Development of the Genital Ducts and External Genitalia Flashcards
What is significant about the first 13 weeks in development?
It is difficult to determine the sex of a baby from their external genitalia
What is the reproductive system derived from?
Intermediate mesoderm
This forms the urogenital ridge
What develops from the urogenital ridge?
- urinary system
2. reproductive system
What are primitive sex cords and what are they derived from?
Thickened epithelium (derived from mesoderm) of the genital ridge produces primitive sex cords
The primitive sex cords migrate into the underlying mesenchyme
What is the alternative name for primitive sex cords?
Medullary cords
What is significant about primordial germ cells?
They migrate from the yolk sac and contain the autosomes that will determine male or female development
In the presence of primordial germ cells carrying XY chromosomes, what happens to the primitive sex cords?
The somatic support cells in the medullary cords differentiate into Sertoli cells
What will the Sertoli cells go on to form?
The primordial germ cells are invested by the Sertoli cells to form testes cords
These are solid cords
What do the testes cords go on to form?
The testes cords develop further and separate from the overlying epithelium to form seminiferous tubules
What happens to the seminiferous tubules at puberty?
They are solid until puberty
At puberty they recanalise, allowing the male to produce sperm
How does TDF influence the male gonadal development?
TDF maintains the medullary cords
How do the primordial germ cells influence female gonadal development?
The PGCs migrate to the posterior wall and stimulate breakdown of the medullary cords
Cortical cords develop
What do the cortical cords go on to form?
The somatic support cells in the cortical cords invest the PGCs
This leads to formation of primordial follicles
What duct system remains in male development?
TDF maintains the mesonephric ducts
The paramesonephric duct is broken down
What duct system remains in female development?
Absence of TDF causes the mesonephric duct to break down
The paramesonephric duct continues to develop
What are the alternative names for the mesonephric and the paramesonephric ducts?
The paramesonephric duct system is the Müllerian system
The mesonephric duct system is the Wolffian system
What encodes TDF and drives development of male genital ducts?
The SRY gene encodes for TDF
This is testis determining factor (TDF)
What happens after the somatic support cells invest the PGCs and differentiate into Sertoli cells?
Sertoli cells secrete anti-müllerian hormone (AMH)
What is the role of anti-müllerian hormone (AMH)?
It actively causes the breakdown of the paramesonephric (Müllerian) ducts
How do the Sertoli cells affect the underlying mesenchyme of the gonad?
They cause differentiation of the mesenchyme into Leydig cells
What is the role of Leydig cells?
They produce testosterone
This promotes maintenance and further development of mesonephric ducts
What 4 structures will the mesonephric duct develop into?
- efferent ductules
- epididymis
- vas deferens
- seminal vesicle
What is formed from the remnants of the paramesonephric duct in males?
- appendix testis
2. utriculus prostaticus
Why do the paramesonephric ducts develop in females?
There is no anti-müllerian hormone
The paramesonephric ducts remain
What happens to the mesonephric duct system in females and why?
The mesonephric ducts degenerate as there is no testosterone to maintain them
How is the uterine canal (uterus) formed from the paramesonephric duct system?
The caudal tips of the duct system fuse together to form the uterine canal
What are the 3 parts of the paramesonephric duct and what do they form?
- cranial
- horizontal
- caudal
Cranial and horizontal parts become the uterine tubes
Caudal parts fuse to form the uterine canal
What structures are formed from the remnants of the mesonephric duct in females?
- Epoophoron
- Paraoophoron
- Gartner’s cyst
Once the caudal tips of the paramesonephric duct system fuse together, what must happen?
The uterine septum between them must break down to form a single uterine cavity
How does the cloaca divide?
What are the 2 parts formed?
The urorectal septum continues to develop to separate the cranial and caudal parts of the cloaca
Cranial part - urogenital sinus
Caudal part - anorectal canal
What are the sinovaginal bulbs and where do they develop from?
