Podocytes and Podocytopathies

Kidneys play a vital role in excretion and water/fluid volume regulation. Glomeruli are the filtration units of nephrons in the kidneys and these contain cellular and non cellular components in addition to capillary space and urinary space. Podocytes (cells with pedicles or feet) are post-mitotic epithelial cells resting in the urinary space of glomeruli. The number, size and morphology of podocytes are influenced by biochemical, immunological, therapeutic and genetic factors. According to the old classification of renal disorders, the patients having nephrotic syndrome can be grouped into two groups: (1) Non-immune complex mediated nephrotic syndrome, and (2) Immune complex mediated nephrotic syndrome. Now, patients with non-immune complex mediated nephrotic syndrome may have three possible diagnoses:

1. Minimal change disease: Wherein morphologic evaluation of the renal biopsy (kidney biopsy) by light microscopy does not exhibit any glomerular damage. However, extensive effacement of foot processes of podocytes can be revealed by electron microscopy.
2. Focal segmental glomerulosclerosis (FSGS): Wherein segmental sclerosis/solidification of the glomerular tuft, along with hyalinosis and adhesion of tuft to the Bowman's capsule is exhibited on the renal biopsy (kidney biopsy) evaluation by light microscopy. In these cases, variable degree of foot process effacement can be revealed by electron microscopy.
3. Collapsing glomerulopathy: FSGS associated with the rapid deterioration of renal function was described as "Malignant FSGS" in 1978. During HIV pandemic in 1980's the associated nephropathy showing collapse of glomerular capillary wall along with increased cellularity in the urinary space was termed as HIV associated nephropathy (HIV-AN). Collapsing glomerulopathy was first time described in non-HIV patients in 1986 by Weiss and associates.

Now we know that podocyte number and effacement of their foot processes due to genetic or biological factors are very much associated with the primary nephrotic syndrome or proteinuric renal disorders. The etiology and pathogenic mechanisms are known to influence the morphologic diagnosis of podocytopathies. Podocytopathies are proteinuric renal disorders caused due to intrinsic or extrinsic podocyte injury exhibited by variable degree of foot process effacement and altered genotypic and/or phenotypic expression. Podocytes may reorganize their foot processes (altered cell morphology without change in cell count/number). There may be decreased number of podocytes (podocytopenia) if the injured podocytes die. There may be podocyte developmental arrest as seen in congenital nephrotic syndrome of Finnish type (CNF). Podocytes may dedifferentiate and proliferate under genetic, immunological, viral or therapeutic insult and re-enter the cell cycle despite the fact that podocytes are post-mitotic cells. Two electron micrographs are exhibited below to illustrate the normal (Figure-1) and increased number(Figure-2) podocytes in the urinary space of glomeruli from different cases.

Figure-1: Electron micrograph through a portion of glomerulus from a case of minimal change disease showing normal number of podocytes. (GBM: glomerular basement membrane, CL: capillary lumen, EnC: Endothelial cell, US: urinary space and Pc: podocyte)

Figure-2: Electron micrograph through a portion of glomerulus from a case of podocytopathy showing increased number of podocytes. (GBM: glomerular basement membrane, CL: capillary lumen, US: urinary space and Pc: podocytes)

End Stage Renal Disease: Management Issues

The patents with end stage renal disease (ESRD) need regular hemodialysis or renal replacement (kidney transplantation) for survival. Both the hemodialysis and kidney transplantation are very costly procedures for the patients and their families. The patients with chronic kidney disease (CKD) are at high risk of developing end stage renal disease (ESRD). Chronic kidney disease (CKD) is diagnosed on the basis of persistently high level of serum creatinine (more than 1.8 mg/dl). As a rough estimate one person in every 150 people may be suffering from CKD and around 3% of CKD cases are sure to develop ESRD. In a country with 500 million population there could be more than 100,000 patients with ESRD and around 3.5 million patients with CKD. Half of the projected figures could be annual incidence.

