Embryology of the Urogenital System: Sexual Differentiation Flashcards
What decides genetic sex?
Presence of Y chromosome due to the presence of the SRY gene.
How can XX males form?
Error during crossing over can take place where the SRY gene is added to an X chromosome.
What is the SRY protein called?
Testis Determining Factor (TDF)
What genes are expressed when SRY is absent?
WNT4 on chromosome 1
FOXL2 on chromosome 3
These genes are expressed by default unless the SRY gene is active.
What does the SRY gene produce?
Testis with sertoli cells, Leydig cells, and spermatogonia
What do WNT4 and FOXL2 produce?
Ovaries with granulosa cells, theca cells and oogonia.
What is the gonad made from embryologically?
2 tissue types:
Mesenchyme which forms the matrix of the gonad
Primordial germ cells which forms the gametes.
Where do primordial germ cells originate?
Endoderm of the embryonic yolk sac and then migrates at about 3 weeks via hindgut to genital ridge.
Epithelial cells of genital ridge become somatic support cells which are differentiated eventually into sertoli cells and granulosa cells (homologous cells)
At this stage the gonads are undifferentiated.
What does the sry gene do to somatic support cells?
Converts them into sertoli cells.
Sertoli cells then form testis cords enclosing PGC.
What do the testis cords form?
Seminiferous tubules and rete testes.
Where do leydig cells arise from and when does this happen?
Leydig cells differentiate from mesenchyme cells about week 9 - 10
What is AMH?
A hormone produced by sertoli cells (Antimullerian hormone) which is essential for formation of male genitalia.
How is testosterone produced in foetuses?
From HCG rather than LH
How are males and females different in gonad formation?
No Sry gene leads to Primordial Germ Cell differentiation into oogonia and are surrounded by somatic support cells.
Oogonia -> Primary oocyte and enter meiosis 1
Somatic support cells -> Follicle granulosa cells
Follicle cells surround oocyte to form primordial follicle.
What is the follicle homologous to?
The seminiferous tubule
What do the paramesonephric duct and mesonephric duct give rise to?
The internal genitalia (epididymis vas deferans and seminiferous tubules in males and uterine tube, the uterus and the upper third of the vagina)
What hormones does development of male internal and external genitalia depend on in foetal testis?
Antimullerian hormone (AMH)
Testosterone
What does AMH do in fetal testis?
Inhibits paramesonephric (female) duct system
What does testosterone do in foetal testis?
Facilitates development of mesonephric (male) duct system
What enzyme converts testosterone into 5alpha dihydrotestosterone (DHT)?
5alpha-reductase
What does DHT doin foetus?
Development of male external genitalia
What happens in the absence of testosterone and AMH to developing reproductive system?
Absence of T: The mesonephric duct system degenerates
Absence of AMH: Paramesonephric duct system is maintained and forms the uterine tube, uterus, and upper third of vagina.
What happens in the presence of testosterone and AMH in developing reproductive system?
Mesonephric duct is maintained and forms the epididymis the vas deferans and the seminal vesicle.
What is the glans penis homologous to in females?
The clitoris
