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Reproductive Biology > Sex Determination and Genitalia Development (Lec 11/12) > Flashcards

Sex Determination and Genitalia Development (Lec 11/12) Flashcards

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1
Q

Sex Determination: Genetics

A

In humans, sex is determined at fertilization by Y chromosome:
XY (heterogametic) = male
XX (homogametic) = female

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2
Q

Y Chromosome

A

Sex Determining Region (SRY) gene codes for Testes Determining Factor (TDF) -> TDF directs embryonic gonads to develop into testes -> secretion of Testosterone and Mullerian Inhibiting Substance

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3
Q

Crossing Over in Male Meiosis

A

Tips of X and Y pair in Meiosis in Psuedo-autosomal regions (PAR1 and PAR2). Cross over of PAR1 essential for proper segregation of chromosomes. Non-recombining region of Y (NRY) unique to Y chromosome and gets passed on without crossing over

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4
Q

Compensation Mechanism

A

Inactivation of all X chromosome through silencing. X-inactive specific transcript (Xist) causes not all genes to be silenced.

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5
Q

X-Inactivation

A

In females, 1 X-chromosome randomly inactivated around 16 days post fertilization. The inactive X forms a discrete body within the nucleus (Barr Body).

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6
Q

Development of Gonads - Indifferent Gonads

A

Initial development of gonads occur during 5th week. Primordial germ cells originate in the endodermal epithelium of the yolk sack and migrate along the mesentry of the hindgut to the Gonadal Ridge (precursor of gonads)

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7
Q

Indifferent Gonads

A

During early stages of fetal development, 2 duct systems arise: Wolffian and Mullerian ducts. Initially, both sexes have both ducts.

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8
Q

Wolffian Ducts (Mesonephric Ducts)

A

Progenitors of the upper male genital tract

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9
Q

Mullerian Ducts (Paramesonephric Ducts)

A

Progenitors of the upper female genital tract

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10
Q

Pre-Week 7: Indifferent Gonads

A

Gonads of the 2 sexes are identical in appearance. Indifferent gonads consist of an outer cortex and inner medulla

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11
Q

Post-Week 8: Early Ovaries and Testes

A

Gonads changed to male phenotype if genes on Y chromosome are expressed.

Females: ovaries develop from the cortex of the indifferent gonad, medulla regresses
Males: Testes develop from medulla of the indifferent gonad, cortex regresses

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12
Q

The Fate of Wolffian Ducts

A

With TDF:

  • > Testes develop from the medulla, cortex regresses
  • > Testes produce Testosterone and Mullerian-inhibiting factor (MIF)

Testosterone Present:
-> Wolffian Ducts turn into male reproductive system

MIF Present:
-> Mullerian ducts regress

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13
Q

The Fate of Mullerian Ducts

A

Without TDF:
-> Ovaries develop from the cortex, medulla regresses

If NO Testosterone and MIF:

  • > Default female development occurs
  • > Mullerian ducts turn into female reproductive system
  • > Wolffian ducts degenerate
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14
Q

Development of the Gonads

A

Differentiation of Ovaries or Testes from the bipotential gonadal ridge is fully achieved by week 13-14 (fetal life)

Males: Medullary cords develop, no Cortical cords
Females: Cortical cords develop, no Medullary cords

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15
Q

Development of External Genitalia

A

Pre-week 8: sex neutral, undifferentiated external genitalia
Post-week 8: distinguishing sexual features begin to appear
12th week: external genitalia fully differentiated

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16
Q

Differentiation of Male External Genitalia

A
  • > Fetal Leydig cells produce testosterone
  • > Enzyme 5α-reductase-2 converts testosterone into dihydrotestosterone (DHT)
  • > Complete masculinisation of external genitalia by DHT by the 14th week
17
Q

Descent of Testes

A

Occurs after 3rd month of pregnancy. Testes transferred from lumbar area of body into the future scrotum through the Inguinal Canal. Transfer due to a combination of growth processes and hormonal influences (androgens)

18
Q

Differentiation of Female External Genitalia

A

Primary female sexual differentiation occurs slowly in the fetus and does not depend on hormones. Growth of primordial phallus gradually ceases and becomes Clitoris, Labia majora are homologous with male scrotum

19
Q

Disorders of Sex Development (DSD)

A

Conditions resulting in discordance between genetic, gonadal, or anatomic sex (internal & external structures)

20
Q

Ambiguous Genitalia

A

Rare condition where infant’s genital phenotype do not appear to be male or female. Genitals either incompletely developed or both sexes present

21
Q

Sex Reversal Studies: SRY Translocation

A

SRY locus translocated onto X chromosome during crossing over. If sperm containing SRY-containing X chromosome fertilises ooctye -> Karyotype of individual will not match phenotypic expression

22
Q

Swayer Syndrome

A

Affects 1:80,000. 46, XY

  • Y chromosome present, genetically male
  • However, Y chromosome does not have SRY gene, therefore female reproductive tract exists
23
Q

XX Male Syndrome

A

Affects 1:20,000. 46, XX

  • No Y chromosome, genetically female
  • SRY translocated onto X chromosome which causes formation of both Ovaries and Testes in same individual
24
Q

Androgen Insensitivity Syndrome (AIS)

A

Affects 2-5:100,000. 46, XY

  • Functional SRY gene, TDF expressed, however due to Androgen insensitivity, female characteristics occur
  • Testosterone produced from testes but target cells lack receptors for Androgens, no masculinizing effect
25
Q

Testosterone Deficiency

A

46, XY
Functional SRY gene, TDF produced, female characteristics due to:
- Leydig cells in testes not secreting testosterone
- Internal structures male, external genitalia female

26
Q

5α-reductase Deficiency

A

46, XY
Functional SRY gene -> TDF expressed, female characteristics due to:
- 5α-reductase Deficiency not producing enough DHT
- Male gonads with ambiguous genitalia

27
Q

True Cryptochidism

A

Absence of one or both testes from the scrotum.

  • Spermatogenesis is greatly influenced by temp
  • > risk of infertility, testicular torsion and cancers
28
Q

Sex Change

A

Female sexual characteristics are substituted for Males, or vice versa.

29
Q

Protandrous Hermaphrodites

A

Organisms that are born male but at some point in life change sexes and become female

30
Q

Protogyous Hermaphrodites

A

Organisms that are born female but at some point in life change sexes and become male

31
Q

Sex Determination in Animals

A

Drosophilia Melanogaster: determined by number of X’s

  • Male: XY, XO (sterile)
  • Female: XX, XXY

Arachnids: 1 sex chromosome (X)

  • Male: XO
  • Female: XX

Birds: Designated Z & W (sex-determining) chromosomes

  • Male: homogametic ZZ
  • Female: heterogametic ZW

Reproductive Biology (16 decks)

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