Mesangial cells are another type of cell, found between capillary tufts of the glomerulus. Mesangial cells are modified pericytes (flat cells occasionally found embracing capillary endothelial cells). Mesangial cells can contract and relax to modify glomerular filtration, they help in the production and breakdown of the basement membrane, unclog the membrane of unfiltered particles, and proliferate or remodel the basement membrane in response to injury. Mesangial cells surround themselves with a glycoprotein that stains magenta in PAS.
Filtration barrier: Afferent arterioles bring blood in, efferent out. The hydrostatic pressure of glomerular capillaries is higher than for most other capillaries. This enhances filtration. Principal filtration barrier is the basement membrane, made both by podocytes and capillary endothelial cells. Basement membrane is the thickest in the body (320-340nm), and has three layers (central electron dense layer –lamina densa – and two less dense layers – lamina rara). The membrane acts like a size and charge barrier. It is negatively charged, so keeps negative stuff in blood.
Substances filtered must pass through three structures: The fenestrations of the endothelium, the glomerular basement membrane, and the slits between pedicels of podocytes.
Glomerular diseases are among the chief kidney pathologies. Chronic glomerular nephritis is one of the most common causes of kidney failure. Different parts of the glomerulus may show damage.
Immune glomerulonephritis – thickening of basement membrane. Attributed to immunologic origin, involving antibody formation. May be result of infection or autoimmunity. Complexes may be desposited on basement membrane, causing damage. Also, circulating molecules may interact with the filtration barrier to create novel antigens and induce antibody formation.
Membranoproliferative glomerulonephritis – thickening of basement membrane and mesangial cell proliferation.
Minimal change glomerular disease – irregular podocyte foot processes, or lack thereof. Most commonly seen in children, characterized by lots of proteinuria.
The renal corpuscle has a vascular pole (arterioles) and a urinary pole (PCT originates). Bowman’s capsule opens in the to the PCT. PCT is the longest tubule in the cortex and most frequently seen. It has a wide lumen with cuboidal epithelium having a prominent brush border. In PAS, apical regions appear magenta due to glycoproteins of brush border.
PCT reduces volume of filtrate. The apical microvilli increase SA. 80% of water and salt, 100% of glucose and AAs are absorbed, and some acids/bases are secreted. In salt reabsorption, Na enters the cell via channels/cotransporters/exchangers, and it is pumped out of the basal/lateral borders. To provide energy for this, PCT cells have abundant mitochondria (arranged parallel to the long axis of the cell). Endocytosis and lysosomal digestion take care of bigger proteins/particles that leak through the filtration barrier (VERY efficient, so that normal urine has very little protein). PCT cells interdigitate a lot, so much so that they obscure the cells’ lateral borders.
Henle’s loop: U shaped structure with a thick descending, thin descending, thin ascending, and thick ascending limbs. Thick descending limb is much like PCT, whereas thick ascending is much like DCT. Thin limbs are lined with squamous epithelium. Thick limbs are cuboidal. The function of the loop of henle varies depending on the location of the