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INTRODUCTION

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The clinical management of kidney diseases is divided primarily between specialized internists (nephrologists) and specialized surgeons (urologists). Similarly, the pathologic evaluation of kidney diseases is divided between specialized pathologists (nephropathologists) who specialize in kidney diseases that are managed by nephrologists, and pathologists (urologic pathologists) who specialize in kidney diseases that are managed by urologists. Nephrologists and nephropathologists focus on so-called medical renal disease, which includes non-neoplastic diseases that affect the renal parenchyma. Urologists and urologic pathologists focus on diseases that can affect any level of the urinary system from kidneys to urethra, including congenital, infectious, and neoplastic diseases. This chapter will deal with the many forms of medical renal disease, and Chapter 13 with urologic kidney disease.

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WHAT WE DOMedical Renal Disease Versus Urologic Disease and the Role of Nephropathologists

Nephrologists work closely with nephropathologists (renal pathologists) in the management of medical renal disease that requires renal biopsy. Many medical renal diseases, especially glomerular diseases, can only be definitively diagnosed by evaluating renal biopsy specimens. Further, repeat renal biopsies are important in the follow-up management of some patients to assess response to therapy, and to determine the degree of disease activity versus chronicity.

Kidney biopsies are most often obtained by percutaneous needle biopsy. The biopsy needle, usually with ultrasound guidance, is inserted into the patient's flank. Because most medical renal diseases have diagnostic features only in the cortex, the goal is to obtain enough renal cortex for a definitive pathologic diagnosis. Renal biopsy specimens are routinely evaluated by light microscopy, immunofluorescence microscopy (or immunohistochemistry), and electron microscopy. Renal biopsies are the only major type of surgical pathology specimen that is routinely processed for electron microscopy. Immunofluorescence microscopy uses fluorochrome-labeled reagent antibodies to detect deposition of IgG, IgA, IgM, C3, C1q, kappa light chains, lambda light chains and fibrin in renal tissue, especially glomeruli. Special immunohistochemistry is required for some specimens, for example, to detect genetically determined protein abnormalities [e.g., abnormal glomerular basement membrane (GBM) collagen caused by mutations of collagen 4 genes], protein deposits other than immunoglobulin (e.g., amyloid A and myoglobin), and infectious pathogens (e.g., polyoma virus). Electron microscopy is able to detect ultrastructural changes that are not discernible by light microscopy but are essential for precise diagnosis. The importance of immunofluorescence microscopy and electron microscopy will become apparent as the diagnostic features of renal diseases are presented in this chapter.

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GLOMERULAR AND VASCULAR DISEASES

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QUICK REVIEWNormal Glomerular and Vascular Anatomy and Histology

The kidneys are highly vascularized organs with numerous vessels of many types that are the targets for multiple diseases.

At the hilum of the kidney, the main renal artery branches into anterior and posterior branches that feed into interlobar arteries that enter the renal parenchyma on either side of each papillary tip. The interlobar arteries feed into the arcuate arteries that run between the outer medulla and the cortex and give rise to interlobular arteries ...

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