JPC SYSTEMIC PATHOLOGY

URINARY SYSTEM

December 2023

U-M09 (NP)

Signalment (JPC 2017771): 11-year-old Wirehaired Fox Terrier

HISTORY: Tissue from an 11-year-old Wirehaired Fox Terrier admitted for the regulation of diabetes mellitus, persistent lipemia, and management of cystitis. The dog was euthanized after two cystectomies for transitional cell carcinoma and continued elevation of the BUN.

HISTOPATHOLOGIC DESCRIPTION: [H&E] Kidney: Diffusely, glomeruli have one or more of the following changes: Global expansion of the mesangial matrix by hyalinized, eosinophilic, fibrillar material with decreased cellularity (glomerulosclerosis), that occasionally replaces entire glomerular tuft; moderate to marked thickening of the basement membrane of Bowman’s capsule and glomerular capillary basement membranes; periglomerular fibrosis; occasional hyperplasia of parietal epithelial cells; and multiple 10-40 µm diameter, irregularly round, clear vacuoles (lipid) within capillaries. Multifocally, the interstitium is expanded by plump fibroblasts and increased collagen (fibrosis) with moderate numbers of lymphocytes and plasma cells. Surrounding tubules are often atrophied or mildly ectatic with attenuated epithelium and luminal protein or hyaline casts, and mildly thickened basement membranes. Tubule epithelial cells multifocally contain bright eosinophilic, cytoplasmic, 2-6µm granular protein droplets. Within the fibrous connective tissue underlying the pelvic urothelium, there are few neutrophils, lymphocytes and plasma cells, which occasionally transmigrate the urothelium. Multifocally, blood vessels are lined by plump endothelial cells, and there are vacuolated cells within the tunica media. The renal capsule is mildly thickened by fibrous connective tissue; and within the medulla.

[Osmium tetroxide] Kidney: Multifocally, vacuolated cells expanding the glomerular tufts contain dark homogenous to granular material (osmiophilic fat emboli).

MORPHOLOGIC DIAGNOSES: Kidney: Glomerulosclerosis, diffuse, moderate, with lipid emboli, thickening of Bowman’s capsule, capillary and tubular basement membranes, and chronic interstitial nephritis and subacute pyelitis, Wirehaired Fox Terrier, canine.

ETIOLOGIC DIAGNOSIS: Diabetic nephropathy (DN)

CONDITION: Diabetes mellitus

GENERAL DISCUSSION:

Diabetic nephropathy (DN) is a major, often fatal complication of both type 1 and type 2 diabetes mellitus in humans, dogs, and cats
In human diabetes, complications manifest as nephropathy, vasculopathy, and neuropathy
In dogs and cats, these types of complications are uncommon, but blindness is strongly predictive of the development of fatal diabetic nephropathy
Glomerular lesions in diabetic dogs are usually generalized thickening of the glomerular basement membrane (GBM) and mild mesangial expansion, often associated with lipid in the mesangial matrix or cell cytoplasm

PATHOGENESIS:

The exact pathogenesis of canine diabetic nephropathy is unknown
In Zucker diabetic fatty (ZDF) rats (animal model):
1. Renal podocytes play a crucial role in maintaining the structure and function of the glomerular filtration barrier and depletion of the podocytes may represent one of the earliest cellular lesions of diabetic nephropathy
2. Hyperglycemia causes increase in methylglyoxal (MGO) concentration which is cytotoxic causing DNA damage in the podocytes
Diabetic neuropathy is the result of oxidative stress within tissues from excess glucose which also stimulates proinflammatory protein kinase C signaling pathway
Diabetic retinopathy, infrequent in canine (often five years after onset of diabetes)
1. At the posterior pole, retinal vascularity is most often severely dilated and tortuous, with saccular capillary microaneurysms and focal hemorrhage
Chronic hyperglycemia > increased glomerular hypertension and renal hyperperfusion and hyperfiltration > derangement of local hemodynamic regulation (NO & RAS pathway) > increased deposition of protein in the mesangium > mesangial expansion encroaches on subendothelial space and glomerular capillary lumina > decreased glomerular blood flow and filtration > glomerulosclerosis > renal failure

TYPICAL CLINICAL FINDINGS:

