HISTOLOGY OF KIDNEY

2nd year Med. Students of UniKL – RCMP ( Universiti Kuala Lumpur – Royal College of Medicine Perak, Malaysia. )

DINI SYAZWANI BINTI MOHD LOTFIFATIN NABILA BINTI ISMAIL

Pre-clinical

Nephron:

• is the (fx)al & structural unit of kidney. There are approx. 1 million nephrons in the human kidney that perform the (fx)s of osmoregulation and excretion

• consists of a renal corpuscle which is a dense rounded structure : the glomerulus, surrounded by narrow Bowman's spaces also it consists of a long folded renal tubules

Renal Cortex :• consists mainly of PCT & smaller number of DCT, CT also 2

interlobular arteries and veins.

Glomerulus:

• glomerular capillaries some of which contain RBC with their nuclei bulging into the capillary lumina

• Spaces between capillary loops in each glomerular lobule are filled by basement membrane-like material: mesangium containing mesangial cells

Mesangial cells:

• modified pericytes which are contractile and thus able to modify diameter of glomerular capillaries

• important in the control of glomerular fx & BP , as they contain receptors for vaso-constrictors : e.g. angiotensin II

• secrete mesangial matrix & may have phagocytic fx as particulate matter from blood may pass into mesangium where they will be eventually degraded by the messangial cells

There are Podocytes @ the capillary loops exposed to the Bowman's Space.They have large,round nuclei.Bowman's capsule lined by squamous epithelium (flatten nuclei)

Blood Supply:• Interlobular artery are seen branching to form aff.arteriole of glomerulus.Eff.arteriole which diameter is much smaller

than the AA leave the glomerulus.The variation of the diameter maintains the pressure within glomerular capillaries

Eff. Arteriole:• give rise to netw. of capillaries which surround the renal tubules• towards the medullas,it give rise to the Vasa Recta

Molecules reabsorbed from glomerular filtrate are returned to the general circulation via those capillary netw. that drains into the renal venous system.

During filtration of plasma from GC into RT, the filtrate passes through 3 layers:

1st layer: Capillary Endothelium which contains numerous large,round fenestrations.Its luminal surface is -vely charged due to podocalyxin

2nd layer: GBM. It consists of 3 layers:

• lamina densa, lamina rara interna(under the endothelium) & lamina rara externa that support the podocytes

3rd layer: Podocytes which have long cytoplasmic extensions : primary processes ---> short secondary foot processes aka pedicels.

• Pedicels are bound to lamina rara externa. Gaps between adjacent pedicels known as Filtrarion Slits which are bridged by Slit Diaphragms

• Slit diaphragms + -vely charged podocalyxin found in the luminal suface of capillary endothelium earlier cover the urinary surface of Podocytes.

• Intracellular component of Nephrin(transmembrane protein) inside slit diaphragms are bound to the actin cytoskeleton of the podocytes

• Slit Diaphragms blocks the passage of any large molecules

• Hb & smaller molecules pass freely through glomerular filter

• Albumin & larger molecules are blocked by -vely charged endothelial cell coat

• Meshwork of lamina densa of the basement membrane is very particular on the basis of molecular size & configuration

Proximal 'Convoluted‘ Tubules :

• Coiled tubes measuring approx. 14 mm in length

• 65% of glomerular filtrate is reabsorbed from PCT

• almost 100% of filtered glucose & aa is reabsorbed by PCT

• surrounded by netw. of capillaries arising from eff.arterioles of glomerulus.They returns the molecules reabsorbed from glomerular filtrate back into the general circulation

• lined by simp. cuboidal epith. with brush border: tall microvilli projecting into the lumen & they are coated with glycocalyx

• epith. cells of PCT form multiple lateral processes which combined with each other to form a complex Lateral Intercellular Space which is separated from the lumen of PCT by a ring of junctional complexes

• contains many pinocytotic vesicles & lysosomes in their cytoplasm where both reabsorb & degrade small amounts of protein that have leaked from the glomerular filter

Distal Convoluted Tubules :• Forms the 3rd segment of renal tubule. Its 1st part forms the MD & the remaining makes up the DCT• In DCT, Na+ ions are reabsorbed frm tubular fluid with 1 H+ / K+ ion being secreted in exchange. Aldosterone controlled that

adjustment of acid-base balance

Characteristics :• absence of brush border• larger & more clearly defined lumen• more nuclei per cross section• paler cytoplasm due to fewer organelles • much shorter than PCT• nuclei that tend to bulge into lumen• large number of tiny pinocytic vesicles

Juxtaglomerular apparatus (JGA)

• Comprises:1. Juxtaglomerular cells- Modified smooth muscle cell in afferent

arteriole - Regulating blood pressure via renin

angiotensin aldosterone mechanism

2. Macula densa- Modified distal convoluted tubule (DCT)- Detect osmolarity of fluid in DCT

3. Extraglomerular mesangial cells- Lacis/Polkissen/Polar cushions

• Point of contact between distal convoluted tubule & afferent arterioles

Juxtaglomerular apparatus• Juxtaglomerular cells- Large rounded myoepithelial cells in

afferent arterioles

• Macula densa- Densely packed columnar cells in DCT- Monitor NaCl in tubular fluid

• Extraglomerular mesangial (Lacis)- Outside renal corpuscle- At the vascular pole- Connects macula densa & JG cells

Loop of Henle• U- shaped epithelial loop

- Simple cuboidal- simple squamous epitheliu• Thick and thin descending limb• Thick and thin ascending limb• Closely associated with vasa recta

• Function:- Establish & maintain osmotic gradient in the interstitial fluid in

medulla- 30% Na+ & Cl- ions reabsorption

Renal Medulla

Thin tubule• Simple squamous

epithelium• Regular, rounded shape

on transverse section• Nuclei protrude slightly

into the lumen• Absence of RBCs in

lumen

Collecting Tubule & Duct

Vasa Recta

Collecting tubule

Collecting tubule & duct cont.• Simple Cuboidal in smaller

tubules• Simple Columnar in larger

ducts• Clear & sharp cell

boundaries, cytoplasm stain poorly • Thick basal lamina• Surrounded by thin loop of

Henle & Vasa Recta

Collecting TubuleCollecting Duct

Collecting tubules -----> collecting ducts ----> ducts of Bellini (open into tip of papilla)

• Principal cells- Plain-stained cells, few organelles- Contains receptor for ADH

• Intercalated/dark cells- Microvilli, more mitochondria- Maintenance of acid-base

homeostasis (secrete H+ into filtrate)