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What is the capsule of the kidney? What does it consist of?

Capsule – a covering of the entire kidney that is made of dense irregular connective tissue and also myofibroblasts.


What is contained within the cortex?


Cortex- outer region of kidney that contains renal corpuscles and tubules (convoluted, straight and collecting ducts-in medullary rays) and associated vascular structures. 


What is contained within pyramids, renal columns and lobes?

Pyramids are conical regions that contain convoluted tubules and their associated vessels. Renal columns are tissue in the cortex between the pyramids. They contain primarily collecting ducts and their accompanying blood vessels, notably the interlobular arteries. A lobe is the tissue in both the cortex and medulla that is located from the center of one renal column to the center of the next adjacent renal column.


What is contained within the medulla?

What is the function of the calyces and renal pelvis?

Medulla – region between cortex and calyces. Contains straight tubules, collecting ducts and vasa rectae.

Calyces and renal pelvis – Inner region formed of funnel like structures that convey urine from the medulla to the ureter.


What kind of epithelium does Bowman's Capsule consist of?

What kind of epithelial cells does Bowman's capsule contain?

Bowman’s Capsule: A hollow spherical structure formed by simple squamous epithelium indented by the glomerulus (a looping capillary between the afferent and efferent arterioles) Visceral layer which invests the glomerulus is formed of specialized epithelial cells called podocytes. The podocytes and endothelial cells of the glomerulus form the filtration apparatus of the kidney.


What are pedicels? (within podocytes)

What do primary and secondary branches of podocytes form? 

What do filtration slits of podocytes contain? What is their function?

Podocytes – are epithelial cells with extensive branching cytoplasmic processes (pedicels or foot processes) that completely envelop the loops of glomerular capillaries. The scanning electron micrograph below (Figure 20.12 from the Ross and Pawlina text) shows podocytes wrapped around a loop of glomerulus capillary. Primary and secondary branches then form the small pedicels, with fine slits between them. The slits contain a protein complex linked to the actin cytoskeleton of the podocytes which together contribute to the regulation of protein passage into the urine. 


What makes up the glomerular basement membrane?

What are pedicels connected by?

The glomerular basement membrane is an attachment site of the endothelial cells and podocytes and is the principal barrier to protein entering the urine. It contains type IV and XVIII collagens as well as other basement membrane components (laminin, fibronectin, entactin, proteoglycans, etc.). The glycosaminoglycans give a strong negative charge to this membrane which blocks movement of proteins of about 70 kDa and larger. Changes in the composition of this basement membrane, such as in diabetes, can radically alter the composition of the fluid passing through this membrane.

pedicels are connected by nephrin – abnormality is basis for nephrotic syndrome


Where are mesangial cells found?

What functions do they perform?

Mesangial Cells are found within Bowman’s capsule and also on the periphery of the capsule forming the juxtaglomerular cells of the JG apparatus (which will be addressed below). Within Bowman’s capsule they endocytose immune complexes and plasma proteins which maintains normal function of the glomerular basement membrane filtration function. They increase in number and secrete growth factors, interleukins and prostaglandins and play a role in glomerular inflammation in a variety of disease states.


What kind of epithelium is the proximal convoluted tubule (PCT) composed of?

What are the differentiating features of the PCT?

Why do they contain a lof of mitochondria?

What molecules are primarily reabsorbed at the PCT?

The proximal convoluted tubule is composed of simple cuboidal epithelial cells that have many microvilli on their apical surface and extensive folding of their basal surface plasma membrane. They are also characterized by a large number of mitochondria. Both of these latter mentioned features are to provide the energy and surface area for the extensive ion movement that occurs across these cells (see the EM on Blackboard). Primary site of reabsorption of sodium, glucose, amino acids, and bicarbonate. 


What kind of epithelium does the thick ascending and descending loops of Henle contain? What are distinguishing features?

What kind of epithelium do thin ascending and descending loops of Henle contain?

What is the function of Loops of Henle?

a. Thick Descending – simple cuboidal epithelium with reduced microvilli and basal surface specializations than proximal tubules

b. Thin Descending and Ascending – simple squamous epithelium that varies depending on the type of nephron (cortical vs juxtamedullary)

c. Thick Ascending – simple cuboidal, similar in appearance to thick descending

Loops of Henle function in the regulation of osmolarity of urine. 


How does the distal convoluted tubule (DCT) compare to the PCT? What are the features of the DCT?

The distal convoluted tubule is about one third the length of the proximal tubule. It is also made of a simple cuboidal epithelium with microvilli much reduced in size and number as compared to the proximal tubule. Mitochondria and basal membrane folding are still numerous to serve the functions of sodium, bicarbonate and ammonium transport.

The lumens of DCTs are smaller than PCTs.


What does increased fluid flow stimulate secretion of from macula densa cells?

What does decreased fluid flow stimulate secretion of from granular cells?

What pole of the Bowman's capsule (vascular or urinary) is the JGA (juxtaglomerular apparatus) adjacent to?

