Sex determination Flashcards
LO
- To explain the fate of the genital ridges
- Be able to describe the fate of germ cells in the ridges.
- Be able to explain why fate of germ cells leads to lifelong fertility in men, but menopause in women.
- Explain the importance of retinoic acid in sex determination.
Tell me about germ cell development in mice and when the determination between males and females can be seen?
- Up to this point there is no observable difference between male and females… things now change!
- The genital ridges are unique- all other organ primordia can form only one structure –they are bipotent the choice between the two is determined by the SRY gene in mammals
- After 6 weeks the difference between males and female can be seen
The genital ridge forms at the same time as what other ridge?
Mesonephric ridge
What can the Wolffian act as?
An immature kidney at this early stage
How long is it for the Müllerian duct to form?
it forms after 6 weeks and above it is the wolffian duct
Can you start to identify between males and females after 6 weeks?
yes
The Y-chromosome gene is essential for male development
Tell me about the location of the SRY gene?
Right at the end of the p arm of the Y chromosome
The SRY gene is a member of what family?
A member of the Sox family of transcription factors
What does the SRY gene show?
Massive evolutionary changes
SRY gene
Tell me about conservation in the SRY gene?
- Only High mobility group box- conserved at ~60 % mouse to man (about same level that HMG conserved between Sox family)
- Outside the HMG there is 0 conservation
What cells is the SRY gene expressed in?
Tell me about the time line for this
SRY is expressed in the somatic cells of the male gential ridge
- first at ~day 11 in mice – it is needed for only 6-12 hrs (in mice) as it turns on expression of Sox9 -another transcription factor
- Sex determination is not a highly conserved mechanism in the animal kingdom
- In situ hybridization
- Note chords start forming at 13.5 d in male – still express Sox9
- Difference between the two is due to the expression of SOX9
In mammals, what does the SRY drive?
Sox9 expression
Tell me about the sex genes in birds and the shared evolutionary male gene with mammals
- In birds, the homogametic sex is male (ZZ) and the female (ZW) -high levels of DMRT1 present on the Z chromosome drive expression of Sox9
(a gene dosage)
Tell me about the reptiles expression of Sox9 and what its related to?
- In some reptile’s expression of Sox9 is related to temperature
- Alligator eggs incubated at
- 33˚C 100% male (high temperature causes SOX9 to be released)
- 30˚C 100% females
Is there SRY selection in monotremes?
no
Tell me about Sox9 expression between animals?
Mammals: SRY gene drive Sox9 expression
Birds: DMRT1 gene drive Sox9 expression
Reptiles: Sox9 expression relates to temperature
Snake/ Turtles/ Alligators: no X/Y/Z but temperature dependent expression of Sox9
What type of transcription factor is Sox9?
Tell me about its feedback loop
Sox9 is an autosomal transcription factor
its expressed in a positive feedback loop with its own gene (this is relatively rare)
What does Sox9 block?
It blocks ovary formation by the genital ridge (stops function of the paracrine Wnt/β-catenin pathway)
What does Sox9 activate the expression of?
- Activates expression of Anti-Mullerian Hormone (by the male genital ridge)- more a paracrine then a hormone but it’s given the name of hormone
- Activates expression of FGF9 (in another positive feedback loop with Sox9- makes more of its self and SOX9) in the (male) genital ridge
What type of messenger system is beta-catenin?
A secondary messenger system
What is FGF9?
Paracrine signalling molecule formed by the somatic cells of the genital ridge
What does FGF9 cause?
Causes proliferation of some genital ridge cells to form Sertoli cells and formation of chords of cells – gives the typical tubular structure of the testis later (seminiferous tubules)
What does FGF9 repress?
Represses (with Sox9) the Wnt/β-catenin path so blocks ovary formation
What does FGF9 coordinate?
Coordinates differentiation pathway of PGCs
What do sertoli cells act as?
nurse cells for male PGCs
The path of the male genital ridge
(piecing together everything discussed before this)
Where is testosterone produced?
produced by the intestinal cells of the testes
What does testosterone allow?
The growth of the Wolffian (mesonephric) duct to form the epididymis and the vas deferens
What does testosterone drive?
The secondary sex determinants
Tell me about what the loss of the androgen receptor produces?
- Loss of the androgen receptor produces female appearance but with retained testes
- Intersex- failure in genetic and anatomic correlation
Tell me about secondary sex determinants
secondary sex determinants – in early development formation of penis etc so if no testosterone female external phenotype
Testostermone more information…
Often (not always) immature testes but always failure of Wolffian duct so no (or vestigal)- vas deferens/epididymis
What signal remains high in the female genital ridge?
