Lecture 4 - Development of the Reproductive Tracts Flashcards
Summarise the steps of embryology up until gastrulation in week 3
- Fertilisation in ampulla of uterine tube
- After 30 hours, fertilised oocyte splits into two equal size blastomeres
- After 3 more divisions (16 cells), group of cells called morula
- Morula re-organised into cavity, known as blastocyst, comprised of outer cell mass (trophoblast) and inner cell mass (embryoblast)
- Week 3, gastrulation takes place, two cell layers becomes three layered disc
What does gastrulation involve?
What occurs after gastrulation in week 4?
- Cellular rearrangement involving migration, invagination and differentiation of the epiblast (inner cell mass comprised of epiblast and hypoblast). Controlled by primitive streak
- Folding (cranio-caudal & lateral) of the embryo leading to formation of body compartments. Loss of the yolk sac, production of peritoneal cavity, primitive gut tube and anterior body wall.
What caudal opening does the urinary tract, reproductive tract and GI tract initially share?
What does this structure become?
- The cloaca, responsible for the anal and urogenital openings
- This then becomes the urogenital sinus, formed from the cloaca in the 4-7th week
How & when does differentialisation of sex occur during development?
- M & F morphological characteristics don’t begin till 7th week, the two genital systems until this point are similar so is an ‘indifferent stage of sexual development’
- Chromosomal sex of embryo determined at fertilisation by kind of sperm (either X or Y), female is always X.
- Y (male) chromosome contained sex-determining region (SRY) on its short arm, protein product of this gene is transcription factor testis-determining factor (TDF) which initiate cascade of downstream genes that determine fate of sexual organs.
- So in presence of TDF, male development occurs
- In absence of TDF, female development occurs
What cell drives the production of reproductive organs in both males and females?
How does it achieve this?
- Primordial germ cells
- Migrate along retroperitoneum to the gonad at the genital ridge
- Karyotype of fertilised egg (XX or XY) determined karyotype of the primordial germ cells which determined whether gonad differentiates into male or female
Both male and female internal genetalia are formed from ductal system, how are these 2 different ductal systems triggered to develop & what are their names?
In Males:
- Primordial germ cells in males carry Y chromosome, cause expression of SRY genes.
- This causes production of gonad (testis), production of testosterone by leydig cells (driving mesonephric duct system) and release of MIH by sertoli cells (causing degeneration of paramesonepheric duct system).
In Females:
- Absence of Y chromosome on primordial germ cells causes development of female gonad (ovary) and internal genetalia via the paramesonephric ductal system (due to absence of testosterone)
Do both ducts develop in male and female embryos during the indifferent stage?
Where do both these ducts end?
What are other names for the mesonephric and paramesonephric ducts?
- Yes!
- Both end at the urogenital sinus part of the cloaca
- Mesonephric (male) = Wolffian duct
- Paramesonephric (female) = Mullerian duct
What determines the fate of these two ducts in both males and females?
- Whether there is functioning testis
If Yes:
- Testis produce MIH suppressing paramesonephric duct system and produce androgens to support development of mesonephric duct.
If No:
- No testis producing MIH, so no suppression of paramesonephric duct system, no testis producing androgens so no development & degeneration of mesonephric duct
What is the final step in the development of the 2 ductal systems?
Mesonephric = duct converted in vas deferens and epididymis, migrates with testis as it descends
Paramesonepheric = appear as invaginations of epithelium of urogenital ridge, caudally make contact with cloaca (urogenital sinus) and cranially open into abdominal cavity.
Therefore provide similarities and differences between the formation of the male and female reproductive systems (typical exam Q)
Similarities:
- Germ cells migrate & drive development of gonad in both systems
- Gonads both descend in both systems
- Duct systems both responsible for formation of internal genetalia
Differences:
- Gonads develop in medulla in males, cortex in females
- Gonads become testis in males, ovary in females
- Internal genetalia formed from androgens + MIH in males, in the absence in females
- Males have open duct system, females have closed duct system
- Mesonepheric duct system in males, Paramesonepheric in females
What are the 3 basic components of external genetalia in M&F during the indifferent stage?
1) Genital tubercle
2) Genital folds
3) Genital swelling
How is the external genetalia formed in males & females?
Males:
- Genital tubercle elongates & genital folds fuse to form spongy urethra
- Genital tubercle develops into glands penis, under influence of testis-derived androgens (dihydrotestosterone)
Females:
- In absence of testosterone, genital swelling spread out to form labia majora and labia minora
- Genital tubercle becomes clitoris, urethra opens into the vestibule
How does the testis + ovary descend?
Testis:
- Gubernaculum attached to gonad inferiorly at labio-scrotal folds, gonads begin descent inferiorly.
Ovary:
- Gubernaculum attached to ovary inferior to labio-scrotal folds, ovary descends to pelvis, but uterus has developed and prevents and further descent
Summary slide for development of reproductive system.
Safe 1 geez thanks
What is Turner’s Syndrome & Klinefelter syndrome?
Turner’s syndrome = absence of one X chromosome (45X0), leads to degeneration of the ovaries. This is an example of gonadal dysgenesis, patient have short stature, high-arched palate, webbed neck, cardiac and renal abnormalities and inverted nipples.
Klinefelter syndrome = an extra X chromosome in a male embryo (47XXY), most common sex chromosome disorder (1 per 1000 males), patients may have small testes, decreased fertility, decreased testosterone and gynecomastia.
