بسن هلال الرحون الرحين

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2 بسن هلال الرحون الرحين Today we will continue talking about the histology of the kidney, but before we start we will go over a quick review of what we took yesterday which s about the different histological divisions of the structures within the kidney and we said that when we look under the microscope we can distinguish two embryologically different structures : 1) the nephron (which's a greek word that means kidney) 2) the collecting tubules *the nephron having the uriniferous tubule as a functional unit is the main part of the functional kidney. It ( the nephron) has two parts : 1) a small tiny body called the renal corpuscle which s located only in the cortex So if you see in (fig.1) the border between the medulla and the cortex the renal corpuscle is within the cortex The renal corpuscle is further subdivided into : A) the blood capillaries inside called the glomerulus ( the wool tuft الصوف (كرة it s actually condensed blood capillaries *down in (fig.1) the red part is the glomerulus and it s surrounded by a capsule (bowman s capsule) with a cavity inside it. B) the second part of the corpuscle is the bowman s capsule ( named after Sir William bowamn s a British physician who described this capsule) with a cavity inside this capsule And its further subdivided into: I) a visceral layer enclosing the blood capillaries and its made of special cells called podocytes ( podo:feet, cytes:cells, the cells with feet). II) an outer parietal layer made of simple squamous epithelium. Now, between the two layers there's the bowman s space which s very important, because it contains the filtrated blood which has ions and the plasma fluid. 2 P a g e

3 2) getting out of the corpuscle are the renal tubules and they're divided according to their location with respect to the corpuscle, the ones which are continues with the bowman s space are called the proximal tubules, the most proximal part is highly coiled thus called the convoluted part of the proximal tubules, then as they become closer to the medulla they straighten up so we call them the recta part of the proximal tubules Again>> pars convoluta which s the convoluted part and pars recta which s the straight part The proximal tubules are lined with simple cuboidal epithelium, But once they enter the medulla they become lined with simple squamous epithelium and they suddenly become very narrow their diameter decreases from 60 µm down to 20 µm and we will call them the loop of Henle because once they enter they re actually descending then they will loop back and ascend thus we called them a loop As they ascend they widen again and we will call them distal tubules starting with a straight part ( pars recta distalis) and continuing as the distal convoluted part, here s the end of the nephron, and it will be draining in the collecting tubules. Quick review: the nephron is made up of : 1) renal corpuscle with its parts the glomerulus and bowman s capsule 2) and renal tubules with its parts proximal, the loop of Henle and the distal tubules 3 P a g e

4 4 P a g e Figure 1 Now, the collecting tubules have three parts which are : a) cortical collecting ducts that are located in the cortex b) medullary collecting ducts c) papillary duct of bellini (named after Lorenzo Bellini an Italian anatomist who was a prof. of medicine at pisa university) which s a very large duct ( µm) made up of the union of all the medullary collecting ducts and its located in the inner part of the medulla close to the papilla. Notice that its called papillary duct since its close to the renal papilla. In the previous lecture we talked about the renal corpuscle and the mechanism of filtration and we said there s a specific kind of filter made up of three layers the fenestrae in the endothelium of the blood capillary,basal lamina which s made up of lamina densa and 2 lamina rara and the filtration slits (or clefts) between the pedicles of the podocytes. Today we will be talking about the renal tubules starting with the proximal tubules. Proximal tubules In a cross-section across them (figure 2) we can see a wide opening with empty lumen around 60 m in diameter that is surrounded by simple cuboidal epithelium.

5 They re 14 mm long. Figure 2 The histological distinguishing features of these tubules are : A)**the Eosinophilic cytoplasm which s due to their richness in mitochondria. They need the mitochondria to produce energy for the active transport of ions as the proximal tubules are responsible for 80% of the resorption of Na and Cl ions from the urine and returning them back into the blood circulation within the kidney interstitium. B)**Striated apical surface (apical microvilli or brush boarder) to increase the surface area (figures 3 and 4) in addition to them we can see a network of canaliculi in the Electron micrograph below (fig.4). Figure 3 5 P a g e

6 Figure 4 Notice in this figure the tubular mitochondria which are larger than the normal mitochondria, and the ion channels (Na channels) which are responsible for active transportation of Na from the lumen to the interstitium are located at the basal membrane of these cells. C) They have well defined basement membrane. D) **Few basally placed nuclei (fig 5), because the cells are wide so the nuclei appear to be few. 6 P a g e

