How does kidney develop during gestation ?

The students of renal pathology, medicine, nephrology and urology would always like to imbibe knowledge about the origin and development of kidney to understand the pathogenesis of developmental kidney diseases. The urogenital system is derived and developed from the intermediate mesoderm and the primitive urogenital sinus of the cloaca. The ureteral bud (UB) develops from the Wolffian duct (WD) at approximately 28 days of gestation. The ureteral bud (UB) initiates the epithelialization/tubulogenesis of the metanephric mesenchyme (MM) while itself undergoes branching to form and adult kidney. Much knowledge about the development of kidney could be gathered from the experimental studies using mouse embryo. In the mouse the ureteral bud (UB) invaginates from the caudal end of the Wolffian duct (WD) and grows out into the adjacent metanephric mesenchyme cells.

The metanephric mesenchyme cells comprise of tubule precursors, endothelial precursors and stromal cells. These loose metanephric mesenchyme cells aggregate to form "pre tubular aggregate" which undergoes structural change to form a 'tear-drop' like structure called renal vesicle (RV). The renal vesicle rapidly undergoes mesenchymal-epithelial transformation (MET) to form a comma-shaped structure. The comma-shaped structure formed by the mesenchymal-epithelial transformation of RV undergoes series of tightly controlled transformations and form a very complex S-shaped body. The lower part of the "S" (of S-shaped body) gives rise to podocytes and Bowman's capsule. The upper part of the "S" (of S-shaped body) forms the distal convoluted tubule of the nephron. The middle segment of the "S" (of S-shaped body) gives rise to the proximal convoluted tubule and the loop of Henle of the nephron. Each tip of the ureteral bud gives rise to a nephron. After 20-22 weeks of gestation in humans, the ureteral bud stops branching but the nephron induction continues for other 8 to10 weeks, leading to arcade formation wherein each tip of the ureteral bud has 9-11 nephrons attached to it. The arcade formation is considered as the last step of the nephron induction in humans. However, in mice the process of nephron induction continues for about two weeks after birth. The mouse kidney has single papilla and carries around 35,000 nephrons/kidney. Human kidneys roughly contain 6x10⁵ to 1.1x10⁶ nephrons/kidney.

The knowledge of developmental stages of kidney is important to understand the pathogenesis of developmental cystic kidney diseases like renal agenesis (no kidney developed), renal hypoplasia (under developed kidneys) and renal dysplasia (abnormally developed kidneys). Renal hypoplasia (reduction in total number of nephrons), renal dysplasia (abnormally developed kidney due to failure of UB to induce formation of nephrons) and segmental hypoplasia are the major developmental cystic kidney diseases. Simple hypoplasia leads to very small sized kidney called miniature kidney. Miniature kidney would represent reduction in renal calyces with glomerular disarray and reduction of tubules, medulla & cortex. Segmental hypoplasia presents during the late childhood with renal insufficiency associated with hypertension and recurrent urinary tract infection (UTI). Patients with segmental hypoplasia have small sized kidney (s) with a transverse groove on the capsular surface at the upper pole, overlying an area of marked parenchymal thinning. Areas of segmental hypoplasia represented by scarred zones with no glomeruli, atrophic tubules and thick walled blood vessels could be revealed under light microscopy of histological sections.

Kidney Diseases caused by Plasma Cell and B-Cell Disorders

A wide spectrum of clinical manifestations may result from the renal involvement with disorders of B-cells (B lymphocytes) and plasma cells. B lymphocytes and plasma cells are responsible cells for providing acquired and active immunity to our body through production of antibodies (immunoglobulins) against infectious organisms. But the disorders related to the function and number of B-cells and plasma cells lead to excessive or incomplete production of immunoglobulin molecules leading to deposition of immunoglobulins or their components in the kidneys. Deposition of immunoglobulins or their light or heavy chains cause a variety of renal disorders affecting glomeruli, extraglomerular blood vessels, tubules and interstitium. Two major classes of such diseases are as under:

A) Glomerular and vascular diseases

Glomerular and vascular diseases caused by B-cell and plasma cell disorders include amyloidosis (AL, AH and AHL type), light chain deposition disease (LCDD), heavy chain deposition disease (HCDD), light & heavy chain deposition disease (LHCDD), cryoglobulinemic glomerulonephritis (type I & II), monoclonal immunotactoid glomerulopathy and proliferative glomerulonephritis with monoclonal IgG deposits.

