Embryology of the Reproductive System Flashcards

1
Q

How is genetic sex determined?

A
  • genetic sex is determined at fertilisation by the sperm
  • the SRY (sex-determining region of the Y chromosome) gene initiates production of Testis-determining factor (TDF)
  • if there is no Y chromosome, there is no TDF and female development commences
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2
Q

What is the reproductive system derived from?

A

intermediate mesoderm

  • the intermediate mesoderm gives rise to the urogenital ridge
  • the urogenital ridge gives rise to the urinary system and reproductive system
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3
Q

How do the gonads first appear?

What type of cell appears in the gonads?

A
  • gonads appear as a pair of longitudinal ridges (genital / gonadal ridges)
  • gonadal ridges are formed by proliferation of the epithelium and condensation of the underlying mesenchyme
  • the germ cells** do not appear in the genital ridges until **week 6
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4
Q

Where do primordial germ cells originate?

When do they enter the genital ridges?

A
  • PGCs originate in the epiblast and migrate through the primitive streak
  • they arrive at the primitive gonads at the beginning of week 5 and invade the genital ridges in week 6
  • if PGCs fail to reach the genital ridges, the gonads do not develop
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5
Q
A
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6
Q

What is meant by the indifferent gonad?

How and when does this develop?

A
  • shortly before and during arrival of the PGCs, the epithelium of the genital ridge proliferates and penetrates the underlying mesenchyme
  • the epithelial cells form irregularly shaped cords - the primititve sex cords
  • in male and female embryos, the cords are connected to surface epithelium
  • at this stage (weeks 5-6) it is impossible to determine between the male and female gonad so it is the “indifferent gonad”
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7
Q

How can the indifferent gonad be split into 2 different regions?

A
  • the indifferent gonad can be split developmentally into 2 distinct regions:
  1. outer cortex
  2. inner medulla
  • the fate of medullary and cortical regions is dependent on TDF production
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8
Q

What is formed from the primitive sex cords if an embryo is genetically male?

What is carried by the PGCs?

A
  • the PGCs carry an XY sex chromosome complex
  • under the influence of the SRY gene on the Y-chromosome, which encodes TDF, the primitive sex cords continue to proliferate and penetrate deep into the medulla
  • they form the testis** or **medullary cords
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9
Q

Following formation of the medullary cords, what else develops in pursuit?

A

Rete testis:

  • towards the hilum of the gland, the cords break up into a network of tiny cell strands
  • these will later give rise to the tubules of the rete testis

Tunica albuginea:

  • this is a dense layer of fibrous connective tissue that separates the medullary cords from the surface epithelium
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10
Q

What happens to the testis cords in the 4th month?

What cell types are they composed of at this stage?

A
  • the testis cords become horseshoe-shaped
  • their extremities are now continuous with those of the rete testis
  • the cords are composed of primitive germ cells and Sertoli cells
    • Sertoli cells are derived from the surface epithelium of the gland
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11
Q

What cell type lies between the testis cords?

When do they begin development and what is their function?

A

interstitial cells of Leydig

  • they are derived from the mesenchyme of the gonadal ridge
  • they begin development shortly after onset of differentiation of the testis cords
  • by week 8, they begin production of testosterone
  • the testis is able to influence sexual differentiation of the genital ducts and external genitalia
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12
Q

What happens to the testis cords at puberty?

What does this structure enter and join with?

A
  • testis cords remain solid until puberty, when they canalise to form the seminiferous tubules
  • the seminiferous tubules join the rete testis tubules, which in turn enter the ductuli efferentes
    • these are the remaining parts of the excretory tubules of the mesonephric system
  • the ductuli efferentes link the rete testis and the Wolffian (mesonephric duct), which becomes the ductus deferens
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13
Q

What happens to the primitive sex cords in female embryos?

A
  • female embryos have an XX chromosome complex and no Y chromosome
  • the primitive sex cords dissociate into irregular cell clusters, which contain groups of primitive germ cells
  • the cell clusters occupy the medullary part of the ovary
  • later on they disappear and are replaced by a vascular stroma that forms the ovarian medulla
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14
Q

What happens to medullary cords in a female embryo?

What type of cord develops?

A
  • the medullary cords degenerate
  • the surface epithelium continues to proliferate (unlike in males) and gives rise to cortical cords in week 7
  • cortical cords penetrate the underlying mesenchyme but remain close to the surface
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15
Q

What happens to the cortical cords in month 3?

