Development of the Urogenital System Part 2 Flashcards
When is the sex of an embryo determined
- At the time of fertilization
- The genetic sex of the embryo depends on whether an X or Y-bearing sperm fertilizes an X-bearing oocyte
Why is the initial genital period referred to as the indifferent stage of sexual development?
- Because at the beginning, the early genital systems in the two sexes are similar as we cannot differentiate between a a male and a female
- The appearance of the two gonads are indifferent up until the 7th week
The gonad are derived from what?
3 sources:
- the mesothelium, lining the posterior abdominal wall
- the underlying mesenchyme
- Primordial germ cells
Describe the point of indifferent gonads
- There are cortexes and medullas in the gonads of both males and females
- In embryos with an XX chromosome, the cortex differentiates to an ovary and the medulla regresses
- In embryos with an XY chromosomes, the medulla differentiates into a testis and the cortex regresses
Explain the migration of the primordial germ cells
- Originally, these cells are found among endodermal cells of the yolk sac in the caudal region of trilaminar disc in the 4th week.
- Then the cells migrate along the dorsal mesentery of the hindgut to the gonadal ridges which is complete in week 6
- The cells enter the underlying mesenchyme to incorporate into the gonadal cord
- The migration is regulated by BMP4, stella and fragilis
What determines the sex of an embryo
- The sex-determining region on the Y chromosome gene (SRY) expresses the testis-determining factor (TDF) that allows for testicular differentiation
- The absence of the Y chromosome results in the formation of an ovary due to the lack of the SRY gene
describe the development of the testes from the indifferent gonad
- In the presence of TDF
- The gonadal cords become the seminiferous tubules by the involvement of Sox9 ad FGF9 that allow the mesonephric ducts to integrate to the gonadal ridge
- The mesonephric ducts/tubules are incorporated into the rete testes and tubuli as they branch and fuse together
- A thick tunica albuginea grows and no cortical cords form (meant for females)
What does the walls of the seminiferous tubules include
- Sperm from the primordial germ cells
- Sertoli cells from the surface mesothelium
Where are leydig cells found and what is their function
- Leydig cells lie in between the seminiferous tubules
- They secrete testosterone in the 8th week to influence the development of indifferent ducts and external genitalia
describe the development of the ovary from the indifferent gonad
- In the absence of TDF
- The gonadal cords degenerate and the cortical cords form which incorporates primordial germ cells
- The primordial germ cells form clusters = follicular cells and oogonia that divide to produce primary oocytes
Explain the genetic signals between a male and a female
- An embryo has the SRY and Wnt4 gene
- In males, the SRY gene will produce Sox9 which will inhibit Wnt4
- In females, Wnt4 will produce Dax1 (and other genes) which will inhibit Sox9
- Sox9 will upregulate SF1 with other genes that help develop the testes
- SF1 develops sertoli cells and leydig cells
- Dax1, TafII and other genes help develop the ovaries
What is the function of Sertoli cells
- Sertoli cells produce the antimullerian hormone and mullerian inhibiting factor
- These two hormones inhibit the development of the paramesonephric (or Mullerian) duct which develops the uterine tube
What is the function of Leydig cells
- These cells produce testosterone which acts on the mesonephric tubules/ducts to develop the epididymus, vas deferens and seminal vesicle
explain the development of the genital ducts
- the mesonephric and paramesonephric ducts run parallel from each other and fuse on the cranial end to form the uterovaginal primordium which gives rise to the uterus
- In males, the mesonephric ducts are developed into genital ducts and the reproductive organs when the testes releases testosterone
- A hormone is also produced by the sertoli cells to prevent the paramesonephric ducts from developing
- In females, because there’s no testosterone, the paramesonephric ducts develop uterine tubes and the mesonephric ducts regress
describe the development of external genitalia in males
- The indifferent external genitalia develops in the 4th week
- because of the presence of testosterone, the external genitalia masculinizes
- the phallus becomes the penis
- the urogenital folds fuse to form the urethra
- the labioscrotal swellings fuse to form the scrotum
- the corpora cavernous a and spongiosum develop the mesoderm for the penis