Embryology - External Genitalia Flashcards
Where is the reproductive system derived from?
Intermediate mesoderm
What is the urogenital ridge?
Thickening of epithelium that starts to produce primitive sex cords
Where do the primordial germ cells come from?
Yolk sac (see embryology 1)
What do the primordial germ cells contain?
Autosomes (XX or XY chromosomes)
Briefly describe order of male gonadal development
- Medullary cords develop
- Sertoli cells
- Testes cords
- Seminiferous tubules
When are seminiferous tubules solid up until?
Puberty
Tubes hollow and allow male to produce sperm
Briefly describe order of female gonadal development
- Medullary cords break down and cortical cords develop
- Follicle cells
- Primordial follicles
Why do females already contain eggs ready for development at birth?
Cortical cords invest primordial germ cells
Form follicle germ cells which stimulates germ cells to change –> undergo 1st stage of meiosis
In the first weeks of development, what two pairs of ducts do all embryos contain?
Where do both of these ducts end?
- Mesonephric duct
- Paramesonephric duct
Both end at the cloaca
Which duct develops in males?
Presence of TDF and testosterone (produced by Leidig cells) causes the mesonephric ducts to develop
What do the mesonephric ducts develop to form in males?
The primary male genital ducts
What happens to the paramesonephric ducts in males? What causes this?
They regress due to presence of anti-Mullerian hormone
What is anti-Mullerian hormone produced by?
Sertoli cells in the testes
Which ducts degenerate in females? Why?
The mesonephric ducts degenerate due to absence of testosterone
Which ducts develop in females?
Paramesonephric ducts develop due to absence of anti-mullerian hormone
What remnant of the mesonephric ducts are left behind in females?
Gartner’s duct (a vestigial remnant)
What are mesonephric ducts sometimes called?
Wolffian ducts
What are paramesonephric ducts sometimes called?
Mullerian ducts
What does the SRY gene encode for?
Testis Determining Factor (TDF)
When the somatic support cells invest the primordial germ cells with the SRY gene, what forms?
Differentiate into Sertoli cells
What hormone does Sertoli cells secrete?
Anti-mullerian hormone (AMH)
What does AMH cause?
Degeneration of paramesonephric ducts
Sertoli cells cause the differentiation of the mesoderm of the gonad into what?
Leydig cells
Where are the Leydig cells located?
In between testis cords
What do Leydig cells produce?
Testosterone
What is job of testosterone in development?
Promotes maintenance and further development of mesonephric ducts
What does the mesonephric duct in males develop into?
- Efferent ductules
- Epididymis
- Vas deferens
- Seminal vesicle
What are the remnants of the paramesonephric duct in males?
- Appendix testis
- Utriculus prostaticus
Describe the appendix testis
A small portion of tissue located on the upper pole of each testicle, which has no physiological function.
Where does the reproductive system empty into urinary system in males?
Prostatic urethra
Describe journey of sperm to prostatic urethra
- Sperm made in testis cords/seminiferous tubules
- Sperm passed to rete testis
- Passes to efferent ductules
- Passes to epididymis
- Vas deferens carries to prostatic urethra (meets wth seminal vesicles before)
What happens to the caudal tips of the 2 paramesonephric ducts in females?
They fuse together to form the uterus, cervix and upper 1/3 of vagina
What does the cranial part of the paramesonephric duct form?
Uterine tubes
Initially, what 3 are the paramesonephric ducts described as having?
- Cranial
- Horizontal
- Caudal
What does the horizontal part of the paramesonephric ducts form?
Uterine tubes
What remnants of the mesonephric ducts persist in females?
- Epophoron
- Paraoophoron
- Gartners cyst
What does the cloaca divide into? What is it separated by?
- Urogenital sinus
- Anus
Separated by urorectal sinus
What forms the lower 2/3 of the vagina?
Sinovaginal bulbs
Where do the sinovaginal bulbs develop from? How do they form the vagina?
