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HLTH3310 > Sex Determination and Genitalia Development > Flashcards

Sex Determination and Genitalia Development Flashcards

1
Q

What determines genotype, phenotype and biochemical aspects of sex determination / genitalia development

A
  • Genotypic: Number and type of sex chromosomes, absence or presence of sex determining genes (SRY)
  • Phenotypic: Type of gonads (ovaries or testicles), internal reproductive anatomy (ducts or uterus) and external genitalia
  • Biochemical: Sex hormone levels
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2
Q

What id sex determination and differentiation

A
  • Sex Determination: Chromosomal and genetic sex is determined at fertilisation, Y chromosome is the genetic determinant of sex in humans
  • Male: XY, heterogametic
  • Female: XX, homogametic
  • Differentiation: Differentiation and dimorphism arise from circulation of sex steroids (testosterone, oestrogen, progesterone and 5aDHT)
  • X (large, 1500 genes, unrelated to sex)
  • Y (83 genes, sex determination / spermatogenesis)
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3
Q

What are 2 important aspects of the Y chromosome

A
  • SRY: Sex determining region, codes for testes determining factor (TDF) transcription factor, absence of a Y chromosome or no SRY = embryo will develop female structures
  • TDF: Directs embryonic gonads to develop into testes, secretion of male hormones testosterone and mullerian inhibiting substance, only passed through males without mixing of parental genes (Y linked)
  • Presence of TDF directs bi-potential gonad to turn into testes
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4
Q

Describe crossing over of the X and Y chromosomes

A
  • Crossing Over: Tips of X and Y pair in meiosis in pseudo-autosomal regions (PAR1 and PAR2), crossover in PAR1 necessary in male meiosis for proper segregation of chromosomes
  • Non-Recombining Region: Region of Y unique to Y chromosome, passed on without crossing over
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5
Q

What is dosage compensation of X chromosomes and the different genetic female types

A
  • Compensation Mechanism: Inactivation of more than one X chromosome (lionisation)
  • Through silencing, packaging into heterochromatin, high levels of DNA methylation, low levels of histone acetylation
  • X-Inactive specific transcript (Xist), not all genes silenced
  • XX Female: One X randomly inactivated ~16 days post fertilisation, inactive X forms a discrete body within nucleus (barr body), all descendent cells keep same X inactive
  • XO Female: No Barr bodies are observed in Turner (XO) females
  • XXY Male: One Barr body is observed in Klinefelter (XXY) males
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6
Q

What are indifferent gonads

A
  • Initial stages of gonadal development occurs during 5th week
  • Development of 2 duct systems arise
  • Genetically males & females possess initially both pairs of ducts
  • Before the 7th week, gonads of the two sexes are identical in appearance (indifferent)
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7
Q

What do the gonads develop from

A
  • Primordial Germ Cells: Gamete precursors, originate in endodermal endothelium of yolk sac, migrate along mesentery of hindgut to gonadal ridges (precursor of gonads)
  • Gonadal Ridge: Primitive sex cords for nutritional support to germ cells & regulation of development
  • Wolffian Ducts: Mesonephric ducts, progenitors of upper male genital tract
  • Mullerian Ducts: Paramesonephric ducts, progenitors of upper female genital tract
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8
Q

When do internal gonads differentiate and what is gestation

A
  • Ovaries / Testes: After 8th week, gonads change to male phenotype if genes on Y chromosome expressed
  • Ovary develops from the cortex of the indifferent gonad, medulla regresses
  • Testis develop from the medulla of the indifferent gonad, cortex regresses
  • Gestation: Default pathway of sexual development in mammals is female, unless chemical signals are present that indicate it should develop as a male
  • If loss of direction then subsequent development will follow female pathway
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9
Q

What are the fates of the wolffian and mullerian ducts

A

TDF:
- Testis develop from medulla of indifferent gonad, cortex regresses, testes start producing testosterone and mullerian inhibiting factor (MIF)
- Testosterone present, wolffian ducts change into male reproductive system
MIF present, mullerian ducts regress
No TDF:
- Ovary develops from cortex of indifferent gonad, medulla regresses, if no testosterone and no MIF, default is female development
- MIF absent, mullerian ducts change into female reproductive system, wolffian ducts degenerate

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10
Q

Provide a summary of sexual development in males

A
  • Genes: 46, XY, SRY and SOX9
  • Wolffian Ducts: Testes / testosterone, progenitors of the upper male genital tract, give rise to epididymis, vas deferens, and seminal vesicles
  • Mullerian Ducts: Presence of MIF causes regression
  • Other Embryonic Structures: Testes / testosterone, arise from the urogenital sinus and genital tubercle, progenitors of lower portions of the genital tracts, give rise to penis & scrotum
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11
Q

Provide a summary of sexual development in females

A
  • Genes: 46, XX, WNT4 and DAX1
  • Wolffian Ducts: Ovaries, absence of MIF causes regression
  • Mullerian Ducts: Ovaries, absence of testosterone, progenitors of the upper female genital tract, develop into fallopian tubes, uterus and upper part of the vagina
  • Other Embryonic Structures: Arise from the urogenital sinus and genital tubercle, progenitors of lower portions of the genital tracts, give rise to lower portion of vagina, clitoris, and labia
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12
Q

How does differentiation of male external genitalia occur

A
  • Masculinise at 8-9 weeks
  • Foetal leydig cells produce testosterone
  • Complete by week 14
  • 5a-reductase-2 converts testosterone into androgen dihydrotestosterone (DHT), stimulates development of penis
  • Medullary cords develop from mesonephric duct to epididymis / vas deferens, no cortical cords
  • Testes transferred into scrotum, reach inguinal ring by week 24
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13
Q