7 Figure 5 E) Indistinct lateral membrane because they re interdigitated with each other. *the main distinguishing features are A, B and D which are again the eosinophillic cytoplasm, the brush boarder and the relatively fewer basal nuclei compared to the distal tubules. As we said the proximal tubules can be structurally divided into two parts: 1)Pars Convoluta (PCT) Near renal corpuscle in the cortex 2) Pars Recta (descending thick limb of Henle s loop) This part extends from the inner cortex until the junction between the outer stripe and the inner stripe of the outer zone of the medulla (fig.6) where it ends and the loop of Henle begins. 7 P a g e

8 8 P a g e Figure 6 The functions of cells in the proximal tubules : I) Resorption of 65-80% of Na +, Cl -, & water 1. Na+ is actively pumped out of the cells 2. Followed by Cl- to maintain electrical neutrality 3. Followed by water to maintain osmotic equilibrium II) Resorption of all glucose & AAs III) Elimination of organic solutes, drugs, & toxins Loop of Henle (Ansa Nephroni) (refer to the box) U-shaped thin (12 20 m) tube consisting of simple sq. epith. Divided into 3 regions - Descending thin limb highly permeable to water

9 - Henle s loop - Ascending thin limb low permeability to water * the length and extension of Henle s loop depends on the type of nephron. Remember we have two types of nephrons : a) cortical nephrons accounting for 80-85% located in the cortical zone of the cortex. named after Friedrich Gustav Henle whose an anatomist, histologist and pathologist in the university of Heidelberg in Germany,he s a very famous person because of his anatomical discoveries upon of which the loop of Henle being the most important,however he also discovered a common tendon between two muscles in the abdomen the transverses abdominus and the internal oblique that joins the pubic bone and he called it Henle s ligament but the name was changed later into conjoint tendon, and he also discovered a small spine in the external auditory meatus at the upper posterior boarder he called it Henle s spine. Henle was the supervisor for a famous student during his PHD his name is Robert Koch, Robert s PHD thesis was about the germ theory of disease, and since Henle was his supervisor he was considered as a co-founder of this theory. In these nephrons the length of the loop of Henle which will be in the outer zone of the medulla is no more than 1-2 mm and even sometimes they re totally absent. (Fig.7) 9 P a g e Figure 7 b) juxtamedullary nephrons accounting for 15-20%

10 The length of the loop of Henle in these nephrons is 9-10 mm it extends until the renal papilla deep in the medulla. This is very important in determining the volume and the concentration of urine because the descending part of the loop of Henle is highly permeable to water however the permeability decreases as you go forward to the ascending part until reaching the distal tubules which are impermeable to water. How to distinguish the loop of Henle in a histological section? As we said they re made up of simple squamous epithelium similar to the cells of the endothelium of the blood capillaries. We know squamous cells are flat so the nuclei will appear bulging into the lumen similar to what we see in capillaries (fig.8). The only distinguishing feature between the blood capillary and the loop of Henle is that the former has blood cells in their lumen (fig 9). Figure 8 10 P a g e

11 Figure 9 Distal tubules As the loop of Henle ascend up again it starts to widen however it will not be as wide as it was in the proximal tubules, it only widens to about µm and it will be called the distal tubule. The distal tubule starts at the border between the inner and outer zones of the medulla. They re narrower and much shorter than the PT. Their convoluted part is much less convoluted than that of the PT and that s why they re shorter. They re divided into 3 parts: -Pars recta (Ascending thick limb of Henle s loop) As this part ascends up to the cortex it ll go to become in close proximity to its own renal corpuscle specifically the vascular ball in the area between the efferent and afferent arterioles that enter and leave the corpuscle (fig.10) and this area is called the Macula Densa. -Macula Densa -Pars convoluta distalis (DCT) 11 P a g e

12 Figure 10 Pars Recta (Ascending Thick Limb of Henle s Loop) Its diameter is 40 µm It starts at the junction between the outer and inner zones of the medulla and ascends up into the cortex. Its impermeable to water and urea, but it has pumps for active transportation of Cl and Na follows passively. Remember in the proximal tubules the Na was actively transported. Now, at the end of the Pars Recta distalis the urine will be hypotonic (low concentration of Cl and Na) but it will have high concentration of urea and water. As it (distal tubules) ascends up in the area between afferent and efferent arteriole we will call this part the Macula Densa (macula= spot, densa= dense) why it is dense? Figure P a g e