Sinovaginal bulbs develop from urogenital sinus that fuse with caudal tips of paramesonephric ducts to form vagina
What type of structure does:
- Distal 2/3 of vagina
- Uterus, cervix, upper 1/3 of vagina
develop from?
Distal 2/3 of vagina: endoderm (from sinovaginal bulbs)
Uterus, cervix, upper 1/3 of vagina: intermediate mesoderm (from paramesonephric ducts)
What is the hymen?
Membrane between sinovaginal bulbs developing cranially and paramesonephric ducts developing caudally
What can cause abnormalities of uterus?
- Failure of paramesonephric ducts to fuse
- Failure of uterine septum to degenerate
- Failure of one paramesonephric duct to elongate
What can failure of paramesonephric ducts to fuse cause?
Double uterus, double vagina
What is bicornate uterus?
Incomplete fusion of paramesonephric ducts so there are 2 distinct uterine horns entering into single vagina
What are dangers of uterus abnormalities?
Higher rates of miscarriage, premature delivery, dystocia (difficult birth)
What is persisten mullerian duct syndrome?
Genetic male
Mutation in AMH gene so no AMH production
Paramesonephric ducts persist
Both mesonephric duct and paramesonephric duct derivatives develop (uterine tubes and primitive uterus too)
Normal male external genitalia but associated with cryptorchidism
What is cryptorchidism?
One or both of the testes fail to descend from the abdomen into the scrotum.
When does the development of the external genitalia begin?
In 3rd week of development
What happens in the ‘indifferent stage’ in development of external genitalia?
Either side of urogenital sinus, mesoderm/mesenchymal cells from primitive streak migrate to surround cloacal membrane to form cloacal folds (elevations)
What then happens to these cloacal folds (cranially)?
Fuse together cranially to form genital tubercle
What will the genital tubercle go on to form?
Future penis or clitoris
What do these cloacal folds divide into caudally?
Anal folds (posteriorly) and urethral folds (anteriorly)
What appears next to the urethral folds?
Genital swellings
What will the genital swellings go on to form?
Future scrotum or labia majora
What is the development of male external genitalia driven by?
Androgens, namely dihydrotestosterone (DHT).
Describe the development of the genital tubercle in males?
Elongates to form phallus
What is found between the urethral folds in males?
Urethral folds pulled together to form urethral groove (urethral folds form lateral walls of urethral groove)
What is the urethral groove lined by?
Lined by epithelium and form urethral plate (thickening)
Which side of the penis is ventral/dorsal when it is flaccid?
The bit you see is the dorsal (back) surface. Need to lift us penis to see ventral (front ) surface.
What is the penis like in the anatomical position?
Erect
What does the urethral groove go on to form?
Genital tubercle elongates and pulls urethral groove with it.
This extends along the caudal aspect of the phallus. The folds close over at end of 3rd month, forming the penile urethra.
What do the genital swellings in males become?
Scrotal swellings
How do scrotal swellings migrate? What then happens?
Migrate caudally and fuse in midline to form scrotum (separated by scrotal septum)
What is hypospadias? What is it caused by?
Urethra opening on underside (ventral side) of penis instead of at tip
What are complications of hypospadias? How can it be repaired?
- Difficulty urinating
- Can be repaired surgically using foreskin
What is epispadias? What is it caused by?
Urethra opening on dorsal surface of penis (upper aspect)
Caused by improper location of genital tubercle posterior to urogenital sinus –> urethral groove located on dorsal surface of penis
What is epispadias often associated with?
Bladder exstrophy
What is bladder exstrophy?
A congenital abnormality that occurs when the skin over the lower abdominal wall does not form properly. The bladder is open and exposed on the outside of the abdomen.
What is development of female external genitalia driven by?
Oestrogens
How does genital tubercle develop in females?
Elongates slightly to form clitoris
Describe development of urethral folds in females?
Urethral folds don’t fuse –> form labia majora
Urogenital groove remains open