What is formed at the end of this process?

A
  • the cortical cords split into isolated cell clusters
  • cells in the clusters continue to proliferate and begin to surround each oogonium with a layer of epithelial cells - follicular cells
  • the oogonia and the follicular cells together constitute a primordial follicle
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16
Q

What is the difference in the structures that develop and regress depending on whether the spermatocyte carries an X or Y chromosome?

A

In presence of Y chromosome (44 + XY):

  • medullary cords develop
  • there are no cortical cords
  • there is a thick tunica albuginea

In absence of Y chromosome (44 + XX):

  • medullary cords degenerate
  • cortical cords develop
  • there is no tunica albuginea
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17
Q

During which week do the gonads become distinguishable?

A

the male and female gonads become distinguishable at 7 weeks

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18
Q

At the stage of the indifferent gonad, what pairs of genital ducts are present?

A
  • both male and female embryos initially have 2 pairs of genital ducts:
  1. mesonephric (Wolffian) ducts
  2. paramesonephric (Müllerian) ducts
  • the paramesopnephric duct arises as a longitudinal invagination of epithelium on the anterolateral surface of the urogenital ridge
  • cranially, the duct opens into the abdominal cavity with a funnel-like structure
  • caudally, it comes into contact with the paramesonephric duct from the opposite side in the midline
  • the 2 ducts project into the posterior wall of the urogenital sinus to form the sinus tubercle (small swelling)
  • the mesonephric ducts open into the urogenital sinus on either side of the sinus tubercle
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19
Q

What is the fate of the genital ducts in the male?

What hormones / substances drive these changes?

A

Sertoli cells:

  • secrete anti-Müllerian hormone (AMH) which suppresses development of the paramesonephric ducts
  • a small portion of the paramesonephric ducts remain as the appendix testis

Leydig cells:

  • secrete testosterone which stimulates development of the mesonephric ducts, which will go on to form:
  1. efferent ductules
  2. epididymis
  3. vas deferens
  4. seminal vesicles
  • dihydrotestosterone stimulates development of the external genitalia, including growth of the penis, scrotum and prostate
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20
Q

What happens to the genital ducts in the female?

What structures do they go on to form?

A
  • in the presence of oestrogens (maternal and placental sources) and absence of AMH and testosterone
  • the paramesonephric ducts are stimulated and go on to form:
  1. uterine tube
  2. uterus
  3. upper portion of the vagina
  • the external genitalia are stimulated:
  1. labia
  2. clitoris
  3. lower portion of vagina
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21
Q

At the indifferent stage, what does the external genitalia consist of?

A
  • in week 3, mesenchyme cells migrate around the cloacal membrane to form a pair of elevated cloacal folds
  • cranial to the cloacal membrane, the folds unite to form the genital tubercle
  • caudally, the folds are subdivided into urethral folds and anal folds
  • the genital swellings appear on either side of the urethral folds
    • these will go on to form the scrotal swellings and labia majora
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22
Q

Under the influence of androgens, what happens to the genital tubercle?

A
  • the genital tubercle is now called the phallus
  • it rapidly elongates and pulls the urethral folds forwards so that they form the lateral walls of the urethral groove
  • the urethral groove extends along the caudal aspect of the phallus but does not reach the glans (most distal part)
  • the epithelial lining of the groove forms the urethral plate
23
Q

How are the penile urethra and external urethral meatus formed?

A
  • at the end of month 3, the 2 urethral folds close over the urethral plate to form the penile urethra
  • the penile urethra does NOT extend to the tip of the phallus
  • the most distal portion of the urethra is formed in month 4 when ectodermal cells from the tip of the glans penetrate inwards and form a short epithelial cord
  • the cord later obtains a lumen to form the external urethral meatus
24
Q

What is the fate of the genital swellings in the male?

A
  • they are known as the scrotal swellings and they arise in the inguinal region
  • they move caudally and each swelling makes up half of the scrotum
  • the 2 swellings are separated by the scrotal septum
25
Q

Under the influence of oestrogens, how do the genital tubercle and genital swellings develop?

A
  • the genital tubercle elongates only slightly to form the clitoris
  • the urethral folds do not fuse and develop into the labia minora
  • the genital swellings enlarge to form the labia majora
26
Q

Why must the testes descend during development?

