lecture 21: sex and differentiation

Question 1

What are different aspects of sexual differentiation?

Answer 1

• genetic sex - XX vs XY
  
  • SRY → SOX9 → testis formation
  • non SRY genetic sex differences
• somatic sex
  
  • internal and external genitalia
• brain sex
  
  • behaviour and neuroendocrine
• disorders of sexual differentiation (DSD)

Question 2

Is there sexual dimorphism in the early embryo?

Answer 2

• in mammals male and female embryos initially look alike
• identical indifferent gonads form on the embryonic kidney early in development

Question 3

Who was Alfred Jost?

Answer 3

• pioneer of foetal endocrinology
• Jost’s experiments in the 1940s and 50s laid the foundation of our knowledge about the hormonal control of sexual differentiation

Question 4

What were Alfred Jost’s experiments?

Answer 4

• gonad
• mesonephros
• mullerian duct
• wolffian duct
• urogenital sinus
• intact male → WD
• female or castrate → MD
• female + testosterone crystal → MD + WD on side with testosterone crystal

Question 5

What can determine sex?

Answer 5

• environmental - e.g. temperature
• genetic - XX/XY mammals
  
  • ZW/ZZ birds, some snakes
• X:autosome ratio - insects
• other strategies…
  
  • hermaphrodite
  • parthenogenote

Question 6

What is genetic sex determination?

Answer 6

• sex chromosomes evolved from autosomes
• pseudoautosomal region

Question 7

What is genetic regulation of gonad differentiation?

Answer 7

• SRY is actually not a very interesting gene
• what it does do is regulate a whole cascade of other genes which do the work for it
• notably SOX9, → turns on various other genes
• SRY turns off genes in the female pathway
• genes in the female pathway (e.g. Wnt4) turn off the SRY pathway
• helps prevent hermaphrodites

Question 8

Where do the gonads form?

Answer 8

• on the mesonephros

Question 9

What are the mesonephric and paramesonephric ducts?

Answer 9

• mesonephric = wolffian duct - urinary duct
• paramesonephic = mullerian duct - induced beside the WD

Question 10

What is testis differentiation in the mouse?

Answer 10

• SRY → SOX9 → multiple genes
• proliferation of coelomic epithelium
  
  • → (less than E11.5) sertoli cells
    
    • →AMH
    • → + germ cells → formation of testis cords (E12.5)
  • → (more than E11.5 → interstitial cells) Leydig cells (~E13)
    
    • →androgens
• after birth cords hollow out to become seminiferous tubules
• male specific vasculature → endothelial cells ingrowing from mesonephros
  
  • vasculature needed for testis cords to form (in mouse)

Question 11

What is ovary differentiation in the mouse?

Answer 11

• timing later than testis differentiation
• primordial germ cells → meiotic arrest
• growth of cortex vs medulla
• PGCs in medulla → apoptosis
• follicle cells (granulosa) probably form from coelomic epithelium

Question 12

What is gonadal differentiation in the mouse?

Answer 12

• overview of mouse gonad morphogenesis
• the expression of SRY directs development of the bipotential gonad toward the testis pathway
• characteristic testis morphology includes formation of testis cords, the coelomic arterial vessel, and Leydig cells
• characteristics of the ovary include entry of germ cells into meiosis, establishment of cortical and medullar domains, and folliculogenesis

Question 13

How do we go from indifferent gonads to male and female gonads?

Answer 13

• XY = male
  
  • AMH → regressed mullerian duct
  • testosterone stimulated wolffian duct → external genetalia
  • wolffian duct will make an epididymis
  • will cause differences in the brain
  • act on urogenital sinus
  • prostate gland
• XX = female
  
  • mullerian duct develops (fallopian tubes and uterus)
  • regressing wolffian duct
  • in the absence of male hormone production
• genetic switch → gonad
• gonadal hormones → body
• indifferent gonad
  
