Embryology (URINOGENITAL)

I will explain every part of this embryology material using only the terms from your text, without analogies, as if you're learning it for the first time.

URINOGENITAL SYSTEM - BASIC EXPLANATION

WHAT IT IS:

· This is one system in the embryo that will become two different systems in the adult

· Urinary system: makes urine, removes waste

· Genital system: reproductive organs

· They start developing together from the same tissue (mesoderm) in the same place (posterior abdominal wall)

· Both have ducts that empty into the same chamber: cloaca

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THE THREE KIDNEY SYSTEMS THAT DEVELOP ONE AFTER ANOTHER

1. PRONEPHROS - THE FIRST ATTEMPT

· Appears in week 4 of development

· Located in the neck region (cervical)

· Just 7-10 clumps of cells called nephrotomes

· These cells never become functional - they disappear completely by end of week 4

· Only important thing it does: starts forming the mesonephric duct (a tube) that will be used by the next kidney system

2. MESONEPHROS - THE TEMPORARY KIDNEY

· Forms right after pronephros disappears

· Located from chest to lower back (thoracic to lumbar)

· Actually works for a short time during early fetal period

· Structure:

· Forms S-shaped tubules

· Each tubule gets a ball of capillaries at one end = glomerulus

· The tubule wraps around glomerulus = Bowman capsule

· Glomerulus + Bowman capsule = renal corpuscle (basic urine-making unit)

· All tubules drain into mesonephric duct (also called Wolffian duct)

· What happens to it:

· Forms big bulges on each side of embryo called urogenital ridge (kidney + gonad together)

· Starts disappearing after 2 months

· In males: some parts stay to become part of reproductive system

· In females: completely disappears

3. METANEPHROS - THE PERMANENT KIDNEY

· Forms in week 5 and becomes your lifelong kidneys

IT HAS TWO PARTS THAT DEVELOP SEPARATELY:

A. COLLECTING SYSTEM (drainage pipes)

· Comes from ureteric bud - a tiny outgrowth from the mesonephric duct

· This bud grows into kidney tissue

· Expands and branches to form:

· Ureter (tube from kidney to bladder)

· Renal pelvis (funnel at kidney center)

· Calyces (smaller funnels)

· Collecting tubules (millions of tiny drainage tubes)

· Forms 12+ generations (branchings) of tubules

B. EXCRETORY SYSTEM (urine-making units)

· Comes from metanephric tissue (different from collecting system)

· Collecting tubules tell this tissue to form nephrons (urine-making units)

· Steps:

1. Tissue forms vesicles (small sacs)

2. Vesicles become S-shaped tubules

3. Blood vessels grow into one end to form glomerulus

4. The tubule wraps around glomerulus = Bowman capsule

5. Tubule elongates to form: proximal convoluted tubule → loop of Henle → distal convoluted tubule

· Each nephron connects to a collecting tubule

· 1 million nephrons form in each kidney, all formed before birth

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HOW GENES CONTROL KIDNEY DEVELOPMENT

TWO TISSUES TALK TO EACH OTHER:

1. Ureteric bud epithelium (from collecting system)

2. Metanephric mesenchyme (tissue that becomes nephrons)

SIGNALS BACK AND FORTH:

From tissue to bud:

· Tissue makes GDNF and HGF chemicals

· Bud has RET and MET receptors that receive these chemicals

· This makes the bud grow and branch

From bud to tissue:

· Bud makes FGF2 and BMP7 chemicals

· Tissue receives these → doesn't die, keeps growing

· Tissue keeps making WT1 protein (important for development)

Making nephrons:

· Bud makes WNT9B and WNT6 proteins

· These tell tissue to make PAX2 and WNT4

· PAX2: clumps tissue together

· WNT4: changes tissue into tubules

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WHAT CAN GO WRONG - CLINICAL PROBLEMS

KIDNEY TUMORS:

Wilms tumor: kidney cancer in children

· Caused by WT1 gene mutations

· Can be part of syndromes:

