Reproductive System Week 1 Flashcards
Which chromosome is the key to sexual dimorphism?
The Y chromosome
Which gene on the Y chromosome is responsible for male development?
SRY (Sex-determining region on Y) gene on Yp11
Which protein is produced from SRY and what is its function?
Testis-determining factor - transcription factor initiating a cascade of downstream genes that cause male development
In what week of development do the gonads acquire male or female characteristics?
Week 7
How do the gonads initially appear in the embryo?
As a pair of longitudinal ridges - ‘genital or gonadal ridges’
How are the genital ridges formed?
By proliferation of the epithelium and a condensation of the underlying mesenchyme
In what week of development do germ cells appear in the genital ridges?
Week 6
Describe the journey of the primordial germ cells to the genital ridges
Originate in the epiblast
Migrate through the primitive streak
By week 3 reside among endoderm cells in the wall of the yolk sac, close to the allantois
In 4th week migrate by ameboid movement along the dorsal mesentery of the hindgut
Arrive at the primitive gonads at the beginning of the 5th week
Invade the genital ridges in the 6th week
How do the germ cells affect the development of the gonads?
If they fail to reach the ridges, the gonads do not develop
They have an inductive influence on the development of the gonad into ovary or testis
Describe the development of the primitive sex cords
Shortly before and during arrival of the primordial germ cells, the epithelium of the genital ridge proliferates and epithelial cells penetrate the underlying mesenchyme
Here they form a number of irregularly shaped cords
These cords are connected to surface epithelium in both female and male embryos
Is it possible to differentiate between the male and female gonad in this early stage?
No
They are known as the indifferent gonads
What sex chromosomes are carried in the primordial germ cells of an embryo that is genetically male?
XY (can have multiple X)
What sex chromosomes are carried in the primordial germ cells of an embryo that is genetically female
XX (no Y)
What effect does testis-determining factor have on the primitive gonads?
The primitive sex cords continue to proliferate and penetrate deep into the medulla to form the testis/medullary cords
What happens towards the hilum of the gland?
The cords break up into a network of tiny cell strands that later give rise to tubules of the rete testis
What is the tunica albuginea?
A dense layer of fibrous connective tissue that develops and separates the testis cords from the surface epithelium of the gonad
What happens to the testis cords in the 4th month of development?
The testis cords become horseshoe-shaped and their extremities are continuous with those of the rete testis
What are the testis cords composed of in the 4th month of development?
Primitive germ cells and sustentacular cells of Sertoli (from the surface epithelium of the gonad)
What are the names of the cells that lie between the testis cords?
Interstitial cells of Leydig
Where are the interstitial cells of Leydig derived from?
From the original mesenchyme of the gonadal/genital ridge
When do the interstitial cells of Leydig develop?
They begin development shortly after onset of differentiation of the testis cords (7th week)
What do the interstitial cells of Leydig produce and what is the consequence of its production?
Testosterone
Influences the sexual differentiation of the genital ducts and external genitalia
What happens to the testis cords during puberty?
They acquire a lumen, forming the seminiferous tubules
Describe the structures you would pass through when travelling from the seminiferous tubules to the ductus deferens
Seminiferous tubules
Rete testis tubules
Ductuli efferentes
Ductus deferens
From what do the ductuli efferentes develop?
The excretory components of the mesonephric system
From what did the ductus deferens differentiate?
The mesonephric or Wolffian duct
What is the fate of the primitive sex cords in the female embryo?
They dissociate into irregular cell clusters, containing groups of primitive germ cells and occupying the medullary part of the ovary
Later replaced by vascular stroma/connective tissue that forms the ovarian medulla
What is different about the surface epithelium of the female gonad compared to the male gonad?
It continues to proliferate and in the 7th week gives rise to cortical cords, which penetrate the underlying mesenchyme but remain close to the surface
Describe the fate of the cortical cords
In the 3rd month the cords split into isolate cell clusters
Cells in these clusters proliferate
They surround each oogonium with a layer of epithelial cells - “follicular cells”
What constitutes a primordial follicle?
The oogonium and surrounding follicular cells
What is the indifferent stage with respect to the genital ducts?
Both male and female embryos start with two pairs of genital ducts: mesonephric (Wolffian) ducts and paramesonephric (Mullerian) ducts
Describe the paramesonephric duct
Arises as a longitudinal invagination of the epithelium on the anterolateral surface of the urogenital ridge
Cranially the duct opens into the abdominal cavity with a funnel-like structure
Caudally it runs lateral to the mesonephric duct, crosses ventrally to grow caudomedially
Comes into contact with the paramesonephric duct from the opposite side in the midline
The caudal tip of the combined ducts projects into the posterior wall of the urogenital sinus
Causes a small swelling - sinus tubercle
Where do the mesonephric ducts open into?
The urogenital sinus on either side of the sinus tubercle
What effect does testosterone have on the genital ducts?
The epigenital tubules (the excretory tubules of the mesonephros) contact the rete testis and form the efferent ductules
The excretory tubules at the pole of testis (paragenital tubules) do not join the rete testis - become a remnant known as paradidymis
The mesonephric duct persists (apart from appendix epididymis) and forms the main genital duct
Immediately below the efferent ductules the mesonephric duct elongates and becomes highly convoluted forming the ductus epididymis
Between the epididymis and the outbidding of the duct (seminal vesicle) obtains a thick muscular coat - forming ductus deferens
Region of the duct beyond seminal vesicle - ejaculatory duct
What hormone is produced by Sertoli cells and what is its action?