13 Looking into a histological section (fig.11) we can see part of the distal tubule, they call it macula densa because the cells are becoming narrow and tall, as they are changing to become taller and narrower the nuclei will become closer to each other giving the dense appearance so when you look under a histological staining you will see that the nuclei are densly packed appearing as a densely stained spot called the macula densa. So the macula densa are the cells of the distal renal tubules located near their own renal corpuscle in the vascular ball trapped between the afferent and efferent arterioles. The cells are tall and narrow so the nuclei become closer to each other. It is part of a system called the juxtaglomerular apparatus (juxta= close to, glomerular= glomerulus).this apparatus is very important because it s involved in monitoring blood pressure through detecting sodium concentration. The last part of the distal tubule is the convoluted part, when they become convoluted they will widen a little bit more (from 40 µm in pars recta to 45 µm in diameter in the convoluted part) however they are much shorter than the proximal convoluted tubules, for every 7 turns of the proximal part there is a single turn of the distal part. 13 P a g e Figure 12 Therefore looking into a cross section (fig.12) you will see so many proximal tubules compared to distal tubules (pale in color and more nuclei). The ratio in a histological section between the proximal and distal tubules profiles is 7:1, so for every 7 proximal tubules there is 1 distal tubule, why is that? Because the distal

14 convoluted tubules are much shorter. Look at the asterisk in the histological section, what are these? this is simple squamous with bulging nuclei they can be either loop of Henle or blood capillaries but if they were blood capillaries they must contain blood cells but those are empty. So they are actually loop of Henle but why they are seen in the cortex? This is to tell you that what you study in medicine it's not always 1+1=2, what we study is the most common scenario so we can always find variations, remember when we spoke about the renal arteries we said that 30% of the population there is extra accessory renal artery therefore in some kidney sections you can see part of the loop of Henle in the cortex, in these cases instead of the pars recta, the loop of Henle will continue until the cortex. There are two main things ultra-structurally to distinguish cells of distal convoluted tubules from the proximal: 1- No brush border (microvilli) 2- Nuclei are located on the apical surface instead of being central or basal as in the proximal part and contain much less mitochondria Figure 13 In the ascending part there are chloride pumps pumping chloride outside and sodium will follow but it s impermeable to water, this is the case in the pars recta. After the macula densa, in the distal convoluted part it s a different story it is only 14 P a g e

15 functional in ion transportation when influenced by aldosterone hormone (a mineralocortcoid secreted by zona glomerulosa of adrenal cortex related to reabsorption of minerals) When aldosterone is secreted by the adrenal cortex it will influence the cuboidal epithelial cells in the distal convoluted tubules to do resorption of the remaining sodium through Na/K pumps in their basal membrane. Why does this happen? This is related to the juxtaglomerular apparatus. As the urine ascends in the pars recta and reaches the macula densa all the chloride and most of the sodium are gone with a little amount of sodium only remaining there, so the macula densa cells are specialized in detecting a specific threshold of sodium. If the concentration of sodium is above this threshold the macula densa cells will be stimulated through their receptors for the concentration of sodium, the cells will send signals through their basal part to the juxtaglomerular cells which are modified smooth muscle cells in the tunica media of afferent and efferent arteriole but mainly in the afferent arteriole. They are modified because they contain specific secretory granules that are rich in an enzyme called renin. When sodium concentration goes above the threshold it will be detected by the macula densa cells which are in a very close intimate contact with the juxtaglomerular cells as there is no basal lamina because we need direct communication between the cells. The basal membrane of macula densa cells are in close contact with the apical membrane of the juxtaglomerular cells, the stimulation in macula densa will stimulate juxtaglomerular cells which will then secrete renin into the circulation through their basal membrane. 15 P a g e

16 Figure 14 So the juxtaglomerular apparatus is made up of macula densa cells in the distal tubule, juxtaglomerular cells in the arterioles and in between them there are extraglomerular mesangial cells also called lacis cells (lacis from the word (رباط= lace because they tie the glomerular cells with the macula densa, they are forming like a lace as you can see in the figure above. They are also called polar cushion cells ( (هخدة because they are forming a cushion for the vascular ball of the glomerulus separating between the afferent and efferent arteriole. The exact function of these cells until now is unknown however some theories and some published articles revealed that they contain secretory granules for renin also and other secretory granules containing erythropoietin hormone which is involved in producing RBCs in the bone marrow. We have two types of mesangial cells 1- Extraglomerular mesangial cells : the ones at the vascular ball 2- Intraglomerular mesangial cells : the ones which are inside within the glomerulas between the blood capillaries Where does the word mesangial come from? Mes from the greek word mesos which means in the middle, angial from the greek word angio which means blood vessels because they are in the middle of blood vessels Intraglomerular are between the blood capillaries, the extraglomerular are between 16 P a g e

