29. The anatomy, histology and development of the kidney.

* Kidney is one of the 2 primary retroperitoneal organs, and is not part of the primary gut.

Anatomy of Kidney

Location: In middle compartment of retroperitoneal space, within the renal fascia, along with suprarenal gland. Located in paravertebral gutter against psoas major m.

SP – B/w T123 – L3/4, beside IVC/ab aorta B/c R lobe of liver is larger than the left, the R kidney is situated slightly lower than the left. The left kidney projects at the 11-12rib, and R is the 12th rib only.

Topography:

• R Kidney – (ant) – Liver, gallbladder, duodenum, R suprarenal gland, R colic flexure, asc colon, loops of SI

• L Kidney – (ant) suprarenal gland, stomach, spleen, pancreas, L colic flexure, SI, desc colon

• Both Kidneys – (post) – (sup) diaphragm, (inf, from lat to med) transverse m aponeurosis, quadratus lumborum m, and psoas major m.

Fxn – prod and excrete urine, filter waste from blood, and maintain electrolyte balance, and regulate BP by producing vasoactive substances.

Parts:

External structures:

Kidney has 3 coverings:

Renal fascia – that covers both kidneys like a huge tent like structures, separates the retroperitoneal cavity into 3 compartments.

This renal fascia also divides the fat that surrounds the kidney into two layers =

• inside the renal fascia, is perirenal fat, between the capsule of the kidney fascia.

• Outside the renal fascia is the pararenal fat. (Peri means around it , like perimeter, and Para means next to, or almost in but not quite, like paranormal, or paraplegic)

Each kidney then as a separate adipose layer, within the fascia is perirenal fat.

Lastly, each kidney has a fibrous capsule, called renal capsule. This capsule, unlike in other organs, does not send CT septa into the kidney and is therefore removable easily.

Renal sinus – is the inner cavity of the kidney (not part of parenchyme, the tissue stuff, just an empty space), which leads the hilum of the kidney. Structures that enter/ leave here = renal a/v, minor calices, major calices, renal pelvis, lymph vessels, renal fat, ANS fibers

Renal hilum – entrance into the renal sinus. Ant–> post is located the renal v, a, then the ureter = VAU

Internal Structure:

Within each kidney, there is a an outer cortex, and inner medulla. Structures within the cortex and medulla will be discussed in further detail in the histology section.

Cortex: The cortex is the outer part of the kidney and projects into the inner medulla region, between the renal pyramids, as renal columns.

Medulla: The medulla is arranged into triangle shaped renal pyramids, that end in a renal papilla, which then lead into the minor calyx (each minor calyx drains one lobe of the kidney)

• 3-4 minor calices join to form the major calices.

• The major calices all join to form the renal pelvis –> that becomes the ureter.

• The function unit of the Kidney is called the nephron. It is made up of a renal corpuscle ( made up of capillaries surrounded by the Bowman’s capsule, which has visceral and parietal layers. The renal corpuscle leads into a series of tubules surrounded by a arterial network, that aids in filtering urine.

Urine flow: From the urinary pole of the renal corpuscle –> prox convoluted tubule –> prox straight tubule –> desc limb, or desc part of thin segment–> Loop of Henle –> asc limb, or asc part of thin segment –> distal straight tubule –> distal convoluted tubule –> collecting duct –> papillary duct (at base of renal papilla) –> minor calyx –> major calyx –> renal pelvis –> ureter –> urinary bladder –> urethra

kidney 1: A longitudinal section, B nephron and adjacent blood vessels; 1 renal papilla, 2 renal column, 3 capsule, 4 renal pyramid, 5 calyx, 6 ureter, 7 renal pelvis, 8 renal vein, 9 renal artery, 10 interlobar artery, 11 arcuate artery, 12 interlobular artery, 13 interlobar vein, 14 cortex, 15 interlobular vein, 16 renal sinus, 17 arcuate vein, 18 medulla, 19 vasa recta, 20 loop of Henle, 21 collecting duct, 22 arcuate vein, 23 arcuate artery, 24 proximal convoluted tubule, 25 glomerulus, 26 Bowman’s capsule, 27 distal convoluted tubule

Blood Supply

• Arteries: Renal a supplies the kidney, located @ L1 of ab aorta, the 2nd paired visceral branch from the aorta

  • Renal a splits into 5 segmental branches –> each segmental br has ant/post br –> interlobar arteries, running within the renal columns –> splits into 2 arcuate arteries, which arch over the base of the renal pyramids –> interlobular a, within the cortical labyrinth between the medullary rays (will talk about these structures in histo, dont worry about them now) –> have arteriolar system that goes to the renal corpuscle and tubular system –> interlobular v –> rest of the v follow the arteries mentioned prev.

