L5: Sex Differentiation Flashcards
What is sex determination?
Progress of development of sex differences, a natural event by which an organism is set become either a male or female.
Defined as the phenotypic development of structures consequent upon the action of hormones produced following gonadal determination.
When and how is sex determined?
Sex is determined at fertilization by the sperm. 50% chance for XX and XY
What is unique about gonadal rudiments?
All rudiment cells have just one fate, exception: gonadal rudiment. It is bipotential, can become either ovary or testis. Two organs with very different tissue architecture. At bipotential ‘indifferent’ stage Mullerian and Wolffian ducts are present.
What is the bipotential stage? How long does it last in humans?
It’s decision time in embryo develpment, gonads are indifferent and it lasts for 7 weeks.
What are the main germ cells in gonads?
- oocyte
- sperm
What are the somatic cells in gonads?
supporting cells:
- granulosa cells in oocytes
- sertoli cells in sperm development
steroidogenic cells:
- theca cells in oocytes
- leydig cells in sperm development
What are Primordial Germ Cells (PGCs)? Where are they located?
- Precursors of sperm and eggs
- located outside the body in yolk sac
When does specification of PGCs begin?
Specification of PGCs begins around 3 weeks of human gestation
How does gonadal ridge form?
PGCs migrate to gonadal ridge. Proliferation of epithelium and mesenchyme on medial side of mesonephros.
What is mesonephros?
Fetal ‘interim’ kidney
How long are gonads identical in appearance?
Until 7th week
How does PGC migration and proliferation happen at week 4-6?
PGCs migrate from yolk sac –> going up through hindgut –> dorsal mesentery (fold of tissue that attaches organs to body wall) –> genital ridges (primitive gonad)
What kind of divisions do PGC cells undergo during migration?
Undergo mitosis while migrating, from ~100 cells to 25,000
How long do PGCs proliferate in humans?
Until ~10th week
What do PGCs require for motility?
- chemotactic signals coming from gonads
- gradients of extracellular matrix glycoproteins
- PGC-PGC contact
divide into two populations –> colonise R & L gonads
What are the chemotactic signals needed for PGCs to migrate?
- TGF-beta (transforming growth factor beta)
- Kit ligand/SCF (stem cell factor)
- bFGF (basic fibroblast growth factor)
When does sex specific differentiation of PGCs begin?
During migration, germ cells are sexually bipotential.
Sex specific differentiation of PGCs begins only after colonisation of gonad (dependant on the cells that are found within the gonad)
What happens after colonisation of gonad by PGCs?
PGCs become surrounded by somatic cells, PGCs continue to proliferate
How do PGCs change in males and females?
- in males to prespermatogonia / prospermatogonia
- oogonia
What is one of the most important genes regulating gonadal development? what is the evidence?
SF1 (NR5A1) KO mice: gonads fail to develop beyond early indifferent stage
What are the genes regulating gonadal development? What are the experiments that prove it?
- Wilms tumour suppressor gene (WT1) – WT1 KO mice fail to develop kidneys and gonads
- Empty spiracles homeobox (EMX2) – EMX2 KO mice fail to develop kidneys, ureters, gonads and genital tracts
- Steroidogenic factor 1 gene (SF1) – SF1 (NR5A1) KO mice: gonads fail to develop beyond early indifferent stage
- Bone morphogenetic protein 7 (BMP7) – BMP7 KO mice: reduction in PGC numbers of both XX and XY at E11.5 due to a reduction in germ cell proliferation
What are the experiments proving Jost paradigm?
If ovaries removed –> female phenotype
if testis removed –> female phenotype
when testosterone administered to female rabbit –> develops into male
sexual dimorphisms arise because of testicular hormone production: Testosterone and Antimullerian hormone.
So, it is thought that the default development of indifirent gonads is female development.
What is Jost paradigm?
Female pathway is a default pathway, that happens without any active input (don’t need any active compound)
What’s the part that forms actual kidneys called?
Metanephric kidney
How does development of gonads and ducts in females happen?
- Mullerian ducts remain, which have AMH receptors
- they remain if they do NOT see AMH
- then they develop into female secondary sex organs (uterus, oviduct, cervix, upper vagina)
- Wolffian duct degenerates, as it needs testosterone for survival
How does development of gonads and ducts in males happen?
- Wolffian ducts remain
- they need testosterone for survival
- they develop into male secondary sex organs: vas deferens, epdidymis, seminal vesicles
- Mullerian duct degenerates
What’s the pathway of testis somatic cell differentiation?
somatic cell differentiation –> sertoli cells –> Testis cords (early seminiferous tubules)
Sertoli cells produce AMH and drive sperm differentiation, makes Mullerian duct regress.
