, and contained foci of scar-like fibrosis in the centre of the nodules. Microscopically, they contained portal tracts and fulfilled the criteria of AH. A large number of arteries were present in the central scarlike fibrosis as well as in the parenchyma of the nodules. There were foci of mildly atypical hepatocytes in one nodule but no cellular atypia in the other. Morphometric analysis showed that the cumulative luminal area of arteries per unit area was much greater in the nodules than in the extranodular liver tissues, while the cumulative luminal area of portal veins per unit area was much less in the nodules than in the extranodular liver tissues. Although the pathogenesis is unclear, these nodules might have developed through localized vascular changes associated with chronic liver disease, may have arisen from pre-existing arterial malformation, or may represent the early stages of angiogenesis in hepatocarcinogenesis.     

Mallory body formation in hepatic nodules of mice ingesting dieldrin

Mallory bodies (MBs) were observed in hepatic nodules induced by long-term administration of a diet containing 10 p.p.m. of dieldrin to C57BL/6 mice. MBs were first detected after 46 weeks and were seen in 26 of 41 mice which developed hepatic nodules. The MBs were limited to the nodules in 25 mice. Twenty-one of the nodules were carcinomas and 20 of those contained MBs. Our observations suggest that MBs may be a marker for neoplastic transformation and may prove useful in experimental studies of carcinogenesis.     

Changes in atrial biopsies in chronic rheumatic heart disease. I: Cellulo-vascular and mesenchymal reaction

The cellular, vascular and connective tissue changes were studied in 32 atrial biopsy specimens from patients with chronic rheumatic heart disease (RHD). 15 of these 32 specimens showed some inflammatory reaction, 7 with small mononuclear cells only and 8 with macrophage reaction amidst increased but necrotic collagen, especially in the subepicardial and subendocardial regions. Most cellulonecrotic foci were histologically consistent with a stage of Aschoff nodule. Acid phosphatase activity in frozen sections was seen in the cytoplasm of the macrophages. Fine structural examination showed membrane-bound vacuoles and lipofuscin bodies rather than ingested material in the macrophages. By light and electronmicroscopy, these macrophages were not different from those encountered in other granulomatous or necrotic conditions. There was moderate proliferation of blood vessels, with prominence of endothelial cells and pinocytotic vesicles, or fibrosis of media, or proliferation of basal laminae. The presence of Aschoff nodules in the right atrium, the least affected chamber in RHD, suggests a diffuse and smouldering pathology on the basis of a persistent subclinical cell-mediated immune (CMI) reaction.     

Two populations of cells with differing proliferative capacities in atypical acinar cell foci induced by 4-hydroxyaminoquinoline-1-oxide in the rat pancreas

A single intravenous injection of 4-hydroxyaminoquinoline-1-oxide in male Wistar rats at a dose of 6 mg. per kg. of body weight induced atypical acinar cell foci in 100 per cent of the animals. Atypical acinar cell foci could be classified histologically as basophilic and acidophilic foci and acidophilic nodules. Cells in baseophilic foci were large, contained an irregular nucleus and a markedly basophilic cytoplasm with a few to small number of zymogen granules (zymogen-poor cells). By transmission electron microscopy, these cells showed markedly irregular plasma membranes, a well-developed rough endoplasmic reticulum and a few zymogen granules. Cells in acidophilic foci and nodules contained an intensely eosinophilic granular cytoplasm (zymogen-rich cells) and a large oval to round nucleus. By transmission electron microscopy, the cells showed zymogen-rich cytoplasm and irregular lateral plasma membranes. Mitotic activity was completely absent or very rarely observed in normal pancreas or basophilic foci, in contrast to acidophilic foci and nodules in which a mean value of 2.75 +/- 1.27 per 1000 cells was found. Autoradiography confirmed these differences between the proliferative capacity of cells in basophilic foci (1 +/- 1 labeled nuclei per 1000 cells) and acidophilic foci (23.2 +/- 3.15 labeled nuclei per 100 cells). These studies indicate that 4-hydroxyaminoquinoline-1-oxide induces two types of atypical acinar cell foci with different morphologic features and proliferative capacity.     

Comparative morphologic and immunohistochemical studies of estrogen plus alpha-naphthoflavone-induced liver tumors in Syrian hamsters and rats.

