Sexual determination and differentiation

Question 1

What is sex determination?

Answer 1

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

Question 2

What controls sex determination?

Answer 2

• Genetic in mammals

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

Question 3

What is sexual differentiation?

Answer 3

• Phenotypic sex establishment

- Development of internal external genitalia

Question 4

What controls sexual differentiation?

Answer 4

Products of the gonad (endocrine)

Question 5

What are the 3 levels of sexual dimorphism?

Answer 5

• Genetic (sex determination)
• Gonadal (sexual differentiation)
• Phenotypic (sexual differentiation)

Question 6

How do the 3 levels of sexual dimorphism link?

Answer 6

Genetic determines gonadal which determines phenotypic sex

Question 7

What is the genetic basis for sex determination in mammals?

Answer 7

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

Question 8

What is SRY?

Answer 8

Y chromosome sex determining region

- “master switch” in regulating testis pathway

Question 9

Describe sexual differentiation in mammal males

Answer 9

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

Question 10

Describe sexual differentiation in mammal female

Answer 10

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

Question 11

What is the genetic basis for sex determination in birds?

Answer 11

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

Question 12

What is the basis for sex determination in reptiles?

Answer 12

• Environmental (in some)

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

Question 13

Compare sex determination in crocodiles and turtles

Answer 13

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

Question 14

Give the phases of normal morphological sexual development

Answer 14

• Pregonadal phase
• Bipotential gonad phase
• Primary sexual differentiation
• Secondary sexual differentiation

Question 15

What occurs in the pregonadal phase?

Answer 15

Germ cells start to migrate from yolk sac

Question 16

What occurs in the bipotential gonad phase?

Answer 16

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

Question 17

What occurs in the primary sexual differentiation phase?

Answer 17

• Differentation of reproductive tissues

- Reflects nature of gonads

Question 18

What occurs in the secondary sexual differentiation phase?

Answer 18

• Development of secondary sexual characteristics

- Marks sexes as phenotypically distinct

Question 19

How does normal morphological sexual development occur?

Answer 19

• 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

Question 20

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

Answer 20

• Some mitotic activity continues

- Meiosis only in puberty

Question 21

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

Answer 21

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

Question 22

From what renal tissue do the gonads develop?

Answer 22

The mesonephros

Question 23

What develops from the paramesonephric ducts in females?

Answer 23

• Mullerian duct

- Female urogenital system

Question 24

What develops from the mesonephric duct in males?

Answer 24

• Wolffian duct
• Future epididymis and ductus deferens
• Male urogenital system

Question 25

Describe the process of the male reproductive system development

Answer 25

- 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

Question 26

Describe the development of the female reproductive system

Answer 26

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

Question 27

Describe the development of the ovaries

Answer 27

- 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

Question 28

Describe the formation and closure of the tubes in testis development

Answer 28

- 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

Question 29

Where do the testes descend from and to?

Answer 29

From retroperitoneal position at level of the ribs to the scrotum

Question 30

What is the ligamentous structure attached to the testis called?

Answer 30

Gubernaculum

Question 31

Describe the structure of the gubernaculum

Answer 31

- In contact with the peritoneum | - Fusion of 2 cell types (gonadal and peritoneal)

Question 32

Describe the normal descent of the testis

Answer 32

- 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

Question 33

When does testicular descent occur in horses?

Answer 33

Late pregnancy or a few days after birth

Question 34

When does testicular descent occur in dogs?

Answer 34

Up to 8 days after birth

Question 35

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

Answer 35

- Elephants | - Whales

Question 36

How does sexual differentiation of the brain occur?

Answer 36

Different hormones secreted by gonads

Question 37

Describe the sexual differentiation of the male brain

Answer 37

- Testosterone secreted into blood - Crosses BBB - Converted to oestradiol (by aromatase) and dihydrotestosterone - Oestradiol masculinises brain

Question 38

Describe teh sexual differentiation of the female brain

Answer 38

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

Question 39

What is the difference between the male and female brain?

Answer 39

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

Question 40

What is freemartinism?

Answer 40

- Sterile bovine | - Both male and female development has taken place

Question 41

How does freemartinism occur?

Answer 41

- Male and female twins - Common blood supply between male and female - Female exposed to some AMH and testosterone

Question 42

What are the effects of freemartinism?

Answer 42

- 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

Question 43

Give examples of male development problems

Answer 43

- Cryptorchidism - Inguinal herniation - Hypospadias

Question 44

What is cryptorchidism?

Answer 44

Failure of testes to descend into scrotum

Question 45

What is inguinal herniation?

Answer 45

Portion of intestine passes through inguinal canal into the vaginal cavity

Question 46

What is hypospadia?

Answer 46

Urethral opening at base of penis