UROGENITAL EMBRYOLOGY

Pronephros = forekidney

develops from mesoderm during 3^rd week of gestation; involutes during 4^th week of gestation → vestigial remnant / complete absence

Mesonephros = midkidney

develops during 4^th week of gestation immediately caudal to pronephros, functions as interim kidney; degenerates around 8 weeks of gestation

(a)  mesonephric tubules

→  paradidymis, epididymis, efferent ductules (M); epinephron (F)

(b)  mesonephric (wolffian) duct

→  appendix epididymis, vas deferens, ejaculatory duct, seminal vesicles (M); vanishes (F)

[Caspar Friedrich Wolff (1733–1794), German anatomist, biologist, and embryologist in Berlin and St. Petersburg, cofounder of embryology, established doctrine of germ layers]

mnemonic:   Gardener’s SEED

»  female:  Gartner duct, cyst

»  male:  Seminal vesicle

Epididymis

Ejaculatory duct

Ductus deferens

Paramesonephric (Müllerian) Duct

(grows alongside mesonephric duct)

[Johannes Peter Müller (1801–1858), German physiologist and comparative anatomist in Bonn and Berlin]

Male:   degenerates ← production of müllerian inhibiting factor (MIF) by Sertoli cells of testis at about 6 weeks GA

→  prostatic utricle + appendix testis

Female:   induced by wolffian duct at 5 weeks GA; grows caudally → joins in midline → fuses with outgrowth of urogenital sinus

→  uterus, fallopian tubes

Metanephros = hindkidney = permanent kidney

(1)  metanephric diverticulum (ureteric bud) buds from mesonephric duct near its entry into the cloaca at 4^th week; it lengthens + grows toward nephrogenic cord which becomes the metanephric blastema that divides and forms

→  ureter (mesonephric duct)

→  renal pelvis (first 4 dividing generations of duct)

→  calices (second 4 dividing generations of duct)

→  collecting tubules (10–12 generations of duct)

(2)  metanephric blastema (= nephrogenic mesoderm) forms nephrons under the influence of ureteral bud → the end of collecting tubules induce clusters of metanephric blastema cells located at the periphery and along the sides of the medullary ray (= pyramid) except around the papilla

(3)  metanephric vesicles form within clusters of metanephric blastema cells + elongate into S-shaped tubules which, by 12^th week of gestation, result in

→  glomerulus

→  proximal convoluted tubule

→  loop of Henle

[Friedrich Gustav Jakob Henle (1809–1885), German pathologist and anatomist in Berlin, Zürich, Göttingen]

→  distal convoluted tubule

→  connective tissue

◊  Polycystic kidney disease is believed to be a failure of linkage!

Urogenital Sinus

forms from cloaca

→ develops into bladder + urethra (+ prostate)

Bladder

develops in 2^nd–4^th embryonal month

Urachus

=  narrowed apex of fetal bladder continuous with allantoic stalk at the umbilicus

→ forms median umbilical ligament

(a)  supravesical portion

(b)  intramural portion

(c)  intramucosal portion

SEX DEVELOPMENT

Indifferent Stage of Sexual Differentiation

Period:   until 7^th week of GA

Composition of undifferentiated gonad:

(1)  Mesenchyme
condensation of mesenchyme forms genital ridges on both sides of midline between 6^th thoracic and 2^nd sacral segments; differentiates into interstitial (Leydig) cells within seminiferous tubules (= supporting stromal cells of testicular interstitium, including endothelial cells and vascular smooth muscle cells)

(2)  Mesothelium
genital ridges are covered by proliferating mesothelium (coelomic epithelium); differentiates into Sertoli / supporting cells of the seminiferous tubules; forms tunica albuginea (vaginalis) as the serous covering of the testes

(3)  Germ cells
form in wall of yolk sac and migrate along hindgut into genital ridge; differentiate into spermatogonia within seminiferous tubules; give rise to most testicular tumors

Chromosomes

›  determine genotypic sex

›  issue complex signaling pathways + hormones → modify primordial mesodermal cells → phenotypic sex

Formation of Testis

Period:   around 8–12 weeks GA

•  sex-determining factor localized on short arm of Y chromosome (SRY gene) → initiates male sex differentiation = seminiferous tubules

•  Leydig cells secrete testosterone → stimulating growth of mesonephric (wolffian) structures

•  Sertoli cells secrete müllerian-inhibiting factor → leading to regression of paramesonephric (müllerian) duct

Testicular Migration

Prenatal Evaluation of Sex

◊   Imaging of genitalia recommended only when
(a) medically indicated
(b) in twin gestation