Syrian hamsters were treated with ethinylestradiol and maintained on a diet containing alpha-naphthoflavone (alpha NF), a regimen that produces a high incidence of liver tumors. Morphologic analyses (light microscopy, immunoperoxidase studies, and electron microscopy) were performed on livers of these animals. After 4 months of hormone plus alpha NF treatment, marked hepatocyte cell changes were already present, as demonstrated by loss of eosinophilic staining of hepatocyte cytoplasm. Large multinucleated hepatocytes exhibiting frequent mitoses were observed around central veins. After 5 months of treatment, there was proliferation of bile ducts, and small cells with eosinophilic cytoplasm resembling hepatocytes appeared surrounding these bile ducts. At 7 to 8 months, the first tumor nodules (foci) were seen. Tumor foci in the portal area consisted of small clusters of large cells resembling hepatocytes with irregular nuclei. At the same time, dysplastic glands were identified among proliferating bile ducts. By 8 to 10 months, large tumors were present. These were trabecular hepatocellular carcinomas with widely varying individual cell morphology. Compared with adjacent liver, dysplastic glands in the portal areas, microcarcinomas, and large tumors all showed intense immunostaining for cytokeratin. Rats treated with the same regimen also developed hepatic tumors, but the light and electron microscopy results and immunohistochemical profiles were very different. Altered hepatic foci composed of small hepatocytes were typically prominent; however, malignant tumors did not arise from the portal area. Neither altered foci nor tumors stained significantly for cytokeratin. These data suggest that the biochemical events giving rise to these liver tumors differ between the species studied, despite the animals being exposed to the same treatment regimens.     

A Histopathological Analysis of Five Cases of Adenomatous Hyperplasia Containing Minute Hepatocellular Carcinoma

We present five cases of adenomatous hyperplasia (AH) containing minute hepatocellular carcinoma (HCC) in cirrhotic liver. All the patients were Japanese, four males and one female, ranging in age from 60 to 80 years. Two of the specimens were obtained at surgery and the others at autopsy. The AH specimens ranged from 2.0 to 3.0 cm in diameter, and the maximum diameter of HCC foci in the AH was 2.0 cm. Histologically, apart from the HCC foci, the AH specimens showed intrinsic atypia, suggesting preneoplastic change. These features included an increase of both cellularity and the nucleo cytoplasmic ratio, distortion of cord structure and pseudoacinar formation. Ail of the AH specimens contained typical portal triads. Details of diagnostic imaging were also obtained in four cases. The findings of the present study support the possibility that AH with intrinsic atypia is a preneoplastic lesion of HCC. The sequence of "adenomatous hyperplasia with intrinsic atypia HCC foci" would thus represent part of the early phase of hepatocarcinogenesis in humans.     

A histopathological analysis of five cases of adenomatous hyperplasia containing minute hepatocellular carcinoma.

We present five cases of adenomatous hyperplasia (AH) containing minute hepatocellular carcinoma (HCC) in cirrhotic liver. All the patients were Japanese, four males and one female, ranging in age from 60 to 80 years. Two of the specimens were obtained at surgery and the others at autopsy. The AH specimens ranged from 2.0 to 3.0 cm in diameter, and the maximum diameter of HCC foci in the AH was 2.0 cm. Histologically, apart from the HCC foci, the AH specimens showed intrinsic atypia, suggesting preneoplastic change. These features included an increase of both cellularity and the nucleo-cytoplasmic ratio, distortion of cord structure and pseudoacinar formation. All of the AH specimens contained typical portal triads. Details of diagnostic imaging were also obtained in four cases. The findings of the present study support the possibility that AH with intrinsic atypia is a preneoplastic lesion of HCC. The sequence of "adenomatous hyperplasia with intrinsic atypia-HCC foci" would thus represent part of the early phase of hepatocarcinogenesis in humans.     

