Signalment:  

20-year-old male white-lipped mud turtle, (Kinosternon leucostomum).This turtle was from a large regional aquarium collection and presented to veterinary clinicians with coelomic distension and periocular swelling. Ultrasound examination confirmed coelomic effusion that was classified as a transudate following fluid analysis. Radiographs demonstrated diffuse, bilateral mineral opacities within the kidneys. Bloodwork showed a moderate anemia and severe elevation in uric acid. With a presumptive diagnosis of chronic renal failure in an elderly and fractious turtle, the animal was maintained on hospice care without further diagnostics. The turtle died 3.5 months after initial presentation.

Gross Description:  

Grossly, there was severe coelomic distention and subcutaneous edema of the neck and limbs. Both kidneys were diffusely off-white and hard, with no normal renal tissue apparent. The liver was friable and enlarged with rounded edges and multifocal green-tan mottling. 

Histopathologic Description:

Approximately 80% of the renal parenchyma is replaced by anastomosing trabeculae of well-differentiated mature bone. The bone contains scattered osteocytes and the trabecular surface is lined by flattened to cuboidal osteoblasts and occasional multinucleated osteoclasts. In non-ossified parenchyma, there is moderate, multifocal expansion of the interstitium by increased clear space (edema) and loosely arranged, fusiform to stellate cells (fibroblasts), as well as scattered infiltrates of mixed inflammatory cells including lymphocytes, plasma cells, heterophils, and azurophils. Remaining renal tubules are frequently ectatic, and/or lined by variably enlarged epithelial cells with abundant indistinct cytoplasmic vacuolization (hydropic degeneration). Increased cytoplasmic basophilia and occasional mitotic figures are also observed among tubular epithelial cells, suggestive of tubular regeneration. Flocculent, lightly eosinophilic proteinaceous material is often present within tubular lumens along with rare cellular debris. Glomeruli are multifocally affected by mild segmental thickening of the mesangial matrix and capillaries by lightly eosinophilic material and mild hypercellularity (membranoproliferative glomerulonephritis). 

Morphologic Diagnosis:  

Kidneys, nephropathy, diffuse, chronic, severe, with marked osseous metaplasia, fibroplasia, tubular degeneration, mild membranoproliferative glomerulonephritis, mild lymphocytic and granulocytic interstitial nephritis. 

Condition:  

Renal osseous metaplasia

Contributor Comment:  

Osseous metaplasia (ectopic ossification), the formation of non-neoplastic bone in soft tissues, has been reported to occur in numerous extra-skeletal organ systems and in the setting of many, clinically disparate disease processes. The ectopic osseous matrix can be mineralized and is typically associated with osteocytes, osteoclasts, osteoblasts, and, in some cases, hematopoietic cells and adipocytes. It is frequently encountered in veterinary medicine as an incidental lesion in the pulmonary connective tissue of dogs and cattle and in the canine dura mater (ossifying pachymeningitis). Ectopic ossification has also been reported in a spectrum of human and animal neoplasms, notably in canine mammary tumors, where it is a common feature.(1) For reasons that are not entirely clear, non-mammalian species empirically are more prone to developing osseous metaplasia than mammals.

Ectopic bone either arises from embryonic cell rests or by differentiation of adult, pluripotent mesenchymal cells into osteoblasts (osseous metaplasia).(2) Osseous metaplasia can occur anywhere uncommitted mesenchymal cells reside, including skeletal muscle, perivascular tissue, and connective tissue or sites of tissue regeneration and repair. It requires the influence of local osteogenic signals in an environment conducive to bone production.(3) Chronic ischemia, trauma, persistent hematoma, chronic inflammation, neoplasia, hypercalcemia, and hypervitaminosis D are among the factors known to stimulate osseous metaplasia.(4,5) While the pathophysiologic mechanisms leading to bone formation is not completely understood, paracrine signaling leading to the expression of bone morphogenetic proteins (BMPs) is likely a common key factor.(3) Most BMPs are members of the TGF beta superfamily and are critical signaling agents in normal development and differentiation and in the formation of new bone during fracture healing.(6) BMP expression has been demonstrated to play a role in neoplastic processes associated with ectopic bone formation, as well as experimental models of chronic inflammation.(7) In addition to non-committed mesenchymal cells, vascular endothelial cells(8) and pericytes(9) have also been implicated as potential cells of origin for osseous metaplasia. Interestingly, inactivating germline mutations of the α-subunit of the stimulatory G protein gene leads to subcutaneous and sometimes deeper ectopic bone formation in humans (Albright hereditary osteodystrophy) and in mice.(10) Osseous metaplasia is also seen in association with dystrophic cardiac and pulmonary mineralization in particular strains of mice where early events involve abnormal cellular calcium, mitochondrial alterations, and myocyte injury in the absence of elevation of serum calcium.(12,13)