  • SRY → testis → androgen + anti-mullerian hormone
  • ovary → no hormones (until puberty)
• WD regresses in females because it needs androgen (testosterone)
• mullerian duct derivatives (fallopian tube, uterus, cervix, top 1/3 vagina) → tissues sensitive to progesterone and oestrogen, after puberty progestrone and oestrogen are driving their development
• MD regresses in males because of AMH from testis
• androgen sensitive tissues e.g. prostate, penis
• wolffian duct derivatives e.g. seminal vesicles, etc
  
  • within the wolffian duct, local connection, high level testosterone
  • testosterone is not a very good androgen
  • 5alpha-reductase can convert testosterone to 5a-dihydro-testosterone
  • these tissues have high levels of 5a-reductase
  • aromatase in brain converts testosterone to oestrogen (oestradiol-17B)

Question 14

What is external genital development?

Answer 14

• XX = ovary
  
  • gential tubercle → glans → clitoris
  • genital fold → urethral fold → urethral orifice and vaginal opening
  • genital swelling → labial swelling → labia minora and majora
• XY = testis
  
  • T → DHT
  • genital tubercle → glans → prepuce and urethral orifice
  • genital fold → urethral groove → scrotal raphe
  • genital swelling → scrotal swelling → scrotum

Question 15

What is testicular descent?

Answer 15

• testes migrate (descend) from abdomen to scrotum via inguinal canal
  
  • androgens, INSL3 and AMH from testes
  • CGRP from the genitofemoral nerve
• scrotal location → 2-3 degrees C cooler
• failure of descent (cryptorchidism)
  
  • no sperm production
  • high risk of testicular cancer
• embryonic position → transabdominal phase (6-15wk) - anchoring → inguinoscrotal phase (25 - 35 weeks) - migration → elongation of cord (0-10 years)

Question 16

What is the brain-sex LH surge?

Answer 16

• male → adding E2 does not result in surge
• femlae → surge
• female with foetal androgen exposure → no surge
• section of the anteroventral periventricular nucleus of the POA in situ for ER-beta mRNA
• female → densly packed
• male → v sparse
• female treated with oestrogen at d1-10 after birth → sparse
• male castrated at birth → present and more densly packed

Question 17

What is the relationship between oestrogens and brain sex?

Answer 17

• location of the sexually dimorphic nucleus in the preoptic area (SDN POA) and the anteroventral-periventricular nucleus of the POA (AVPv-POA) of the rat
• male SDN-POA bigger than female (behav?_
• male AVPv-SDN smaller than female (LH surge?)
• aromatase enzyme in many brain areas
• non-aromatisable androgens ike DHT do not have same effect on brain as testosterone
• oestrogens masculinise brain
• anti-oestrogens can block effect of T on brain sex
• effects via cell divisions in some cells and apoptosis in others
• design: 5 day old female rats; control or treated with E2 and killed at 10 or 24 h after
  
  • TUNEL labeled cells in the POA (stains apoptotic cells)

Question 18

What are sex differences independent of gonadal hormones?

Answer 18

• marsupials
  
  • sex dimorphic formation of mammary and scrotal primordia BEFORE gonadal differentiation
  • depends on one vs two X chromosomes
  • see lec on marsupials

Question 19

What are sexual dimorphisms before gonadal differentiation?

Answer 19

• mouse
• collect and sort blastocysts
• take large and smaller
• transfer to foster mothers
• larger = mostly males
• both = 50:50
• smaller = mostly female

Question 20

What is the bilateral gynandromorph zebra finch?

Answer 20

• fascinating case
• gyn = female, andro = male, morph = form
• boy finches have very striking plumage, females very plain
• this bird had boy plumage on one side and girl on the other
• sort of sang like a boy
• half of the brain was male, half female (i.e. half stained with W chromosome and half didn’t)
• half of the body was ZW, half ZZ
• song centre was bigger on one side than the other → must be related to chromosomes and not influence of hormones in blood

Question 21

What is gonadal differentiation?

Answer 21

Question 22

What is female gonadal development?

Answer 22

Question 23

What is male hormonal development of the testis?

Answer 23

• testicular descent to scrotum → several testicular hormones