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Repro > Sex Determination & Genitalia Development > Flashcards

Sex Determination & Genitalia Development Flashcards

1
Q

biological sex determination (genotypic vs phenotypic)

A
  • Variety of different structures or characteristics present at birth:

Genotypic sex determination:
o Number and type of sex chromosomes
o Absence and presence of sex determining genes (SRY, etc.)

Phenotypic sex determination:
o Type of gonads –> ovaries or testicles
o Internal reproductive anatomy including ducts or uterus
o External genitalia

Biochemical sex determination:
o sex hormone levels

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2
Q

sex determination in humans - genetics

A
  • Chromosomal and genetic sex is determined at fertilization
  • In placental mammals and marsupials Y chromosome is determining male sex and phenotype
  • In humans:
    o XY: male: Heterogametic
    o XX: female:Homogametic
  • Differentiation and dimorphism arises from circulation of sex steroids including Testosterone, Estrogen, Progesterone, 5ɑDHT
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3
Q

genetic determinant of sex in humans in Y chromosome

A
  • Presence of Y chromosome –> male
  • Absence of Y chromosome –> female

46, XX female
45, X0 female
47, XXX female

46, XY male
47, XYY male
47, XXY male

Traditionally, the symbol ♀ designates female and the symbol ♂ designates male

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4
Q

mapping of genes X and Y chromosomes

A
  • Half of the genes on Y chromosome also on the X chromosome –> common evolutionary origin
  • X chromosome large –> ~ 1500 genes, most unrelated to sex
  • Y chromosome got smaller –> 83 genes, most related to sex determination and spermatogenesis
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5
Q

Y chromosome

A
  • SRY codes for testes determining factor TDF = transcription factor
  • TDF directs embryonic gonads to develop into testes –> secretion of male hormones testosterone and Mullerian Inhibiting Substance
  • Only passed on through males without any mixing of parental genes –> Y linked inheritance
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6
Q

crossing over in male meiosis

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
  • SRY located on boarder of PAR1
  • Non-recombining region of Y unique to Y chromosome
  • -> gets passed on without crossing over
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7
Q

dosage compensation of X chromosomes

A
  • Additional X chromosome in females would lead to more gene products expressed compared to males
  • Compensation mechanism: Inactivation of all X chromosome more than one (lionization) through silencing:
  • Packaging into heterochromatin, high levels of DNA methylation, low levels of histone acetylation
  • Not all genes silenced i.e. X-inactive specific transcript (Xist)
  • Xist RNA = 17kb transcript is not translated –> thought to be a structural component of the inactivation process by physically associating with the inactive chromosome
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8
Q

X inactivation example

A
  • Fur pigmentation gene in tortoiseshell or calico cats is X-linked
  • Male cats have only one X chromosome –> express only one coat color e.g. black
  • Females are genetic mosaics, depending on which copy of the X chromosome is inactivated –> patchy orange/black coat colour
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9
Q

development of the gonads - indifferent gonads

A
  • Initial stages of gonadal development occur during 5th week
  • Primordial germ cells migrate to the gonadal ridges = precursor of gonads
  • Primitive sex cords for nutritional support to germ cells & regulation of development
  • During early stages of fetal development 2 duct systems arise
  • Genetically males & females possess initially both pairs of ducts
  • Wolffian ducts = mesonephric ducts: Progenitors of the upper male genital tract
  • Müllerian ducts = paramesonephric ducts: Progenitors of the upper female genital tract
  • Before the 7th week, gonads of the two sexes are identical in appearance = indifferent gonads
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10
Q

Barr Body

A
  • In XX females –> one X randomly inactivated ~16 days post fertilization
  • Inactive X forms a discrete body within the nucleus = Barr body
  • All descendent cells keep same X inactive (mitotic divisions)
  • No Barr bodies are observed in Turner (XO) females
  • One Barr body is observed in Klinefelter (XXY) males
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11
Q

early ovaries and testes

A

after 8th week, gonads change to male genotype if genes on Y chromosome expressed

