Daniel Delgado Pineda

Chronic kidney disease progression is closely linked to tubulointerstitial inflammation, fibrosis, and tubular atrophy, which often predict renal outcomes better than the initiating glomerular lesion. This chapter reviews the cellular, molecular, histopathological, and biomarker evidence connecting interstitial injury with kidney disease. Fibroblasts derived from metanephric stromal progenitors retain substantial phenotypic plasticity. After injury, inflammatory mediators, TGF-β, platelet-derived growth factor, Toll-like receptor signaling, and mechanotransduction induce myofibroblast transition, extracellular matrix deposition, and loss of erythropoietin production. Chronic inflammation may also generate tertiary lymphoid structures that sustain local T- and B-cell activation, autoantibody production, and tissue damage. Intrarenal renin–angiotensin–aldosterone system activation, proteinuria-induced lysosomal and mitochondrial stress, cellular senescence, and alternative complement pathway activation further amplify tubular inflammation and fibrosis. In IgA nephropathy, lupus nephritis, ANCA-associated vasculitis, monoclonal gammopathy of renal significance, and kidney transplantation, interstitial fibrosis and tubular atrophy consistently predict treatment resistance, declining estimated glomerular filtration rate, kidney failure, and reduced graft survival. Urinary biomarkers may noninvasively capture tubular injury or dysfunction (KIM-1, NGAL, NAG, α1- and β2-microglobulin), inflammation (MCP-1/CCL2, CXCL9), stress and fibrosis (DKK-3), and functional tubular mass and repair (uEGF). However, assay heterogeneity, disease-specific performance, age-related variation, and absent universal thresholds limit routine use. Future studies should test whether longitudinal biomarker trajectories detect reversible inflammation before fixed scarring and whether biomarker-guided treatment improves outcomes. Integrating histopathology, clinical variables, and validated biomarker panels may refine prognosis and enable earlier, mechanism-directed interventions.

Podocytes are specialized epithelial cells that play a crucial role in maintaining the structure and function of the glomerular filtration barrier (GFB). Their unique morphology, characterized by interdigitating foot processes connected by slit diaphragms, allows for precise size and charge selectivity during filtration. The slit diaphragm, composed primarily of nephrin, podocin, and CD2AP, forms a mechanosensitive signaling platform that links the plasma membrane to the actin cytoskeleton, ensuring both mechanical resilience and dynamic adaptability to hemodynamic stress. Podocytes integrate biomechanical, metabolic, and paracrine cues through pathways such as the PI3K/Akt, Rho GTPase, and integrin pathways, thereby coordinating cytoskeletal remodeling, adhesion, and survival. Disruption of these networks—whether due to genetic mutations, oxidative stress, hyperglycemia, or immune injury—leads to foot process effacement, loss of slit diaphragm integrity, and the onset of proteinuria. Persistent podocyte injury contributes to glomerulosclerosis and progressive chronic kidney disease. Emerging evidence suggests that targeting podocyte-specific pathways, including TRPC6, Piezo1, and epigenetic modulators, may preserve filtration barrier integrity. Various immunosuppressive treatments have proven to be effective in modulating the structure and function of the podocyte.

Renal function refers to the combined actions of the glomerulus and tubular system to achieve homeostasis in bodily fluids. While the glomerulus is essential in the first step of urine formation through a coordinated filtration mechanism, the tubular system carries out active mechanisms of secretion and reabsorption of solutes and proteins using specific transporters in the epithelial cells. The assessment of renal function usually focuses on glomerular function, so the tubular function is often underestimated as a fundamental part of daily clinical practice. Therefore, it is essential to properly understand the tubular physiological mechanisms and their clinical association with prevalent human pathologies. This review discusses the primary solutes handled by the kidneys, including glucose, amino acids, sodium, potassium, calcium, phosphate, citrate, magnesium and uric acid. Additionally, it emphasizes the significance of physicochemical characteristics of urine, such as pH and osmolarity. The use of a concise methodology for the comprehensive assessment of urine should be strengthened in the basic training of nephrologists when dealing with problems such as water and electrolyte balance disorders, acid-base disorders, and harmful effects of commonly used drugs such as chemotherapy, antibiotics, or diuretics to avoid isolated replacement of the solute without carrying out comprehensive approaches, which can lead to potentially severe complications.