Assessment of proliferative activity by anti-PCNA monoclonal antibodies in formalin-fixed, paraffin-embedded samples and correlation with mitotic index

A daily rhythm of cell proliferation in a songbird brain

Neurogenesis is an active process of creating new neurons in the neurogenic zone. It is influenced by many factors, including the circadian system, which is synchronized by light. Neurogenesis in laboratory rodents peaks at night, and the rodents are nocturnal, contrary to humans that are active during the day. Here, we studied whether proliferation and apoptosis exhibit a daily rhythm in the brain of the diurnal songbird zebra finch (Taeniopygia guttata) and whether the cell proliferation peaks during the dark phase of the day, as in rodents. We injected the birds with the cell proliferation marker 5-ethynyl-2´-deoxyuridine (EdU; thymidine analog), quantified the number of dividing cells in the neurogenic ventricular zone (VZ), and measured mRNA expression of clock genes as well as genes indicating cell proliferation or apoptosis. First, we confirmed the daily rhythms of the clock genes. Next we found that proliferation along the whole VZ did not exhibit a daily rhythm. However, proliferation in the central ventral part of the VZ, i.e. "the hot-spot" area, showed a daily rhythm of proliferation. The highest number of newborn cells was detected in the dark phase of the day. The relative expression of the apoptotic genes caspase 3, Bcl-2, and Bax as well as the proliferating cell nuclear antigen (PCNA) did not show any rhythm. In summary, our results show that cell proliferation in the "hot-spot" region of the VZ in diurnal songbirds shows rhythmic activity over a period of 24 h and that the maximum cell proliferation occurs in the passive phase. This study may have implications for understanding the mechanisms underlying the daily regulation of brain cell proliferation in different species.

Modelling human placental villous development: designing cultures that reflect anatomy

The use of in vitro tools to study trophoblast differentiation and function is essential to improve understanding of normal and abnormal placental development. The relative accessibility of human placentae enables the use of primary trophoblasts and placental explants in a range of in vitro systems. Recent advances in stem cell models, three-dimensional organoid cultures, and organ-on-a-chip systems have further shed light on the complex microenvironment and cell-cell crosstalk involved in placental development. However, understanding each model's strengths and limitations, and which in vivo aspects of human placentation in vitro data acquired does, or does not, accurately reflect, is key to interpret findings appropriately. To help researchers use and design anatomically accurate culture models, this review both outlines our current understanding of placental development, and critically considers the range of established and emerging culture models used to study this, with a focus on those derived from primary tissue.

Detection of Cell Proliferation Markers by Immunofluorescence Staining and Microscopy Imaging in Paraffin-Embedded Tissue Sections

This unit describes a step-by-step protocol to detect and quantify proliferating cells in paraffin-embedded tissue sections. Two well-established markers of proliferation (incorporation of BrdU into newly synthesized DNA and expression of the nuclear protein Ki67) are detected after antigen-retrieval and subsequent immunofluorescence staining and confocal microscopy. © 2016 by John Wiley & Sons, Inc.

Relationship between DNA Ploidy and Proliferative Cell Nuclear Antigen Index in Canine Hemangiopericytoma

The mitotic index is reported to be correlated with recurrence, mean patient survival, and metastasis of canine hemangiopericytoma (CHP). However, to the authors' knowledge, studies investigating the parameters that can predict recurrence or metastasis of CHP with low mitotic index have not been done. To evaluate growth kinetics of CHP with low mitotic index, a retrospective analysis of the proliferative activity by antiproliferative cell nuclear antigen monoclonal antibody and DNA contents by flow cytometry (FCM) was performed with 21 formalin-fixed and paraffin-embedded CHP samples. Of the 21 tumors evaluated by FCM, 6 (26.6%) were aneuploid tumors, and 15 (71.4%) were diploid tumors. There was significant correlation between the PCNA index and ploidy pattern. The diploid group had 39.1 ± 9.2 PCNA index, whereas the aneuploid group's proliferative cell nuclear antigen (PCNA) index was 63.1 ± 8.2. The diploid group had mean mitotic index value of 1.140 ± 0.855, and the aneuploid group had a mean value of 1.067 ± 0.767. From these results, the CHP samples with low mitotic index were classified into either the aneuploid group with higher PCNA index or the diploid group with lower PCNA index, suggesting that DNA ploidy and proliferative activity may give an indication about malignancy of CHPs with a low mitotic index.

