In situ techniques in diagnostic pathology.

Many rapid advances are being made in understanding the molecular pathogenesis of tumors and infectious diseases. This knowledge is being used in diagnostic pathology as well as in making decisions about targeted therapies for specific diseases. Many new techniques such as RNA profiling, genomics, proteomics, and laser capture microdissection have contributed to these rapid advances [1-4]. For anatomic pathologists, the ability to visualize these changes in protein and nucleic acid levels in tissues under the microscope extend theirabilitytomakepathologicdiagnosisatthemolecularlevel. In situ techniques have usually combined histopathologic analyseswithinnovationsinchemistryandbiochemistryover many decades. In the past 2 decades, advances in molecular biology have been joined to histological methods leading to the application of immunohistochemical and in situ hybridization techniques for tissue diagnoses. With the combination of these methods,the pathologist can readily observe changes in abnormal tissue and compare them with normal tissue in the same section under the microscope. These approaches have resulted in greater insights into the development and progressionof diseaseprocessesthan couldbeappreciated by morphologists in the past. One obvious advantage of this approach is that the trained morphologist does not have to assume which cell type in the tissues is contributing to the amplified or abnormal signal that is detected. Although some indirect techniques such as laser capture microdissection have also tried to bridge the gap between measuringmoleculareventsandattributingthesechangestoa particular cell type, laser capture microdissection is at best an indirect approach, hence not as powerful as direct in situ observations. Four articles in this special section of this issue combine molecular techniques such as fluorescent (FISH) and chromogenic in situ hybridization (CISH) and in situ polymerase reaction with direct histopathologic observations using moleculartechniquesthathavedirectclinicaldiagnosticapplications. The article by Lambros et al [5] examines CISH and FISH in breast cancer, the most widely used clinical application of these techniques. These authors show how in situ molecular genetic analyses are changing diagnostic and prognostic approaches to breast cancer in the analysis of HER2 amplification, 12p13 amplification, and analysis of the basal-like phenotype in breast cancer. These studies illustrate how FISH and CISH are helping to unravel the complexity of breast cancer molecular genetics. The review by Nuovo [6] illustrates how routine histopathology is often insufficient in diagnosing the specific etiologic agents of infections. These studies show how in situ polymerase chain reaction amplification is required to localize some viruses in intact cells. These studies also illustrate the flexibility of in situ approaches for the detection of human papilloma virus in lesions such as cervical squamous intraepithelial lesions and cervical cancer. The in situ detection of Epstein Barr virus in a wide variety of pathological states, including posttransplant lymphoproliferative disorders and AIDS lymphoma, is illustrated. Similarly, localization of HIVin various tissues has provided new insights into the role of this virus in infection. The article by Halling and Kipp [7] illustrates how diagnostic cytology has rapidly progressed from observation of single cells or cell clusters to diagnosing some malignancies with molecular tools. These principles are illustrated in carcinomas of the bladder, biliary tract, and lung as well as dysplastic changes in Barrett esophagus and in esophageal carcinoma.