AHE II 4.5 - URINARY SYSTEM (PART 2)

• This lecture focuses on recalling how the body produces and expels filtrate; identifying gross and microscopic structures of the urinary system and tracing its pathway

Lecture Learning Objectives

• Visually identify the kidney, ureter, urinary bladder, and urethra
• Describe the urogenital ridge, including its location, embryonic tissue, and portions that form the urinary and reproductive systems
• Explain the three successive sets of kidneys that develop in the embryo, noting which parts remain
• Describe the ureteric bud and metanephrogenic blastema
• List the portions of the adult kidney formed by the ureteric bud and the metanephrogenic blastema
• Explain the relocation of the kidneys
• Describe the development of the urinary bladder and urethra, focusing on the portions of the urogenital sinus and embryonic tissue derivatives that develop each component
• Compare the development of the male and female urinary system

Kidney Review

• The kidney has a renal/fibrous capsule, cortex, and medulla
• The cortex contains specific structures
• The medulla has specific structures present in the pyramids of the medulla

Kidney General Structures

• Capsule
• Cortex
• Medulla
• Renal Pelvis
• Hilum
• Renal pyramid
• Renal column

Nephron Review

• Afferent arteriole
• Renal corpuscle component is the glomerulus
• The glomerular capsule has a visceral and parietal layer, plus a capsular space
• Proximal convoluted tubule
• Nephron loop with thick descending arm, thin arm and thick ascending arm
• Distal convoluted tubule has a macula densa

Renal Corpuscle (RC)

• Glomerulus contains a bundle of vessels within RC
• Glomerular capsule with visceral layer and parietal layer
• Capsular space is created by two layers of the glomerular capsule

Nephron Histological Characteristics - Cortex

• Proximal convoluted tubule (PCT) has a “fuzzy” lumen due to numerous microvilli
• Distal convoluted tubule with a clear lumen and very limited microvilli

Nephron Histological Characteristics - Medulla

• In the medulla, note these nephron loop characteristics:
• Thick descending limb has a fuzzy lumen and is part of the PCT
• Thin limb (ascending/descending) with thin walls
• Thick ascending limb has a clear lumen
• Collecting duct with prominent dark staining nuclei and light staining cytoplasm
• All three portions of the nephron loop can be in the same image because they are components of different nephrons

Kidney Cortex-Nephron

• Renal corpuscle with glomerulus
• Proximal convoluted tubule
• Distal convoluted tubule
• Glomerulus with a renal corpuscle
• Glomerular capsule with a visceral and parietal layer
• Capsular space
• Macula densa in the distal convoluted tubule

Kidney Medulla-Nephron

• Thick descending limb and thin limb
• Thick ascending limb

Kidney Medulla-Collecting Duct

• Collecting duct to compare to PCT and DCT

Ureter

• Look for transitional epithelium, thick lamina propria, two-layered muscularis (inner longitudinal layer and outer circular layer), and adventitia (A)
• Mucosa with transitional epithelium and lamina propria
• Muscularis includes an inner longitudinal layer and an outer circular layer
• Adventitia

Urinary Bladder

• Look for transitional epithelium and thin lamina propria
• Three layers of muscularis, including inner longitudinal, middle circular, and outer longitudinal layers
• Mucosa is followed by muscularis

Urethra

• Look for transitional epithelium (female) or stratified/squamous columnar epithelium (male)
• Also look for thick lamina propria and two layered muscularis, surrounded by additional tissue, such as:
  • skeletal muscle in females
  • erectile tissue in males
• Male: stratified columnar epithelium (spongy portion of male urethra) and lamina propria
• Female: transitional epithelium (upper most portion of female urethra) and lamina propria

Urinary Tract Embryology

• Closely linked to the genital system (urogenital system), derived from the intermediate mesoderm on the developing embryo's dorsal wall
• The urogenital ridge is a tissue ridge on either side of the primordial aorta
• The nephrogenic cord forms urinary system components
• The gonadal ridge forms reproductive systems

Development of the Kidneys

• Three sets of successive kidneys develop embryonically
• Pronephroi are rudimentary and nonfunctional
• Mesonephroi are well-developed and function briefly
• The metanephroi form the permanent kidney

Pronephroi

• Appear in the 4th week and are represented by a cluster of cells near the neck
• Connected to pronephric ducts that open into the cloaca
• Pronephroi degenerate with a remaining portion of pronephric ducts becoming mesonephric ducts

Mesonephroi

• Large, elongated excretory organs create urine between weeks 6-10, until the metaphrenos can function
• ~40 glomeruli and mesonephric tubules open into the mesonephric ducts (pronephric duct remnants)
• Mesonephroi degenerate around 3 months
• In males, mesonephroi and mesonephric duct remnants form portions of the adult reproductive system

Metanephroi

• Primordia of the permanent kidney.
• Begin to develop early in the 5th week and begin to function around the 9th week
• Urine continuously forms throughout fetal life, excreted into amniotic fluid
• Permanent kidneys form from the ureteric bud (outgrowth from the mesonephric duct) and the metanephrogenic blastema

Metanephrogenic Blastema

• The mass of cells surrounds the ureteric bud
• Some cells form nephrons (~2 million by 36 weeks)
• The ureteric bud penetrates metanephrogenic blastema.
• The root forms ureter
• Branches within blastema form the renal pelvis, calices, and collecting tubules
• Collecting tubules induce mesenchymal cells in metanephrogenic blastema to begin the formation of nephrons

Kidney Relocation

• Early in development, kidneys are close together in the pelvis
• Kidneys relocate in the abdomen. As the abdomen elongates, rotate medially 90 degrees
• Kidneys ascend cranially and move further apart, taking a vascular supply and maintain a connection with the ureter
• Eventually contact suprarenal/adrenal glands around the 9th week of development

Development of the Urinary Bladder and Urethra

• Formed from the urogenital sinus in the embryo's caudal part
• The urogenital sinus can be divided into three parts:
• Vesical: forms the bladder; continuous with the allantois (projects into the umbilical cord).
  • The connection with the allantois degenerates and becomes the median umbilical ligament
  • Pelvic: forms the proximal urethra portion (males) and the entire urethra (females)
  • Phallic: forms the spongy urethra (males)
• The urinary bladder epithelium forms from endoderm of urogenital sinus
• Surrounding muscle and CT layers are derived from surrounding mesenchyme (mesoderm)
• The distal portion of the ureters forms the trigone
• Urogenital sinus separates from the developing GI tract/rectum
• The Connective tissue and smooth muscle that surround the urethra in both sexes are formed in mesenchyme (mesoderm) Epithelium of the Urethra
• Females have an epithelium of entire urethra, formed from urogenital sinus endoderm
• Males have an epithelium in most of the urethra that comes from urogenital sinus endoderm
• Epithelium in the glans penis (distal portion) is via external ectoderm