Sexual determination and differentiation Flashcards
What is sex determination?
The developmental decision that directs the “bipotential” gonad to develop as a testis or ovary
What controls sex determination?
- Genetic in mammals
- Envionrmental in some species (e.g. temperature and alligators)
What is sexual differentiation?
- Phenotypic sex establishment
- Development of internal external genitalia
What controls sexual differentiation?
Products of the gonad (endocrine)
What are the 3 levels of sexual dimorphism?
- Genetic (sex determination)
- Gonadal (sexual differentiation)
- Phenotypic (sexual differentiation)
How do the 3 levels of sexual dimorphism link?
Genetic determines gonadal which determines phenotypic sex
What is the genetic basis for sex determination in mammals?
- Males heterogametic
- Male: XY
- Female: XX
- Y must be present for male gender
What is SRY?
Y chromosome sex determining region
- “master switch” in regulating testis pathway
Describe sexual differentiation in mammal males
- XY
- Testosterone causes unisex genitalia to develop to penis and scrotum
- Testosterone converted to dihydrotestosterone
- Causes external appearance of penis and scrotum
Describe sexual differentiation in mammal female
- XX
- Absence of testosterone causes development of clitoris, labia and vaginal opening
- Cells without SRY gene express female genes and inhibit male genes
What is the genetic basis for sex determination in birds?
- Z and W are sex chromosomes
- ZZ = male
- ZW = female
What is the basis for sex determination in reptiles?
- Environmental (in some)
- Incubation temperature of egg determines development of gonad towards ovary or testis
Compare sex determination in crocodiles and turtles
- Both environmental (temperature)
- Crocodiles: high temp = male
- Turtles: high temp = female
Give the phases of normal morphological sexual development
- Pregonadal phase
- Bipotential gonad phase
- Primary sexual differentiation
- Secondary sexual differentiation
What occurs in the pregonadal phase?
Germ cells start to migrate from yolk sac
What occurs in the bipotential gonad phase?
- Development of gonadal precursor tissue near kidneys (genital ridge)
- Germ cells migrate to genital ridge
What occurs in the primary sexual differentiation phase?
- Differentation of reproductive tissues
- Reflects nature of gonads
What occurs in the secondary sexual differentiation phase?
- Development of secondary sexual characteristics
- Marks sexes as phenotypically distinct
How does normal morphological sexual development occur?
- 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
What happens to the proliferative activity of the germ cells in males after birth?
- Some mitotic activity continues
- Meiosis only in puberty
What is the difference between proliferative activity of the germ cells in males and females?
- Meiosis initiated in female, but then arrests until puberty
- In males, meiosis only occurs in puberty
From what renal tissue do the gonads develop?
The mesonephros
What develops from the paramesonephric ducts in females?
- Mullerian duct
- Female urogenital system
What develops from the mesonephric duct in males?
- Wolffian duct
- Future epididymis and ductus deferens
- Male urogenital system
Describe the process of the male reproductive system development
- 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
Describe the development of the female reproductive system
- No SRY protein
- Ovaries develop
- No AMH
- Paramesonephric ducts become ovidcuts, uterus, cervix and part of the vagina
- Complete female tract
Describe the development of the ovaries
- 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
Describe the formation and closure of the tubes in testis development
- 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
Where do the testes descend from and to?
From retroperitoneal position at level of the ribs to the scrotum
What is the ligamentous structure attached to the testis called?
Gubernaculum
Describe the structure of the gubernaculum
- In contact with the peritoneum
- Fusion of 2 cell types (gonadal and peritoneal)
Describe the normal descent of the testis
- 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
When does testicular descent occur in horses?
Late pregnancy or a few days after birth
When does testicular descent occur in dogs?
Up to 8 days after birth
In what species do the testes remain in their original position?
- Elephants
- Whales
How does sexual differentiation of the brain occur?
Different hormones secreted by gonads
Describe the sexual differentiation of the male brain
- Testosterone secreted into blood
- Crosses BBB
- Converted to oestradiol (by aromatase) and dihydrotestosterone
- Oestradiol masculinises brain
Describe teh sexual differentiation of the female brain
- Alpha-fetoprotein binds to oestradiol
- Complex formed cannot cross BBB
- Protects female brain from masculinising effects of oestradiol
What is the difference between the male and female brain?
Females have surge centre, males do not (inhibited by oestrogen)
What is freemartinism?
- Sterile bovine
- Both male and female development has taken place
How does freemartinism occur?
- Male and female twins
- Common blood supply between male and female
- Female exposed to some AMH and testosterone
What are the effects of freemartinism?
- 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
Give examples of male development problems
- Cryptorchidism
- Inguinal herniation
- Hypospadias
What is cryptorchidism?
Failure of testes to descend into scrotum
What is inguinal herniation?
Portion of intestine passes through inguinal canal into the vaginal cavity
What is hypospadia?
Urethral opening at base of penis