How does differentiation of female external genitalia occur

A
  • Does not depend on hormones, occurs if ovaries are absent
  • Complete around 10th week
  • Growth of the primordial phallus gradually ceases and becomes clitoris
  • Labia majora are homologous to the scrotum
  • Cortical cords develop from mullerian duct to oviduct / uterus, medullary cords degenerate
  • X chromosome = ovarian development
  • Primordial follicle develop at 16 weeks, proliferate
  • Oestrogen involved in feminisation of genitalia
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14
Q

What are disorders of sex development and examples

A
  • Conditions resulting in discordance between genetic, gonadal, or anatomic sex (internal & external), intersex individuals, mutations of any of the genes involved in typical sex development or differentiation
  • Ambiguous genitalia
  • True hermaphrodite
  • SRY translocation
  • Swayer syndrome
  • XX male syndrome
  • AIS
  • Testosterone deficiency
  • 5a-reductase-2 deficiency
  • MIS deficiency
  • True cryptorchidism
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15
Q

What are the differences between XY, X0 and ZW systems of sex determination in other species

A 
XY:
- Number of X chromosomes determines sex
- Female (XXY / XX), male (XY) and sterile male (XO)
- Drosophila melanogaster
XO:
- Only X chromosome 
- Female (XX) and male (XO)
- Arachnids 
ZW:
- Presence of W determines sex
- Female (ZW) and male (ZZ)
- Birds (no SRY)
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16
Q

How can the environment influence sex determination in some species

A
  • Environmental: Some reptiles environmental factors determine sex of progeny
  • Turtles: Eggs incubated above 32°C develop into females, eggs below 28°C become male and eggs between 28°C-32°C develop into either gender
  • Alligator: Eggs incubated below 30°C all females, eggs incubated above 34°C all males, temperature sensitive period days 7-21, natural sex ratio 5 females : 1 male
17
Q

What is the difference between protandrous and protogynous hermaphrodites

A
  • Sex Change: Female sexual characteristics are substituted for male ones, or vice versa, may occur naturally as part of sequential hermaphroditism e.g. many species of coral reef fish
  • Protandrous:Organism born male, at some point in change sex to female (clown fish)
  • Protogynous:Organisms born female, at some point change sex to male, most common form of hermaphroditism in fish, ~75% sequentially hermaphroditic fish
18
Q

What is ambiguous genitalia (DSD)

A
  • Infant’s genital phenotype (external genitals) do not appear to be male for female
  • Genetic basis, incompletely developed or characteristics of both sexes present
19
Q

What is a true hermaphrodite (DSD)

A
  • Organisms with both male / female reproductive organs
  • Common in plant and animal kingdoms, and usually fertile
  • Mechanism to avoid self fertilisation in plants, male and female gametes mature at different times, less likelihood of self-pollination
20
Q

What is SRY translocation (DSD)

A
  • SRY located adjacent to PARS region
  • SRY locus translocated onto X chromosome during crossing over
  • Karyotype does not match phenotypic expression
  • SRY makes male factor and phenotypic expression, person is genetically female
21
Q

What is swayer syndrome (DSD)

A
  • Affects 1:80,000, male karyotype 46, XY
  • Absence of SRY gene in crossing over or SRY mutation making it non-functional
  • No active SRY gene, which encodes TDF, phenotypical female, even though person is genetically male
  • Testes hidden in abdominal cavity and a female phenotype
22
Q

What is AIS (DSD)

A
  • Affects 2-5:100,000, male karyotype 46, XY, functional SRY gene
  • TDF expressed, but have outwardly female phenotype due to androgen insensitivity syndrome
  • Testes, testosterone secreted but target cells lack receptors for androgens, no masculinising effects occur
  • SRY has been shown to interact with the androgen receptor
  • Female body / gender identity, sterile, complete or partial AIS
23
Q

What is testosterone deficiency (DSD)

A
  • Male karyotype 46, XY, functional SRY gene
  • TDF expressed,can have an outwardly female phenotype due to Leydig cells in testes not secreting testosterone
  • Internal structures and gonads will develop into male structures, but the external genitalia will be female
  • No masculinising effects, individuals are sterile and will not go through puberty
24
Q

What is 5a-reductase-2 deficiency (DSD)

A
  • Male karyotype 46, XY, functional SRY gene, TDF expressed
  • Outwardly female phenotype due to 5-α-reductase deficiency, do not produce enough dihydrotestosterone (DHT)
  • Male gonads but born with female or ambiguous genitalia
25
Q

What is MIS deficiency

A
  • If no gene for mullerian inhibiting substance, mullerian female ductal structures form
  • External genitalia will be male due to presence of testosterone
  • Sterile, testes do not develop normally and presence of female ducts interferes with sperm transport
26
Q

What is true cryptorchidism (DSD)

A
  • Absence of one or both testes from scrotum, undescended testicle in 3-4% - Production of viable sperm affected, production of testosterone is not
  • Untreated leads to risk of infertility, testicular torsion, cancers if not descended by 6 months operation
27
Q

What is XX male syndrome (DSD)

A
  • Affects 1:20,000, female karyotype, 46, XX
  • Inherit the SRY gene on the X chromosome (or translocation to an autosome)
  • Formation of both testicles and ovaries in the same individual
  • Incomplete penetrance possible (X inactivation), no MIF (female internal structures)

HLTH3310 (12 decks)

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