ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 11, No. 4 Copyright 1981, Institute for Clinical Science, Inc. Diagnosis of Tumors of the Kidney: Ultrastructural Classification BERNARD GONDOS, M.D. Department o f Pathology, University o f Connecticut, Farmington, CT 06032 ABSTRACT Diagnosis of tumors of the kidney depends principally on clinical evaluation and radiologic studies. The role of the pathologist is most important in the classification and characterization of the tumors. In addition, morphologic studies employing the use of electron microscopy may aid in the understanding of histogenesis and the recognition of tumor sub-types. Electron microscopic evaluation indicates specific cellular features characteristic of renal adenocarcinoma, nephroblastoma, and sarcomas. Ultrastructural examination can be particularly useful in distinguishing between sarcomas and sarcomatoid carcinomas and in identifying the origin of metastatic tumors. For these reasons, the use of electron microscopy for the evaluation of renal tumors is recommended in selected cases. Introduction Cancer of the kidney is a relatively uncommon form of neoplasia when compared to other sites in the body. Nevertheless, there has been a steady increase in the age-adjusted mortality rate for renal cancer in men. The most recent figures indicate a death rate of 3.7 per 100,000 in the male population, approximately twice the rate in women in whom the mortality rate has remained relatively stable.3 The reason for the increase in renal cancer in men is unknown. Diagnosis of tumors of the kidney generally depends on utilization of radiologic 308 techniques. The role of the pathology laboratory in the initial detection of renal tumors has been limited. There are no laboratory tests which specifically indicate the presence of a renal cancer, although some nonspecific methods are available and other potentially useful tests are currently under investigation. Cytologic examination of the urine has been used, but the most successful application of this technique is in the diagnosis of tumors of the lower urinary tract. Urine cytology has proven to be only rarely of value in confirming or excluding the presence of carcinoma of the renal parenchyma. Urinary enzyme assays also do not 0091-7370/81/0700-0308 $01.20 Institute for Clinical Science, Inc.
DIAGNOSIS OF TUMORS OF THE KIDNEY 309 appear to contribute significantly to the diagnosis of tumors of the kidney. While the urine levels of lactic dehydrogenase and alkaline phosphatase may be elevated in the presence of renal cancer, they are also found to be elevated in patients with p yelonephritis and other benign conditions. Nevertheless, the pathologist can make important contributions to the diagnosis and understanding of renal tumors. An area of special interest in recent years has been the ultrastructural classification of renal tumors. While the use of the electron microscope has not been applied to the early detection of such tumors, it has helped to characterize and evaluate the diverse forms of renal neoplasia. The present report reviews the current classification of renal tumors, with emphasis on information derived from ultrastructural studies. Major Types of Renal Tumors The principal categories of tumors involving the renal parenchyma are indicated in table I. The renal adenocarcinoma is the most common, representing between 80 and 90 percent of all kidney tum ors. Among children, th e n ep h ro blastoma, or Wilms s tumor, is the most frequent and constitutes about 20 percent of all m alignant tum ors in children. M esenchymal tumors, particularly sarcomas, are rarely seen in the kidney but can occur. Transitional cell carcinoma arising in the renal pelvis is excluded from the present analysis. These tumors are generally considered separately from renal parenchymal tumors because of their radiologic presentation as filling defects in the collecting system which clearly distinguishes them from parenchymal tumors and because of their origin from transitional epithelium which relates them more closely to other tumors of urothelial origin arising in the ureter and bladder than to tum ors of renal parenchyma TABLE I Major Types of Malignant Tumors of the Kidney Peak Age Relative Incidence Frequency* Sex Ratio Type Year Percent (M:F) Renal adenocarcinoma > 40 85.7 2:1 (hypernephroma; clear cell carcinoma; renal cell carcinoma) Nephroblastoma < 10 11.6 1:1 (Wilms' tumor; embryonal nephroma; adenosarcoma) Sarcomas 40-60 2.7 1:1 Data from California Tumor Registry.