  • ovary develops from the cortex of the indifferent gonad; the medulla regresses
  • testis develop form the medulla of the indifferent gonad, the cortex regresses
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12
Q

sexual development during gestation

A
  • Default pathway of sexual development in mammals is female, unless chemical signals are present that indicate it should develop as a male
  • Male development in mammals is directed at every step
  • If loss of direction –> subsequent development will follow female pathway
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13
Q

sex determination in mammals

A
  • testicular differentiation is controlled by time and dosage-sensitive genes
  • Sexual differentiation begins when sex determining region Y (SRY) on Y chromosome produces testis determining factor (TDF)
  • Presence of TDF directs the bipotential gonad to turn into testes rather that ovaries
  • In the absence of a Y chromosome or no SRY the embryo will develop female structures
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14
Q

fate of wolfian and mullerian ducts (male)

A
  • With TDF: Testis develop from the medulla of the indifferent gonad -> cortex regresses
  • Testes start producing testosterone and Muellerian inhibiting factor (MIF)
  • Testosterone present –> Wolffian ducts change into male reproductive system
  • MIF present – > Muellerian ducts regress
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15
Q

fate of wolfing and mullerian ducts (female)

A
  • Without TDF: Ovary develops from the cortex of the indifferent gonad –> medulla regresses
  • If no testosterone and no Muellerian inhibiting factor –> default is female development
  • MIF absent –> Müllerian ducts change into female reproductive system, Wolffian ducts degenerates
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16
Q

sexual development in humans - gonads

A
  • Differentiation of ovaries or testes from the bipotential gonadal ridge tissue in humans is fully achieved by 13 to 14 weeks, of fetal life
  • In the male: Medullary cords develop, no cortical cords
  • In the female: Cortical cords develop, medullary cords degenerate
17
Q

sexual development in humans - external genitalia

A
  • Up till 8th week, Sex neutral, undifferentiated external genitalia
  • Distinguishing sexual characteristics begin to appear during 9th week
  • External genitalia not fully differentiated until the twelfth week
18
Q

differentiation of male external genitalia

A
  • Genitalia start to masculinize at 8-9 weeks of gestation
  • Fetal Leydig cells in testes produce testosterone
  • Enzyme 5α-reductase-2 required to convert testosterone to thn androgen dihydrotestosterone (DHT)
  • Complete masculinization of external genitalia by DHT by week 14
  • In absence of either testosterone or 5α-reductase-2 –> sex neutral external genitalia develop along female lines
19
Q

descent of male testes

A
  • Between 3rd month of pregnancy and its end, testes become transferred from the lumbar area into the future scrotum
  • Transfer due to a combination of growth processes and hormonal influences -> androgens are an absolute requirement for this migration
  • Testes reach internal inguinal ring by week 24
  • Testis passing through inguinal canal to reach the scrotum
20
Q

differentiation of female external genitalia

A
  • Primary female sexual differentiation occurs slowly in the fetus and does not depend on hormones –> occurs even if ovaries are absent
  • X chromosomes bear genes for ovarian development and an autosomal gene also appears to play a role in ovarian organogenesis
  • Ovary is not identifiable histologically until about the 10th week
  • Primordial follicles develop at ~ 16 weeks: Each primordial follicle consists of an oogonium, derived from primordial germ cell
  • Oogonia proliferate > than 7 million primary oocytes produced
    o many degenerate before birth
    o ~2 million are left at birth
  • Estrogen produced by the placenta and fetal ovaries appear to be involved in feminization of indifferent external genitalia
  • Growth of the primordial phallus gradually ceases and becomes clitoris
  • Labia majora are homologous to the scrotum
21
Q

disorders of sexual development (DSD)

A
  • Conditions resulting in discordance between genetic, gonadal, or anatomic sex (internal & external structures) –> intersex individuals
  • DSD result from mutations of any of the genes involved in typical sex development or differentiation
  • Adequate expression of these genes & proper timing of their expression is important: e.g. normal but delayed androgenic effects result in incomplete masculinization of external genitalia while the persistent presence of Müllerian structures results from delayed MIS action
22
Q

ambiguous genitalia

A
  • Rare condition where infant’s genital phenotype (external genitals) do not appear to be clearly either male for female
  • Many causes have a genetic basis
  • Genitals either incompletely developed or characteristics of both sexes present
  • Previously surgery straight after birth –> often into female
  • Intersex surgery has long-term consequences for affected individuals
  • Assigned or genetic sex not always psychological sex –> gender identity
  • Now delaying genital surgery on hermaphrodite babies - patient consent
23
Q

true hermaphrodites

A
  • Organisms with both male and female reproductive organs
  • Common in plant and animal kingdoms, and usually fertile
  • Mechanism to avoid self fertilization in plants: Male and female gametes mature at different times –> less likelihood of self-pollination
24
Q

sexual reversal studies (SRY translocation)

