Sex differentiation and determination

Question 1

what is sex determination?

Answer 1

A developmental decision that directs the bipotential gonad to develop as either testes or ovaries

Question 2

In mammals how is sex determination controlled?

Answer 2

It is genetically determined

Question 3

What is sexual differentiation?

Answer 3

The phenotypic sex is established by development of the external and internal gonads, it is driven by products from the gonads

Question 4

What are the three levels of sexual dimorphism and how are they caused

Answer 4

Genetic - sex determination is lost

Gonadal and phenotypic where sex differentiation is lost

Question 5

In birds which gender is heterogametic?

Answer 5

The female, females are ZW and males are ZZ

Question 6

In reptiles environmental sex determination can occur, in crocodiles and turtles high temperatures will produce which gender?

Answer 6

Crocodiles: high temp is male promoting
Turtles: high temp is female promoting

Question 7

In mammals which gender is heterogametic?

Answer 7

The male is heterogametic XY and the female is XX

Question 8

Will a male with XY chromosomes but no testes show male characteristics?

Answer 8

No, testes must be present for the formation of male characteristics as must the Y chromosome be present for it to be of the male gender (regardless of how many X chromosomes it has)

Question 9

What is the SRY gene?

Answer 9

The Y chromosome sex determining region and is the master switch to the testis pathway

Question 10

Cells without the SRY gene…..

Answer 10

Express female genes and inhibit male genes

Question 11

How does male sex differentiation occur in the brain?

Answer 11

Testosterone is secreted into the blood
It crosses the blood brain barrier
It is converted to oestradiol and dihyrotestosterones
Oestradiol defeminises the brain and the surge centre does not develop

Question 12

How does female sex differentiation occur in the brain?

Answer 12

The fetal ovary produces oestradiol
Oestradiol binds to alpha fetoprotein and the complex cannot cross the blood brain barrier
This protects the brain from masculinisation and the surge centre develops

Question 13

Describe the four main stages of sexual differentiation

Answer 13

1. Pregonadal phase: germ cells migrate from the yolk sac to the hind gut
2. The bipotential gonad: development of gonadal precursor tissue near the kidneys (at the genital ridge) as the germ cells migrate there
3. Primary sexual differentiation: production of the different reproductive tissues for example when cells arrive at the gonadal ridge in males then connective tissue proliferates and cords are produced
4. Secondary sexual differentiation: marks sexes as phenotypically different

Question 14

Does all this sexual differentiation occur before birth?

Answer 14

No, In males the germ cells arrest at the genital ridge and gonad (in G0 phase of the cell cycle) where they stay in arrest until after birth where they resume some mitotic action but only resume meiosis at puberty.

In females the germ cells migrate to the genital ridge and they arrest at prophase I of meiosis and they remain in arrest until puberty

Question 15

How does the early renal system develop into the reproductive system?

Answer 15

The mesonephros produces urine which travels down the mesonephric tubules to the urogenital sinus
from the caudal mesonephric duct the metanephros develops and this is what will become the kidney and ureters
The gonads grow and either the mesonephros breaks down into the wolffian duct to become the male reproductive system or the paramesonephric duct breaks down into the mullerian duct to becoe the female system

Question 16

What role does the SRY gene play in development of the reproductive system from the early renal system?

Answer 16

The SRY protein causes the testes to develop and the testes produce anti-mullerian hormone (AMH)
AMH causes leydig cells to differentiate and produce testosterone so the tract develops (and dihydrotestosterone so the external genitalia develop) and also causes degeneration of the paramesonephric duct.

Question 17

The mesonephric tubules become the …………………..

The mesonephric ducts become the …………………

Answer 17

mesonephric tubules = efferent ducts

mesonephric ducts = epididymis and the ductus deferens

Question 18

When does testicular descent generally occur?

When does it occur in dogs and horses?

Answer 18

Middle to end of foetal development
Horses - Late pregnancy to a few days after birth
Dogs - Up to 8 days after birth

Question 19

Which species have testes which do not descend?

Answer 19

Elephants and whales

Question 20

What is cryptorchidism?

Answer 20

Failure of testes to descend into the scrotum

Question 21

What is inguinal herniation?

Answer 21

A portion of the intestines passes through the inguinal canal and into the vaginal cavity

Question 22

What is hypospadias?

Answer 22

The urethral opening is at the base of the penis

Question 23

How is the development of the female ovary different to the testes?

Answer 23

Active development occurs much later than the testes
There is little hormone production in the first two thirds of gestation
Unlike the testes there is no tube connecting the primary follicles of the ovary to the uterus so they are caught by the infundibulum and channeled down

Question 24

How is freemartinism caused?

Answer 24

When a heifer is born twin to a bull
There is an anastomosis across the placentas which causes the female calf to be exposed to be exposed to AMH and testosterone

The paramesonephric ducts dont develop completely and the ovaries do not develop complementary to the germ cells. The CNS is programmed to exhibit male behaviour.
The female is sterile.