In Vitro Influence of Mycophenolic Acid on Selected Parameters of Stimulated Peripheral Canine Lymphocytes

Mycophenolic acid (MPA) is an active metabolite of mycophenolate mofetil, a new immunosuppressive drug effective in the treatment of canine autoimmune diseases. The impact of MPA on immunity is ambiguous and its influence on the canine immune system is unknown. The aim of the study was to determine markers of changes in stimulated peripheral canine lymphocytes after treatment with MPA in vitro. Twenty nine healthy dogs were studied. Phenotypic and functional analysis of lymphocytes was performed on peripheral blood mononuclear cells cultured with mitogens and different MPA concentrations– 1 μM (10⁻³ mol/m³), 10 μM or 100 μM. Apoptotic cells were detected by Annexin V and 7-aminoactinomycin D (7-AAD). The expression of antigens (CD3, CD4, CD8, CD21, CD25, forkhead box P3 [FoxP3] and proliferating cell nuclear antigen [PCNA]) was assessed with monoclonal antibodies. The proliferation indices were analyzed in carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled cells. All analyses were performed using flow cytometry. The influence of MPA on apoptosis was dependent on the mechanism of cell activation and MPA concentration. MPA caused a decrease in the expression of lymphocyte surface antigens, CD3, CD8 and CD25. Its impact on the expression of CD4 and CD21 was negligible. Its negative influence on the expression of FoxP3 was dependent on cell stimulation. MPA inhibited lymphocyte proliferation. In conclusion, MPA inhibited the activity of stimulated canine lymphocytes by blocking lymphocyte activation and proliferation. The influence of MPA on the development of immune tolerance–expansion of Treg cells and lymphocyte apoptosis–was ambiguous and was dependent on the mechanism of cellular activation. The concentration that MPA reaches in the blood may lead to inhibition of the functions of the canine immune system. The applied panel of markers can be used for evaluation of the effects of immunosuppressive compounds in the dog.

Hepatic Lesions Caused by Large Granular Lymphocyte Leukemia in Fischer 344 Rats: Similar Morphologic Features and Morphogenesis to Those of Nodular Regenerative Hyperplasia (NRH) in the Human Liver

To characterize the hepatic lesions in Fischer 344 (F344) rats afflicted with large granular lymphocyte (LGL) leukemia, the livers of rats with LGL leukemia at various stages were examined histopathologically and immunohistochemically. The morphologic features in the livers of rats afflicted with LGL leukemia were diffuse, uniform-sized, granular, or micronodular lesions consisting of hepatocytes showing centrilobular atrophy and perilobular hypertrophy (CAPH) without fibrosis. With progression in the stage of the LGL leukemia, the severity of the CAPH of hepatocytes increased resulting in fatty change and/or single-cell necrosis, along with compensatory hyperplasia of the hepatocytes, finally resulting in lesions similar to those seen in nodular regenerative hyperplasia (NRH) in the human liver. The CAPH of hepatocytes was a nonspecific tissue adaptation against ischemia or hypoxemia and/or imbalance in blood supply due to disturbance in the portal circulation and hemolytic anemia induced by the leukemia cells. In addition, direct and/or indirect hepatocellular injuries by leukemia cells were considered to be necessary for the formation of human NRH-like lesions. Morphogenetic investigation of the livers of rats afflicted with LGL leukemia may be helpful to clarify the pathogenesis of NRH in the human liver.

Expression of transferrin receptor 1, proliferating cell nuclear antigen, p27(Kip1) and calbindin in the fetal and neonatal feline cerebellar cortex

Cerebellar cortices from feline fetuses with estimated gestational ages of 40-66days and from kittens aged 2days to 2months, all negative for feline panleukopenia virus (FPV) infection, were analysed for expression of the transferrin receptor 1 (TrFR1), proliferating cell nuclear antigen (PCNA), p27(Kip1) and calbindin. TrFR1, the receptor used by FPV to enter target cells, was expressed in capillary endothelial cells in the cerebellum at all fetal stages investigated and in Purkinje cells of a 3-week-old kitten, but not in the neuroblasts in the external granule layer (EGL). PCNA was expressed in cells of the superficial layer of the EGL. The cyclin-dependent kinase inhibitor p27(Kip1) was expressed in cells of the deep layer of the EGL. Purkinje cells expressed calbindin from the earliest fetal stage investigated. Co-expression of PCNA and calbindin could not be demonstrated, indicating that feline Purkinje cells are post-mitotic from at least 40days gestation.

