Normal Sexual Differentiation

Question 1

Define Sexual Determination & Sexual Differentiation

Answer 1

Sexual determination – Genetically controlled process dependent on the ‘switch’ on the Y chromosome. Chromosomal determination of male or female.

Sexual differentiation – The process by which internal and external genitalia develop as male or female.
The two processes are contiguous and consist of several stages.

Question 2

What is the SRY

Answer 2

The SRY gene creates the testis

In the absence of the Y chromosome, ovaries develop

The SRY causes the undifferentiated gonads to develop into testis, so the products of the testis affect the rest of the development (influence further gonadal and phenotypic sexual development)

• Sex determining region Y (SRY) switches on briefly during embryo development (>week 7) to make the gonad into a testis.
• Testis develops cells that make 2 important hormones which are:
o anti-Mullerian hormone (AMH) – made by Sertoli cells
o testosterone – made by Leydig cells

Products of the testis influence further gonadal and
phenotypic sexual development.

Question 3

Mechanisms of SRY action

Answer 3

• SRY is a DNA binding protein that acts as a transcription factor.
• The critical site of DNA binding is upstream of SOX9 which is also a transcription factor.
• SOX9 can bind its own upstream activation site so its levels remain high even if SRY declines. This is a positive feedback mechanism.
• SOX9 then acts to stimulate the expression of further downstream genes.

Question 4

How does SOX9 achieve its effects?

Answer 4

1. Stimulates production of prostaglandin D2 (PGD2) by the pre-sertoli cells.
   PGD2 acts as a paracrine hormone to stimulate further SOX9 production in these cells.
   This second positive feedback mechanism serves to re-enforce all precursor cells along the sertoli pathway.
2. SOX9 stimulates production of fibroblast growth factor 9 (FGF9).
   FGF9 acts as a chemotactic factor for cell migration into the developing testis.
   It also further stimulates the production of SOX9 a third positive feedback mechanism.
3. SOX9 stimulates the production of anti-Mullerian hormone (AMH).
   AMH causes the regression of the mullerian ducts which develop into the uterus and fallopian tubes in the female.
4. Inhibition of female transcription factors WNT4 and FOXL2

Question 5

Gonadal Development: What is the Genital Ridge

Answer 5

• After fertilisation, a pair of Gonads develop which are bipotential.
• The genital ridge is the somatic precursor of gonads derived from common somatic mesenchymal tissue precursors called the genital ridge primordia (3½ - 4½ weeks) on posterior wall of lower thoracic lumbar region.

The genital ridge will either become ovaries or testis.
In order for this to occur, 3 waves of cells invade the genital ridge…
1. Primordial Germ Cells – become Sperm (male) or Oocytes (female).
2. Primitive Sex Cords – become Sertoli cells (male) or Granulosa cells (female).
3. Mesonephric Cells – become blood vessels and Leydig cells (male) or Theca cells (female).

Question 6

Explain Primordial Germ Cell Migration in the genital ridge

Answer 6

• An initially small cluster of cells in the epithelium of the yolk sac expands by mitosis at around 3 weeks.
• They then migrate to the connective tissue of the hind gut, to the region of the developing kidney and on to the genital ridge – completed by 6 weeks.
• By 6 weeks: they’re ready to be triggered into oocytes or sperm

Question 7

Explain primitive sex cords in the genital ridge

Answer 7

Cells from the germinal epithelium that overlies the genital ridge mesenchyme migrate inwards as columns called the primitive sex cords.

In males, the cells express SRY, and therefore, penetrate the medullary mesenchyme and surround primordial germ cells to form the testis cords. This is the precursor of the seminiferous tubules.

These cells eventually become the Sertoli Cells which express AMH.

(AMH causes the regression of mullerian hormones )

Question 8

Explain Mesonephric cells in the genital ridge

Answer 8

These originate in the mesonephric primordium which are just lateral to the genital ridges.

In males, they act under the influence of pre-Sertoli cells (which themselves express SRY) to form…
• Vascular tissue
• Leydig cells (synthesize testosterone, do not express SRY)
• Basement membrane – contributing to formation of seminiferous tubules and rete-testis

In females, without the influence of SRY, they form…
• Vascular tissue
• Theca cells (synthesis androstenedione which is a substrate for oestradiol production by the granulosa)

Question 9

What are the Mullerian Ducts

Answer 9

They give rise female internal reproductive system (the oviducts, uterus, cervix and upper vagina)
It is inhibited by AMH.

Question 10

What are the Wolffian ducts

Answer 10

Give rise to the male internal reproductive system (testes, spermatic cord)
It is stimulated by testosterone.

In females, the lack of testosterone causes the regression of the wolffian ducts.

Question 11

What causes the external differentiation in males.

Answer 11

• Testosterone is converted in the genital skin to the more potent androgen DHT (dihydrotestosterone) by 5-alpha-reductase.
• DHT binds to the testosterone receptor but is more potent than testosterone.

DHT causes differentiation of the male external genitalia:
• Clitoral area enlarges into penis
• Labia fuse and become ruggated to form scrotum
• Prostate forms

• In females there is still 5-alpha reductase but no testosterone substrate, so no DHT and the external genitalia develops into clitoris and labia.