  • Renal a also gives inf supra renal a, and possibly branches to ureter.

• Veins: Veins within kidney, follow arteries, until getting to renal v –> IVC R renal v is a little lower, and much shorter than L renal v, b/ c IVC is located on the right side of vert. column.

Lymph Drainage : Lymph vessels in the kidneys follow the arteries –> renal sinus –> hilum –> para-aortic and common iliac l.n. –> lumbar trunks –> cisterna chyli

Renal innervation: N. of kidney from the renal n plexus = PNS/SNS fibers, from ab/pelvic splanchnic n.

• PNS = @ origin of renal a, is the location of the aortico-renal plexus, with pre-ggl fibers from all 3 splanchnic n (Greater, Lesser, Least) –> post ggl fibers –> organ

• SNS = From T12 – L2 run through sympathetic trunk to least splanchnic n and lumbar splanchnic n, comes the pre ggl fibers –> renal ggl (not the same as aortico renal plexus) –> post ggl fibers –> kidney/ ureters

• Most supply renal a/v and branches, to decrease urine production

• PARS – PNS = Aortico-renal, Renal = SNS

Ureters

• Pathway of Ureters = from renal hilum (lies most post. ) –> runs in retro peritoneal space, ant to psoas m, and crosses the bifurcation of the common iliac a, and rec. blood from those arteries.

• Topography of ureter

  • R ureter – behind desc part of duod, post to root of mesentery, post to gonadal a/v

  • L ureter – post to left colic a/v, next to gonadal vessels, post to sigmoid a and sup rectal a (in sigmoid mesocolon)

  • Both = pass behind Ductus deferens (or cardinal ligaments/ uterine a/v in females), and enter bladder on the lower/post side.

• Blood supply = branches from various a it passes

• Innervation = W/in renal pelvis = least splanchnic n from T 12 Abdomen/pelvis = lumbar splanchnic n (SNS), Pelvic splanchnic n give PNS innervation along the entire ureter.

Suprarenal glands:

• Location: on top of each kidney, medially, below diaphragm, is pyramid shaped on R side, and semilunar shaped on L side, also covered by renal fascia, and has a fibrous capsule.

• Topography:

  • R = r lobe of liver (bare area), R kidney

  • L = Lesser sac, pancreas, L kidney

• Parts:

  • Cortex –> prod steroid hormones, including mineralocorticoids (aldosterone), glucocorticoids (cortisone), and sex hormones

  • Medulla –> derived from embryonic neural crest cells, rec pre-ggl SNS fibers directly and secrets epinephrine and norepinephrine.

• Blood Supply:

  • Arteries:

    • a) Sup suprarenal a – from inf phrenic a (ab aorta)

    • b) Mid suprarenal a – from aorta directly

    • c) Inf suprarenal a – from renal a

  • Veins

    • R suprarenal v –> IVC

    • L suprarenal v –> L renal v –> IVC

Histology = Kidney Slide # 60 * H&E

Structures to Identify:

• renal corpuscles

• renal column

• renal pyramids

• cortical labyrinth

• medullary rays

• Cortex

• Medulla

• Cortex corticis

• a/v

• renal corpuscles

• tubular system

• urinary pole

• vascular pole

• Juxta glomerular apparatus = macula densa, JG cells, (dont have to identify mesengial cells)

General Info:

• Main organ of urinary system

• conserve body fluids and electrolytes

• remove metabolic wastes like urea, uric acid, and other debris

• produce urine as ultrafiltrate of blodd that is modified by selective resorption /secretion by kidney tubule cells

• 10cm long, bean shaped, between T12-L3

• about 20-30 renal pyramids

Nephron is fxn’l unit of the kidney . There are two types of nephron, classified depending on their location within the kidney.
1. Cortical nephron – located anywhere in the cortex, long thick segments, short thin segments
2. Justamedullary nephron – located at the cortio-medullary border, near base of pyramid, where arcuate arteries are.  have short thick segments and long thin segments that go almost to the bottom of the renal pyramid. have more hypotonic enviroment, to produce a concentrated urine.

* Look the same with in slide.

Parts of nephron, in order of flow :
Renal corpuscles –> proximal convoluted tubule, proximal straight tubule, descending limb, loop of Henle, ascending limb, distal straight tubule, distal convoluted tubule.