Molecular carcinogenesis of canine mammary tumors: news from an old disease

Studies focusing on the molecular basis of canine mammary tumors (CMT) have long been hampered by limited numbers of molecular tools specific to the canine species. The lack of molecular information for CMT has impeded the identification of clinically relevant tumor markers beyond histopathology and the introduction of new therapeutic concepts. Additionally, the potential use for the dog as a model for human breast cancer is debatable until questions are answered regarding cellular origin, mechanisms, and cellular pathways. During the past years, increasing numbers of canine molecular tools have been developed on the genomic, RNA, and protein levels, and an increasing number of studies have shed light on specific aspects of canine carcinogenesis, particularly of the mammary gland. This review summarizes current knowledge on the molecular carcinogenesis of CMT, including the role of specific oncogenes, tumor suppressors, regulators of apoptosis and DNA repair, proliferation indices, adhesion molecules, circulating tumor cells, and mediators of angiogenesis in CMT progression and clinical behavior. Whereas the data available are far from complete, knowledge of molecular pathways has a significant potential to complement and refine the current diagnostic and therapeutic approach to this tumor type. Furthermore, current data show that significant similarities and differences exist between canine and human mammary tumors at the molecular level. Clearly, this is only the beginning of an understanding of the molecular mechanisms of CMT and their application in clinical patient management.

Clinocopathologic and immunohistochemical findings of multiple genital leiomyomas and mammary adenocarcinomas in a bitch

A 13-year-old female Pointer dog was presented for evaluation of mammary tumours and bloody vaginal discharge at the Veterinary Teaching Hospital of Mehmet Akif Ersoy University, Faculty of Veterinary Medicine. On the history, owner complained of mammary tumours and bloody vaginal discharge. Three mammary tumours and lymphadenopathy at the mammary lymph nodes were observed at the clinical examination. A big, firm, palpable mass was found in the abdominal cavity. Vaginal cytology revealed numerous pleomorphic and anaplastic cells. Abdominal ultrasonography demonstrated a large mid-abdominal mass at the distal part of the left uterine horn. Also multiple masses in the cervix and vagina were found. Because of the poor prognosis and the desire of the owner, the bitch was killed. At the necropsy numerous masses were seen at the vagina and cervix and one big mass seen at the left cornu uteri. Histopathological diagnosis was leiomyoma. Multiple metastases of mammary tumours were seen at the lungs. Histopathologically, mammary tumours were diagnosed as complex type tubulopapillary adenocarcinoma. The objectives of this study were to measure the proliferation indices in canine complex type mammary adenocarcinoma and genital leiomyomas using immunohistochemical detection of Ki-67 and proliferating cell nuclear antigen to determine the relationship of these antigens to clinical and pathologic variables; and to examine the immunoreactivity of these tumours with different markers. Pan-cytokeratin and S100 were negative, desmin and glial fibriler acidic protein were slight positive and the other markers (carsinoembryogenic antigen, proliferating cell nuclear antigen, vimentin, smooth muscle actin, p53, fibronectin, Ki67) were found strong positive at the genital tumours. Only desmin were negative; the other markers were strong positive at the mammary tumours.

Proliferative response by the ostrich proventriculus in idiopathic gastric stasis: a case report

In this report, the late clinical, necropsy, and histopathological findings of idiopathic gastric stasis in an ostrich are described, with special attention to the proventriculus. An ostrich with clinical signs of gastric stasis was brought to our clinic in a state of impending death that could not be reversed with emergency medical care. At necropsy, the koilin layer of the gizzard was uneven without ulcerations. The proventriculus was distended with food items without impaction and mucosal lesions. None of the most typical aetiological factors including foreign objects, Macrorhabdus ornithogaster and parasitic infection with Libyostrongylus douglassii was present. Histologically, the proventriculus was characterised by hyperplastic mucosa with luminal buds and necrotic cells in the lumen. Immunochemistry for proliferating cell nuclear antigen (PCNA) revealed a high rate of proliferation in epithelial cells of the glandular and mucosal lining in the proventriculus neighbouring the affected gizzard: approximately 88.6% of the cells were immunoreactive as compared to 34.4% of the cells in control ostriches used for comparison. In conclusion, in the absence of gizzard contraction, the mucosal lining of the proventriculus is hyperplastic with a high rate of proliferation that may help compensate the distension due to the accumulation of food items.