B) Tubulointerstitial diseases

Cast nephropathy and light chain proximal tubulopathy are the tubulointerstitial diseases caused due to renal involvement in multiple myeloma (Plasma cell disorder).

Important Investigations

Routine urine examination along with microscopy, blood biochemistry to ascertain renal functions and kidney biopsy evaluation by light, fluorescence and electron microscopy is required to establish an accurate diagnosis of renal disorder in patients affected by B-cell and plasma cell disorders.

Disseminated Intravascular Coagulation: Pathophysiology and Diagnosis

Disseminated intravascular coagulation (DIC) should be recognized as consumptive coagulopathy since it is not a primary disease. It is always a complication of an underlying disease that not only triggers it but also fuels it. Disease or trauma associated tissue injury with a release of thromboplastic material into the circulation is the major cause of DIC. The clotting system as well as the fibrinolytic system (bleeding system) are involved in the pathophysiology of disseminated intravascular coagulation. Clinically, coagulopathy could be recognized as acute hemorrhagic DIC and subacute or chronic DIC. A third type of consumptive coagulopathy could be recognized with fibrinolysis. Disseminated intravascular coagulation is an acquired coagulation disorder in which formation of microthrombi, consumption of coagulation factors, activation of fibrinolysis and a bleeding tendency may occur consecutively or simultaneously. In brief, it is a systemic pathologic process characterized by a disseminated (generalized) activation of clotting and/or fibrinolytic systems in the circulatory system of the patient. The common pathway of all inciting causes (independent of etiologies) is the formation of thrombin and plasmin (fibrinolysin).

Thrombin plays a vital role in DIC. The alterations of coagulation system detected in the laboratory during DIC reflect the multiple actions of thrombin. Thrombin cleaves fibrinogen to release fibrinopeptide-A (FPA) and fibrinopeptide-B (FPB). Subsequently the remaining fibrin monomers may combine with fibrinogen and circulate as soluble fibrin monomer complexes (SFMC) or polymerize to form fibrin microthrombi. Thrombin also activates factor XIII (fibrin stabilizing factor) to form factor XIIIa, and the factor XIIIa creates bridges, linking any two adjacent monomers of fibrin. Thrombin activates procoagulant cofactors, factors VIII and V, to participate in the process of its own generation. Thrombin also plays a regulatory role by activating protein-C, which acts as an anticoagulant to inactivate factors VIIIa and Va. In brief, thrombin alone accounts for decreased levels of fibrinogen and factors II, V, VIII & XIII and decreased count of platelets in patients with DIC.

Screening tests for DIC are: Prothrombin time (PT), Partial thromboplastin time (PTT), Fibrinogen assay and Platelet count. Platelet count, PT, Fibrinogen assay and Determination of Antithrombin-III (AT-III) should always be done to diagnose consumptive DIC.

Confirmatory tests for DIC are: Fibrin monomer assay (it measures thrombin cleaved fibrinogen), Detection of fibrin split products (i.e. detection of plasmin-cleaved fibrinogen or fibrin) and Detection of D-dimer (i.e. detection of plasmin-cleaved cross-linked fibrin). Activation of coagulation could be assessed by the detection of soluble fibrin monomer complexes(SFMC). Detection of fibrinogen degradation products (FDPs) is indicative of reactive fibrinolysis.

Medical conditions which may lead to 'Acute Hemorrhagic DIC':

• Infections: Typhoid fever, Gram-positive and Gram-negative septicemia, viremia, parasites etc.
• Tissue injury: Renal allograft rejection, snake bite, heat stroke, brain injury, crush injury, necrotizing enterocolitis, hemolytic transfusion reaction etc.
• Malignancy: Acute promyelocytic leukemia.
• Obstetric: Amniotic fluid embolism, eclampsia, abruptio placentae, hypertonic saline abortion.
• Other causes: Severe liver disease.

