Development of the Urogenital System Part 2

Question 1

When is the sex of an embryo determined

Answer 1

• 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

Question 2

Why is the initial genital period referred to as the indifferent stage of sexual development?

Answer 2

• 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

Question 3

The gonad are derived from what?

Answer 3

3 sources:
- the mesothelium, lining the posterior abdominal wall
- the underlying mesenchyme
- Primordial germ cells

Question 4

Describe the point of indifferent gonads

Answer 4

• 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

Question 5

Explain the migration of the primordial germ cells

Answer 5

• 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

Question 6

What determines the sex of an embryo

Answer 6

• 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

Question 7

describe the development of the testes from the indifferent gonad

Answer 7

• 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)

Question 8

What does the walls of the seminiferous tubules include

Answer 8

• Sperm from the primordial germ cells
• Sertoli cells from the surface mesothelium

Question 9

Where are leydig cells found and what is their function

Answer 9

• 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

Question 10

describe the development of the ovary from the indifferent gonad

Answer 10

• 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

Question 11

Explain the genetic signals between a male and a female

Answer 11

• 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

Question 12

What is the function of Sertoli cells

Answer 12

• 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

Question 13

What is the function of Leydig cells

Answer 13

• These cells produce testosterone which acts on the mesonephric tubules/ducts to develop the epididymus, vas deferens and seminal vesicle

Question 14

explain the development of the genital ducts

Answer 14

• 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

Question 15

describe the development of external genitalia in males

Answer 15

• 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

Question 16

What does the indifferent external genitalia include

Answer 16

• the genital tubercle → phallus
• the labioscrotal swellings
• The urogenital folds

Question 17

describe the development of external genitalia in females

Answer 17

• the last of testosterone causes the phallus becomes the clitoris
• the urogenital folds become the labia minora
• the labioscrotal swellings becomes the labia majora