What structure must they pass through in order to do this?

A
  • the testes develop retroperitoneally in the abdominal region
  • they must move caudally and pass through the abdominal wall to reach the scrotum
  • passage through the abdominal wall is via the inguinal canal
  • entry to the inguinal canal is via the deep (internal) inguinal ring and exit is via the superficial (external) inguinal ring
27
Q

How are the testes pulled towards the deep (internal) inguinal ring?

How long do they reside here for?

A
  • the extra-abdominal gubernaculum shortens and pulls the testes towards the anterior abdominal wall in weeks 7-12
  • the testes arrive at the deep (internal) inguinal ring and remain here from 3rd to 7th month
28
Q

What is the inguinal canal formed by?

A
  • the inguinal canal is formed by the processus vaginalis
  • this is an invagination of the parietal peritoneum
29
Q

What happens to the testes during month 7?

What aids this process and what happens by month 9?

A
  • the gubernaculum begins to shorten again, which pulls the testes through the inguinal canal of the abdominal wall
  • this process is aided by pressure from growing abdominal organs
  • the testes reach the scrotum by month 9, just before birth
  • within the 1st year, the processus vaginalis obliterates and leaves a remnant around the testes called the tunica vaginalis
30
Q

During descent of the testes, what happens to their blood supply?

A
  • during descent, blood supply to the testes from the aorta is maintained
  • testicular vessels extend from their original lumbar position to the testis in the scrotum
31
Q

What 3 muscles make up the abdominal wall?

Where are the deep and superficial inguinal rings located?

A
  1. transversus abdominis (and transversalis fascia)
  2. internal oblique
  3. external oblique
  • the deep ring is the entrance to the inguinal canal and is located in the transversalis fascia
  • the superficial ring is the exit of the inguinal canal and is located in external oblique
32
Q

As it passes through the abdominal wall, how does the testis become ensheathed in layers and what do these go on ot form?

A
  • the testes are accompanied by peritoneal layers derived from the processus vaginalis
  • as they descend, they become ensheathed in layers derived from the anterior abdominal wall:
  • the transversalis fascia forms the internal spermatic fascia
  • the internal oblique muscle forms the cremasteric fascia and muscle
  • the external oblique muscle forms the external spermatic fascia
  • the transversus abdominis does not contribute a layer as it arches over this region and does not cover the path of migration
33
Q

What can happen if there is a patent processus vaginalis?

A
  • parts of the processus vaginalis may remain unfused, resulting in formation of small cysts
  • the cysts can produce fluid and swell, resulting in hydrocele
  • if the processus vaginalis remains open to the abdominal cavity, intenstines may protrude through the inguinal canal and produce an indirect inguinal hernia
34
Q

How can hydrocele be easily recognised?

What are the symptoms and treatment?

A
  • as the hydrocele is formed by clear fluid, light passes through it easily (transillumination)
  • solid tissue of a tumour or hernia would block the light
  • hydrocele is benign but larger ones may cause discomfort
  • they can be removed surgically
35
Q

What is cryptorchidism and how common is it?

Why does it occur?

A
  • in 97% of cases, the testes are present in the scrotum before birth
  • in most of the remainder, descent will happen in the first 3 months postnatally
  • in <1% of infants, one or both of the testes will fail to descend - this is cryptorchidism
  • it may be caused by decreased androgen (testosterone) production
36
Q

What is the treatment for cryptorchidism?

What is it associated with?

A
  • those that remain undescended are surgically corrected at 4-6 months
  • the undescended testes fail to produce spermatozoa, so it is associated with infertility
  • it is associated with a 3-5% incidence of renal anomalies and increased risk of testicular cancer
37
Q

How do the ovaries descend during development?

A
  • the ovaries descend to the pelvic brim
  • the gubernaculum passes through the inguinal canal to insert onto the labia majora
  • it persists in the adult as the ovarian ligament proper and the round ligament of the uterus
38
Q

What is produced by Sertoli cells and what does this result in?

A
  • Sertoli cells secrete anti-Müllerian hormone (AMH) which causes degeneration of the paramesonephric ducts
  • Sertoli cells differentiate under the influence of TDF, which is encoded for by the SRY gene
39
Q

Differentiation of what cell type is influenced by Sertoli cells?

