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REPRO > Sexual determination and differentiation > Flashcards

Sexual determination and differentiation Flashcards

1
Q

What is sex determination?

A

The developmental decision that directs the “bipotential” gonad to develop as a testis or ovary

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2
Q

What controls sex determination?

A
  • Genetic in mammals

- Envionrmental in some species (e.g. temperature and alligators)

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3
Q

What is sexual differentiation?

A
  • Phenotypic sex establishment

- Development of internal external genitalia

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4
Q

What controls sexual differentiation?

A

Products of the gonad (endocrine)

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5
Q

What are the 3 levels of sexual dimorphism?

A
  • Genetic (sex determination)
  • Gonadal (sexual differentiation)
  • Phenotypic (sexual differentiation)
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6
Q

How do the 3 levels of sexual dimorphism link?

A

Genetic determines gonadal which determines phenotypic sex

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7
Q

What is the genetic basis for sex determination in mammals?

A
  • Males heterogametic
  • Male: XY
  • Female: XX
  • Y must be present for male gender
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8
Q

What is SRY?

A

Y chromosome sex determining region

- “master switch” in regulating testis pathway

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9
Q

Describe sexual differentiation in mammal males

A
  • XY
  • Testosterone causes unisex genitalia to develop to penis and scrotum
  • Testosterone converted to dihydrotestosterone
  • Causes external appearance of penis and scrotum
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10
Q

Describe sexual differentiation in mammal female

A
  • XX
  • Absence of testosterone causes development of clitoris, labia and vaginal opening
  • Cells without SRY gene express female genes and inhibit male genes
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11
Q

What is the genetic basis for sex determination in birds?

A
  • Z and W are sex chromosomes
  • ZZ = male
  • ZW = female
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12
Q

What is the basis for sex determination in reptiles?

A
  • Environmental (in some)

- Incubation temperature of egg determines development of gonad towards ovary or testis

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13
Q

Compare sex determination in crocodiles and turtles

A
  • Both environmental (temperature)
  • Crocodiles: high temp = male
  • Turtles: high temp = female
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14
Q

Give the phases of normal morphological sexual development

A
  • Pregonadal phase
  • Bipotential gonad phase
  • Primary sexual differentiation
  • Secondary sexual differentiation
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15
Q

What occurs in the pregonadal phase?

A

Germ cells start to migrate from yolk sac

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16
Q

What occurs in the bipotential gonad phase?

A
  • Development of gonadal precursor tissue near kidneys (genital ridge)
  • Germ cells migrate to genital ridge
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17
Q

What occurs in the primary sexual differentiation phase?

A
  • Differentation of reproductive tissues

- Reflects nature of gonads

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18
Q

What occurs in the secondary sexual differentiation phase?

A
  • Development of secondary sexual characteristics

- Marks sexes as phenotypically distinct

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19
Q

How does normal morphological sexual development occur?

A
  • Migration of promordial cells
  • Migrate by amoeboid movement from hind to mid gut
  • Lodged in genital/gonadal ridge
  • In male, stimulate proliferation of connective tissue and production of cords
  • Pushes gonadal ridge towards development of testes
  • Germ cells undergo mitosis during migration
20
Q

What happens to the proliferative activity of the germ cells in males after birth?

A
  • Some mitotic activity continues

- Meiosis only in puberty

21
Q

What is the difference between proliferative activity of the germ cells in males and females?

A
  • Meiosis initiated in female, but then arrests until puberty
  • In males, meiosis only occurs in puberty
22
Q

From what renal tissue do the gonads develop?

A

The mesonephros

23
Q

What develops from the paramesonephric ducts in females?

A
  • Mullerian duct

- Female urogenital system

24
Q

What develops from the mesonephric duct in males?

A
  • Wolffian duct
  • Future epididymis and ductus deferens
  • Male urogenital system
25
Q

Describe the process of the male reproductive system development

A
  • SRY protein
  • Testes develop
  • Sertoli cells secrete anti-mullerian hormone (AMH)
  • Degeneration of paramesonephric duct
  • AMH causes Leydig cells to differentiate
  • Testosterone and dihydrotestosterone produced
  • Testosterone for development of male duct system
  • Dihydro for development of penis, scrotum and accessory glands
26
Q

Describe the development of the female reproductive system

A
  • No SRY protein
  • Ovaries develop
  • No AMH
  • Paramesonephric ducts become ovidcuts, uterus, cervix and part of the vagina
  • Complete female tract
27
Q

Describe the development of the ovaries

A
  • Similar structure to male gonads
  • Tubules in middle regressing, nephric duct also beginning to regress
  • Germ cells and paramesonephric duct remain
  • Paramesonephric duct will form oviduct
28
Q

Describe the formation and closure of the tubes in testis development

A
  • Mesonephric tubules become efferent ducts
  • Mesonephric ducts become epididymis and ductus deferens
  • Ducts align in correct position
  • Start to join up, paramesonephric duct begins to generate
  • Tubes form and connect
29
Q

Where do the testes descend from and to?

A

From retroperitoneal position at level of the ribs to the scrotum

30
Q

What is the ligamentous structure attached to the testis called?

A

Gubernaculum

31
Q

Describe the structure of the gubernaculum

A
  • In contact with the peritoneum

- Fusion of 2 cell types (gonadal and peritoneal)

32
Q

Describe the normal descent of the testis

A
  • Testis moves towards inguinal ring as foetus grows
  • Gubernaculum through inguinal ring
  • Expands
  • Expansion pulls testis through canal as consequence
  • As moves through, testis encapsulated in layer of peritoneum
  • Gubernaculum contracts, pulls testis into scrotum
33
Q

When does testicular descent occur in horses?

A

Late pregnancy or a few days after birth

34
Q

When does testicular descent occur in dogs?

A

Up to 8 days after birth

35
Q

In what species do the testes remain in their original position?

A
  • Elephants

- Whales

36
Q

How does sexual differentiation of the brain occur?

A

Different hormones secreted by gonads

37
Q

Describe the sexual differentiation of the male brain

A
  • Testosterone secreted into blood
  • Crosses BBB
  • Converted to oestradiol (by aromatase) and dihydrotestosterone
  • Oestradiol masculinises brain
38
Q

Describe teh sexual differentiation of the female brain

A
  • Alpha-fetoprotein binds to oestradiol
  • Complex formed cannot cross BBB
  • Protects female brain from masculinising effects of oestradiol
39
Q

What is the difference between the male and female brain?

A

Females have surge centre, males do not (inhibited by oestrogen)

40
Q

What is freemartinism?

A
  • Sterile bovine

- Both male and female development has taken place

41
Q

How does freemartinism occur?

A
  • Male and female twins
  • Common blood supply between male and female
  • Female exposed to some AMH and testosterone
42
Q

What are the effects of freemartinism?

A
  • Paramesonephric ducts do not develop properly
  • Ovaries do not develop complement of germ cells
  • Gonad may produce testosterone and androstenedione
  • CNS programmed so male behaviour
  • Overdeveloped clitoris (penile appearance)
  • Objects that look like testes on underside, but no scrotum
43
Q

Give examples of male development problems

A
  • Cryptorchidism
  • Inguinal herniation
  • Hypospadias
44
Q

What is cryptorchidism?

A

Failure of testes to descend into scrotum

45
Q

What is inguinal herniation?

A

Portion of intestine passes through inguinal canal into the vaginal cavity

46
Q

What is hypospadia?

A

Urethral opening at base of penis

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