L5: Sex Differentiation

Question 1

What is sex determination?

Answer 1

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.

Question 2

When and how is sex determined?

Answer 2

Sex is determined at fertilization by the sperm. 50% chance for XX and XY

Question 3

What is unique about gonadal rudiments?

Answer 3

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.

Question 4

What is the bipotential stage? How long does it last in humans?

Answer 4

It’s decision time in embryo develpment, gonads are indifferent and it lasts for 7 weeks.

Question 5

What are the main germ cells in gonads?

Answer 5

• oocyte
• sperm

Question 6

What are the somatic cells in gonads?

Answer 6

supporting cells:
- granulosa cells in oocytes
- sertoli cells in sperm development

steroidogenic cells:
- theca cells in oocytes
- leydig cells in sperm development

Question 7

What are Primordial Germ Cells (PGCs)? Where are they located?

Answer 7

• Precursors of sperm and eggs
• located outside the body in yolk sac

Question 8

When does specification of PGCs begin?

Answer 8

Specification of PGCs begins around 3 weeks of human gestation

Question 9

How does gonadal ridge form?

Answer 9

PGCs migrate to gonadal ridge. Proliferation of epithelium and mesenchyme on medial side of mesonephros.

Question 10

What is mesonephros?

Answer 10

Fetal ‘interim’ kidney

Question 11

How long are gonads identical in appearance?

Answer 11

Until 7th week

Question 12

How does PGC migration and proliferation happen at week 4-6?

Answer 12

PGCs migrate from yolk sac –> going up through hindgut –> dorsal mesentery (fold of tissue that attaches organs to body wall) –> genital ridges (primitive gonad)

Question 13

What kind of divisions do PGC cells undergo during migration?

Answer 13

Undergo mitosis while migrating, from ~100 cells to 25,000

Question 14

How long do PGCs proliferate in humans?

Answer 14

Until ~10th week

Question 15

What do PGCs require for motility?

Answer 15

• chemotactic signals coming from gonads
• gradients of extracellular matrix glycoproteins
• PGC-PGC contact

divide into two populations –> colonise R & L gonads

Question 16

What are the chemotactic signals needed for PGCs to migrate?

Answer 16

• TGF-beta (transforming growth factor beta)
• Kit ligand/SCF (stem cell factor)
• bFGF (basic fibroblast growth factor)

Question 17

When does sex specific differentiation of PGCs begin?

Answer 17

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)

Question 18

What happens after colonisation of gonad by PGCs?

Answer 18

PGCs become surrounded by somatic cells, PGCs continue to proliferate

Question 19

How do PGCs change in males and females?

Answer 19

• in males to prespermatogonia / prospermatogonia
• oogonia

Question 20

What is one of the most important genes regulating gonadal development? what is the evidence?

Answer 20

SF1 (NR5A1) KO mice: gonads fail to develop beyond early indifferent stage

Question 21

What are the genes regulating gonadal development? What are the experiments that prove it?

Answer 21

• 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

Question 22

What are the experiments proving Jost paradigm?

Answer 22

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.

Question 23

What is Jost paradigm?

Answer 23

Female pathway is a default pathway, that happens without any active input (don’t need any active compound)

Question 24

What’s the part that forms actual kidneys called?

Answer 24

Metanephric kidney

Question 25

How does **development of gonads and ducts in females** happen?

Answer 25

- 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

Question 26

How does development of gonads and ducts in males happen?

Answer 26

- Wolffian ducts remain - they need testosterone for survival - they develop into male secondary sex organs: vas deferens, epdidymis, seminal vesicles - Mullerian duct degenerates

Question 27

What's the **pathway of testis somatic cell differentiation**?

Answer 27

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

Question 28

What's the pathway of **testis germ cell development**?

Answer 28

germ cells develop within testis cords --> differentiate into sperm (puberty)

Question 29

What do **interstitial cells** in the testis develop to?

Answer 29

Leydig cells

Question 30

What do germ cells become interspersed with in ovary?

Answer 30

Germs cells become interspersed with somatic cells and then germ cells become enclosed by somatic cells (granulosa)

Question 31

What do **mesenchyme cells** in the ovary develop to?

Answer 31

Theca cells

Question 32

What is **AMH**? Where is it produced and what does it do?

Answer 32

Anti-mullerian hormone is produced by **Sertoli cells** and r**egresses Mullerian Duct** in males

Question 33

What is **SRY** gene?

Answer 33

- 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

Question 34

Where is **SRY** gene expressed?

Answer 34

Expressed in male somatic cells: sertoli cells

Question 35

What are the targets of **SRY** gene?

Answer 35

**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

Question 36

what are the studies proving importance of **SRY**?

Answer 36

- 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

Question 37

What is the example of twin sisters found who were XY and had SRY on Y? How is that possible?

Answer 37

- 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

Question 38

What is **DAX1**?

Answer 38

Nuclear hormone receptor. Involved in many hormone production pathways, including ovary and testis

Question 39

What is **beta-catenin**? Where is it involved?

Answer 39

- 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

Question 40

What are the three types of **Disorders of sex development**?

Answer 40

- Sex chromosome DSDs - 46 XX DSDs - 46 XY DSDs

Question 41

What is sex chromosome DSDs? what are their examples?

Answer 41

i) Numerical/structural abnormalities of sex chromosomes ii) Examples: Turners (XO), Klinfelter (XXY)

Question 42

What is 46 XX DSDs? what are their examples?

Answer 42

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

Question 43

What is 46 XY DSDs? what are their examples?

Answer 43

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

Question 44

What are the examples of sex chromosome abnormalities? give genotype and phenotype

Answer 44

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

Question 45

How is sex determined in crocodiles and turtles?

Answer 45

- 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)

Question 46

how is sex determined in birds, flies and hymenopterans?

Answer 46

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)