Practical Murine Hematopathology: A Comparative Review and Implications for Research

Hematologic parameters are important markers of disease in human and veterinary medicine. Biomedical research has benefited from mouse models that recapitulate such disease, thus expanding knowledge of pathogenetic mechanisms and investigative therapies that translate across species. Mice in health have many notable hematologic differences from humans and other veterinary species, including smaller erythrocytes, higher percentage of circulating reticulocytes or polychromasia, lower peripheral blood neutrophil and higher peripheral blood and bone marrow lymphocyte percentages, variable leukocyte morphologies, physiologic splenic hematopoiesis and iron storage, and more numerous and shorter-lived erythrocytes and platelets. For accurate and complete hematologic analyses of disease and response to investigative therapeutic interventions, these differences and the unique features of murine hematopathology must be understood. Here we review murine hematology and hematopathology for practical application to translational investigation.Abbreviations: GEM, genetically engineered mouse; NMB, new methylene blue; nRBC, nucleated RBC; RDW, RBC distribution width; TNCC, total nucleated cell countHematology is an important adjunct to both clinical medicine and biomedical research, with more than 1700 currently funded NIH projects¹⁰⁹ and more than 3400 research articles published over the past 5 years using mouse models.¹²⁰ There are now more than 6000 genetically engineered mouse (GEM) models of disease, with 500 new GEM created each year at the Jackson Laboratory alone, and several large projects are underway to thoroughly phenotype each new mutant mouse strain (https://www.komp.org/).^13,176 A mouse tumor database (http://tumor.informatics.jax.org/mtbwi/index.do) is available to provide information regarding mouse models of human cancer, and the Mouse Phenome Database at the Jackson Laboratory provides links to phenotypic data for many GEM models (http://phenome.jax.org/).⁸ The defined components to complete the phenotyping of GEM models have been recently reviewed.^13,157,176 In addition, 21 inbred strains of mice are commonly used for investigations into such topics as response to infectious and genetically induced disease and dietary and pharmacologic therapies. These commonly used laboratory mouse strains have, for example, inherent differences in immunology or iron trafficking, which can affect research outcomes.^16,47,137 These interstrain differences are important to recognize and understand as a component of effective study design and prior to strain selection for laboratory investigations, especially when hematologic responses to disease need to be considered.^13,16,137For any appropriately designed experiment, concurrent age-, sex-, and strain-matched control mice must be included to accurately compare the effects of a disease, genetic manipulation or therapeutic intervention;^13,155 alternatively, individual mice can be used as their own controls in some studies. Several important guidelines exist to ensure that appropriate numbers of experimental and control mice are incorporated into a study design to maximize statistical power yet minimize waste.^{13,40-42,71,72,176} During and between studies, consistent blood collection methods are essential for accurate comparative analyses. Species-appropriate hematologic instrumentation and timely analysis of fresh blood are necessary to minimize preanalytic hematologic errors.^3,37,71 Especially important for mice and their restricted available blood volume are the use of practical, accurate, species-specific, and up-to-date hematologic methods.Here we comprehensively review murine hematology and hematopathologic responses to disease in the context of biomedical research, discovery, and phenotyping studies. To maximize the opportunity for detecting phenotypes, disease, and responses to therapeutic interventions in mice, we focus on providing a practical summary of methods and analysis for accurate hematologic studies and on describing the morphologic assessment of mouse hematopathology in peripheral blood and bone marrow in ways that will be useful to those—veterinarians and researchers alike—who work with murine species.