Assessment of proliferative activity by AgNOR and PCNA in prostatic tissues of ram lambs implanted with zeranol

The aim of this study was to assess cellular proliferation using silver-stained nucleolar organizer regions (AgNOR) and the proliferating cell nuclear antigen (PCNA) in various tissues in the prostate of ram lambs implanted with increasing zeranol doses and to compare the sensitivity of different tissues of lamb prostate to zeranol. Twenty-four Akkaraman lambs were implanted with increasing zeranol doses, including 12 mg (n = 8), 24 mg (n = 8) and 96 mg (n = 8), with eight lambs serving as controls. After 33 days, the prostate tissues of the lambs were stained using AgNOR and PCNA techniques. The prostate tissues were divided into two compartments--the epithelial tissues, including glandular acinus, collecting duct and penile urethra, and the non-epithelial tissues, including interstitial tissue and striated muscle. AgNOR dots and PCNA index on each prostatic tissue were counted under a light microscope and were evaluated statistically. AgNOR staining in the treatment groups showed a higher score in the non-epithelial tissues than the epithelial components, whereas the PCNA index was significant in the epithelial tissues and non-epithelial tissues had very low PCNA immunostaining. According to the PCNA index, collecting duct epithelium showed more sensitivity to increasing zeranol doses and according to AgNOR counts, there was no difference of sensitivity to zeranol among tissues of the same origin. Both AgNOR counts and PCNA indexes seem to be valuable proliferating markers for the epithelial components of ram prostate, but PCNA index had no significance in relation to the non-epithelial components in contrast to AgNOR counts. Therefore, the controversial results arising from the combined use of both techniques as proliferating markers for the ram prostate should be considered in further studies.

Evaluation of cell proliferation in rat tissues with BrdU, PCNA, Ki-67(MIB-5) immunohistochemistry and in situ hybridization for histone mRNA

The standard method for assessment of cell proliferation in paraffin-embedded tissue sections is 5-bromodeoxyuridine (BrdU) immunohistochemistry (IHC). BrdU can be administered to laboratory animals via IP injections, is readily incorporated into nuclei during the DNA synthetic phase of the cell cycle, and is detected with an anti-BrdU antibody. This method has several disadvantages, and an accurate method for evaluation of proliferative activity that can substitute for BrdU IHC, when necessary, is of great interest to investigators. Alternative methods for detection of proliferating cells in tissue sections are proliferating cell nuclear antigen (PCNA) IHC, Ki-67 IHC, and in situ hybridization (ISH) for histone mRNA. To determine the optimal choice, we analyzed the correlation of anti-PCNA, anti-Ki-67(MIB-5), and histone mRNA labeling indices (LIs) with anti-BrdU LI in rat highly replicative (renewing) tissues. The correlation between anti-BrdU and histone mRNA LIs, as well as the correlation between anti-BrdU and anti-Ki-67 LIs, was statistically significant. There was no significant correlation between anti-BrdU and anti-PCNA LIs. These results suggest that both ISH for histone mRNA and IHC with MIB-5 are preferable techniques for assessment of cell proliferation in rat paraffin-embedded renewing tissues compared to PCNA IHC. They can substitute for BrdU IHC when necessary.

Multiparametric survival analysis of histological stage and proliferative activity in feline mammary carcinomas

Mammary tumours are among the most frequent malignant neoplasms in the cat and determination of prognosis on histological grounds alone can be unsatisfactory because it does not always correspond to the clinical behaviour of the neoplastic disease. The aim of this two-year post-mastectomy survival study is to relate the histological stage or invasiveness (the most commonly used histological parameter to grade malignancy) to several parameters assessing the proliferative activity-mitotic index, MIB1 index, and AgNOR index. Invasiveness was graded as local and vascular invasion whilst values of the parameters expressing proliferative activity, all quantified by image analysis, have been classified into low and high proliferative activity groups according to their median values, (0.719 for mitotic index, 12.11 for MIB1 index, and 3.19 for AgNOR index). For each group, mean survival (months+/-SD) was calculated. Histological stage (local invasion 21.83+/-7.83 months, blood vessels and/or lymphatics invasion 13.38+/-8.99,P<0.01), mitotic index (low 22.43+/-88.78, high 12.37+/-7.49,P<0.001), and AgNOR index (low 21.86+/-10.68, high 13.82+/-7.11,P<0.05) revealed a significant association with survival in univariate analysis and had an independent prognostic value in multiparametric survival test (P<0.001).