It has been worked out that the cost of annual dialysis is much more than the renal replacement therapy (RRT). Though the renal transplantation (kidney transplantation) is the more effective and sustainable therapy but the economic factors, availability of kidney and facilities retard its scope. The annual cost of dialysis may range from US$5000 to 10,000 depending on condition of the patient; whereas the one-time cost of renal transplantation at government funded hospitals in most of the developing countries ranges from US$1500 to US$2000 and annual cost of immunosuppressive therapy would be around US$3000 to 4000. As compared to patients on dialysis, the quality of life for the patients of renal transplantation is extremely better. A renal transplantation at an optimum time minimizes the graft maintenance costs and maximizes the graft survival. The patient can return to productive life within a year after renal transplantation. My friend AB, who got renal transplantation around 10 year ago, is living a normal life.

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.

The Role of Lymphatic System in Cellular Nutrition and Immunity

Every single cell in our body tissues and organs needs nutrition and clearing away of its waste products for survival and vital functioning. Lymphatic system plays a vital role in the circulation and regulation of interstitial fluid or tissue fluid. As the blood passes through blood capillaries in the tissues; plasma or tissue fluid oozes out through the porous walls of blood capillaries. The tissue fluid or the interstitial fluid fills the spaces or interstices between the cells of different tissues and organs. The blood circulates only through the blood vessels but the tissue fluid circulates through the actual tissue and carries food, oxygen and water from the blood stream to each individual cell and carries away its waste products like carbon dioxide, water and urea and pours out all these in the blood stream for final disposal. Lymphatic system is pump less system and runs parallel to the circulatory system and is comprised of following components.

Components of the Lymphatic System:

1. Lymphatic capillaries: These are hair like fine vessels in the spaces in the tissues and gather up excess fluid from the tissues. Lymphatic capillaries unite to form lymphatic vessels.
2. Lymphatic vessels: These are similar to veins in structure but carry lymph instead of blood. They are finer and more in number than the veins and are provided with unidirectional valves, to prevent the back flow of lymph or the tissue fluid. Lymphatic vessels are present in all tissues except the central nervous system. These run in the subcutaneous tissue and pass through one or more lymphatic nodes.
3. Lymph nodes or lymphatic nodes: Lymph nodes are numerous in number and vary in size from a pinhead to an almond. Lymphatic vessels which bring lymph to them are called afferent vessels. afferent vessels divide up within the node and discharge the lymph into the mesh of the lymph node. The lymph is collected again into a fresh vessel known as efferent vessel, which ultimately empties into a lymph duct. Lymph nodes consist of cells similar to white blood cells and are encapsulated by connective tissue. Lymph nodes filter out bacteria, provide fresh lymphocytes for the circulation and also produce some antibodies and antitoxins and boost up immunity.
4. Lymphatic ducts: These are major lymph channels. There are two lymphatic ducts, the thoracic duct and the right lymphatic duct. The thoracic duct is larger and all the lymphatic vessels from the lower limbs, and abdominal and pelvic organs empty into it. The thoracic duct empties into the left subclavian vein. The right lymphatic duct is comparatively small vessel formed by union of lymphatic vessels from the right side of the head, thorax and the right upper limb at the root of the neck. The right lymphatic duct is about one centimeter long and empties into the right subclavian vein.
5. Spleen, the master lymphatic organ: The spleen is the largest nodule of the lymphoid tissue. It is deep purplish red in color and lies high up at the back of the abdomen, on the left side behind the stomach and is enclosed in a capsule of connective tissue. It is composed of fibrous meshwork filled with pulp like material known as splenic pulp. It is a source of fresh lymphocytes for the blood stream, an area for the destruction of worn red blood cells (RBCs) and a legendary organ for fighting out circulatory infections.