17 the afferent and efferent arterioles so both are mesangial cells but the difference is that the extraglomerular cells are part of the juxtaglomerular apparatus while the intraglomerular cells have two functions: 1- They are phagocytic cells involved in the regeneration of the basal lamina A student asked how do the structures pass through the collagen fibers in the basal lamina? The collagen fibers in the basal lamina are not closing the way totally there are spaces and if a large molecule sometimes pass between it can destroy these collagen fibers so we need to repair them and sometimes large proteins like the albumin (more than 69 kilodalton) gets stuck in the basal lamina so we need cells that can phagocytose the large proteins and the broken collagen fibers in addition to regenerating the basal lamina, and that is the function of the intraglomerular cells. 2- They are pericyte-like cells (peri= around, cyte=cells) located around the podocytes and the blood capillaries containing contractile proteins and once they contract they constrict the blood capillaries reducing the blood flow through the glomerulus The juxtaglomerular apparatus works through the RAAS signaling pathway which indicates the renin-angiotensin-aldosterone system. As we said when the urine reaches the macula densa with a higher concentration of sodium (above the threshold) and after being detected by their apical membrane, the basal membrane of the macula densa will stimulate the juxtaglomerular cells which will secrete renin, renin now goes to the blood where there is a protein called angiotensinogen, in blood the angiotensinogen will be broken down into angiotensin-i an inactive decapeptide (10 aminoacids) by the action of renin (that's why the old name of renin was angiotensinase).as angiotensin-i is circulating in the blood it will reach into the endothelial cells in the lungs which will secrete another enzyme called angiotensin-converting enzyme (ACE) that will break two aminoacids from the decapeptide forming an octapeptide called angiotensin-ii which is the active form of angiotensin. Angiotensin-II will go and do two things : 1- Goes to the zona glomerulosa of the adrenal cortex stimulating the cells to secrete aldosterone, aldosterone will go to the distal convoluted tubules and stimulate the cells to resorb sodium through Na/K pump and chloride will follow. 2- Goes to the smooth muscles in tunica media of blood vessels and stimulate them to contract causing constriction which will increase the speed of blood flow raising the blood pressure. 17 P a g e

18 Collecting tubules are of different embryological origin from the nephrons however they fuse together later on in the development. Collecting tubules are of 3 types : 1- Cortical collecting ducts (medullary rays) 2- Medullary collecting ducts 3- Papillary ducts of bellini (formed by the joining of the medullary collecting ducts) Collecting tubules are impermeable to water however under the influence of antidiuretic hormone (ADH) also called vasopressin (produced by hypothalamus and stored in the posterior pituitary gland) they will become permeable to water and will allow it to flow back instead of being excreted by the urine. Figure 15 Collecting tubules whether they are cortical (shown as rays in the figure) or medullary they are made up of simple cuboidal epithelium. Cortical collecting ducts: Have two types of cuboidal cells 1- Principal (main) cells with few mitochondria and short sparse microvilli, their function is unknown. 2- Intercalated cells containing many microvilli, numerous apical vesicles and abundant mitochondria because they are involved in active transportation of hydrogen ions against high concentration gradient. If the hydrogen concentration was more in the urine it will be transported from inside the cells into the urine and if it was more in the cells it will be resorbed from the urine. So they have contain 18 P a g e

19 two types of active channels out or in depending on the concentration of hydrogen ions that's why they are involved in the regulation of acid-base balance in the body. Medullary collecting ducts : 1- Larger caliber than cortical tubules 2- Located in the outer zone of medulla 3- Contain similar types of cells Figure 16 Figure 17 Papillary Ducts : 1- Formed by union of medullary tubules µm in diameter 3- Formed by tall columnar principal cells only 19 P a g e

20 Figure 18 As these collecting tubules become close to the renal papilla they will enlarge and join together to become the papillary ducts of bellini, moving to the papillary ducts the cells change from simple cuboidal to become simple columnar epithelium, the papillary ducts will open to the minor calyx and the epithelium will become transitional epithelium. In the minor calyx, major calyx, renal pelvis, ureter, and bladder the epithelium is transitional epithelium. Figure P a g e

21 A quick review of the different types of epithelium : *Visceral layer of bowman's capsule is made up of podocytes, parietal layer of simple squamous *Proximal tubules- simple cuboidal *Loop of Henle- simple squamous *Distal tubules- simple cuboidal except in the area of macula densa *Collecting ducts ( cortical and medullary)-simple cuboidal *Papillary ducts- simple columnar *Minor calyx until the bladder- transitional epithelium Remember as we go down the cells become paler,smaller and the nuclei become closer to each other. All slides are included. If you found any mistake don't think of tagging us و سالنتكم Made with love by: Sarat Al-shaba an and Lina Bshara 21 P a g e

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