Kidney histology

Definitions & Key takeaways

The kidneys are a pair of bean-shaped organs located on either side of the spine, below the ribcage. The functions of the kidneys are to remove waste products and excess water from the blood, to regulate blood pressure, and to produce erythropoietin (EPO), which helps to produce red blood cells.

The functional tissue of the kidney, or parenchyma, has an outer renal cortex and inner renal medulla. The medulla is organized into cone-like structures called renal pyramids and renal columns in between the pyramids, which are extensions of the renal cortex. At the junction between the cortex and medulla are millions of functional units called nephrons. Each nephron can be divided into its major parts: The renal corpuscle, proximal convoluted tubule, the loop of Henle, distal convoluted tubule, and collecting duct.

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Introduction0:00–1:30

Kidneys are large, bean-shaped organs that are approximately 12 cm long, 6 cm wide, and 3 cm thick in adults. The kidneys perform a lot of different functions, such as the excretion and removal of metabolic waste and foreign substances through urine.
The kidneys also activate vitamin D when needed, and help maintain the balance of fluid volume, pH, blood pressure, and electrolytes in the body.
The kidneys also secrete important hormones, such as erythropoietin, which increases the production of red blood cells. The medial border of each kidney has a concave area called the hilum.
This is where the ureter exits the kidney and renal artery, renal vein, and lymph vessels enter and exit the kidney. The functional tissue of the kidney, or parenchyma, has an outer renal cortex and inner renal medulla.
The medulla is organized into cone-like structures called renal pyramids and renal columns in between the pyramids, which are extensions of the renal cortex.
At the junction between the cortex and medulla are millions of functional units called nephrons. Each nephron can be divided into its major parts: The renal corpuscle, proximal convoluted tubule, loop of henle, distal convoluted tubule, and collecting duct.Each nephron starts with a renal corpuscle, which is a spherical structure in the cortex that has a diameter of about 200 um.

Gross anatomy1:30–3:27

The corpuscle consists of the glomerulus as well as the surrounding double-layered epithelial capsule called the glomerular or Bowman’s capsule.
The glomerulus is a bundle or tuft of capillaries that supplies the blood that’s filtered to become a fluid called the glomerular filtrate or ultrafiltrate, which typically contains no blood cells or large proteins.
The ultrafiltrate initially drains into the capsular or Bowman’s space, which is actually the space in between the two layers that make up Bowman’s capsule.
The outer layer of the capsule is a parietal layer of flat simple squamous epithelium; and the inner layer is a visceral layer of complex epithelial cells called podocytes.
The podocytes have prominent large oval nuclei and cell bodies that wrap around each of the capillaries. The podocytes form narrow slits between the cells that filter the blood to form the ultrafiltrate.
The glomerulus also has large cells present called mesangial cells that have irregularly shaped nuclei and can be difficult to distinguish from podocytes in a standard slide stained with hematoxylin and eosin; but they’ll often have nuclei that look darker than podocyte nuclei in the image.
The mesangial cells have contractile properties to help regulate the amount of blood flowing to the glomerular capillaries.
They’re also involved in immune defense and repair of the glomerulus.In some images of the renal corpuscle, you’ll also be able to see the vascular pole, which is where the afferent and efferent arterioles enter and exit the glomerulus in order to supply and drain the glomerular capillaries.
The urinary pole is where the ultrafiltrate is drained from the capsular space into the next part of the nephron, the proximal convoluted tubule.Typically, the proximal convoluted tubule, or PCT for short, can only be seen in cross-sections because of the winding or “convoluted” path it takes through the cortex.

Proximal convoluted tubule3:27–4:23

This part of the nephron reabsorbs many of the substances the body wants to keep such as organic nutrients, proteins, as well as most of the water and electrolytes in the ultrafiltrate.
The PCT also secretes anions and cations, such as hydrogen and ammonium into the lumen of the tubule for excretion. The walls of this tubule consist of simple cuboidal or columnar epithelial cells with long microvilli.
The cytoplasm in each cell is more eosinophilic than the cells that make up the distal convoluted tubules and collecting ducts, which is why the PCT cells will typically stain a slightly darker pink.
Also, their lumen will often form a star-like or “stellate” appearance. Next is the loop of Henle, which can be split into three parts, the proximal straight tubule or thick descending limb; the thin descending and ascending limbs; and the distal straight tubule or thick ascending limb.

Loop of Henle4:23–6:03

The thick descending limb starts in the renal cortex and moves down into the renal medulla, which is where this cross section and longitudinal section of the tubule were taken from.
The cells of this portion of the loop of henle have very similar morphology to the previous section of the nephron, with simple cuboidal or columnar epithelial cells that have long microvilli and a dark pink or eosinophilic cytoplasm.The thin descending and ascending limbs of Henle’s Loop consist of a simple squamous epithelium and passively reabsorbs water.
The wall of the tubule is noticeably thinner compared to the thick descending limb, which is seen in the bottom right of this image.
The thick ascending limb starts in the medulla and ascends into the cortex. This tubule is responsible for reabsorbing electrolytes such as sodium, potassium, and chloride.
In both this cross section and longitudinal section of the tubule, we can see that the walls consist of simple cuboidal epithelial cells.
These cells commonly have unclear or indistinct borders between the cells. In the longitudinal section, there are neighboring tubules from the thick descending limb that can be differentiated based on the thick ascending limb’s more neutral staining, meaning it will have a noticeably lighter stain or color to the cells.At the junction between the thick ascending limb and the distal convoluted tubule is a portion of the tubule with tightly packed cells that face the arterioles entering the glomerulus.

Macula densa6:03–6:39

This area is called the macula densa. The cells here often have prominent nuclei and taller cell bodies.
The cells are able to detect changes in sodium chloride levels within the tubule. This allows them to trigger an autoregulatory response when needed in order to help maintain normal blood pressure and blood volume by increasing or decreasing the amount of water and ions the kidney reabsorbs.Similar to the proximal convoluted tubule, the distal convoluted tubule typically can only be seen as cross sections because of their convoluted path through the cortex.

Distal convoluted tubule6:39–7:06

The main differentiating features of the distal convoluted tubule include its lighter stain and shorter microvilli when compared to the proximal convoluted tubule.
This portion of the nephron will reabsorb sodium and chloride, which makes the urine hypotonic.The collecting duct is the final portion of the nephron that drains urine from the nephron into a minor calyx at the apex of a renal pyramid.

Collecting duct7:06–7:39

The walls of the collecting duct consist of simple cuboidal and columnar epithelial cells that have well defined borders and are also lightly stained.
The collecting duct is the last part of the nephron that’s able to actively help regulate the body’s electrolyte and fluid balance.
It does this by reabsorbing water from the filtrate as needed; and secreting or absorbing various electrolytes as well.