Previously identified comparable morphological features of human and rat hepatoproliferative lesions were identified, including hepatocellular carcinomas (HCCs). In this study we identified similarities and differences in immunohistochemical (IHC) detection of some key proteins important in carcinogenesis that may link pathogenesis pathways of human and rat HCC. The comparative features of six IHC markers (Ki-67, beta-catenin, CD34, glutamine synthetase (GS), c-myc, and transforming growth factor alpha (TGF-alpha)), previously shown to be positive or altered in rodent and human HCC when compared to normal hepatocytes, were investigated. Glutamine synthetase (a hepatocellular enzyme) was strongly positive in 5/5 (100%) human and 4/5 (80%) rat HCCs examined. CD34 (an endothelial marker) and Ki-67 (a cell proliferation marker) were consistently positive in five human HCCs (5/5 for both markers) but weakly positive in only 2/5 and 3/5 rat HCCs, respectively. Beta-catenin, c-myc, and TGF-alpha were infrequently altered, with immunopositivity found in only one or two of either rat or human HCCs (a total of five each examined; beta-catenin: 1/5 rat samples, c-myc: 2/5 human samples, TGF-alpha: 1/5 rat samples positive). It was concluded that Ki-67, CD34, and GS, are most likely to be useful in studying the pathogenesis of HCC in rats and humans.

In this comparative review, histomorphological features of common nonneoplastic and neoplastic hepatocyte lesions of rats and humans are examined using H&E-stained slides. The morphological similarities and differences of both neoplastic (hepatocellular carcinoma and hepatocellular adenoma) and presumptive preneoplastic lesions (large and small cell change in humans and foci of cellular alteration in rats) are presented and discussed. There are major similarities in the diagnostic features, growth patterns and behavior of both rat and human hepatocellular proliferative lesions and in the process of hepatocarcinogenesis. Further study of presumptive preneoplastic lesions in humans and rats should help to further define their role in progression to hepatocellular neoplasia in both species.

The diagnosis of a non-neoplastic lesion is usually based on subjective morphologic characteristics of the lesion, which are published and/or based on the pathologist’s training and experiences. Naturally occurring lesions in most animal species typically follow an age-related progression of extent and severity in a specific tissue. Induced lesions often follow doseand timerelated progression. The changes in a lesion with time or dose are often reported according to general guidelines at the pathologist’s place of employment for reporting of the grading of lesions, namely, extent of the lesion in the tissue and severity of the lesion itself. The reporting usually depends on the computer system used and requirements for the study. Yet few publications have established guidelines for the grading of natural or induced lesions (Shackelford et al. 2002), although many have reported graded lesions in animals and humans. We are not aware of any government regulatory agency that requires grading of lesions. The Society of Toxicologic Pathology best practices guidelines for toxicologic histopathology (Crissman et al. 2004) recommend that ‘‘for common lesions in a species or strain of animal, it is important to know if the experimental treatment alters severity. The pathologist should use a severity grading system that allows for an appropriate severity classification, as treatment may affect incidence or severity. Toxicologic lesions are found in a continuous spectrum of severity and are often on a borderline in any classification system. Therefore, severity grading systems should be: (1) definable, (2) reproducible, and (3) meaningful.’’ The lack of any requirement or guidelines on the use of grading can result in difficulty in a clear understanding of the pathology report, especially if read by nonpathologists. The question arises as to whether or not lesions in all tissues are always graded in a report, or are those mentioned only in specific tissues such as kidney and liver also graded? Should grading only be used selectively and not always, or should it be standard for all tissues? Is the extra work involved of use, and is it cost effective? How does grading contribute to the overall quality of the pathology report? The grading of lesions should depend on the organ or tissue, anatomic structure, type of lesion, natural progression of the lesion, and dose relatedness of the lesion. Some organs would require more than one type of grading system for natural or induced lesions. The kidney has various anatomic subunits, such as glomeruli, tubules, and blood vessels, each of which can require a different type of grading system. Pathologists can be lumpers or splitters for diagnosing lesions in an organ. For example, aging nephropathy can be an average grading for the extent and severity of involvement of any portion of the nephron and kidney. Splitters can grade the glomerular lesion, tubular lesions, and grades of inflammation, as examples. Our new ‘‘INHAND’’ liver nomenclature gives an example of grading for non-neoplastic liver lesions (Thoolen et al 2010). There are many examples of grading of spontaneous or induced lesions in the kidney and other tissues in both animals (Bruner et al. 2010; Grim et al. 2009; Hard et al. 2011; Hard and Khan 2004; NTP 2007) and humans (Caballero et al. 2001). Suggested methods for statistical analysis of subjectively graded lesions have been reported in these and many other papers. Holland and Holland (in this issue) review various methods for grading and its statistical evaluation. They suggest that the best approach may be an ‘‘ordering method,’’ whereby the slides are all coded and placed in groups of grades, which are subsequently decoded. The pathologist then makes a judgment on the lesions after a statistical test is performed. It seems to be a valuable method, but it perhaps requires more work than the usual, less formalized methods of grading. Computerized image analysis may in some cases represent the ultimate method for reporting and grading lesions of any type (Boyce et al. 2010; Maximova et al. 2009; Potts et al. 2010). It is not subjective or based on the pathologist’s opinion of the diagnosis and grading of a lesion, but rather, it is based on the computerized appearance of the lesion and its cell components, using computer analysis possibly supplemented by immunohistochemistry. Presently, regulatory agencies in many nations do not require the grading of lesions, and little is defined for this step with regard to quality in pathology reporting (Isaacs 2007). When regulators read reports of the toxicologic pathology of a chemical in animals and then read about the grading of lesions, they often ask questions as to the methods used and the significance of the grading results. They can also be confused

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP); Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally-accepted nomenclature for proliferative and non-proliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature and differential diagnosis for classifying microscopic lesions observed in the hepatobiliary system of laboratory rats and mice, with color microphotographs illustrating examples of some lesions. The standardized nomenclature presented in this document is also available for society members electronically on the internet (http://goreni.org). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous and aging lesions as well as lesions induced by exposure to test materials. A widely accepted and utilized international harmonization of nomenclature for lesions of the hepatobiliary system in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.