A
  • SRY located adjacent to PARS region
  • In rare cases, SRY locus translocated onto X chromosome during crossing over
  • If sperm containing SRY-containing X chromosome fertilises oocyte –> Karyotype will not match phenotypic expression i.e. SRY makes male factor and phenotypic expression but person is genetically female
25
Q

sexual reversal studies (swayer syndrome)

A
  • Male karyotype 46, XY
  • Y chromosome present –> genetically male
  • Y chromosome has accidently lost the SRY gene in crossing over, or SRY mutation making it non-functional
  • If no active SRY gene, which encodes TDF –> Development into a phenotypical female, even though person is genetically male
  • Testes hidden in abdominal cavity and a female phenotype
26
Q

sexual development studies (XX male syndrome)

A
  • Female karyotype: 46, XX
  • No Y chromosome –> genetically female
  • Offspring who 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 produced so female internal duct structures present
27
Q

androgen insensitivity syndrome (AIS)

A
  • Affects individuals who are genetically male
  • Male karyotype 46, XY
  • Functional SRY gene –> TDF expressed, but can have outwardly female phenotype due to an underlying androgen insensitivity syndrome (AIS)
  • Testes develop, testosterone is secreted but target cells lack receptors for the androgens – no masculinizing effects occur
  • SRY has been shown to interact with the androgen receptor
  • Female body type and gender identity but sterile
  • Individuals can have complete or partial AIS
28
Q

testosterone deficiency

A
  • Male karyotype 46, XY
  • Functional SRY gene –> TDF expressed, but 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 masculinizing effects occur
  • Individuals are sterile and will not go through puberty
29
Q

5-α-reductase deficiency

A
  • Male karyotype 46, XY
  • Functional SRY gene -> TDF expressed, but can have an outwardly female phenotype due to 5-α-reductase deficiency
  • Do not produce enough dihydrotestosterone (DHT)
  • Male gonads
  • Born with female or ambiguous genitalia
30
Q

mullerian inhibiting substance deficiency

A
  • If no gene for Mullerian Inhibiting Substance, Mullerian female ductal structures will form, but the external genitalia will be male due to presence of testosterone
  • An affected individual is usually sterile because the testes do not develop normally and the presence of female ducts interferes with sperm transport
31
Q

true cryptochidism

A
  • Absence of one or both testes from the scrotum
  • Undescended testicle in 3-4% of boys
  • Spermatogenesis: Production of viable sperm, is greatly affected by the temperature of the testicle but production of testosterone is not affected by temperature
  • Untreated: Risk of infertility, testicular torsion, cancers
    o If not descended by 6 months operation
32
Q

complications of sexual reversal issues

A
  • olympic committee gender verification for females or males with sexual reversal issues
    eg SRY gene, XXY males
33
Q

environmental sex determination

A
  • Some reptiles environmental factors determine sex of progeny
  • Turtles:
    o Eggs incubated above 32°C develop into females
    o Eggs below 28°C become male
    o Eggs between 28°C-32°C develop into either gender
  • Some reptiles environmental factors determine sex of progeny
  • Alligators:
    o Eggs incubated below 30°C all females
    o Eggs incubated above 34°C all males
    o Temperature sensitive period days 7-21
    o Natural sex ratio 5 females : 1 male
34
Q

sex change

A
  • Female sexual characteristics are substituted for male ones, or vice versa
  • Sex change may occur naturally as part of sequential hermaphroditism e.g. many species of coral reef fish
  • Protandrous hermaphrodites: Organisms that are born male and at some point in their lifespan change sex to female e.g. clown fish
  • Protogynous hermaphrodites: Organisms that are born female and at some point in their lifespan change sex to male
    o Most common form of hermaphroditism in fish in nature
    o ~75% of the 500 known sequentially hermaphroditic fish species are protogynous

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