Cells regulate renal function in two ways. Increase fluid flow (via detection of NaCl) triggers a paracrine stimulation of afferent arteriole constriction. Decreased flow triggers stimulation of renin secretion by the granular cells which will result in increased blood pressure.

The JGA is adjacent to the vascular pole. 


What kind of epithelium are collecting ducts composed of? How does this epithelium change? 

What are collecting ducts condensed in the cortex called?

What do collecting ducts turn into as they become larger?

What is the function of epithelial cells of collecting ducts?

Collecting Ducts – the distal convoluted tubules empty into collecting ducts. They are formed of a simple cuboidal epithelium, small microvilli and a primary cilium. The epithelial becomes columnar as the ducts become larger. Ducts are clustered together in the cortex in regions termed medullary rays and continue from this region to run down into the medulla, merging together and becoming larger as they end at the tips of the pyramids, the papilla, at which point they are referred to as papillary ducts. As with other kidney tubules, the epithelial cells are active in ion movement to adjust the concentration and pH of urine.


Explain the flow of renal blood starting with the renal artery.

What are the two options for the branching of efferent arterioles?

The blood supply of the kidney begins at the renal artery. The renal artery branches when entering the renal pelvis into interlobar arteries which pass between the calyxes of the pelvis. The interlobar arteries reach the corticomedullary border and branch sideways forming arcuate arteries which run parallel to this border. Smaller interlobular arteries project from the arcuate arteries into the cortex. These vessels form the borders of kidney lobules. The blood supply to the glomerulus is a branch of the interlobular artery called the afferent arteriole. Blood leaving the glomerulus does so through the efferent arteriole. The blood supply into and out of the glomerulus is tightly regulated since the glomerular filtration rate (how much fluid leaves the glomerulus capillary into the lumen of Bowman’s capsule) will vary with this blood flow. The efferent arterioles then do one of two things. They either feed a capillary bed throughout the cortex that surrounds the convoluted tubules (peritubular capillaries), or if they are coming from the glomerulus of nephrons near the corticomeullary border the feed the vasa rectae. These are long looping capillaries that run in parallel with the Loops of Henle in the medulla. As you will hear they play an important role in fluid movement in the medulla to adjust the composition of urine. The venous drainage flows via capillaries to interlobular, arcuate and cortical veins called stellate veins.


What layers do ureters consist of? What kind of epithelium do they contain?

Urine collects through the minor and major calyxes which form a funnel like arrangement emptying into a single ureter. Ureters are muscular tubes lined by transitional epithelium. Transitional epithelium is unique to the urinary system (renal pelvis, ureters, bladder). Similar to other tubular organs, the epithelium has an underlying lamina propria and then layers (2-3) of smooth muscle. This is covered by a serosa for most of the length of the ureters. 

Has a characteristically star-shaped lumen. Outer circular-inner longitudinal musclularis.  


What structure is the bladder located posteriorly to?

Describe the layers of the bladder. 

What are the effects of parasympathetic innervation of the gallbladder? 

The bladder is a largely muscular organ directly posterior to the pubic symphysis that is highly distensible for the storage of urine. The outer surface that projects into the pelvic cavity is covered by a serosa while other portions are covered by a connective tissue adventitia. The muscularis of the bladder wall consist of three smooth muscle layers that converge toward the neck of the bladder. These constitute the detrusor muscle. These fibers are intermingled but are roughly inner and outer longitudinal and middle circular. Parasympathetic innervation, which is activated by stretch receptors in the bladder, stimulates contraction of the detrusor muscle to cause voiding of the bladder. This is done in concert with CNS regulation of the sphincter muscles which may or may not override the contraction of detrusor muscle. The mucosa of the bladder is folded in its relaxed state. The lining of the bladder, like the ureter, is transitional epithelium. This is a stratified epithelium whose cells appeared dome shaped in a contracted state. As the bladder stretches, the cells are able to greatly increase their surface area by unfolding extensive invaginations of the plasma membrane, a bit like an accordion opening up. In the distended state, the epithelium appears squamous and only several cells in thickness. 


What is similar btwn the male and female urethra?

Explain the epithelium transitions of the female urethra. Where does the urethra terminate in the female?

What are 2 important histological feature of the male urethra?

The single urethra leaving the inferior portion of the urinary bladder differs in the male and female. In both cases upon leaving the bladder there is a short segment that is lined by transitional epithelium. In the female, the urethra is a short tube terminating in the vestibule superior to the vagina. The epithelium goes through a change from transitional epithelium, usually to columnar or cuboidal to stratified squamous as it approaches the urethral orifice. In the male, the urethra serves as a duct for both the urinary and reproductive systems. The various portions of the urethra will be discussed in the context of the male reproductive organs.

The male urethra contains corpus spongiosum and glands of Littre. Corpus spongiosum is a column of erectile tissue consisting of venous sinuses that surround the urethra. Glands of Littre lubricate the mucosal surface.