What is produced by these cells?

A
  • Sertoli cells cause differentiation of the mesoderm of the gonad into Leydig cells
  • Leydig cells produce testosterone
  • testosterone promotes the maintenance and further development of the mesonephric ducts
40
Q

What are the derivatives of the mesonephric duct in the male?

A
  1. efferent ductules
  2. epididymis
  3. vas deferens
  4. seminal vesicle
41
Q

What do parts of the paramesonephric duct persist as in the male?

A
  1. appendix testies
  2. prostatic utricle
42
Q

What drives development of the female genital ducts?

A
  • development of the female genital ducts occurs in the absence of the SRY gene
  • as there is no anti-Müllerian hormone, the paramesonephric ducts remain
  • as there is no testosterone, the mesonephric ducts degenerate
43
Q

How can the paramesonephric duct be divided in the female?

What structures are derived from it?

A
  • the paramesonephric duct can be divided into 3 parts:
  1. cranial part
  2. horizontal part
  3. caudal part
  • the cranial and horizontal parts become the uterine tubes
  • the caudal parts fuse together to form the uterine canal
44
Q

What are the remnants of the mesonephric duct in the female?

A
  1. epophoron
  2. paraoophoron
  3. Gartners cyst
45
Q

How is the urogenital sinus involved in the formation of the vagina?

A
  • the cloaca divides into the anus and the urogenital sinus
  • the solid tip of the paramesonephric ducts contacts the urogenital sinus and 2 solid evaginations grow out from the pelvic part of the sinus - the sinovaginal bulbs
  • the sinovaginal bulbs proliferate to form a solid vaginal plate
  • proliferation continues at the cranial end of the plate, increasing the distance between the uterus and urogenital sinus
  • by month 5, the vaginal outgrowth is completely canalised
46
Q

What are the 2 origins of the vagina?

A
  • the upper portion is derived from the uterine canal
  • the lower portion is derived from the urogenital sinus
47
Q

Why might duplications of the uterus arise?

A
  • duplications of the uterus result from a lack of fusion of the paramesonephric ducts in a local area or throughout their normal line of fusion
  • the most severe form is uterus didelphys (double uterus)
  • the least severe form is uterus arcuatus in which there is a slight indentation in the centre
  • uterus bicornis is relatively common and involves the uterus having 2 horns but entering a common vagina
48
Q

What can happe if there is complete or partial atresia of one of the paramesonephric ducts?

A
  • the rudimentary part lies as an appendage to the well-developed side
  • the lumen does not communicate with the vagina, so complications are common
  • if atresia involves both sides, atresia of the cervix may result
49
Q

What can happen if the sinovaginal bulbs fail to fuse or do not develop at all?

A
  • if the sinovaginal bulbs fail to fuse, this results in a double vagina
  • if the sinovaginal bulbs do not develop, this results in atresia of the vagina
50
Q

What are the 3 main causes of abnormalities of the uterus and what are they associated with?

A
  1. failure of paramesonephric ducts to fuse
  2. failure of uterine septum to degenerate
  3. failure of one paramesopnephric duct to elongate
  • uterine abnormalities are associated with higher rates of miscarriage, premature delivery or dystocia (difficult birth)
51
Q

Who is affected by persistent Müllerian duct syndrome?

Why does this occur?

A
  • it affects genetic males and involves a mutation in the AMH gene
  • there is no AMH production, so the paramesonephric ducts persist
  • both mesonephric and paramesonephric derivatives develop
  • there is normal male external genitalia, but it is associated with cryptochidism
52
Q

What is hypospadias and why does it occur?

What is the main symptom and how can it be repaired?

A
  • fusion of the urethral folds is incomplete, resulting in abnormal openings along the inferior aspect of the penis
  • abnormal openings are usually near the glans, along the shaft or near the base of the penis
  • it causes difficulty urinating and can be repaired surgically using the foreskin
53
Q

What is epispadias and why does it occur?

What other abnormality is this associated with?

A
  • this occurs when the urethral meatus is located on the dorsal surface of the penis
  • it can occur in isolation but is often associated with exstrophy of the bladder and abnormal closure of the ventral body wall
  • it results from improper location of the genital tubercle posterior to the urogenital sinus
  • this results in the urethral groove being located on the dorsal surface of the penis
54
Q
A