Fil & Shef in…The Magical World of

Reproductive Development

A Shefinski ProductionDirected by Filipino

“Fantastic!” – Aleiks, the Holyhead Herald

“What’s for lunch?”- Anonymous

The Gonads - Ovary or Testicle? (I)

• How and when is the genetic sex is determined?• By the presence of the Y chromosome

• At fertilisation

• What is a germ cell?• Diploid cells that give rise to gametes (i.e. sperm/ovum)

• Arise in the yolk sac at 3 weeks (primordial germ cells)

• What tissue does the genitourinary system develop from?• Genital ridges of the intermediate mesoderm

Endoderm

Ectoderm

Intermediatemesoderm

The Gonads - Ovary or Testicle? (II)

• The indifferent gonads• Surface Epithelium of the genital ridge

proliferates and penetrates the mesenchyme to form primitive sex cords at 6 weeks.

• At the same time (5-6w) germ cells migrate from the yolk sac and invade the genital ridges

• The male and female gonads• Ovary: Germ cells of XX genotype colonise the

cortex

• Testis: Germ cells of XY genotype colonise the medulla

Ovarian Development

• XX Germ cells colonise the cortex of the gonad while the surface epithelium proliferates further

• Primordial follicles develop as mesenchymal cells surround these germ cells

• Notes:• Remaining cords degenerate

• Cortex develops even in absence of germ cells

Testicular Development (I)

• XY Germ cells colonise the medulla of the gonad

• Y chromosome gene expression influences masculinisation

• Primitive sex cords continue to proliferate and colonise the medullary region, forming definitive sex cords • These will acquire a lumen and

become seminiferous tubules at puberty

Testicular Development (II)

• Secretion of testosterone by 8th week further aids sexual differentiation of the embryo’s genitalia (discussed later)

• Notes:• Mesenchymal cells differentiate into

Leydig cells, which secrete testosterone

• Surface epithelial cells differentiate into Sertoli cells

Now what?

• Descent of the gonad

• Development of the internal genitalia

• Development of the external genitalia

Descent of the Gonad

• Where do the gonads develop?• On the posterior abdominal wall

• Where do the gonads end up?• Ovaries in the pelvis

• Testes in the scrotum (~28-33 weeks)• Processus vaginalis descends into the

labioscrotal folds through the inguinal canal, and pushes layers of the abdominal wall with it

• Gonad follows as it is attached to the abdominal wall by the gubernaculum (a fibrous cord)

• What descends with the gonad?• Neurovasculature

Development of the Internal Genitalia

• Foetus has a primordial bipotential for both male and female internal ducts:• ♂ Mesonephric = Wolffian duct

• ♀ Paramesonephric = Müllerian duct

• Müllerian duct is dominant in the absence of stimulus, causing the spontaneous regression of the Wolffian duct

• Y chromosome gene expression is essential for development of the Wolffian duct• Müllerian Inhibitory Hormone (MIH) secreted by Sertoli cells

• Regression of the Müllerian duct

The Wolffian Duct

• The Wolffian duct gives rise to structures that include:• Epididymis

• Vas deferens

• Seminal vesicle

The Müllerian Duct

• The Müllerian duct gives rise to structures that include:• Oviducts (fallopian tubes)

• Uterus

• Cervix

• Upper part of the vagina

External Genitalia

• Function to transfer gametes

• Begin as bipotential tissue with three main structures

Structure Male Female

Urethral Folds Shaft of penis Labia minora

Genital Swellings

Scrotum Labia majora

Genital Tubercle

Glans of penis Clitoris♂: Enlargement, Elongation & Fusion

♀: Slight Enlargement; no fusion

Quiz

• Why do the testes descend into the scrotum unlike the ovaries?• Maturation of sperm requires a lower temperature than that offered in the pelvis

• What is the lymph drainage of the ovaries & testes?• Travel up to the para-aortic nodes (~L2)

• What hormone is essential in determining the differentiation of the internal genitalia?• MIH (Mullerian Inhibitory Hormone)