Pathogenetic role of the stromal cells in endometriosis and adenomyosis

Ten cases of endometriosis of bowel, ovaries, uterine serosa and 10 cases of adenomyosis were studied. Blocks of tissue with areas of interest were submitted for serial sectioning of the entire block. Some sections were immunostained for oestrogen receptor, vimentin, Ber-EP-4 and cytokeratins. The common finding was the presence of type 1 nodules, consisting of isolated nodules of endometrial stromal cells without endometrial glands, along blood or lymphatic vessels. The stromal cells showed positive immunoreactivities for oestrogen receptor and vimentin, and negative reactivities for cytokeratins. Due to the absence of connection with adjacent endometriosis or adenomyosis, it is likely that these endometrial stromal nodules arise from the multipotential pericytes. In addition, in serosa of all cases of endometriosis, type 2 nodules, having adjacent mesothelium (Ber-EP4 ?) changing into epithelium (Ber-EP4 +) and type 3 nodules, with non-endometrial epithelium (oestrogen receptor ?) changing into endometrial gland (oestrogen receptor +) were identified. We believe that the formation of type 1 nodules from the pericytes and the transformation of the mesothelium into endometrial glands in type 2 and 3 nodules are accomplished through the process of induction by the endometrial stroma, and the proliferation is controlled by genetic, hormonal and immunological factors. Type 1, 2 and 3 nodules are likely to represent a histological continuum in the development of early endometriosis. Subsequent to the formation of endometriosis in the serosa, the pathway of development of endometriosis and adenomyosis is similar. Through the processes of induction and proliferation there is an increase in size of the stroma of type 1 nodules and that of endometrial tissue with subsequent fusion of the stroma of type 1 nodules and that of foci of adenomyosis or endometriosis. Consequently, there is enlargement of the stroma of the foci of adenomyosis or endometriosis. The ‘newly enlarged stroma’ serves as ‘new soil’ for further growth of the endometrial glands in the endometrial tissue.     

Morule-like features and tumor-associated lymphoid proliferation in gallbladder carcinoma

A unique case of gallbladder carcinoma with morule-like features and tumor-associated lymphoid proliferation in a 53-year-old man is presented. The surgically resected gallbladder demonstrated a polypoid tumor with a thin stalk, measuring 1.3 x 0.5 cm. Histologically, a well-differentiated tubular adenocarcinoma was accompanied by multiple spindle cell nodules. The stroma of the tumor showed dense lymphocytic infiltrate. Immunohistochemically, the spindled cell nodules were diffusely positive for cytokeratin AE1/AE3 but negative for S-100, NSE, and chromogranin A. The immunohistochemical results considered to be spindle cell nodules were morule-like features. Intranuclear expression of beta-catenin was observed in morule-like features and carcinoma cells. Ki-67 labeling index was 16.7% of carcinoma cells, but Ki-67 immunoreactivity was negative in spindle cell nodules; therefore, morule-like features were considered to represent metaplastic foci of carcinoma cells and not nodular growth of carcinoma cells. Although the exact pathogenesis of marked lymphoid proliferation in the stroma remained unknown, tumor-produced substances may derive from lymphoid proliferation.     

Argyrophilic nucleolar organizer regions and alpha-fetoprotein in adenomatous hyperplasia in human cirrhotic livers.

Recently, adenomatous hyperplasia (AH) of the liver has been suspected as a precancerous lesion in human hepatocarcinogenesis. The authors examined 75 cases of AH from 42 cirrhotic livers, using staining of argyrophilic nucleolar organizer regions (AgNORs). These reflect proliferative cell activity. Findings in AH were compared with those seen in hepatocellular carcinoma (HCC) and other chronic liver diseases. Expression of alpha-fetoprotein (AFP) was also examined immunohistochemically. The authors classified AH into three types: ordinary (OAH), atypical (AAH), and AH with focal malignancy (FM). OAH implies a lack of atypia; AAH represents AH with structural and cellular atypia but without the features of overt carcinoma; and FM denotes AH with foci of overt HCC. Forty of the 75 cases of AH were categorized as OAH, 19 as AAH, and 16 as FM. The noncancerous areas of FM had features of AAH. The mean number of AgNORs in AH was intermediate between that seen in cirrhosis (2.93) and HCC (6.18) and showed a step-wise increase in the following order: OAH (2.95), AAH (3.89), noncancerous areas in FM (4.58), and malignant foci in FM (5.71). There was no significant difference in AgNOR counts between OAH and cirrhosis. AgNOR counts in AAH and FM were significantly higher than those of OAH, and lower than those of HCC. AFP was positive in 12 of 25 HCCs and in malignant foci of 3 FM lesions, but it was absent in OAH and AAH. These data suggest that OAH has a limited capacity for proliferation but that AAH and FM are much more replicative. The latter two conditions are probably preneoplastic lesions or early forms of HCC.     

