Intralobar Nephroblastematosis in a Nine-week-old Wistar Rat

Abstract

Intralobar nephroblastematosis (ILNB) is a precursor lesion to the development of nephroblastoma (NB) in rats. Unilateral ILNB was observed in the kidney of a nine-week-old female Wistar rat (Crl:WI) from a short-term toxicity study. Clinical pathology and urinalysis did not reveal altered renal function. This microscopic, unencapsulated lesion consisted of basophilic sheets of blastemal cells that did not include a prominent mesenchymal component. These cells expanded in the interstitium, which trapped and compressed few normal renal tubules. The blastemal cells moderately differentiated to form rosettes, primitive tubules, and a glomeruloid body. Multifocally, the lumen of primitive tubules contained eosinophilic secretions with basophilic material in the center. The diagnostic criteria used were compared and differentiated with renal dysplasia, nephrogenic rest, NB in rats, and with that of the identical lesion in children.

Introduction

Fetal kidney is formed by the development of nephrons from fetal metanephric blastema surrounding ureteric bud. Persistent metanephric blastema in infant kidney is known as nephrogenic rest (NR), which represents incomplete maturation of embryonic primitive cells. Diffuse, grossly visible variations of NR that involve a very large portion of the renal cortex in children are referred to as nephroblastomatosis (Lonergan et al. 1998). This lesion is considered to be related to the tumorigenesis of nephroblastoma (NB) in humans (Mitsumori et al. 2002). Nephroblastematous foci in rats are not grossly visible, and it becomes a fully developed neoplastic tissue when the lesion is grossly visible. Owing to this variation in the rat, a diagnostically descriptive terminology nephroblastematosis was coined for the infiltrative blastemal lesions in the renal interstitium (Mesfin 1999). Nephrogenic rest/nephroblastomatosis in children may be located anywhere within the lobe (intralobar), or may be limited to the lobar periphery (perilobar) (Lonergan et al. 1998). These anatomic locations of perilobar and intralobar can also be applicable to the rat kidney, which is unipyramidal in nature. Superficial cortical areas in the rat kidney are perilobar, and the corticomedullary region are intralobar (Frank et al. 1992). Intralobar nephroblastematosis (ILNB) is a precursor lesion to the development of NB in rats. Spontaneous NB is a solid primary renal tumor in children, which occurs rarely in young adult rats with 0%–0.4% incidence, and the occurrence in the strain (Upj:TUC[SD]spf.nb) that was genetically predisposed to this tumor is 6.5% in male rats and 21.6% in female rats (Mesfin and Breech 1996). Nephroblastoma is one of the more common among renal neoplasms of rats (Chandra et al. 1993). Spontaneous cases of immature tissue in the kidneys of adult rats are very uncommon. We did not find any report of such spontaneous preneoplastic conditions in the Wistar rat; hence this finding is presented here.

Case

A nine-week-old female Wistar (Crl:WI) rat from a high-dose group weighing 236.75 g at necropsy was housed and maintained for a short-term toxicity (fourteen-day) study. The rats were housed in polycarbonate cages at a temperature of 18°C–26°C with relative humidity of 30%–70% and a twelve-hour light/dark cycle with twelve air changes per hour. The rats received commercial rodent diet (Harlan, The Netherlands), and purified autoclaved water ad libitum. The rats were maintained according to the national guidelines, and the study was performed with established standard regulatory guidelines. No untoward changes were observed during the in-life phase. Clinical pathology and urinalysis did not reveal altered renal function. No gross lesions were observed at necropsy. Standard study protocol tissues were collected and preserved in 10% neutral buffered formalin. Tissues were processed, embedded in paraffin, sectioned at 4- to 5-μm thickness, and stained with routine hematoxylin and eosin (H&E).