Successful imaging of genitalia:

›  in 60% by 14–18 weeks GA

›  in 80–100% > 20 weeks GA

Errors in imaging of genitalia:

√  adducted fetal legs

√  US beam at wrong angle

Female:

√  3 parallel lines between legs on transverse US

√  3 bumps with clitoris in midline directed caudally

Male:

√  small semicircular structure (= scrotal sac)

√  penis in midline directed anteriorly and superiorly

√  visualization of testes in scrotum in late 2^nd trimester

Ambiguous genitalia:

√  thorough fetal survey to detect associated anomalies

√  repeated US examinations with early GA

√  MRI with present oligohydramnios / multiple anomalies

RENAL ANATOMY

Adult Kidney

›  forms by fusion of superior + inferior subkidneys (= metanephric lobes); the line of fusion runs obliquely forward and upward

√  separation of upper + lower groups of calices

√  indentation of cortical contour + echogenic line (= interrenicular septum = junctional parenchymal defect) delineates junctional parenchyma (often referred to as hypertrophic column of Bertin)

›  consists of 20,000 lobules within 14 lobes (reniculi)

›  initially located in pelvic region ventral to sacrum, ascending cranially at 9 weeks of gestation ← body growth caudal to kidneys + straightening of body curvature

›  renal hilum at first ventrally located, eventually rotating medially by 90 degrees with renal ascent

Reniculus = renal lobe

=  central core of medullary tissue enveloped by

(a)  centrilobar cortex (= cortical arch) that covers the base of the pyramid → subsequently forming the renal cortex with loss of grooves

(b)  mural cortex that wraps around sides of pyramid → and fuses with the mural cortex of adjacent lobe → to form renal septum (= column of Bertin)

›  renal lobes completed by 28 weeks GA

√  ren lobatus (= interlobar surface grooves) present in fetus and infant, rare in adulthood

√  assimilation of independent lobes > 28 weeks GA makes renal surface smoother

›  nephrogenesis completed by 36 weeks GA

Renal Size (in cm)

›   < 1 year of age:   4.98 + 0.155 x age (in months)

›   > 1 year of age:   6.79 + 0.22 x age (in years)

›   adulthood:   R kidney 10.74 ± 1.35 (SD); L kidney 11.10 ± 1.15 (SD)

›   ratio of renal length (RL) to distance between first 4 lumbar transverse processes (4 TP) = 1.04 ± 0.22

Renal Echogenicity

A.  ADULTHOOD

liver ≥ spleen ≥ renal cortex > renal medulla

B.  INFANCY (in neonate up to 6 months of age)

√  cortex may be more echogenic than adjacent normal liver / spleen

Cause:   glomeruli occupy 18% of cortex in neonate compared with 9% in adult

√  increase in corticomedullary differentiation

Cause:   ratio of cortex to medulla 1.64÷1 in neonate compared with 2.59÷1 in adult

√  hypoechoic pyramids appear relatively large

Cause:   thin immature cortex in neonate

N.B.:   may be misinterpreted as dilated calices / renal cystic disease

√  renal sinus echogenicity less prominent

Cause:   paucity of fat

Renal Vascular Anatomy

Hilar Arteries

Entry:   renal hilum

1^st order:   main renal arteries at level of L1 / upper margin of L2

2^nd order:   5 segmental branches = apical, anterior superior, anterior inferior, posterior, basilar

Extrahilar branching:

=  branching of main renal artery prior to reaching hilum

Entry:   renal hilum / direct as polar arteries

›  early branching: within 1.5 cm from aorta

Resistive index:   < 0.70 1 SD of several measurements = 0.04

Renal Polar Artery

Entry:   without going through renal hilum directly into renal parenchyma at the poles

Types:   superior polar artery; inferior polar artery (with supply of upper urinary tract)

Origin:   main renal a., contralateral main renal a., aorta, iliac a., superior mesenteric a., inferior mesenteric a., celiac a., middle colic a., lumbar a., gonadal a., middle sacral a.

Capsular artery

=  tiny vessels perfusing the renal capsule coursing tangentially to renal margin

Origin:   main renal a., branch renal a., other retroperitoneal aa. (lumbar a.)

Anatomic Renal Artery Variants

Multiple renal arteries (25–30%):

unilaterally (32%); bilaterally (12%)

Number of renal arteries per kidney:

(a)  one 71%

(b)  two 24%

›  2 hilar arteries 12%

›  1 hilar + 1 superior polar artery 7%

›  1 hilar + 1 inferior polar artery 5%

(b)  three 4%

(c)  more than three 1%

Separate aortic ostium:

Main renal artery

=  vessel with the greatest diameter

Accessory renal artery

=  vessel with smaller diameter

=  segmental artery originating from aorta / iliac a.