Multipotency of melanoblasts isolated from murine skin depends on the notch signal

Background : Melanoblasts (MBs), derived from neural crest cells, only differentiate into melanocytes (Ms) in vivo. We previously showed that MBs isolated from mouse skin were multipotent, generating neurons (Ns) and glial cells (Gs) together with Ms. Using Sox10‐IRES‐Venus mice and mouse embryonic stem cells, we investigated how MBs expressed their multipotency. Results : MBs generated colonies containing Ns, Gs, and Ms in the presence of ST2 stromal cells, but they generated only M colonies when incubated with keratinocytes or ST2 culture supernatant, thus showing that MBs required contact with ST2 stromal cells for expression of their multipotency. Notch signaling was shown to be one of the important cues for the maintenance and differentiation of MBs through cell–cell contact. When Notch signaling was inhibited, MBs mainly generated colonies that contained just one type of cells, Ns, Gs, or Ms; the number of colonies containing two or three types of cells markedly decreased even on ST2 stromal cells, showing restriction of their differentiation potency. Whereas when Notch signaling was activated, the number of colonies containing two or three types of cells increased, indicating that their multipotency had been maintained. Conclusions : Our results demonstrate that Notch signaling played novel roles in MB multipotency. Developmental Dynamics 245:460–471, 2016. © 2015 Wiley Periodicals, Inc.     

Excretory function in cultured hepatocytes from griseofulvin-treated mice

We determined the role of cytokeratin (CK) intermediate filaments in the excretory function of hepatocytes in cultured hepatocytes containing Mallory bodies (MBs) from the livers of griseofulvin (GF)-fed mice. Hepatocytes for primary culture were obtained from GF-fed and control mice using the 0.1% collagenase perfusion method. Each component of the cytoskeleton in cultured hepatocytes and liver frozen sections was visualized by immunofluorescence. The whole mount extraction of hepatocytes was carried out using 0.5% Triton X-100. To examine the excretory function of the bile canaliculi (BC), fluorescein diacetate and horseradish peroxidase were used as visible excretory products. Thin sections of the cultured cells were made by the "pop-off" method for electron microscopic examination. Frozen sections of livers from the GF-fed mice showed that the MBs were stained with a rat monoclonal antibody to mouse CK, but the CK filaments in the cells containing MBs did not stain. The intercellular BC were reduced in number in the livers of the GF-fed mice compared with the controls. At 3 hours after seeding, hepatocytes with MBs were not stained, but by 24 hours the CK filament network stained normally in cells containing MBs. The loss of staining of the CK filaments was therefore rapidly reversible in the absence of GF in tissue culture. This reversion to normal was prevented by adding 2 x 10(-4) m GF to the culture medium. Thus, the loss of the CK filament antigenic determinants was directly maintained by GF in vitro. The extracted hepatocytes showed spherical canalicular sheaths formed by the CK filaments within the cytoplasm. This was confirmed in "pop-off" sections which revealed that the canaliculi were lined by microvilli and by the localization of actin around the canaliculi as visualized by immunofluorescence. Excretion of fluorescein diacetate into the intracytoplasmic BC was seen both in the cells from GF and control mice but uptake of horseradish peroxidase was markedly reduced by the hepatocytes from the GF-fed mice. The results show that the hepatocytes containing MBs do not form intercellular BC and excretion of fluorescein diacetate into intracytoplasmic BC is not impaired but the uptake of horseradish peroxidase is markedly reduced. The results imply that the rearrangement of the cytoskeleton induced by GF causes both structural and functional deficits in the affected hepatocytes.     

Ultrastructural, biochemical, and immunologic characterization of Mallory bodies in livers of griseofulvin-treated mice. Fimbriated rods of filaments containing prekeratin-like polypeptides.

Mallory bodies (MBs) induced in hepatocytes by long term feeding of mice with griseofulvin were isolated, purified, and examined by electron microscopy in ultrathin sections and negatively stained preparations. The major structural component of the MBs was randomly oriented, unbranched rods of filaments; these were usually 175 to 250 nm. long and 14 to 20 nm. thick and covered by a dense fimbriate coat of laterally projecting 1.5- to 3-nm. thick threads. Such lateral threads could extend for more than 20 nm. and seemed to be involved in the interconnection of adjacent filaments and their association and aggregation into MBs. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified filament material showed six major polypeptide bands with apparent molecular weights ranging from 48,000 to 66,000. When the portion of the MB filament material that was soluble in solutions containing 8 M urea was allowed to reaggregate upon removal of the urea, an enrichment of one of the polypeptide components (approximate molecular weight, 64,000) was observed. When frozen sections of liver tissue MBs were subjected to indirect immunofluorescence microscopy, they were specifically revealed by guinea pig antibodies directed against purified bovine prekeratin. No significant accumulation of MBs was observed with a series of other antisera, including those containing antibodies against tubulin, actin, and vimentin, the major protein of the intermediate sized filaments predominant in mesenchymal cells. The observations suggest that MBs in livers of griseofulvin-treated mice, and probably also of human alcoholic hepatitis, contain large amounts of prekeratin-like polypeptides which are assembled into a special form of fimbriated rods of 14- to 20-nm. filaments. These filaments are morphologically different from other forms of intermediate sized and thick filaments, including the prekeratin-containing 6- to 11-nm. tonofilament-like filaments present in various epithelial cells.     