Dietary restriction alters cell proliferation in rats: an immunohistochemical study by labeling proliferating cell nuclear antigen

Dietary restriction (DR) delays the onset of aging and lowers the incidence of both spontaneous and chemically induced cancers. The inhibition of cell proliferation has been suggested as a possible mechanism for this effect. We examined the effect of DR on cell proliferation in duodenum, forestomach, glandular stomach, and liver tissues of male Fischer 344 rats receiving 60% of the control feed intake for 24 months starting at 16 weeks of age. Rats were sacrificed, when 28 months old. Tissues were collected, histologically prepared, and stained immunohistochemically for proliferating cell nuclear antigen (PCNA). The PCNA-stained nuclei are detected in different phases of the cell cycle. A minimum sample of 2000 cells was counted in liver. The percentage of labeled S-phase cells per total cells counted was used as the labeling index for liver. The number of labeled S-phase epithelial cells per 1.1 mm of basement membrane or muscularis mucosa was used as the labeling index for duodenum, forestomach, and glandular stomach. Cell proliferation in glandular stomach and liver tissues was inhibited in rats DR for 24 months; however, cell proliferation in duodenum and forestomach mucosal tissues was unexpectedly enhanced by DR. These results indicated that while DR inhibits cell proliferation in tissues of rats, it is tissue-dependent. The decreased rate of cell division by DR in the designated tissues could be implicated in lowering the conversion of endogenous DNA damage or lesions to mutation and cancer.

Assessment of proliferative activity of canine dermal mast cells by bromodeoxyuridine and proliferating cell nuclear antigen labelling

Cutaneous mast cells from skin biopsies of three healthy dogs and three dogs with atopic dermatitis were assessed for their proliferative potential using bromodeoxyuridine and proliferating cell nuclear antigen labelling. Mast cells isolated from the skin of two healthy dogs were also studied using bromodeoxyuridine labelling. Mast cells in skin biopsy specimens and mast cells isolated from the skin of healthy dogs did not incorporate bromodeoxyuridine. Two mast cells expressing proliferating cell nuclear antigen were seen around two superficial vessels in the dermis of one atopic dog. Epidermal cells, glandular epithelial cells, fibroblasts and endothelial cells incorporated bromodeoxyuridine and showed positive staining for proliferating cell nuclear antigen. These results suggest that canine mature mast cells do not proliferate in the dermis.

A comparison of lymphatic tissues from cats with spontaneous feline infectious peritonitis (FIP), cats with FIP virus infection but no FIP, and cats with no infection

Lymphatic tissues (spleen, mesenteric lymph nodes, thymus) from 24 cats with spontaneous feline infectious peritonitis (FIP) were examined by light microscopy and immunohistochemistry for cellularity, cellular composition, and degree of cellular turnover. Additionally, the formation of granulomatous lesions in lymphatic tissues in cats with FIP was examined. For comparison, tissues from 14 specific pathogen-free (SPF) cats and seven cats infected with FIP virus (FIPV; as the result of long-term exposure) but free from FIP were examined. In cats with FIP, the precardial mediastinum (including site of the thymus) and mesenteric lymph node parenchyma were often affected by granulomatous-necrotizing processes. In general, lymphoid tissues showed T- and B-cell depletion, often including massive to complete thymic involution or atrophy. In some cases, the number of apoptotic lymphocytes was increased in lymphoid follicles as well as in T-cell zones. The number of macrophages was increased in the splenic red pulp. In contrast, the FIPV-exposed cats without FIP generally showed a distinct lymphoid hyperplasia. The findings indicated that the major difference in lymphatic tissues between FIPV-infected cats with FIP and those without FIP was the development of lymphocyte depletion in the first group and lymphocyte proliferation in the second.

Evaluating mitotic activity in canine and feline solid tumors: standardizing the parameter

Three different methods for evaluating mitotic activity (mitotic count, mitoses/area, mitotic index) were applied to different types of canine and feline solid tumors to determine the method that is most objective and correlates best with other parameters of cell proliferation. Mitotic activity was evaluated on toluidine blue stained histological sections. Slides stained with histochemical (AgNOR proteins) and immunohistochemical (MIB1, PCNA) markers of cell proliferation were available for each case. Quantitation of mitotic activity and cell proliferation parameters was performed with an image analyzer. Mitotic activity assessment was compared with cell proliferation indices and its ability to discriminate tumors grouped on histologically based criteria including the histological type, malignant or benign characteristics, and grade. A significant correlation by linear regression analysis with other parameters assessing cell proliferation revealed that mitotic index correlated 1000% and mitoses/area and mitotic count correlated 40% of the time. In discriminating the proliferative activity of tumors grouped by histological criteria, mitotic index and mitotic count revealed 1000% concordance with the other parameters of cell proliferation, while mitoses/areas showed 80% concordance.