^ Renal Adenocarcinoma The origin of the common carcinomas of the kidney from the epithelial cells of the proximal convoluted tubules is now well established. In the past, a variety of terms was used for these tumors such as hypernephrom a, clear cell carcinoma, and Grawitz tumor, reflecting the confusion regarding histogenesis. The designation of renal adenocarcinoma is now preferred because it clearly indicates the histologic classification and cell of origin and eliminates the need for a variety of confusing synonyms. Electron microscopic observations on renal tumors have been critical in resolving the tissue of histogenesis. Ultrastructural studies of renal adenocarcinoma in the rat induced by dimethylnitrosamine7 and by lead com pounds10 showed the characteristic cellular features seen in the proximal convoluted tubules of the rat. Similar observations have been made in comparing human renal adenocarcinomas w ith norm al proxim al convoluted tubules.11 Im m unologic studies have further strengthened the view that these tumors are derived from cells of the proximal convoluted tubule. Using immunofluorescence methods, it has been shown that antibodies to brush border antigens, which are specific for cells of the proximal convoluted tubule, also react with cells
310 GONDO S from re n a l a d e n o c a rc in o m a, w h ile a n tibodies to T am m -H orsfall antigen, w hich is specific for cells o f th e distal convoluted tu b u le an d loop of H en le, do n o t.16 Ultracytochem ical stu d ies have p ro v id ed a d d i tional ev id en c e for the sim ilarity o f renal ad enocarcinom a cells an d proxim al con vo lu ted tu b u le lin in g ce lls.17 W hile th e p o ssibility rem ains th at som e tum ors or portions o f tum ors m ay contain antigens of distal co n v o lu ted tu b u le type, cu rren t in form ation p o in ts to th e proxim al convo lu ted tu b u le cells as the origin for renal adenocarcinom as. T he d iagnostic application o f th e stu d ies on h isto g en esis or renal carcinom as can be dem onstrated by the use o f electron m icroscopy for exam ining th ese tum ors. U ltr a s tr u c tu r a l s im ila r itie s b e tw e e n tu m o r c e lls a n d ce lls o f th e proxim al co n v o lu ted tu b u le are characteristically fo u n d.11121314,17 T h e features o f sim ilar ity in c lu d e a b ru s h b o rd e r o f tig h tly p ac k ed m icrovilli, pin o cy to tic v esicles, in fo ld in g s o f th e c e ll m e m b ra n e, a n d a b u n d a n ce o f irregularly sh ap e d b ra n ch ing m itochondria. In ad d itio n to th ese features, th e clear cells typically found in renal ad en o car cinom as are see n by electro n m icroscopy to contain large am ounts o f glycogen and lip id m aterial (figure 1). T h ese substances are re m o v e d d u rin g ro u tin e h isto lo g ic p ro cessin g im parting a clear ap p earan ce to th e cytoplasm. In electro n m icroscopic p re p a ra tio n s, th e se o rg a n elles are p re serv ed an d w ell d em onstrated. It should b e n o ted th at th e term clear cell carcinom a has b e e n u sed for a variety o f u n re la te d tum ors occurring in various parts o f the F ig u r e 1. E le ctro n m i crograph o f re n al a d en o c arcinom a sh o w in g a b u n d a n c e o f lip i d d r o p le ts a n d g lycogen p a rticle s in cytoplasm (x 9700).