The lesion was not observed either during gross examination or during trimming, and it was visible to the naked eye as a tiny basophilic focus in the H&E-stained section. Microscopically, the unencapsulated, infiltrative lesion was observed in the interstitium at the corticomedullary junction of the kidney unilaterally (Figures 1 and 2). The lesion was composed of basophilic sheets of blastemal cells with a varying degree of differentiation (Figure 3), and it did not include a prominent mesenchymal component. Blastemal cells expanded into the interstitial space, and trapped, compressed, and displaced few normal renal tubules (Figure 4). These cells were often moderately differentiated to form rosettes and primitive tubules. A single glomeruloid body was also observed. Multifocally, the lumen of primitive tubules contained eosinophilic secretions with basophilic material in the center (Figure 5). The blastemal cells were round to oval, with inconspicuous basophilic cytoplasm and indistinct cytoplasmic borders. The nuclear-to-cytoplasmic ratio was high. The vesicular nuclei were round to oval, with one or two nucleoli and contained finely granular chromatin. Mitotic figures were low (one or two per 40× objective). Subsequent serial sections of the kidney revealed similar morphology, and reduction in the size of the lesion. The architecture of the adjacent parenchyma and the other kidney were not affected. This infiltrative blastemal lesion in the intralobar interstitium was diagnosed as ILNB. This single incidence was noted only in the fourteen-day, repeated dose toxicity study and not in the twenty-eight-day repeated dose study in the rat. There was no such incidence even in the fourteen/twenty-eight-day repeated dose toxicity study conducted in the monkey. The test article was also nongenotoxic in standard battery of genotoxic tests (Ames, micronucleus and chromosomal aberration). Hence the lesion under these circumstances was considered as incidental and not test article related.

Fetal kidney is developed from ureteric bud and metanephric blastema. On day 13 of gestation in the rat, a focal dilation of the mesonephric duct result in the formation of ureteric bud, which develops into collecting ducts, pelvis, and ureters. Fetal tissue around the ureteric bud, called metanephric blastema, is then induced to form nephrons around the collecting ducts in a centrifugal fashion, adding layers of nephrons around each individual medullary ray (Montgomery and Seely 1990; Lonergan et al. 1998). In the rat, nephrogenic blastema is present until postnatal day (PND) 4, kidney morphogenesis is completed at PND 15, and the presence of undifferentiated tissue in the adult rat is uncommon (Ruehl-Fehlert et al. 2003).

Disturbed kidney organogenesis may lead to renal dysplasia, which is characterized by the presence of mesenchymal cells, immature collecting ducts derived from the ureteric bud, and embryonic structures (primitive tubules and glomeruli) that originate from metanephric blastema. The primitive glomerular and tubular structures may be associated with frequent mitotic figures. Disturbed renal growth results in oligonephronia. Blastema-like structures were absent in a recently reported case of renal dysplasia in a Wistar rat (Ruehl-Fehlert et al. 2003).

Nephrogenic rest has been reported in an eleven-week-old male Sprague-Dawley (SD) rat. It was found in the cortical interstitium and was composed of inconsistently distinct basophilic tubules lined by a single layer of cuboidal to columnar cells variably surrounded by blastemal cells, with formation of glomeruloid structures with no apparent compression or evidence of invasion of adjacent parenchyma. The lesion did not show evidence of proliferation, maturation, or involution (Jackson and Kirkpatrick 2002).

Intralobar nephroblastematosis has been demonstrated to progress to NB in a subline (Upj:TUC[SD]spf.nb) of the SD rat genetically predisposed to NB. It was first detected at the age of twenty-eight weeks, and the incidence was greater in females of this subline. The renal alterations were not grossly visible; however, they were visible to the naked eye as basophilic foci in H&E sections. Intralobar nephroblastematosis were invariably located in the deep cortex close to corticomedullary junction and clearly delineated from the normal renal tissue by their intense basophilia. The proliferating cells in this subline were tightly packed blastemal cells expanding in the interstitial space while crowding, compressing, and trapping normal tubules. The blastemal cells displaced the renal tubules and differentiated into rosettes and alveolar structures, and mitotic figures were very rare. This lesion progressed to early NB and was characterized by the presence of blastematous cells and by formation of primitive tubular and glomerular structures. Mitotic activity was high and was noted especially in cells arranged in the tubular pattern (Mesfin 1999).

Nephroblastoma is well-demarcated circumscribed growth, compressing adjacent parenchyma resulting in the formation of pseudoencapsulation, displacing a substantial portion of kidney at the time of evaluation. The features of NB are highly basophilic blastema and attempted organoid differentiation along the epithelial pathway into nephric elements. Blastemal cells densely pack into clusters, balls, columns, anastomosing cords, and alveolar or papillary patterns. Blastemal cells differentiate into primitive tubules and glomeruli. Newly formed tubules range from rosettes to well-formed basophilic tubules. The supporting stroma is not neoplastic, ranging from well developed to relatively structureless stroma with areolar tissue. Nephroblastoma is frequently associated with formation of ciliated columnar duct similar to the epididymis (Hard et al. 1991; Mohr 1992; Mesfin 1999; Mitsumori et al. 2002).