Leydig cells produce testosterone that is required for Wolffian duct to survive
What’s the pathway of testis germ cell development?
germ cells develop within testis cords –> differentiate into sperm (puberty)
What do interstitial cells in the testis develop to?
Leydig cells
What do germ cells become interspersed with in ovary?
Germs cells become interspersed with somatic cells and then germ cells become enclosed by somatic cells (granulosa)
What do mesenchyme cells in the ovary develop to?
Theca cells
What is AMH? Where is it produced and what does it do?
Anti-mullerian hormone is produced by Sertoli cells and regresses Mullerian Duct in males
What is SRY gene?
- A single gene on Y chromosome that determines the sex of an organism - initiates development of testis (instead of an ovary)
- has DNA binding properties
- encodes a transcription factor
- genetic ‘switch’ - switching genes on - these genes often on X chromosome
Where is SRY gene expressed?
Expressed in male somatic cells: sertoli cells
What are the targets of SRY gene?
Sox9:
- autosomal gene
- can induce testis formation
- expressed shortly after SRY
- XX mice transgenic for Sox9 develop testes
SF-1:
- transcription factor
- activates several genes involved in steroid synthesis
- Sry + SF-1 -> AMH
what are the studies proving importance of SRY?
- Knock-out of SRY gene in XY mouse zygote develops to XY female (phenotypic), reduced fertility or infertile
- Knock-in of SRY gene in XX mouse zygote develops to XX male (phenotypic), but no sperm
What is the example of twin sisters found who were XY and had SRY on Y? How is that possible?
- Twin sisters had duplication of part of X chromosome
- When two copies present -> inhibition of SRY
- How is that possible?
- Ovaries form in the absence of SRY, AMH and testosterone
- At present, no known gene that is specific to producing and ovary
- But, there are genes that may affect the amount/pattern of proteins encoded by testis forming genes –> resulting in development of ovary
What is DAX1?
Nuclear hormone receptor. Involved in many hormone production pathways, including ovary and testis
What is beta-catenin? Where is it involved?
- Involved in ovaries formation
- Wnt4 activates beta-catenin signalling
- expressed in indifferent gonad: become undetectable in testis (downregulated by Sry); expression maintained in ovaries
- XY human with duplication of Wnt4 overproduce DAX1 and ovaries form
What are the three types of Disorders of sex development?
- Sex chromosome DSDs
- 46 XX DSDs
- 46 XY DSDs
What is sex chromosome DSDs? what are their examples?
i) Numerical/structural abnormalities of sex chromosomes
ii) Examples: Turners (XO), Klinfelter (XXY)
What is 46 XX DSDs? what are their examples?
i) Androgen excess during pregnancy
ii) Examples: congenital adrenal hyperplasia
iii) Will have XX, but was diverted male developmental pathway, due to being exposed to high levels of androgens
What is 46 XY DSDs? what are their examples?
i) Incomplete intrauterine masculinization caused by abnormal testis development or defects in steroidogenesis/androgens
ii) Example: 5alpha reductase type 2 deficiency – individuals are born phenotypically looking as females, they do not have the enzyme that converts testosterone to dihydrosterone (DHT), however then during puberty they would develop male characteristics (more muscle tone, deeper voice), because puberty is driven by testosterone, which they do have
What are the examples of sex chromosome abnormalities? give genotype and phenotype
genotype - phenotype:
- XO (Turners) - female
- XXX (triple-X syndrome) - female
- XXY, XXXY (Klinfelter) - male
- XYY (XYY syndrome) - male
only monosomy compatible with life (Turners):
- absent ovaries (gonadal dysgenesis)
- short stature
- webbed neck
- skeletal deformities
How is sex determined in crocodiles and turtles?
- sex of most turtles and crocodiles is determined after fertilisation
- temperature regulated
- temperature of the eggs during a specific period in development acts as the deciding factor in determining sex
- small changes in temperature can cause drastic changes in sex ratio
- Expression of Sox9 gene changes in response to temperature (higher expression at lower temp and lower expression at higher temp)
how is sex determined in birds, flies and hymenopterans?
Birds:
- Females – presence of a W chromosome (ZW)
- Males – presence of a second Z chromosome (ZZ)
Flies:
- Females – Y chromosome plays no role
- Males – Ratio of X chromosomes to autosomes determines sexual phenotype
Hymenopterans (‘haplodiploid’ system):
- Females – fertilized diploid eggs
- Males – unfertilized haploid eggs (can have no father)