DIAGNOSIS O F TUMORS O F T H E KIDNEY body. T his has le d to co n sid erab le confu sion reg ard in g n o m en clatu re and evalua tion of m etastatic tum ors w ith u nknow n or in d e fin ite p rim ary sites o f origin. T he clear ap p earan ce m ay b e a re su lt o f intracy to p lasm ic a c c u m u la tio n o f glycogen (clear cell carcinom as o f th e fem ale g en i tal tract), lip id (lipid cell tum ors o f the ovary), or b oth (renal adenocarcinom a). A w areness o f th e ex istence o f such differ ences m akes th e u se o f electro n m icros copy p articu larly h elp fu l in d istin g u ish in g m e ta sta tic tu m o rs o f q u e stio n a b le origin. T h e ap p earan ce o f granular cells seen in som e re n a l ad e n o c a rc in o m a s is ac c o u n te d fo r b y a n a c c u m u la tio n o f m ito c h o n d ria, e n d o p la s m ic re tic u lu m, G olgi form ations, a n d lysosom es. Such c e lls c o n ta in re la tiv e ly little lip id or glycogen. T h e lig h t m icroscopic ap p ear F ig u r e 2. N u m e ro u s m icrovilli are e v id e n t on surface o f c ell from renal adenocarcinom a, in d ic a t ing e p ith e lia l n a tu re c o n siste n t w ith tu b u la r o rigin (x 17,100). 311 ance is a re su lt o f th e density o f organ elles. T h e term gran u lar cell tum or has b e e n u sed for a neoplasm found in a vari ety o f sites, e.g., granular cell tum or of skin, an d it is th erefo re im portant to be aw are th at th e latter is ch aracterized by d e n s e c o lle c tio n s o f co a rse g ra n u le s, a f e a tu r e n o t s e e n in c e lls o f re n a l adenocarcinom as. In th e grading o f renal adenocarcinom a, electro n m icroscopy was found to have n o th in g to ad d to th e light m icroscopic grading on th e basis o f n u clear structure. In a series of 22 cases, no ultrastructural ch aracteristic could b e found th at w ould have b e e n specific for grading p u rp o se s.13 S im ilarly, ele c tro n m icroscopy has no t b e e n h elp fu l in d istin g u ish in g b e tw e e n ren al ad en o m as an d ad en ocarcinom as.6 T h e lack o f clea r histologic d istin c tio n b e tw e e n th e tw o has resu lted in co n sid
312 GONDO S erab le contro v ersy re g ard in g th e in te r p retatio n o f sm all renal cortical tum ors of g lan d u lar type. Some have su g g ested th at in q u estio n ab le cases, tum ors less than th ree cm sh o u ld b e c o n sid ered adenom as. This is clearly an arbitrary separation, p ar ticularly since it is know n th at tum ors of small size h av e b e e n ob serv ed on occa sion to m etastasize. P erhaps th e m ost rea sonable approach, in view o f th e lack of d istin g u ish in g features, is to co n sid er all of th e tum ors adenocarcinom as, w ith the u n d e rsta n d in g th a t sm all asym ptom atic tum ors h ave a g en erally ex cellen t prog nosis b u t do possess the capacity to m etas tasize. T h ese observations are co n sisten t w ith th e g en eral ex p erien ce th at th e ap p lic a tio n o f u ltra s tru c tu ra l an a ly sis in tum or diagnosis is m ost h elp fu l in tum or classification an d id entification o f specific structures (figure 2) and m uch less so in grading a n d d istin g u ish in g b e n ig n an d m alig n an t tum ors. Nephroblastoma T h e n ephroblastom a, com m only know n as W ilm s s tu m o r, is d istin c tiv e in its characteristic occurrence in young ch il d re n a n d its histologic diversity. T h e lat te r feature has re su lte d in a variety o f ap p ro aches to th e evaluation of h istogenesis an d classification. T h e im portance of care ful evalu atio n o f th ese tum ors has tak en on special m ean in g in re c e n t years b ec au se of th e successful use o f ch em o th erap y in certain types o f tum ors and b ec au se o f ev id en c e th at resp o n se to trea tm e n t m ay vary in th e d iffe ren t sub-types. A lthough u ltrastru c tu ral stu d ies have thus far p ro v id ed little new in sig h t into th e etiology o f nephroblastom a, th e y have F ig u r e 3. L ow m agni fic a tio n e le c tr o n m ic ro graph o f n ep h ro b la sto m a sh o w in g e p ith e lia l c e lls above a n d m esen ch y m altype cells b e lo w (x 4600).