Supplementary artery:

Entry:   renal hilum

Origin:   aorta, iliac a., internal spermatic a., SMA, IMA, celiac trunk, middle colic a., lumbar a., middle sacral a., contralateral renal a.

Supply:   lower pole (72%) > upper pole

Aberrant renal artery:

=  segmental artery arising from superior mesenteric artery / internal spermatic artery

Renal Veins

Single right renal vein

without major extrarenal tributaries (85%)

with major extrarenal tributaries:

(a)  right adrenal vein (30%)

(b)  right gonadal vein (7%)

(c)  lumbar + hemiazygos veins (3%)

Single preaortic left renal vein

without major extrarenal tributaries (86%)

with major extrarenal tributaries:

(a)  left adrenal vein

(b)  hemiazygos vein

(c)  lumbar vein

(d)  ascending lumbar vein

(e)  left gonadal vein

Anatomic Renal Vein Variants

MULTIPLE RIGHT RENAL VEINS (15–30%)

(a)  single right renal vein divides just before union with IVC 4%

(b)  right gonadal vein joins the renal vein 6%

(c)  accessory branch of adrenal vein enters right renal vein 31%

(d)  lumbar / azygos vv. enter right renal vein 3%

CIRCUMAORTIC LEFT RENAL VEIN (5–6–17%)

SINGLE RETROAORTIC LEFT RENAL VEIN (2–3%)

LUMBAR VEINS JOINING LEFT RENAL VEIN (75%)

URETER

=  retroperitoneal muscular conduit that carries urine from renal pelvis to urinary bladder

Origin:   ureteropelvic junction at intra / extrarenal pelvis

Terminus:   ureterovesical junction

Length:   ~ 25 cm

Diameter:   < 3 mm

Congenital Ureteral Abnormalities

A.  URETERAL DUPLICATION

1.   Complete ureteral duplication

2.   Partial ureteral duplication

B.  SMOOTH MUSCLE DYSFUNCTION

1.   UPJ obstruction

2.   Primary congenital megaureter

C.  ABNORMAL TERMINATION OF SINGLE URETER

1.   Ectopic ureter

2.   Ureterocele

3.   Vesicoureteral reflux

Extrarenal Pelvis

=  normal anatomic variant in which the renal pelvis lies predominantly outside of the renal sinus

√  may be dilated under normal circumstances

DDx:  UPJ obstruction, distal obstruction

√  NO associated caliectasis

√  normal size of ureter

RETROPERITONEUM

=  space posterior to peritoneal cavity extending from diaphragm to pelvic brim; separated from peritoneum anteriorly by posterior peritoneal fascia; bounded posteriorly by transversalis fascia

(a) anterior border:   posterior peritoneal fascia (posterior to peritoneum)

(b) posterior border:   transversalis fascia (attaches to psoas muscle)

Retroperitoneal Compartments

A.  Anterior pararenal space

Boundaries:   posterior parietal peritoneum (anteriorly), anterior renal fascia (posteriorly), lateroconal fascia (laterally)

→  superiorly joins with posterior renal fascia and attaches to crux of diaphragm

→  in the middle blends with connective tissues of central prevertebral space around great vessels

→  inferiorly joins with posterior renal fascia and attaches to great vessels

(a)  pancreaticoduodenal space

Contents:   pancreas, duodenum, root of small bowel mesentery

(b)  pericolonic space

Contents:   ascending colon + descending colon

B.  Perirenal / perinephric space = capsula adiposa

Boundaries:   fascia renalis = Gerota fascia = anterior (membrane of Toldt) + posterior (Zuckerkandl) renal fasciae

[Carl Toldt (1840–1920), Austrian anatomist, professor of anatomy in Prague and Vienna]

subdivided into multiple compartments by thin fibrous lamellae + incomplete bridging septa (Kunin septa) that attach to anterior + posterior renal fascia

→  forms superiorly closed inverted cone around adrenal gland + perirenal fat + upper half of kidney; fixed to diaphragmatic fascia above; abuts bare area of liver (R) + subphrenic space (L)

→  inferiorly forms cone around perirenal fat + lower pole of kidney blending with iliac fascia below

→  laterally closed by fusion of anterior + posterior sheath

→  medially open communicating with central prevertebral space

Contents:   kidneys, adrenal glands, proximal renal collecting system, proximal ureters, perirenal fat, renal hilar vessels, lymphatic vessels

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