While pathologic mineralization within the kidney is a not an uncommon finding in cases of chronic renal disease, the presence of abundant, trabecular bone in the renal parenchyma of this turtle is remarkable. Reports of spontaneous osseous metaplasia in the kidney are rare in the human medical literature and even rarer in the veterinary medical literature. In humans, ossified tissue in the kidney is associated with chronic interstitial nephritis, chronic ischemia, pyelonephritis, and papillary necrosis, and it is an uncommon nidus for renal calculus formation.(5,11) Contributing factors for renal osseous metaplasia observed in this turtle likely included chronic inflammatory stimulation and possibly calcium/phosphorous imbalance secondary to chronic renal dysfunction. 

JPC Diagnosis:  

1. Kidney: Osseous metaplasia, diffuse, severe, with renal tubular degeneration and necrosis.
2. Kidney: Nephritis, interstitial, lymphoplasmacytic, diffuse, moderate.

Conference Comment:  

We thank the contributor for providing such a thorough summary of osseous metaplasia in veterinary species, and we concur with the proposed explanation that a combination of mineral imbalance due to chronic renal dysfunction (supported by the clinical pathology results and radiographs) and chronic inflammatory stimulation likely contributed to the striking pathologic findings observed in this case. Conference participants briefly considered osteoma as a rule out; however, this benign tumor is generally attached to the periosteum and should not result in the incorporation of renal glomeruli/tubules within bony trabeculae. Additionally, the moderator observed that some tissue sections contain portions of adrenal gland adjacent to the renal hilus, which is a normal anatomic location in some turtle species. Conference participants also noted that the large collecting duct within the hilus appears to contain numerous spermatozoa.

References:

1. Thompson K. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol. 1. 5th ed. Philadelphia, PA: Saunders Elsevier, 2007:1-184.

2. Myers R, McGavin M, Zachary J. Cellular adaptations, injury, and death: morphologic, biochemical, and genetic basis. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Elsevier; 2012:2-59.

3. McCarthy EF, Sundaram M. Heterotopic ossification: a review. Skeletal Radiol. 2005;34:609619.

4. Landim FM, Tavares JM, de Melo Braga DN, da Silva JE, Bastos Filho JBB, Feitosa RGF. Vaginal osseous metaplasia. Arch Gynecol Obstet. 2009;279:381384.

5. Bataille S, Daniel L, Legris T, Vacher-Coponat H, Purgus R, Berland Y, Moal V. Osseous metaplasia in a kidney allograft. Nephrol Dial Transplant. 2010;25:37963798.

6. Wozney JM, Rosen V. Bone morphogenetic protein and bone morphogenetic protein gene family in bone formation and repair. Clin Orthop Relat Res. 1998;346:2637.

7. Rifas L. T-cell cytokine induction of BMP-2 regulates human mesenchymal stromal cell differentiation and mineralization. J Cell Biochem. 2006;98:706714.

8. Medici D, Olsen BR. The role of endothelial-mesenchymal transition in heterotopic ossification. J Bone Miner Res. 2012;27:16191622.

9. Dayoub S, Devlin H, Sloan P. Evidence for the formation of metaplastic bone from pericytes in calcifying fibroblastic granuloma. J Oral Path Med. 2003;32:232236.

10. Huso DL, Edie S, Levine MA, Schindinger W, Wang Y, Harald J, Germain-Lee EL. Heterotopic Ossifications in a mouse model of Albright hereditary osteodystrophy. Plos One. 2011:6:e21755.

11. Fernandez-Conde M, Serrano S, Alcover J, Aaron JE. Bone metaplasia of urothelial mucosa: an unusual biological phenomenon causing kidney stones. Bone. 1996;18:289291. 

12. Percy DH, Barthold SW. Pathology of Laboratory Rodents and Rabbits. 3rd ed. Ames, IA: Iowa State Press; 2007:94-95, 219.

13. Ernst H, Dungworth DL, Kamino K, Rittinghausen S, Mohr U. Nonneoplastic lesions in the lungs. In: Mohr U, Dungworth DL, Capen CC, Carlton WW, Sundberg JP, Ward JM, eds. Pathobiology of the Aging Mouse. Washington, DC: ILSI Press; 1996:298.

Click the slide to view.

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4-1. dorsoventral view

4-2. lateral view

4-3. Kidney and adrenal gland

4-4. Kidney and adrenal gland

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