Cortex

Cortex characterized by renal corpuscles and their tubules including convoluted and straight parts of nephron, the collecting tubules and extensive vascular network.

Renal corpuscle

• Renal corpuscle consists of glomerular capillary bed surrounded by double layer of epith cap (Bowman’s capsule),

• space between glomerulus and capsule is called capsular or urinary space.

• Each capsule has afferent arteriole and efferent arteriole, and has two layers: a inner visceral layer, and outer parietal layer.

• The visceral layer of the capsule surrounds the capillaries of the glomerulus with modified epithelial cells called podocytes.

• There are two poles to the corpuscles = urinary pole, where there tubular system starts from, and the vascular pole, where the arterioles enter/exit

Blood enters corpuscles through specialize mechanism of corpuscle:  Filtration Apparatus, which is located in the area between the podocyte fingers and the endothelium of the capillaries.

Layers in barrier b/w capillaries and epith lining of Bowman’s capsule:

1. Glomerular Basement membrane

2. Podocytes lining capillaries assist GBM in further filtration via unique structures containing finger like structures called pedicles that interweave between each other.

3. The Slit Membrane

• The basement membrane of the endothelium of capillaries and the basement membrane of the podocytes fuse,

• each basement membrane has two layers: an outer fibrous membrane and an inner basal lamina.

• the two fibrous membranes touch each other and fuse, forming one single layer.

• Therefore, the filtration barrier between the capillaries and the podocytes is made up of the following layers:

  • 1. The endothelium of the a/v

  • 2. the glomerular basement membrane = made of three layers (GBM)

    • a) Lamina rara interna = the basal lamina of endothelium

    • b) Lamina densa = the fused 2 fibrous layers of the basement membranes of both endothelium of capillaries and podocytes

    • c) Lamina rara externa = the basal lamina of the podocyte

  • 3. The Slit membrane = CT covering the spaces between podocytes.

• So picture it like this: Put your hands up together, and put your fingers between each other, but without clasping them together or weaving your fingers together. Picture your hands holding a giant messed up ball of yarn and then imagine cling film over your hands, covering the spaces between your fingers. The yarn is the glomerular capillary bed, your hands is the Bowman’s capsule (the palm being the visceral layer and outside skin being the parietal layer), and your fingers are the podocytes interdigitating, with the cling film between your “podocytes” is the slip membrane, covering the spaces between the podocytes.

• Filtration happens in the cling film area, that is to say, that it only occurs between the podocytes, where the slit membrane is located

With in the cortex, one can also the see the next part of the nephron, the proximal convoluted tubule, and its counterpart on the way back from the loop of Henle, the distal convoluted tubule.

You have to be able to show both of them

Prox  convoluted tubule

• simple cuboidal epith

• v. visible BM

• irregular lumen

• border between cells indistinct

• acidophillic cytoplasm = lots of mitochondria

• brush border present

• basal striations

Distal convoluted tubule

• less in # than proximal in cortex

• cytoplasm lighter

• cell boundaries seen

• very smooth wide lumen

• cuboidal cells still

• brush border cannot be seen (doesn’t mean it’s not there)

• striations present, but not seen well

Juxtaglomerular apparatus

Between the renal corpuscle’s vascular pole and the distal convoluted tuble is a structure called the Juxta-Golmerular Apparatus, with 3 parts:.
1. Juxtaglomerular cells.

• @ vascular pole,

• smooth m. cells of tunica media of afferent arteriole is replace by epith-like cells with granules in cytoplasm =  Juxtaglomerular cells.

• secrete renin as part of Renin-angiotensin system

2. Mesengial cells

• Also in apparatus are mesengial cells (no need to locate)

• chemoreceptor cells @ vascular pole

• activated by sensing the ion concentration in blood in macula densa,

• in other regions, help clean glomerular basement membrane

• secrete erythropoeitin.

• found with in and out of glomerulus, so named that  way = Extra glomerular, Intra glomerular mesengial cells

3.Macula Densa

• Lastly, the distal convoluted tuble right next to the vascular pole will have cells next to the JG cells that are a bit different.

• They are a darker row of cells on the side of the distal convoluted tube

• just next to the vascular pole = Macula Densa.

Within the cortex, is also the medullary rays. Contain = proximal straight tubules, distal straight tubules and the collecting ducts.

NOTE – area of cortex above the level of medullary rays is called the cortex corticis.

The characteristics of the straight part of tubules pretty much same as convoluted, Just look for thin eosinophillic tube structures with fuzzy lumen = prox, and very thin tubules with almost no staining in wall = distal.