Wnt 4 (paracrine signal) expression remains high in the female genital ridge (reduced/lost in males)
Tell me about the signals that lead to the production of the epithelium of the ovary to form granulosa cells surrounding the PGCs/ follicles
- Signals through the second messenger β catenin (over-expression in males causes an ovary to form eg TG mice)
- Induces expression of a series of ovary specific transcription factors and paracrine signals – including the secreted signal Follistatin- which induces the epithelium of the ovary to form granulosa cells surrounding the PGCs/follicles
In the female the müllerian duct is not lost, instead what does it differentiate into?
uterus, oviducts and cervix
This is the default pathway in the absence of SRY/Sox9/FGF9
What does Follistatin cause?
Follistatin causes granulosa cells to form surrounding the PGCs rather than chords of Sertoli cells as in male
Follistatin expression
In the absence of testosterone, what is lost (in females)?
What grows instead?
The Wolffian duct
The Mullerian duct grows (no AMH- anti-Mullerian hormone as this is a male specific thing) to develop the ovarian ducts, uterus cervix and top of vagina
Wolffian duct = mesonephric duct
Gender differences- PGCs
Gender differences- how male and females differ in their reproductive lifespan
Tell me about the female gamete lifespan
Women: Born with a FINITE number of eggs. With age these eggs are used up (they mostly die, but some get ovulated). By age 51 years (on average) women reach MENOPAUSE.
Tell me the following species which show menopause?
- killer whales
- short-finned pilot whales
- belugas
- narwhals
Tell me about the lifespan of male gametes
Men: Born with a capacity to continuously produce mature sperm from PUBERTY throughout life.
Why is it thought that the male gamete lifespan is longer than womens?
- Eggs are costly to make, women make enough to last their entire life (true for most of our evolutionary past).
- Older women undergo menopause so as to be able to nurture existing children, and not leave very young children motherless.
- Eggs most die before birth – 1-2 million at birth – a few 100s ovulated (300-400)
- 2000 yrs. ago few women lived to 51
- Human Female reproductive cycle is costly even if no pregnancy occurs
- NB no menopause in other primates
Fate of PGCs is gender dependent
Diagram to illustrate gender dependent PGC fate
Why do females enter meiosis?
- Retinoic Acid (RA) induces meiosis in females. (RA - the active metabolite of vitamin A)
- There are very high levels of RA about the genital ridge
- RA diffuses into the PGCs and causes the expression of Stra8 (Stimulated by retinoic acid)
Tell me about the Stra8
- Stra8 is a transcriptional regulator, modifying the gene expression pattern in the cell.
- Stra8 is ‘the master switch’ for meiosis. It causes PGCs in female embryos to stop dividing mitotically and enter meiosis.
- Difference – male and female fate of PGCs
- Stra- Stimulated by Retinoic Acid- here 9th gene discovered
- All Stra genes have an RRE sequence in their promoter – retinoic response element – RA enters the cell (lipophilic) and binds to promoter
Where is retinoic acid made?
RA is made not in the genital ridge (G), but in the Mesonephros (M).
Where is the Mesonephros located?
The Mesonephros is next to the genital ridge (it goes on to form parts of the epididymis and vas deferens in males, but regresses, in females).
Mesonephric tubules (T) and ducts (D) (D-wolffian duct) connects to what?
What does this mean about the diffusion of RA?
The anterior end of the gonad
Therefore, RA diffuses into the genital ridge from the Mesonephros.
Experimentally created XX/XY chimeras, all germ cells, both XX and XY, enter meiosis when present in what?
fetal ovary
What stops PGCs in male foetal testis entering meiosis?
Stra8 levels do not rise in male PGCs
How do the Stra8 levels not rise in male PGCs?
How: Fibroblast growth factor 9 (FGF9) produced by other male gonad cells
- induces Cyp26b1 which degrades RA before it reaches the PGCs
- represses Stra8 expression so inhibits meiosis entry
in the absence of RA –FGF9 maintains expression of pluripotency genes like NANOG/SOX2
- Cyp-cytochrome enzyme
- Pluripotency genes allows PGCs proliferation in the male later
Cyp26b1 is an enzyme involved in the degradation of retinoic acid
CYP26b1 is only present in male gonads
Experiment showing only male produce Cyp26b1
- Result of Cyp26b1 in the male is RA signal is blocked and no Stra8 – while in the female RA signaling occurs and Stra8 expression is seen
- Loss of Cyp26B1 enzyme in males results in male PGCs entering meiosis in utero this results in an infertile male (has testis but no sperm)
- Notice slight Cyp26b1 present before 11.5 – lost in female but maintained in male by expression of sry-sox9-fgf9 path which starts at E11.5
Summary
- Predetermined (C. elegans) and inductive (mammals) germ cell development.
- Inductive germ cells reach genital ridge following migratory journey. Only in females do germ cells enter meiosis when they reach the genital ridge.
- SRY-SOX9-FGF9 pathway converts the default female genital ridge to a male outcome
- RA causes meiotic entry of PGCs in females by activating Stra8, but Cyp26b1 (under control of SRY/ SOX9/FGF9) degrades RA in males.