The sinovaginal bulbs are a thickening of endoderm from the urogenital sinus
How does the vagina form?
The sinovaginal bulbs fuse with the caudal tips of the paramesonephric ducts to form the vagina
What are the 2 origins of the vagina?
- the distal part of the vaginal canal is formed from the sinovaginal bulbs (endoderm)
- the upper vaginal canal is formed from paramesonephric ducts (mesoderm)
What can cause double uterus and double vagina?
Failure of caudal tips of paramesonephric ducts to fuse together
What causes double uterus?
but only one vagina
The paramesonephric ducts do fuse, but only at the very tip
What causes bicornate uterus?
The paramesonephric ducts have fused, but slightly in the wrong location
What causes cervical atresia?
What is this?
The caudal tips of the paramesonephric ducts only just meet
This leads to the cervix being very thin
What causes unicornate uterus?
Only one paramesonephric duct doesn’t develop properly
This leads to only one uterine tube being present
What is septated uterus and what causes it?
The septum between the 2 paramesonephric ducts may break down
What % of fertile women are affected by abnormalities of the uterus?
What are the risks associated with this?
1%
This leads to higher rates of:
- miscarriage
- premature delivery
- dystocia (difficult birth)
What causes persistent müllerian duct syndrome?
A mutation in the AMH gene
This means there is no anti-müllerian hormone so the paramesonephric ducts persist
What is involved in persistent müllerian duct syndrome?
Both mesonephric and paramesonephric duct derivatives develop
There are developing testes, but a uterus also develops
What are the genitalia like in persistent müllerian duct syndrome?
There is normal male genitalia but it is associated with cryptorchidism
What is the indifferent stage in development of the external genitalia?
Up until 13 weeks in development, the male and female external genitalia is almost the same
What is the first stage in development of the external genitalia?
Mesoderm cells migrate to surround the cloacal membrane
They form elevations called cloacal folds
What do the cloacal folds go on to form?
Cloacal folds unite anteriorly to form the genital tubercle
This is the future penis or clitoris
What will the cloacal folds divide into?
Anal and urethral divisions
The urogenital septum separates the cloacal (urethral) fold from the anal fold
What will appear next to the urethral folds?
Genital swellings
These form the future scrotum or labia majora
What is development of male external genitalia dependent on?
Androgens - testosterone
How does testosterone affect the genital tubercle?
It drives elongation of the genital tubercle to form the phallus
As the genital tubercle elongates, it pulls the urethral groove with it
What is the urethral groove?
It is the space between the urethral folds
What happens to the genital swellings as the genital tubercle develops?
The genital swellings enlarge and move caudally
What is the urethral plate?
A thickening of the urethral groove
What happens to the urethral folds at the end of the 3rd month?
The urethral folds close over the urethral plate to form the penile urethra
What happens to the scrotal swellings at the end of the 3rd month?
They fuse in the midline
They are separated by the scrotal septum
What causes hypospadias?
Incomplete fusion of the urethral folds
The urethra opens on the ventral surface of penis
how many births are affected by hypospadias?
What does it lead to and how can it be repaired?
3-5 in 1,000 births
It causes difficulty urinating
It can be repaired surgically using the foreskin
What is epispadias?
The urethra opens on the dorsal side of the penis
It often is associated with exstrophy of the bladder
How many births are affected by epispadias?
1 in 30,000
What causes epispadias?
Improper location of the genital tubercle posterior to the urogenital sinus
This means the urethral groove is located on the dorsal surface of the penis
What is development of the female external genitalia dependent on?
Oestrogens
What happens to the genital tubercle in development of female external genitalia?
The genital tubercle elongates slightly to form the clitoris
What happens to the urethral folds and genital swellings in development of the female external genitalia?
The urethral folds do NOT fuse and go to form the labia minora
Genital swellings form the labia majora
What happens to the urogenital groove in female development>
it remains open