Collecting ducts, not part of nephron, lumen is wide and the cells making up the wall is much bigger. usually widest lumens in this area. lightly stained cuboidal epithelium

Medulla
location of renal pyramids – remember that the base of the pyramid faces the cortex, and the apex of the pyramid (renal papilla) faces the hilum.

The cortex projects into medulla between renal pyramids as renal columns. (don’t have to show that)

Area between the medullary rays = cortical labyrinth

located where renal corpsucles end *NONE in medulla
contains only straight tubules, and asc and desc limbs of loop of Henle , as well as collecting tubules.

• @ apex of pyramid, (bbttom of slide) on one side of slide, can see the edge of the renal papilla – where it enters the minor calyx. You will see a slit in the slide, with one side’s epith v. thick and pink related to the other side of the slide. This is where the transistion urothelium of the minor calyx turns into the columnar epithelium covering the papilla. The one more lateral (and thicker) is the minor calyx.
• Also the location to find the papillary ducts. – large lumen, lined by tall columnar pale staining cells.
• base of papilla has a stratified columnar epith

• to identify the Loops of Henle

  • look for v. thin  tube like structures.

  • They look like thin capillaries

  • only with a thicker epithelium

  • simple squamous epithelium

  • no RBCs within their lumen.

  • To find them , you have to go alll the way down to the lower edge of the slide.

Blood Supply:

Blood flow in kidney v. different from other organs due its filtration function.

• From renal a –> interlobular a (renal columns) –> arcuate a (at base of pyramid) –> interlobular arteries ( cortical labyrinth) –> give multiple afferent arterioles to renal corpuscle.

• Here’s where it is different. Normally from capillaries you get venules leaving. In the kidney, instead, another arteriole leaves the glomerular capillary bed, the efferent arteriole

• from renal corpuscle –> efferent arteriole –> this then forms a second capillary bed around the tubular system we’ve discussed. –> then venules come from this capillary bed, and the veins follow the arteries.

There are three types of arterial branch of the efferent arterioles:
1. Cortical nephrons – aff a –> glomerulus –> eff a –> 2nd capillary bed local to the first one –> venules

2. Juxta medullary nephrons – aff a –> glomerulus –> eff a –> doesnt break into 2nd capillary bed right away, instead follow tubules all the way down to medulla (long thin segment, remember?) into the pyramid  –> uturn  –> come back up –> venules These long arterioles following the long segments of tubules staight down into medulla are called the vasa recta.

3. Capsular nephron (minor) – located in cortex corticis, the eff. a can form a star shaped capillary bed just under the capsule of the kidney = stellate capillary bed.  Cannot be seen in slide.

Slide # 61 Kidney Tangential section

• Same structures cut the opposite way. can see the cross section of straight tubules.

• characteristics the same.

• the bubble like, more clear area = medullary ray

• Eosinophillic area = cortical labyrinth

• find interlobular a here, about 6 around each column.

• Identify same parts as first slide

Slide #62 Kidney * H&E + Indian Ink

• Ink injected while animal is still alive, and H&E added posthumously

• a/v stained black, so can see all parts of vascular system: glomeruli, arcuate a, vasa recta, stellate capillary bed seen at top, under capsule.

• Star shape not visible due way of cutting.

Embryology of Kidney:

• Intermediate mesoderm from long ridge on post (dorsal) body wall = urogenital ridge.

• Part of this ridge = nephrogenic cord –> becomes urinary system

• Nephrogenic cord  –> splits into pronephros, mesonephros, metanephros

• Pronephros = most cranial, forms pronephric tubules, and pronephric duct, regresses in wk 5, not fxn’l in humans

• Mesonephros = middle structures, from mesonephric tubules and ducts (Wolffian duct), fxn’l for only a short period of time. The duct will still exite, and opens into urogenital sinus.

• Metanephros = most caudal, forms from outgrowth of mesonephric duct = ureteric bud, and from mesoderm grouping within nephrogenic cord called metanephric mesoderm. , forms @ wk 5, and fxn’l @ wk 10

• The ureteric bud penetrates the metanephric mesoderm, and then divides to form: ureters, renal pelvis, major and minor calyces, and collecting ducts.

• Collecting ducts push the metanepheric mesoderm to form vesicles (pouches), that become all the parts of the nephron.

Ascent of Kidneys:

• metanephros located w/in sacral region, but ends up at level of T12-L3.

• B/c of increase amt of growth caudal to metanephros.

• While ascending, kidneys rotate 90 degrees, so hilum faces medially.