Functions of Lymphatic System:

1. Restoration of constant stream of fresh interstitial fluid or lymph in the interstitial spaces as depicted in the diagram given below:

2. Regulation excess proteins in the tissue fluid and passing that back to the blood stream.
3. The lymph nodes filter out the bacterial infection and harmful substances from the lymph before pouring it back into the blood stream.
4. Lymphatic vessels in the abdominal organs assist in the absorption of digested fat.
5. Lymph nodes also produce fresh lymphocytes for the circulation.

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.

Nephritic Syndrome and Urine Examination

Examination of fresh urine as part of routine physical examination may show the presence of protein, blood or pus cells in the patients affected by nephritic syndrome. In many cases the renal disease is occult and is often detected during the routine physical examination. It is important to understand that acute nephritis, nephrotic syndrome and renal failure may occur either as a result of intrinsic disease of the kidneys or in association with a systemic disease. Metabolic and functional disturbances such as hypertension, uremia (elevated level of urea in blood), or anemia may cause occult renal disease.

Hypertension along with mild edema (swelling), apparently as suborbital puffy eyes is characteristic clinical feature of nephritic syndrome. Hematuria (blood in urine) with or without proteinuria (protein in urine), oliguria (low urinary output) and impaired excretory function are other characteristic features. The urine may be red or brownish to smoky brown in color in patients affected by nephritic syndrome. Microscopic examination of fresh urinary sediment in these patients may reveal very high count of dysmorphic red blood cells (dysmorphic RBCs). The detection of RBCs in the urinary casts signifies that the hematuria comes from the kidneys (renal parenchyma). Proteinuria may vary from 0.3g to 3.0g daily. Urinary sodium level (Na⁺ level) tends to be low due to sodium retention as a result of impaired excretory function by the kidneys. Retention of sodium and water in these patients lead to increased circulatory blood volume and cause hypertension. The uncontrolled hypertension may lead to cardiomegaly along with mild renal pain. Patients affected by nephritic syndrome need renal biopsy examination and specialized treatment under the supervision of a nephrologist.

Diagnosis and Type of Kidney Disease – Investigations and interpretations

Correlation of clinical and laboratory features is must for an accurate diagnosis and type of a kidney disease (renal disease) or glomerulonephritis. An experienced nephrologist can make a diagnosis of glomerulonephritis from thorough history, physical examination, urine examination and microscopy of urinary sediment. The assessment of presenting features of the patient, such as nephritic or nephrotic syndrome is important. However, the decision on the type of glomerulonephritis can not be based on the clinical and laboratory features; as the nephrotic syndrome may occur with any histological glomerulonephritis, and nephritic syndrome is the outcome of proliferative glomerulonephritis. So the ultimate diagnostic tool is renal biopsy and its light and fluorescent microscopy as well as ultrastructural study by electron microscope.

The interpretation of clinical features in the light of histological diagnosis of renal biopsy helps the clinician to detect any systemic disease associated with the renal disease (kidney disease). Majority of the patients with suspected glomerulonephritis need renal biopsy evaluation. However, in children with nephrotic syndrome; if there is no microscopic hematuria (blood in urine) and red cells' or granular casts, renal biopsy procedure may be avoided initially. In patients, who do not respond to steroid therapy; renal biopsy investigation is must. There are around one million glomeruli (1x10⁶ glomeruli) in each kidney and at least 5 glomeruli should be included in the renal biopsy evaluated histologically to achieve a diagnosis of glomerulonephritis.

Radiological and laboratory investigations in glomerulonephritis:

The clinical presentation, urine-analysis and microscopy findings, and presence of a normal upper & lower urinary tract on intravenous pyelography (IVP: a radiological investigation) or ultrasonography without any renal scarring could be indicative of glomerulonephritis, but there could be a need for renal biopsy.