The present lesion is different from renal dysplasia, as it did not show the structures arising from the ureteric bud and did not include prominent mesenchymal components; rather, it had predominant blastema with a varying degree of differentiation. This focal lesion did not affect the general architecture of the kidney. The morphologic features of the current lesion were similar to NR described in the rat (Jackson and Kirkpatrick 2002), except for the infiltrative and proliferative nature. Mesfin (1999) reported that, in addition to infiltrative blastemal cells with alveolar and rosette structures, presence of primitive tubules and glomeruli with high mitotic activity, especially in cells arranged in a tubular pattern, is indicative of early NB. However, the presence of primitive tubular and glomerular structures are also the feature of renal dysplasia and NR. Renal dysplasia is also associated with frequent mitotic figures (Jackson and Kirkpatrick 2002; Ruehl-Fehlert et al. 2003). These observations are contrary to Mesfin’s (1999) criteria of early NB. Since ILNB is considered as a precursor lesion of NB, it may be associated with mitotic figures. The available standardized nomenclature systems did not address the diagnostic criteria for NR and nephroblastematosis. The criteria given for early NB in the nomenclature systems are “exclusively blastemal” (Hard et al. 1991) or “focal proliferation of blastematous cells” (Mohr 1992); differentiating criteria for NR with nephroblastematosis are lacking. The current case contained infiltrative blastemal cells that differentiated into rosettes, primitive tubules, and rarely, a glomeruloid body with low mitotic activity; in addition, the blastemal cells trapped, compressed, and displaced few adjacent normal renal tubules, therefore the case is classified as ILNB. It is evident that in time, the lesion would have led to well-developed NB. Nephroblastoma is differentiated from the current lesion by the fact that the lesion did not exhibit the typical compression (mass effect) to the adjacent parenchyma by the expanding neoplastic tissue. The proliferating blastemal cells caused compression of few trapped and displaced tubules. A similar feature was observed in ILNB described by Mesfin (1999).

The blastemal cells with tubuloepithelial differentiation including primitive glomeruli are the common features of NR, nephroblastematosis, and NB. The infiltrative, proliferative and compressive nature of the lesion defines the diagnosis. In view of the different diagnostic criteria used in the past to describe these lesions in the rat, the authors propose that NR in rats may be characterized by the presence of blastematous cells with varying degrees of differentiation into tubuloepithelial components, without the evidence of proliferation and infiltration in to the adjacent parenchyma. Nephroblastematosis is NR that has shown evidence of proliferation and infiltration in the renal interstitium. Nephroblastoma may be diagnosed, in addition to the presence of blastemal, tubuloepithelial structures, by the presence of well-demarcated and circumscribed growth, which cause compression (mass effect) to the surrounding parenchyma. Since ILNB is a precursor to the development of NB in humans and rats, the authors believe that ILNB is the proliferative and infiltrative phase of NR, which may progress to NB. Identification and classification of such preneoplastic conditions may provide insights into the evolution of NB. However, this is only the authors’ perspective, and the current INHAND nomenclature working committee may consider clearly defining the diagnostic criteria for renal dysplasia, NR, nephroblastematosis, and NB.

In humans, based on the proportion of blastemal, stromal and epithelial components, NR is classified as incipient or dormant, sclerosing, hyperplastic, or neoplastic, which indicates that NR may regress, sclerose, or become mitotically active, respectively, giving rise to neoplastic rest (synonymous with NB). Incipient and dormant rests are identical, composed of primarily blastemal or epithelial, and are microscopic. If NR is found in young infants, the term “incipient” is used, and if found in adults, the term “dormant” is used. Sclerosing rests are also microscopic, characterized by stromal elements with few or no mitotic cells and few epithelial elements, and believed to be the end stage of rest regression. Hyperplastic and neoplastic rests are microscopic or macroscopic. Hyperplastic rest represents proliferation of most or all of the cells of the rest. The cells tend to preserve the original shape of the rest. Neoplastic rest results from proliferation of a single cell within a rest. Neoplastic rest tends to be spherical and form expansile masses that are clearly demarcated from and exert mass effect on the surrounding parenchyma. Hyperplastic and neoplastic rest are mitotically active and may show cellular atypia (Beckwith 1993; Lonergan et al. 1998). Incipient NR has been reported in a SD rat (Jackson and Kirkpatrick 2002). Mesfin (1999) reported that ILNB of the rat could be equivalent to dormant NR of humans. However, we consider that ILNB of the rat corresponds to hyperplastic NR of humans owing to its infiltrative and proliferative nature. In the current spontaneous case, ILNB was observed in a female rat that was nine weeks old, which is younger than in the strain genetically predisposed to NB. To the authors’ knowledge, this is the first report of such spontaneous preneoplastic condition in a Wistar rat of this age.