DIAGNOSIS O F TUMORS OF THE KIDNEY b ro u g h t o u t special features useful in u n d ersta n d in g th e h isto g en esis a n d classifi cation o f th e tum ors. S everal d iffe ren t cell ty p es have b e e n d e s c rib e d.1,8>9>14,15 T h ese in clu d e m esen ch y m al-type cells, p rim i tiv e e p ith elial cells, tu b u la r lin in g cells, an d g lo m eru lo id stru ctures (figure 3). T h e m esen ch y m al-type cells are e lo n gated w ith large n u clei co n tain in g p rom i n e n t n u cleo li an d m o derate am ounts of c y to p la s m in c lu d in g fre e rib o s o m e s, rough en d o p lasm ic reticu lu m, scattered elo n g ated m itochondria, an d G olgi e le m ents. T h e re is ev id en c e o f collagen fibril d ep o sitio n an d th e cells g en erally resem b le fibroblasts (figure 4). T h e ep ith elial cells p re s e n t d em o n strate varying degrees o f d iffe re n tia tio n. T h e m ore p rim itiv e ce lls can b e s e e n by e le c tro n m icro s copy to ex h ib it close alig n m en t o f adja F ig u r e 4. Strom al cells o f n e p h ro b la s to m a h a v e c h a r a c te ris tic s o f fib ro blasts (x 14,000). 313 ce n t cell b o rd ers an d form ation o f ju n c tional attachm ents, in clu d in g occasional d e sm o so m e s. O th e r e p ith e lia l g ro u p s show d istin c t lu m en form ation w ith the d e v e lo p m e n t o f apical ju n c tio n a l com p le x e s c h a ra c te ris tic o f tu b u la r lin in g ce lls a n d fo rm a tio n o f a b asal la m in a around th e groups. G lom erulus-like struc tures seen by lig h t m icroscopy are found ultrastructurally to re sem b le norm al ap p e a rin g g lo m e ru li in som e in s ta n c e s. M ore freq u en tly, th e structures show in c o m p le te o r a b e r r a n t d if f e r e n tia tio n ch aracterized by lack o f one or m ore o f the usual elem en ts p re se n t in glom eruli or ir regular arran g e m e n t of th e d ifferen t c e llu lar e le m e n ts a n d b a s e m e n t m em b ra n e m aterial. T he u ltrastru ctu ral findings are g en e r ally co n sisten t w ith ep ith elial d ifferen tia
314 GONDOS tion toward structures normally formed by the m etanephrogenic blastem a. The mesenchymal-type elements noted may also be derived from undifferentiated m etanephrogenic tissue. It has been suggested that the mesenchymal appearing cells may actually be undifferentiated epithelial cells.14 Marked similarities in ultrastructure betw een normal developing m etanephron units and epithelial components of Wilms s tumors at varying stages of differentiation have been demonstrated, supporting origin of the tumor from metanephrogenic blastema.8 Ultrastructural studies of some cases indicating differentiation of cells w ith Z-bands characteristic of skeletal m uscle15 and cells with neurosecretory-type granules9 have raised some questions regarding the precise histogenesis of Wilms s tumors. Further electron microscopic studies may help to clarify the origin of these tumors. Sarcomas Malignant connective tissue tumors of varying types, including leiomyosarcoma, rhabdomyosarcoma, liposarcoma, angiosarcoma, and fibrosarcoma, may occur in the kidney.5 They are quite unusual and the incidence of more than two percent of all malignant tumors of renal parenchyma TABLE II Ultrastructural Characteristics of Tumors of the Kidney Type Cytoplasm Cell Surface Renal adenocar Abundant glycogen. Prominent brush cinoma lipid, few other border, numerous organelles (clear microvilli, well cells); numerous formed glycocalyx, mitochondria, well membrane infoldings. developed Golgi, pinocytotic vesicles, endoplasmic reticu intercellular lum (granular junctions cells) Nephroblastoma Primitive appearance. Prominent microvilli. scant organelles, glomeruloid characteristics of structures, basement epithelial cells; membrane formation may have myofilaments, neurosecretory-type granules Sarcomas Myofilaments, lipid, No specific features, etc., depending on related to type, type generally few microvilli in the California Tumor Registry collection is probably high based on the referral nature of the case m aterial.2 Electron microscopic examination of sarcomas of the kidney would be expected to demonstrate the general features of corresponding tumors in soft tissue locations and elsewhere. Ultrastructural studies have been performed on two leiomyosarcomas of the kidney, showing numerous bundles of myofilaments characteristic of these tumors in other sites.14 A useful application of electron microscopy is in distinguishing true sarcomas from sarcomatoid carcinomas. The latter are variants of renal adenocarcinoma, as indicated by the presence of junctional complexes, microvilli and intracytoplasmic lumina.4 These are features which indicate epithelial origin and which can be clearly dem onstrated by electron microscopy. The characteristic ultrastructural features of the major types of malignant tumors of the renal parenchyma are summarized in table II. Summary The main types of malignant tumors of the kidney are generally diagnosed without difficulty by light microscopy. Electron microscopy has been particularly useful in helping to clarify the histogenesis and classification of these tumors. In some instances, ultrastructural study may be important in distinguishing the sarcomatoid variety of renal adenocarcinom a from a true sarcoma and in evaluating tumors with atypical features. Another potentially important application is in the differential diagnosis of metastatic tumors of unknown origin. When presenting in a site other than the kidney, specific identification of an adenocarcinoma of renal origin may not be possible on the basis of light microscopic findings, but distinctive findings seen by electron microscopy may enable definitive diagnosis.
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