Immune system associated investigations:

Our body is equipped with a multitasking immune system composed on lymphocytes, antibodies and complement system. The immune system always defends our body internally against a variety of infections and pathological conditions; and assessment of its components and abnormal products produced by it helps in diagnostic conclusions. Complement system of our body is composed of 9-components and boosts the body defense in association with cellular components. The blood level of complement components C3, C4 and C1q may be reduced or normal in some renal diseases. Low total serum complement, C3, C4 and C1q levels are observed in glomerulonephritis associated with circulatory immune complex disorders like systemic-lupus erythematosis (SLE), bacterial endocarditis and serum sickness. Normal levels of C4 and C1q but decreased level of C3 is generally observed in membranoproliferative glomerulonephritis (MPGN) and dense deposit disease of the kidney.

Following investigations are considered important to ascertain the diagnosis and type of glomerulonephritis:

Investigations for likely diagnosis of glomerulonephritis:

• Clinical presentation
• Urine analysis (proteinuria, hematuria and electrophoresis)
• Microscopy of urinary sediment
• Intravenous pyelography (IVP: Radiological investigation)
• Abdominal ultrasonography.

Investigations for likely type of glomerulonephritis:

• Estimation of serum complement components' level
• Detection of circulating immune complexes
• Detection of auto-antibodies such as anti-nuclear antibodies (ANA), anti-DNA antibodies and anti-glomerular basement membrane antibodies (anti-GBM antibodies)
• Renal biopsy

Investigations for assessing the implications of glomerulonephritis and monitoring the effect of therapy:

• Determination of 24 hour urinary protein
• Determination of level of serum proteins
• Determination of serum cholesterol and/or lipid profile
• Determination of serum creatinine, blood urea and serum electrolytes.

Diabetic Renal Disease

Diabetes is a multidisciplinary disease, as many systems may need medical care. Optimal control of blood glucose level is essential to prevent diabetic complications like neuropathy, diabetic renal disease (diabetic nephropathy) and diabetic disease of eyes (diabetic retinopathy). There may also vascular and cardiac complications associated with diabetes in some patients. Diabetes may be insulin dependent (type-I) or non-insulin dependent diabetes mellitus (NIDDM or type-II diabetes). Diabetic patients develop progressive thickening of glomerular basement membrane (GBM) of glomerular capillaries along with widening of mesangium in majority of glomeruli of their kidneys. The unusual thickening of GBM and widening of mesangial area of the bundles of glomerular capillaries lead to narrowing down of functional lumen of these capillaries, there by causing pathophysiological change in the glomerular function affecting the glomerular filtration rate (GFR). Ultrastructural features of glomerulus affected by diabetes have been illustrated in Figure-1b below in comparison to normal features depicted in Figure-1a at the same magnification.

Figure-1a: Electron micrograph of a portion of the tuft of a normal glomerulus depicting normal GBM: glomerular basement membrane, Mes: mesangial area, EpC: epithelial cells or podocytes, CL: capillary lumen and US: urinary space.

Figure-1b: Electron micrograph of a portion of the tuft of a glomerulus affected by diabetes, depicting thickened GBM: glomerular basement membrane, Mes: mesangial area (widened), EpC: epithelial cell or podocyte, CL: capillary lumen (narrowed down) and US: urinary space. Note: Just compare the feature with the electron micrograph shown in figure-1a.

Uncontrolled diabetes may lead to global sclerosis of glomeruli resulting in 'end stage renal disease' (ESRD) or renal failure. Retinopathy, neuropathy and vascular and/or cardiac disease accompanying ESRD may complicate the management of prospective patients. So, diabetic patients are advised to comply sincerely with the advice of general physician or diabetologist to avoid diabetes associated complications, otherwise they may require the consultation of a nephrologist, ophthalmologist and cardiologist to manage the complications. The treatment of diabetes associated renal disease should ideally be introduced when 'traces of albumin in urine' (microalbuminuria) and polyuria (increased urine output) are detected in diabetic patients. Optimal control of diabetes by insulin and/or diet and exercise is must to avoid complications. Once massive proteinuria (excretion of >3.5 g protein per 24 hours) is developed in diabetic patients, the cost of reversal of complications may be many times higher. Just be health conscious and stay healthy & live-long.