Embryology of the Reproductive System Flashcards
(54 cards)
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
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
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
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
5
Q
A
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”
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:
- outer cortex
- inner medulla
- the fate of medullary and cortical regions is dependent on TDF production
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
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
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
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
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
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
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
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
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
17
Q
During which week do the gonads become distinguishable?
A
the male and female gonads become distinguishable at 7 weeks
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:
- mesonephric (Wolffian) ducts
- 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
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:
- efferent ductules
- epididymis
- vas deferens
- seminal vesicles
- dihydrotestosterone stimulates development of the external genitalia, including growth of the penis, scrotum and prostate
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:
- uterine tube
- uterus
- upper portion of the vagina
- the external genitalia are stimulated:
- labia
- clitoris
- lower portion of vagina
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
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
Under the influence of oestrogens, how do the genital tubercle and genital swellings develop?
* 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
Why must the testes descend during development?
What structure must they pass through in order to do this?
* 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
How are the testes pulled towards the deep (internal) inguinal ring?
How long do they reside here for?
* 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
What is the inguinal canal formed by?
* the inguinal canal is formed by the **_processus vaginalis_**
* this is an **invagination of the parietal peritoneum**
29
What happens to the testes during month 7?
What aids this process and what happens by month 9?
* 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
During descent of the testes, what happens to their blood supply?
* 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
What 3 muscles make up the abdominal wall?
Where are the deep and superficial inguinal rings located?
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
As it passes through the abdominal wall, how does the testis become ensheathed in layers and what do these go on ot form?
* 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
What can happen if there is a patent processus vaginalis?
* 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
How can hydrocele be easily recognised?
What are the symptoms and treatment?
* 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
What is cryptorchidism and how common is it?
Why does it occur?
* 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
What is the treatment for cryptorchidism?
What is it associated with?
* 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
How do the ovaries descend during development?
* 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
What is produced by Sertoli cells and what does this result in?
* 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
Differentiation of what cell type is influenced by Sertoli cells?
What is produced by these cells?
* 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
What are the derivatives of the mesonephric duct in the male?
1. efferent ductules
2. epididymis
3. vas deferens
4. seminal vesicle
41
What do parts of the paramesonephric duct persist as in the male?
1. appendix testies
2. prostatic utricle
42
What drives development of the female genital ducts?
* 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
How can the paramesonephric duct be divided in the female?
What structures are derived from it?
* 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
What are the remnants of the mesonephric duct in the female?
1. epophoron
2. paraoophoron
3. Gartners cyst
45
How is the urogenital sinus involved in the formation of the vagina?
* 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
What are the 2 origins of the vagina?
* the **upper portion** is derived from the **_uterine canal_**
* the **lower portion** is derived from the **_urogenital sinus_**
47
Why might duplications of the uterus arise?
* 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
What can happe if there is complete or partial atresia of one of the paramesonephric ducts?
* 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
What can happen if the sinovaginal bulbs fail to fuse or do not develop at all?
* 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
What are the 3 main causes of abnormalities of the uterus and what are they associated with?
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
Who is affected by persistent Müllerian duct syndrome?
Why does this occur?
* 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
What is hypospadias and why does it occur?
What is the main symptom and how can it be repaired?
* **_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
What is epispadias and why does it occur?
What other abnormality is this associated with?
* 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