The cell cycle: a review

The cell cycle is a complex process that involves numerous regulatory proteins that direct the cell through a specific sequence of events culminating in mitosis and the production of two daughter cells. Central to this process are the cyclin-dependent kinases (cdks), which complex with the cyclin proteins. These proteins regulate the cell's progression through the stages of the cell cycle and are in turn regulated by numerous proteins, including p53, p21, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRb and E2F. The cell cycle can be altered to the advantage of many viral agents, most notably polyomaviruses, papillomaviruses, and adenoviruses. The cell cycle often is dysregulated in neoplasia due to alterations either in oncogenes that indirectly affect the cell cycle or in tumor suppressor genes or oncogenes that directly impact cell cycle regulation, such as pRb, p53, p16, cyclin D1, or mdm-2. The cell cycle has become an intense subject of research in recent years. This research has led to the development of techniques useful for the determination of the effects of drugs and toxins on the cell cycle. Any drug or toxin with DNA damaging ability would be expected to alter cell cycle progression, and therefore, the cell cycle should be considered in the design of studies using such chemicals. With the appropriate techniques, cell cycle alterations may also be detected in tissue sections. Because of the ubiquitous nature of the cell cycle, it deserves consideration in the design and interpretation of studies in a wide variety of disciplines.

Immunohistochemical detection of Ki-67 and PCNA in canine mammary tumors: relationship to clinical and pathologic variables

The objectives of this study were to measure the proliferation indices in canine mammary tumors using immunohistochemical detection of Ki-67 and proliferating cell nuclear antigen (PCNA), to determine the relationship of these antigens to clinical and pathologic variables, and to investigate the usefulness of these antigens as prognostic indicators. Ninety-six female dogs with 115 primary nonmetastasized spontaneous mammary tumors and dysplasias were included in the study. Immunostaining was performed using MIB-1 and PC10 monoclonal antibodies against Ki-67 and PCNA, respectively. Ki-67 and PCNA proliferation indices were determined. Dogs were followed for 18 months, with clinical examinations every 3-4 months. There was a significant correlation between Ki-67 and PCNA indices in the dogs with dysplasias and benign tumors but not in the dogs with malignant tumors. The clinical stage at first presentation was related to the proliferative index measured with Ki-67 but not to that measured with PCNA. Proliferation indices were significantly lower in the nonmalignant tumors and dysplasias than in the malignant tumors. In malignant tumors, the PCNA index had a positive correlation with the histologic malignant grade and the nuclear grade. High index values of Ki-67 were positively correlated with metastasis, death from neoplasia, low disease-free survival rates, and low overall survival rates. PCNA displayed no significant association with these variables. Multivariate analyses concerning metastasis, disease-free survival, and overall survival revealed that the Ki-67 index had prognostic value.

Detection of proliferating cell nuclear antigen (PCNA) in canine and feline mammary tumours

Quantitation of immunohistochemical staining of the proliferating cell nuclear antigen (PCNA, clone PC10) by image analysis was performed on benign and malignant mammary tumours of dogs and cats. Scoring of the slides was carried out by image analysis to assess the percentage of labelled nuclei (expressed as a ratio of areas). Either the strongly labelled nuclei (SP-PCNA index), or all of the stained nuclei (TP-PCNA index) were counted as positive to determine the growth fraction and its correlation with the histopathological classification and nuclear grade (degree of nuclear differentiation, considered a morphological correlate of tumour aggressiveness). A significant difference in the values of PCNA indices was seen between benign and malignant growths (P < 0.0001, dog; P < 0.05, cat). Neither of the PCNA indices showed correlation with nuclear grade in dogs (P = 0.14 for SP-PCNA index and P = 0.31 for TP-PCNA index) or cats (P = 0.09 for SP-PCNA index and P = 0.07 for TP-PCNA index). A significant difference in the number of mitoses, expressed as mitotic index, was seen between benign and malignant growths in the dog (P < 0.01) but not in the cat (P = 0.078). Good correlation of mitotic index with nuclear grade was revealed in canine malignant growths (P < 0.05), but in feline malignant tumours such correlation (P < 0.05) was shown only when the values of intermediate plus typical forms were compared with the data for atypical forms. It is concluded that quantitation of PCNA-positive nuclear area by image analysis provides an objective method for assessing proliferative activity in benign and malignant mammary tumours of dogs and cats.