1. Fil & Shef in… The Magical World of Reproductive Development
A Shefinski Production
Directed by Filipino
“Fantastic!” – Aleiks, the Holyhead Herald
“What’s for lunch?”
- Anonymous

2. 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
Intermediate
mesoderm
Turner syndrome – XO genotype is female
Gastrulation at day 19

3. Image(s) taken from this video: https://www. youtube. com/watch
Image(s) taken from this video: (many things are in much more detail than you need to know)

4. 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
Turner syndrome – XO genotype is female
Gastrulation at day 19

5. 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
Remaining cords as well as the medulla, degenerate.

6. 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
Dense layer between the cords and the and the epithelium = tunica albuginea
Secretion of testosterone by week 8 helps influence the differentiation of the genitalia

7. 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
Dense layer between the cords and the and the epithelium = tunica albuginea
Secretion of testosterone by week 8 helps influence the differentiation of the genitalia

8. Now what? Descent of the gonad Development of the internal genitalia
Development of the external genitalia

9. 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
Transversalis fascia => internal spermatic fascia
Internal oblique muscle fascia => cremasteric fascia
External oblique muscle fascia => external spermatic fascia
Processus vaginalis is a fold of the parietal peritoneum, and gives rise to two layers (parietal and visceral) of the Tunica vaginalis
Testes migrate through the inguinal canal at 28 weeks, and end up in the scrotum at around 33 weeks

10. 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

11. The Wolffian Duct
The Wolffian duct gives rise to structures that include:
Epididymis
Vas deferens
Seminal vesicle

12. The Müllerian Duct
The Müllerian duct gives rise to structures that include:
Oviducts (fallopian tubes)
Uterus
Cervix
Upper part of the vagina

13. 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
We said it was transport in the session – mistake.
Internal genitalia – storage, maturation and transport.
♂: Enlargement, Elongation & Fusion
♀: Slight Enlargement; no fusion

14. 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)