Vomiting, dehydration, weakness, weight loss, polyuria, and polydipsia
Clinical signs depend on the severity of the glomerulosclerosis (depends on degree of glycemic control) and the kidney's ability to excrete metabolic waste
Initially, severe proteinuria (primarily albuminuria) > progressive impairment of glomerular filtration > azotemia > uremia
With severe glomerular fibrosis > oliguric renal failure > anuric renal failure
With renal involvement - azotemia, hypoalbuminemia, hypoproteinemia, hyperphosphatemia, hyperlipidemia, proteinuria (due to lower urinary tract infection or glomerular disease), hematuria, and pyuria

TYPICAL GROSS FINDINGS:

Initially the kidney is enlarged; over time, the kidney becomes contracted and fibrotic
Principal lesion is a remarkably yellow, fatty liver
Emphysematous cystitis is a rarely reported finding in dogs with hyperglycemia due to bacterial fermentation of glucose with gas production

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Glomerulosclerosis - diffuse or nodular; GBM thickening with accumulation of PAS positive basement membrane material > hyaline thickening of capillary basement membranes and sclerosis of lobules in glomerular tufts > diffuse thickening of mesangium and mesangial proliferation > glomerular obliteration; sclerosis may be limited to the mesangium and may or may not form nodules
Glycogen nephrosis - vacuolar change in renal epithelium (loops of Henle, distal convoluted tubules, inner cortical nephrons) - highly specific for diabetes mellitus
Fat emboli occasionally present in the glomerular capillaries

Fatty degeneration of the epithelium of the proximal convoluted tubules
Lipid may be present in mesangial matrix or cell cytoplasm

In cats, fat vacuoles in tubule epithelial cells are normal

ADDITIONAL DIAGNOSTIC TESTS:

Transmission electron microscopy (TEM): Significant increase in the glomerular basement membrane (GBM) thickness, with marked mesangial matrix expansion
1. Osmium tetroxide (OsO₄): A good fixative and stain used to preserve lipids in membranous structures and vesicles prior to electron microscopy
Clinical pathology
1. Fasting hyperglycemia, hypercholesterolemia, increased ALP and ALT, glucosuria, hyponatremia, hyperkalemia, ketonemia, and ketonuria
Glucose tolerance test - to confirm or exclude a diagnosis of diabetes mellitus
Serum fructosamine (glycated serum proteins) and glycated hemoglobin (GH) tests
1. Less affected by acute glucose concentration changes
2. Serum fructosamine – one to two weeks
  - High sensitivity and specificity for the diagnosis of diabetes mellitus
3. Glycated hemoglobin – two to three months

DIFFERENTIAL DIAGNOSIS:

Glomerular amyloidosis - characteristic birefringence with Congo red stain
Immune‑mediated glomerulonephritis due to various agents - more prominent inflammatory component with less tubular basement membrane involvement

COMPARATIVE PATHOLOGY:

Current animal models of type-1 and type-2 diabetes have limitations correlating renal pathology to human renal pathology
Animal models
1. Baboon: Diabetic baboons exhibited thickened glomerular basement membrane and mesangial matrix expansion
2. Non-obese diabetic (NOD) are established models of spontaneous insulin-dependent diabetes mellitus (IDDM)
  - Early stages: Renal hypertrophy and slight glomerular injury
  - Later stages: Structural alteration of the proximal straight tubules
3. Zucker diabetic fatty (ZDF) and ZSF1 rats are Non-IDDM

REFERENCES:

Barthold, S., Griffey, S. and Percy, D. Pathology of Laboratory Rodents and Rabbits, 4th ed. Ames, IA: John Wiley & Sons; 2016: 4.
Cianciolo RE, Mohr FC. Urinary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 406.
Heinz-Taheny, K., Harlan, S., Qi, Z. and Heuer, J., 2018. Synopsis of sweet mouse models of diabetic kidney disease. Toxicol Pathol. 2018; 46(8): 970-975.
Jubb K, Stent AW. Pancreas. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 372.
Miller MA. Endocrine System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022: 796.
Rincon-Choles H, Abboud HE, Lee S, et al. Renal histopathology of a baboon model with type 2 diabetes. Toxicol Pathol. 2012; 40(7): 1020-1030.
Wilcock BP. Eye. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 472.