Multifocal PEComa (PEComatosis) of the female genital tract associated with endometriosis, diffuse adenomyosis, and endometrial atypical hyperplasia

The authors describe a case of multifocal perivascular epithelioid cell tumor (PEComa) arising in the pelvis of a 39-year-old woman affected by tuberous sclerosis. The tumor presented in the form of multiple fascicular, focally cystic nodules involving the uterine corpus, both ovaries, and the omentum. Microscopically, the nodules were composed of foci of adenomyosis and endometriosis (with focal atypical complex hyperplasia) associated with a stromal spindle cell population immunoreactive for HMB-45, smooth muscle actin, and estrogen and progesterone receptors. We interpret these foci as the result of a widespread proliferation of perivascular epithelioid cells (PEC). Because of the diffuse quality of the process, the designation of PEComatosis seems warranted.     

Effects of hydroxylapatite coating crystallinity on biosolubility, cell attachment efficiency and proliferation in vitro.

A hydroxylapatite (HA) coating with approximately 97% crystalline HA content (MP-1 treated HA coating, MP-HA) was tested in vitro for its biosolubility and cellular biocompatibility. The MP-HA coating was compared with a standard HA coating with approximately 63% crystallinity (SHA) and an amorphous HA coating with approximately 25% crystallinity (AHA), as well as a titanium (Ti) surface without HA coating as a control. The topographic study with scanning electron microscopy indicated that MP-HA appeared more coarse, with projected nodules which altered the shape of cells attached to the substrate. Biosolubility study indicated that MP-HA had the least effect on the culture medium pH, while AHA (P < 0.01) and SHA (P < 0.05) significantly raised the medium pH up to 8.2 and 7.75, respectively. X-ray diffraction (XRD) analysis showed essentially unchanged levels of the total soluble phases of all coatings after incubation with culture medium, except that the CaO phase was rapidly dissolved from AHA coatings and completely eliminated from SHA coatings. Cultures of human gingival fibroblasts on these HA coatings showed that MP-HA and SHA had about the same cell attachment efficiency which was relatively lower than that of AHA coatings. MP-HA generated significant higher cell proliferation rate relative to AHA (P < 0.01) and SHA (P < 0.05). This study indicated that surface chemistry and topography of lower crystallinity might be favorable to cell attachment, but that elevated medium pH might result in a cytotoxic effect that inhibits the proliferation of attached cells on coating surfaces.     

Ovarian mucinous cystic tumor with sarcoma-like mural nodules and multifocal anaplastic carcinoma: a case report

A 48-year-old woman presented with left abdominal pain and fullness. Computed tomography scan revealed a multicystic mass with multifocal mural nodules. Histologic examination showed a mucinous cystic tumor with cystadenoma, borderline malignant cystadenoma and cystadenocarcinoma, which were associated with sarcoma-like mural nodules (SLMNs) and multifocal anaplastic carcinoma. Mural nodules showed a positive reaction for CD56 and vimentin, but were negative for cytokeratin 7 and SMA. She underwent postoperative chemotherapy and is currently under follow-up; no recurrence or metastases were found in the first year of follow-up. Ovarian mucinous cystic tumor with SLMNs and foci of anaplastic carcinoma is extremely rare. To our knowledge, this case reports the most complex neoplastic and reactive components. Our findings shed some light on the pathogenesis of this rather rare carcinoma. We think that the formation of SLMNs may be the result of the reactive proliferation of undifferentiated mesenchymal cells, while the anaplastic carcinoma may be derived from mucinous epithelium. Moreover, because of difficulties encountered in their differential diagnosis, we think that the existence of foci of anaplastic carcinoma along with SLMNs necessitates careful histologic and immunohistochemical analysis of mural nodules for the determination of treatment and prognosis.     