Anti-mullerian hormone or Mullerian inhibiting substance
Causes the paramesonephric ducts to degenerate except for the small portion at the cranial ends (appendix testis)
What causes the paramesonephric duct to become the main genital duct of the female?
The presence of oestrogen and the absence of testosterone and MIS(AMH)
Describe what happens to the paramesonephric duct in the female
With descent of the ovary:
The cranial and horizontal portions develop into the uterine tube
The caudal parts fuse to form the uterine canal
Describe the formation of the broad ligament of the uterus
As the horizontal part of the paramesonephric duct moves mediocaudally, the urogenital ridges come to lie in a transverse plane
After the ducts fuse in the midline a broad transverse pelvic fold is established
Extends from the lateral sides of the fused paramesonephric ducts toward the wall of the pelvis
How is the pelvic cavity divided in the female?
The uterus and broad ligaments divide the cavity into the uterorectal pouch and the uterovesical pouch
What structures do the fused paramesonephric ducts give rise to?
The corpus and cervix of the uterus
And the upper portion of the vagina
What structures of the uterus are formed from mesenchyme?
The myometrium and the perimetrium
What is the consequence of the absence of testosterone in females?
The mesonephric ducts degenerate
In what week of development does the uterine septum disappear?
Week 9
What is the name of the two solid evaginations that grow out from the pelvic part of the sinus shortly after the sinus tubercle is formed?
The sinovaginal bulbs
Describe the fate of the sinovaginal bulbs
They proliferate and form a solid vaginal plate
Proliferation continues at the cranial end - increasing the distance between the uterus and the urogenital sinus
By what month is the vaginal outgrowth entirely canalised?
5th month
What is the name of the wing-like expansions of the vagina that surround the end of the uterus and what is their origin?
Vaginal fornices
Paramesonephric origin
What is the embryonic origin of the vagina?
Has a dual origin :
- upper portion (vaginal fornices) are paramesonephric
- lower portion is derived from urogenital sinus
What separates the lumen of the vagina from the urogenital sinus?
A thin layer of tissue consisting of a thin layer of vaginal cells and the epithelial lining of the sinus - the hymen
Usually develops an opening during perinatal life
What are the names of the remnants of the cranial and caudal excretory tubules found in the mesovarium?
Epoophoron
Paroophoron
Does the entirety of the mesonephric duct disappear?
A small cranial portion can be found in the epoophoron and occasionally a small caudal portion may be found in the wall of the uterus or vagina
What is the consequence of the small caudal remnant of the mesonephric duct that may be found on the wall of the uterus or vagina?
Later in life may form Gartner cyst
What is the pathophysiology behind duplications of the uterus and what are the names of these conditions?
Lack of fusion of the paramesonephric ducts:
In a local area:
- Uterus bicornis (relatively common - uterus has two horns entering common vagina)
- Uterus arcuatus (least severe - slight indentation in middle)
Throughout their normal line of fusion:
- Uterus didelphys (entirely double with double vagina)
What are the consequences of and conditions associated with complete or partial atresia of one or both of the paramesonephric ducts?
One duct:
The rudimentary part lies as an appendage to the well-developed side
It’s lumen usually does not communicate with the vagina - complications are common
Uterus bicornis unicorns with one rudimentary horn
Two ducts:
Atresia of the cervix may result
If the sinovaginal bulbs fail to fuse - double vagina
If sinovaginal bulbs fail to develop - vaginal atresia with small vaginal pouch derived from paramesonephric ducts surrounding uterus
Describe the indifferent stage with respect to the external genitalia
3rd week of development
Mesenchyme cells originating in the primitive streak migrate around the cloacal membrane
Form a pair of slightly elevated cloacal folds
Cranial to cloacal membrane folds unite to form genital tubercle
Caudally the folds are subdivided to into urethral folds anteriorly and anal folds posteriorly
Genital swellings become visible on each side of the urethral folds
Cant distinguish the two sexes at the end of week 6
What is the consequence of androgen secretion for the external genitalia?
Rapid elongation of the genital tubercle - now called phallus
This pulls the urethral folds forward - so they form lateral walls of the urethral groove
The groove extends along the caudal phallus but does not reach the glans
Urethral plate formed from endoderm - epithelial lining of the groove
At the end of 3rd month - urethral folds close over the urethral plate forming the penile urethra (does not extend to phallus tip)
In the 4th month ectodermal cells from the tip of the glans penetrate inward and form a short epithelial cord - later obtains lumen and becomes external urethral meatus
The scrotal swellings arise in the inguinal regions and move caudally with development - each makes up half of the scrotum and they are separated by the scrotal septum
Describe the meaning of the term hypospadia
Fusion of the urethral folds is incomplete
Abnormal openings of the urethra occur along the inferior aspect of the penis - near the glans, along the shaft or near the base, rarely extends along scrotal raphe
When fusion of folds fails entirely - wide sagittal slit along entire length of penis and scrotum - scrotal swellings resemble labia majora
Incidence of 3-5/1000 births
Doubled over the past 15-20 years
Potentially due to rise in environmental oestrogens
What is epispadia?
Rare abnormality (1/30000) Urethral meatus is found on dorsal of penis May occur as isolated defect Most often associated with exstrophy of the bladder and abnormal closure of the ventral body wall