· WAGR: Wilms tumor + Aniridia (no iris) + Gonadoblastomas (gonad tumors) + mental Retardation

· Denys-Drash: kidney failure + ambiguous genitalia + Wilms tumor

KIDNEY MALFORMATIONS:

1. Renal dysplasias/agenesis: kidneys don't form properly

· Multicystic dysplastic kidney: full of cysts, no nephrons

· Bilateral renal agenesis: no kidneys at all (1/10,000 births)

· Causes Potter sequence:

· No urine made → low amniotic fluid → compressed fetus

· Results: flattened face (Potter facies), lung problems, other defects

2. Polycystic kidney disease: kidneys full of fluid-filled cysts

· Autosomal recessive (ARPKD): severe, childhood kidney failure

· Autosomal dominant (ADPKD): milder, adult kidney failure

· Both caused by cilia problems (tiny hair-like structures on cells)

· Other cilia diseases: Bardet-Biedl syndrome and Meckel-Gruber syndrome

3. Duplicated ureter: two ureters from one kidney

· From splitting of ureteric bud

· Sometimes one ureter drains to wrong place (vagina, urethra)

KIDNEY POSITION PROBLEMS:

Kidneys start in pelvis, move up to abdomen during development

· Pelvic kidney: gets stuck in pelvis

· Horseshoe kidney (1/600): lower poles fuse, gets blocked by artery

· Accessory renal arteries: extra blood vessels from development

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BLADDER DEVELOPMENT

CLOACA SPLITS INTO TWO:

· Anterior part → urogenital sinus (becomes bladder and urethra)

· Posterior part → anal canal

· Urorectal septum separates them → becomes perineal body

THREE PARTS OF UROGENITAL SINUS:

1. Urinary bladder - biggest part

· Connected to allantois (early umbilical structure)

· Allantois becomes urachus cord → median umbilical ligament

2. Pelvic part → becomes parts of urethra

3. Phallic part → differs in males/females

BLADDER TRIGONE FORMATION:

· Mesonephric ducts get absorbed into bladder wall

· Ureters now enter bladder separately

· Trigone (triangle at bladder base) starts as mesoderm → becomes endoderm like rest of bladder

URETHRA DEVELOPMENT:

· Lining from endoderm

· Muscle from mesoderm

· In males: buds from urethra form prostate gland

· In females: buds form urethral glands

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BLADDER DEFECTS

URACHAL PROBLEMS:

· Urachal fistula: allantois stays open → urine leaks from umbilicus

· Urachal cyst: portion of allantois stays as cyst

· Urachal sinus: upper allantois stays connected to bladder

MAJOR DEFECTS:

1. Bladder exstrophy (2/10,000):

· Bladder exposed on outside of body

· Always with epispadias (urethra opens on top of penis)

· From body wall not closing

2. Cloacal exstrophy (1/30,000): more severe

· Bladder + rectum both exposed

· Anal malformations + genital defects

· Body wall closure failure + septum problems

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TIMELINE SUMMARY:

· Week 4: Pronephros appears and disappears

· Week 4-8: Mesonephros forms and mostly disappears

· Week 5: Metanephros (permanent kidney) starts

· Week 10: Glomerular capillaries form

· Week 12: Urine production begins

· Birth: 1 million nephrons per kidney

· Infancy: Kidney lobulation disappears

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KEY POINTS TO REMEMBER:

1. Three kidneys in sequence: only last one stays

2. Two parts of permanent kidney: collecting system (from ureteric bud) + excretory system (from metanephric tissue)

3. Kidneys start low, move up: can get stuck in wrong place

4. Bladder from urogenital sinus: splits from anal canal

5. Many things can go wrong: from genes (WT1) to structures (exstrophy)

6. Nephrons only form before birth: 1 million each kidney, no new ones after birth

All information comes directly from your Langman's Embryology text and notes. Each term used is from the medical text provided.