Male UG Embryology (Week 6--Trelease)

Question 1

What are the 3 overlapping kidney systems that form during development?

Answer 1

1) Pronephros
2) Mesonephros
3) Metanephros

Note: these are from intermediate mesoderm

Question 2

Pronephros

Answer 2

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)

Question 3

Mesonephros

Answer 3

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

Question 4

Metanephros

Answer 4

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

Question 5

When are nephrons made?

Answer 5

From week 6 until birth

1,000,000 nephrons per kidney

Question 6

What is an epithelial to mesenchymal transition in kidney development?

Answer 6

When metanephric ureteric bud epithelium interacts with mesenchyme of metanephric tissue cap (blastema)

Question 7

When does the definitive metanephric kidney become functional?

Answer 7

Week 12

Urine is passed into amniotic cavity/fluid, recycled through kidney when it’s swallowed

Question 8

Renal developmental defects

Answer 8

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)

Question 9

Where does kidney move?

Answer 9

Starts out in pelvic region but then shifts upward (cranially) and receives supply from aortic branches along the way

Question 10

Abnormal kidney position and shape

Answer 10

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

Question 11

What does the cloaca divide into?

Answer 11

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

Question 12

How is the bladder formed?

Answer 12

Upper/largest part of UG sinus forms bladder, which is initially continuous with the allantois

Question 13

What happens to the allantois?

Answer 13

Originally attached to bladder, but then is obliterated to form urachus which connects top of bladder to umbilicus (median umpilical ligament in adult)

Question 14

How is the male urethra formed?

Answer 14

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

Question 15

What layer does the epithelium of the bladder and urethra come from?

Answer 15

Endoderm

Derived from cloaca

Question 16

Mesonephric contribution to ureters and ejaculatory ducts

Answer 16

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

Question 17

What layer do the ureters, mesonephric ducts, trigone of the bladder come from?

Answer 17

Mesoderm

Question 18

What layer does the surrounding connective tissue around the urethra come from?

Answer 18

Splanchnic mesoderm

Question 19

Where does the prostate arise from?

Answer 19

Epithelium of prostatic urethra in late month 3

(comes from splanchnic mesoderm)

Question 20

Bladder and urethra developmental defects

Answer 20

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

Question 21

Which gene is most important in determining sexual dimorphism?

Answer 21

SRY gene of Y chromosome: a transcription factor (testis determining factor)

Question 22

When do gonads acquire male or female characteristics?

Answer 22

Week 7

Question 23

General gonad development for males and females

Answer 23

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

Question 24

Development of testis

Answer 24

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

Question 25

What happens to testis cords at puberty?

Answer 25

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)

Question 26

What layers are the Sertoli and Leydig cells derived from?

Answer 26

Sertoli derived from surface epithelium

Leydig derived from gonadal ridge mesoderm

Question 27

What do the Wolffian (mesonephric) ducts become?

Answer 27

Vas deferens (ductus deferens)

Question 28

What does SRY do?

Answer 28

1) Induces mesonephric tubules to penetrate gonadal ridge and induce differentiation
2) Induces Sertoli and Leydig cells to differentiate

Question 29

Early (indifferent) genital ducts

Answer 29

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)

Question 30

Male genital ducts

Answer 30

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

Question 31

Indifferent external genitalia

Answer 31

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

Question 32

How are male external genitalia formed?

Answer 32

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

Question 33

Genitalia developmental defects

Answer 33

Closure defects: hypospadias, epispadias, exstrophy of bladder, micropenis, bifid penis

Kleinfelter’s syndrome (47, XXY), infertility, gynecomastia, underandrogenization

Question 34

Indifferent gonad descent

Answer 34

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

Question 35

How do the testes descend?

Answer 35

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

Question 36

What layers are outside parietal layer of tunica vaginalis?

Answer 36

Internal spermatic fascia, cremasteric fascia, external spermatic fascia

(transversalis fascia, internal abdominal oblique and external abdominal oblique respectively)