Hepatocyte cytokeratins are hyperphosphorylated at multiple sites in human alcoholic hepatitis and in a mallory body mouse model

Alcoholic hepatitis (AH) is associated with cytokeratin 8 and 18 (CK8/18) accumulation as cytoplasmic inclusion bodies, termed Mallory bodies (MBs). Studies with MB mouse models and cultured hepatocytes suggested that CK8/18 hyperphosphorylation might be involved in MB formation. However, no data exist on phosphorylation of CK8/18 in human AH. In this study, antibodies that selectively recognize phosphorylated epitopes of CK8 or CK18 were used to analyze CK8/18 phosphorylation states in normal human and murine livers, human AH biopsies, and livers of 3,5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC)-intoxicated mice, the last serving as model for MB induction. Hepatocyte cytokeratins become hyperphosphorylated at multiple sites in AH and in DDC-intoxicated mice. Hyperphosphorylation of CK8/18 occurred rapidly, after 1 day of DDC intoxication and preceded architectural changes of the cytoskeleton. In long-term DDC-intoxicated mice as well as in human AH, MBs preferentially contain hyperphosphorylated CK8/18 as compared with the cytoplasmic cytokeratin intermediate filament network suggesting that CK8/18 hyperphosphorylation may play a contributing role in MB pathogenesis. Furthermore, the site-specific phosphorylation of cytokeratin in different stages of MB induction provides indirect evidence for the involvement of a variety of protein kinases known to be activated in stress responses, mitosis, and apoptosis.     

Heterogeneity of hepatocyte antigen expression in rat liver carcinogenesis: concordance between neoplastic nodules and tumours

Fischer F344 rats were given a cyclical diet of 0.06% 2-acetylaminofluorene (AAF), which progressively induced oval cell proliferation, cirrhosis and hyperplastic (or neoplastic) nodules. Primary liver tumours developed from 7 months after ceasing the diet. Liver samples taken during and after AAF administration and specimens of primary tumours were processed into frozen sections and examined microscopically for morphological changes in cell populations, stained histochemically for gamma-glutamyl transpeptidase (GGTase) and four phosphatases, and stained by the immunoperoxidase technique for the presence of antigens detected by seven anti-liver cell monoclonal antibodies and monoclonal antibodies to six oncoproteins. During and after AAF treatment several of the anti-liver antibodies revealed foci of aberrantly or heterogeneously-stained cells, although anti-oncoprotein antibodies showed no consistent changes. Foci of cells positive for GGTase and heterogeneous for adenosine triphosphatase (ATPase) were also seen. Nodules invariably showed heterogeneous antigenicity, raised GGTase and abnormal ATPase expression. Primary tumours exhibited varying degrees of positivity, negativity and heterogeneity with the anti-liver monoclonal antibodies, and all were positive for GGTase. Comparison between various parameters and different lesions showed the greatest concordance between nodules and tumours, suggesting that nodules are probably the precursors of malignant tumours in this system.     

Heterogeneity of hepatocyte antigen expression in rat liver carcinogenesis: concordance between neoplastic nodules and tumours.

Fischer F344 rats were given a cyclical diet of 0.06% 2-acetylaminofluorene (AAF), which progressively induced oval cell proliferation, cirrhosis and hyperplastic (or neoplastic) nodules. Primary liver tumours developed from 7 months after ceasing the diet. Liver samples taken during and after AAF administration and specimens of primary tumours were processed into frozen sections and examined microscopically for morphological changes in cell populations, stained histochemically for gamma-glutamyl transpeptidase (GGTase) and four phosphatases, and stained by the immunoperoxidase technique for the presence of antigens detected by seven anti-liver cell monoclonal antibodies and monoclonal antibodies to six oncoproteins. During and after AAF treatment several of the anti-liver antibodies revealed foci of aberrantly or heterogeneously-stained cells, although anti-oncoprotein antibodies showed no consistent changes. Foci of cells positive for GGTase and heterogeneous for adenosine triphosphatase (ATPase) were also seen. Nodules invariably showed heterogeneous antigenicity, raised GGTase and abnormal ATPase expression. Primary tumours exhibited varying degrees of positivity, negativity and heterogeneity with the anti-liver monoclonal antibodies, and all were positive for GGTase. Comparison between various parameters and different lesions showed the greatest concordance between nodules and tumours, suggesting that nodules are probably the precursors of malignant tumours in this system.