Medical conditions which may lead to 'Subacute Chronic DIC':

• Vascular: Chronic renal disease, connective tissue disorders, venous thrombosis, arterial embolization, pulmonary embolus etc
• Obstetric: Retained dead fetus.
• Malignancy: Mucin-producing adenocarcinomas.

Various Causes of Acute Renal Failure

The cause and/or precipitating factor of acute renal failure (ARF) is always responsible for the effectiveness of therapy and supportive care techniques including hemodialysis. A rapid loss of renal function is exhibited through elevated levels of serum creatinine and blood urea due to fall in the clearance of these nitrogenous wastes by the kidneys in all cases of ARF. It has been observed that a loss of 50% of glomerular filtration rate (GFR) leads to significant elevation of the level of creatinine in the blood with a decrease in the urine output (oliguria). There could be three types of causes and implicating factors of acute renal failure: 1) Pre-renal, 2) Renal and 3) Post-renal. In pre-renal type ARF causes are the physiological factors or conditions which lead to poor renal perfusion and severe impairment of renal function. Hemorrhage in gastrointestinal tract (stomach and intestines) and other internal spaces, sepsis, hepatic failure (liver failure), over compliance of antihypertensive drugs or non-steroidal anti-inflammatory drugs (NSAID), arterial or venous thrombosis and intra-vascular hemolysis due to transfusion reactions, are the major pre-renal causes of ARF.

Acute tubular necrosis (ATN), rapidly progressive glomerulonephritis (RPGN), post infection glomerulonephritis and interstitial nephritis are some major renal causes of ARF. Pre-renal factors and use of nephrotoxic drugs may also be associated cause of ATN. Some viral infections, drugs, multiple myeloma, lymphoma and granuloma may cause interstitial nephritis leading to renal type ARF.

Post-renal type ARF is caused by intra-tubular obstruction due to fibrosis, stones or tumors. Every case of acute renal failure needs urgent investigations to establish the cause and efficient mode of supportive care and line of treatment. A comprehensive physical examination is required to look for possible causes of ARF and planning the investigations to classify the type of ARF. By timely diagnosis and treatment, renal function could be restored in majority of cases of pre-renal type acute renal failure.

Amyloidosis: Causes and Detection

Amyloidosis or deposition of amyloid in vital organs could be labeled as chronic pathological state. Amyloid is an abnormal protein derivative and amyloidosis is characterized by extracellular accumulation of this abnormal protein, which could be detected with Congo-Red staining during histological examination of biopsies/tissues. Genesis of amyloid is associated with B-cell (B Lymphocytes) and Plasma-cell disorders or chronic infections like tuberculosis. Renal (kidney) involvement in amyloidosis may affect all compartments of kidneys. Renal glomeruli, extraglomerular blood vessels, uriniferous tubules and even interstitium could be severely affected leading to impairment of renal function and can cause renal failure. Amyloid could be composed of one or more proteins out of around two dozen different monotypic polypeptides, including immunoglobulin light chains (AL type amyloid), immunoglobulin heavy chains (AH type amyloid), amyloid-A-protein (AA type amyloid), prealbumin, b-2 microglobulin, b-amyloid protein, islet amyloid polypeptide, procalcitonin, cystatin-C, apolipoprotein A-1 or A-2, gelsolin, lysozymes etc. Immunoglobulin light chains type (AL type) and amyloid-A-protein (AA type) amyloid mostly affect the kidneys. Almost all the patients with amyloidosis of kidneys have proteinuria (excretion of proteins in urine; >3g/day) and around 70% also have diminished renal function. On electron microscopy amyloid could be resolved as approximately 10 nm thick non branching and randomly arranged fibrils as illustrated in Figure-1.

Figure-1: Electron micrograph showing randomly arranged non-branching fibrils of amyloid in the mesangial area of a renal glomerulus. Original magnification 36000x.

