What are the 3 overlapping kidney systems that form during development?
Note: these are from intermediate mesoderm
Solid cell groups in cervical region form nephrotomes (vestigial excretory units)
At the top (rostral)
Pronephric system disappears by end of 4th week (disappear before next caudal, mesonephric ones, form)
Between pronephros and metanephros
Start forming early 4th week
Develop into glomeruli, Bowman's capsules, renal corpuscules
Longitudinal collecting duct behind develops into Wolffian (mesonephric) duct, which empties into cloaca
Middle of 2nd month, formed 2 developing gonads
Metanephros is what creates entire definitive kidney (pro and mesonephros break down)
Uretic bud forms collecting system: ureter, renal pelvis, major calyces then subdivides to form minor calyces then renal pyramids, collecting tubules (from week 5 to month 5)
Metanephric tissue cap (metanephric mesoderm) forms nephron: glomeruli, Bowman's capsules, joins collecting duct of uretic bud to form DCT, loop of Henle, PCT
When are nephrons made?
From week 6 until birth
1,000,000 nephrons per kidney
What is an epithelial to mesenchymal transition in kidney development?
When metanephric ureteric bud epithelium interacts with mesenchyme of metanephric tissue cap (blastema)
When does the definitive metanephric kidney become functional?
Urine is passed into amniotic cavity/fluid, recycled through kidney when it's swallowed
Renal developmental defects
Wilms' tumor (nephroblastoma)
Renal dysplasias and agenesis (spectrum): multicystic dysplastic kidney, renal agenesis, congenital polycystic kidney
Bifid ureters, some with additional abnormal exit points (into vagina or directly into urethra)
Where does kidney move?
Starts out in pelvic region but then shifts upward (cranially) and receives supply from aortic branches along the way
Abnormal kidney position and shape
Accessory renal arteries that fail to detatch during ascent
If kidneys fail to pass through form of umbilical arteries, can have pelvic kidneys or fusion and horseshoe kidney
What does the cloaca divide into?
Anterior urogenital sinus and posterior anal canal
(during weeks 4 through 7)
Partitioning formed by urorectal septum
Perineal body is formed by inferior tip of urorectal septum
How is the bladder formed?
Upper/largest part of UG sinus forms bladder, which is initially continuous with the allantois
What happens to the allantois?
Originally attached to bladder, but then is obliterated to form urachus which connects top of bladder to umbilicus (median umpilical ligament in adult)
How is the male urethra formed?
Narrow pelvic part of UG sinus gives rise to prostatic urethra and membranous urethra
Note: third phallic part of UG sinus develops differently depending on sex
What layer does the epithelium of the bladder and urethra come from?
Derived from cloaca
Mesonephric contribution to ureters and ejaculatory ducts
Caudal portion of mesonephric ducts incorporated into wall of bladder
Ureters form as buds of distal mesonephric ducts but then move upward (cranially) and have separate attachment to bladder from mesonephric ducts
Mesonephric ducts become ejaculatory ducts
What layer do the ureters, mesonephric ducts, trigone of the bladder come from?
What layer does the surrounding connective tissue around the urethra come from?
Where does the prostate arise from?
Epithelium of prostatic urethra in late month 3
(comes from splanchnic mesoderm)
Bladder and urethra developmental defects
Urachal fistula, cyst, sinus: persistence of remnants of allantois
Exstrophy of the bladder: ventral body wall defect exposing bladder mucosa, with epispadias and open urinary tract
Exstrophy of the cloaca: severe ventral body wall defect (inhibited midline migration of mesoderm) with lack of tail fold progression, resulting in exstrophy of bladder, imperforate anus, omphalocele, spinal defects
Which gene is most important in determining sexual dimorphism?
SRY gene of Y chromosome: a transcription factor (testis determining factor)
When do gonads acquire male or female characteristics?
General gonad development for males and females
1) Longitudinal genital (gonadal) ridges formed by proliferating epithelium and underlying mesenchyme medial to caudal (bottom) mesonephros
2) Primirive germ cells migrate into ridges from endoderm of yolk sac near allantois
3) Germ cells induce gonad development and form primitive sex cords
Development of testis
Primitive sex cords continue to grow and penetrate deep medulla to form medullary (testis) cords
Rete testis form near hilum
Tunica albuginea grows
Month 4 testis cords become horseshoe shaped, contain germ cells, sustenancular cells of Sertoli, interstitial cells of Leydig
What happens to testis cords at puberty?
Before puberty the testis cords remain solid, then hollow to form seminiferous tubules and join rete testis tubules then ductuli efferentes (go into ductus deferens)
What layers are the Sertoli and Leydig cells derived from?
Sertoli derived from surface epithelium
Leydig derived from gonadal ridge mesoderm
What do the Wolffian (mesonephric) ducts become?
Vas deferens (ductus deferens)
What does SRY do?
1) Induces mesonephric tubules to penetrate gonadal ridge and induce differentiation
2) Induces Sertoli and Leydig cells to differentiate
Early (indifferent) genital ducts
All fetuses have 2 pairs of genital ducts: mesonephric (Wolffian) and paramesonephric (Mullerian)
Paramesonephric duct opens into abdominal cavity on top (cranially) and crosses over mesonephric duct below (caudally)
Male genital ducts
Form from persistent mesonephric ducts
Efferent ductules form when mesonephric excretory tubules connect with rete testis
Ductus epididymis forms, ductus deferens forms
Note: vestigial paradidymus forms where paragenital tubules do NOT join rete
Indifferent external genitalia
Mesenchyme from primitive streak migrates to form cloacal folds in week 3
Genital tubercle forms on top (cranial) of cloacal membrane
Get urethral folds (anterior) and anal folds (posterior)
Genital swellings on either side of urethral folds
Week 6, you CANNOT distinguish genitalial sex
How are male external genitalia formed?
Fetal testes secrete androgens and cause elongation in genital tubercle to form phallus
Urethral folds pulled around urethral groove
Urethral plate forms from endodermal epithelium of urethral groove
Urethral folds close over plate to form penile urethra (still doesn't exted all the way to tip
External urethral meatus (distal part of urethra) formed by epithelial canalization by ectodermal cells from glans
Genital swellings move down (caudally) as they develop and become scrotal sacs separated by septum
Genitalia developmental defects
Closure defects: hypospadias, epispadias, exstrophy of bladder, micropenis, bifid penis
Kleinfelter's syndrome (47, XXY), infertility, gynecomastia, underandrogenization
Indifferent gonad descent
In late month 2, gonad and mesonephros is attached to posterior abdominal wall by UG mesentary
Mesonephros disappears and gonadal mesentary becomes caudal genital ligament
Gubernaculum extends from gonads and ends in inguinal abdominal wall
How do the testes descend?
Gubernaculum regresses and intraabdominal pressure increases
Processus vaginalis follows course of gubernaculum and is pulled into scrotum--covers testes as viscreal layer then parietal layer of tunica vaginalis
What layers are outside parietal layer of tunica vaginalis?
Internal spermatic fascia, cremasteric fascia, external spermatic fascia
(transversalis fascia, internal abdominal oblique and external abdominal oblique respectively)