Morphological characteristics of renal tissue damage
In acute pyelonephritis, histological changes predominantly affect the tubular system and the renal parenchyma.
- Glomeruli: typically demonstrate resistance to the infectious process. Their damage is observed only upon the formation of extensive necrosis zones. However, in fungal pyelonephritis (specifically caused by fungi of the genus Candida), the glomeruli are quite frequently affected.
- Renal tubules: the main morphological sign of acute inflammation is necrosis of the renal tubular epithelium. Histological examination of micropreparations stained with hematoxylin and eosin reveals alteration of the tubular epithelium.
- Macroscopic appearance: multiple small abscesses are identified in the renal parenchyma, representing foci of purulent tissue liquefaction.
Pathomorphology of renal pyramid necrosis
Papillary necrosis (necrosis of the renal papillae) is one of the three main complications of the disease.
- Prevalence: most commonly, the pathological process is localized within a single renal pyramid, but in rarer cases, absolutely all renal pyramids undergo necrotic changes.
- Macroscopic changes: the apices or distal two-thirds of the renal pyramids are affected. Necrotic areas acquire a gray-white or yellow color. Visually, these zones resemble changes seen in a renal infarction.
- Microscopic appearance: morphology corresponds to infarct-like coagulative necrosis. The contours of the tubules in the damaged areas remain intact. A leukocytic infiltrate is clearly defined at the border between the necrotic and viable tissue.
Severe destructive complications of obstruction
The development of urinary tract obstruction against the background of inflammation leads to the formation of dangerous destructive processes.
| Complication | Conditions and mechanisms of development | Morphological essence of the process |
|---|---|---|
| Pyonephrosis | Caused by complete or nearly complete obstruction of the urinary tract (primarily in high ureteral blockage). | Due to the inability to drain, purulent exudate accumulates in the renal pelvis, calyces, and ureters, leading to pyonephrosis. |
| Perinephric abscess | Develops when purulent inflammation breaches or spreads beyond the renal capsule. | The paranephric (perirenal) adipose tissue becomes involved in the inflammatory destructive process. |
Thus, the triad of major complications (papillary necrosis, pyonephrosis, perinephric abscess) reflects the progressive purulent melting of renal and perirenal structures.
Briefly: answers to topical questions
What microorganisms are the most frequent causative agents of acute pyelonephritis?
The most common causative agents of acute pyelonephritis are gram-negative bacteria that represent normal gastrointestinal microflora, among which Escherichia coli predominates. The disease is also caused by other representatives of Enterobacteriaceae: Proteus, Klebsiella, and Enterococcus.
- Escherichia coli (Escherichia coli) is the most frequent cause of pyelonephritis.
- Proteus (Proteus) is a Gram-negative microorganism.
- Klebsiella (Klebsiella) is a representative of the intestinal flora.
- Enterobacter (Enterobacter) are frequent causative agents of urinary tract infections.
Through what pathways does infection penetrate the kidney in the development of acute pyelonephritis?
Infection penetrates the kidney during the development of pyelonephritis mainly via three primary pathways, with the ascending pathway predominating. The ascending pathway is realized by the retrograde spread of microorganisms from the lower urinary tract. The hematogenous pathway is relatively rare and is associated with the hematogenous seeding of the pathogen during bacteremia from distant purulent foci or in sepsis. The lymphogenous pathway is also distinguished as a possible retrograde spread of infection via lymphatic vessels.
- Ascending (urogenital) pathway — the main pathway of infection from the lower urinary tract.
- Hematogenous pathway — seeding of the pathogen via the bloodstream during bacteremia.
- Lymphogenous pathway — retrograde spread via lymphatic vessels.
What histological changes develop in the renal interstitial tissue during acute pyelonephritis?
Acute pyelonephritis is based on interstitial purulent inflammation accompanied by pronounced exudative and destructive changes in the organ parenchyma.
- Interstitial edema — an early manifestation of exudative changes.
- Neutrophilic infiltration — a pronounced accumulation of neutrophils in the interstitial tissue of the medulla and perivascular zones.
- Destructive changes — dystrophy and necrosis of the renal tubular epithelium with the destruction of their structure.
What are the possible macroscopic outcomes of acute pyelonephritis on the renal surface?
The macroscopic picture of acute pyelonephritis on the surface and in the renal tissue is characterized by the formation of purulent foci of various sizes and shapes depending on the nature of suppuration.
- Multiple small abscesses — identified in the renal tissue during a generalized process.
- Localized abscesses — may affect one or both kidneys.
- Carbuncle — large wedge-shaped areas of suppuration in the renal parenchyma.
The three main "P"s of acute pyelonephritis complications: Papillary necrosis, Pyonephrosis, Perinephric abscess.
Class and exam questions
Are renal glomeruli involved in the process in acute pyelonephritis?
The glomeruli are generally resistant to infection. They are affected only in the presence of large areas of parenchymal necrosis, or in fungal (candidal) pyelonephritis, where their involvement frequently occurs.
What are the macroscopic and histologic signs of papillary necrosis?
Macroscopically, the tips or distal 2/3 of the pyramids are necrotic (gray-white or yellow, resembling an infarction). Histologically, infarct-like coagulative necrosis is revealed with preserved tubular contours and leukocytic infiltration at the tissue border.
What is the difference between pyonephrosis and perinephric abscess?
Pyonephrosis occurs with complete urinary tract obstruction and the accumulation of pus inside the renal pelvis, calyces, and ureters. A perinephric abscess forms when pus spreads through the renal capsule into the surrounding paranephric adipose tissue.
What else to study on this topic
- Features of glomerular involvement in candidal (fungal) pyelonephritis
- Microscopic differential diagnosis of tubular epithelial necrosis on hematoxylin and eosin staining
- Comparative morphological characteristics of renal infarction and papillary necrosis
- Pathogenesis of pyonephrosis in high ureteral obstruction
- Pathways of purulent process spread into the paranephric adipose tissue in perinephric abscess
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