Amyloid-A-protein type (AA type) amyloidosis is most often associated with chronic inflammatory diseases like tuberculosis, osteoarthritis, or rheumatoid arthritis. Some viral infections can also boost amyloidosis. Production of amyloidogenic light chains is associated with B-cell lymphoma, multiple myeloma or plasma-cell dyscrasia. AL and AA type amyloid have identical physicochemical properties. On renal biopsy evaluation we find acidophilic deposits which stain weakly with Periodic acid Schiff's stain or Silver stain. Amyloid stains bright red with Congo-Red stain and shows apple green birefringence by polarized light microscopy. Amyloid deposits could be revealed in the mesangium and peripheral capillary wall of renal glomerulus depending on the chronicity of the disease process. In advanced stages of amyloidosis, the amyloid deposits could be detected in arteries and interstitial tissue of kidneys in addition to glomeruli, by conventional methods and electron microscopy.

IgA Nephropathy as a cause of End Stage Renal Disease

There are a variety of causes of end stage renal disease (ESRD) in teenagers and adults. Immunoglobulin-A (IgA) nephropathy could be a cause of end stage renal disease (ESRD) in around 25% of cases. There are five types of immunoglobulins in our body for protection against microorganisms and IgA provides defence at mucous membranes. Colostrum and breast milk are rich sources of IgA and protect us during infancy through breast-feeding. However, later in life, chronic mucosal inflammation (inflammation of respiratory, oral, or gastrointestinal mucous membranes) may lead to IgA-nephropathy (IgAN). Viral (including HIV), bacterial, yeast and parasitic infections have been found to be associated with IgAN. Environmental and food antigens have also been implicated in IgAN as these may mimic molecular structure of microbial antigens and lead to excessive IgA production, aggregation and breakdown of mucosal barrier. Patients affected by IgAN may present with hematuria (blood in urine) and/or proteinuria (protein in urine) with or without rise in serum creatinine. The most common initial symptom in children is microscopic hematuria. Some adults may present with acute or chronic renal failure.

IgA nephropathy is a common nephropathy, which could be detected on renal (kidney) biopsy evaluation through light and fluorescence microscopy. However, electron microscopic study of renal biopsy acts as a diagnostic adjunct as the location of immune complexes in the renal glomerulus could be pronounced on electron micrographs. Figures 1 and 2 are the electron micrographs from a proven case of IgAN, illustrating mesangial deposits of IgA.

Figure-1: Electron micrograph of an area of glomerulus of a case of IgAN showing electron dense deposits (D) in the mesangial (Mes) area. Glomerular basement membrane (GBM), capillary lumen (CL), podocyte or epithelial cell (EpC) and urinary space (US) are also exhibited; Original Magnification 4600x.

Figure-2: Electron micrograph of an area of glomerulus of a case of IgAN showing electron dense deposits (D) in the mesangial (Mes) area. Glomerular basement membrane (GBM), capillary lumen (CL), podocyte or epithelial cell (EpC) and urinary space (US) are also exhibited; Original Magnification 6000x.

The pathology of IgAN may be variable depending on underlying cause. Mesangioproliferative glomerulonephritis is the most common pattern in many renal biopsies; however, glomeruli may appear normal on light microscopy in some of the cases. Renal biopsies in a few cases may also show crescent formation in occasional glomeruli. Diagnosis of IgA nephropathy is established by direct immunofluorescence technique on renal biopsies and the pattern may be dominant or co-dominant for IgA staining. The incidence of ESRD has been found to be high in patients presenting with >1g/day proteinuria with increased level of serum creatinine as compared to those having proteinuria <1g/day with increased level of serum creatinine. Pathogenesis of IgAN is very complex. A variety of underlying diseases including hepato-biliary disease can be associated with IgA nephropathy. Defective detection and clearance by liver of polymeric immune complexes of IgA (IgA1) due to abnormal galactosylation of O-linked glycans is probably the major cause of IgAN in addition to loss of mucosal barrier and chronic mucosal inflammation. Recurrent tonsillitis may also lead to IgA nephropathy and tonsillectomy may be helpful in these cases to remove the mucosal foci of infection. Optimal treatment of tonsillitis and other oromucosal infections with antibiotics along with conventional treatment of IgAN would be helpful to put brakes on the progression of IgA nephropathy. Patients with acute or chronic renal failure due to advanced stage of IgAN may need hemodialysis or renal transplantation. Use of anti-oxidants and fish oil as food supplements in some cases of IgA nephropathy have been found beneficial.