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1 How do we (and other animals) become male or female?

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Presentation on theme: "1 How do we (and other animals) become male or female?"— Presentation transcript:

1 1 How do we (and other animals) become male or female?

2 2 Sex Determination The event that determines whether an individual will become male or female Sexual Differentiation The developmental process of becoming male or female Sexual Differentiation: Overview

3 3 Chromosomal sex Chromosomal sex Gonadal sex Gonadal sex Gametic sex Gametic sex Hormonal sex Hormonal sex Morphological sex Morphological sex Behavioral sex Behavioral sex Gender identity Gender identity Gender role Gender role Legal sex Legal sex What is Sex? Sexual Differentiation: Overview

4 4 Josts Model of Sexual Differentiation: Sexual Differentiation: Overview

5 5 What determines gonadal sex (development of testes vs. ovaries)?

6 6 Germinal ridge: Thickened ridge of tissue on surface of each mesonephros (protokidney) Can develop into a testis or an ovary Nelson 2005 Fig. 3.4 Gonadal Differentiation

7 7 Gonadal Differentiation: SRY Gene (Sex-Determining Region of Y Chromosome) TDF binds to DNA and regulates genes controlling development of testes

8 8 Evidence for role of SRY gene: SRY is activated shortly before gonads differentiate. XY phenotypic females often have mutations in SRY (humans, mice). XX phenotypic males often have SRY-containing translocation to X chromosome. Gonadal Differentiation: SRY Gene

9 9 Evidence for role of SRY gene: SRY is activated shortly before gonads differentiate. XY phenotypic females often have mutations in SRY (humans, mice). XX phenotypic males often have SRY-containing translocation to X chromosome. BUT: Autosomal genes also contribute to testis development. XX transgenic mice: introduction of SRY causes testicular development. Gonadal Differentiation: SRY Gene

10 10 Gonadal Differentiation: Ovary-Determining Genes Unclear if any exist. Have been hypothesized to exist and to be switched off in males by activation of SRY gene. Two X chromosomes necessary for normal ovarian development.

11 11 Gonadal Differentiation TDF acts locally – not blood-borne. In mammals, sex steroids have limited effects. Germinal ridge is initially indifferent/bipotential. TDF expression Human fetal ovaries testes no TDF expression

12 12 What determines differentiation of the internal reproductive tracts?

13 13 Nelson 2005 Fig duct systems initially present in each embryo: Wolffian ducts Müllerian ducts Reproductive Tract Differentiation

14 14 Reproductive Tract Differentiation 1.Wolffian ducts 2.Müllerian ducts T No T Müllerian inhibitory hormone (MIH) from testes No MIH

15 15 Reproductive Tract Differentiation Silverthorn 2009 Fig. 26-3

16 16 Wolffian ducts Müllerian ducts TestesNo Testes T MIH vas deferens epididymis seminal vesicles regression No T No MIH fallopian tubes uterus cervix regression Reproductive Tract Differentiation

17 17 What determines differentiation of the external genitalia?

18 18 Differentiation of External Genitalia Nelson 2005 Fig. 3.7

19 19 Differentiation of External Genitalia Bipotential anlagenBipotential anlagen Genital folds, genital tubercle Identical in males and females until week 6-8 Effects of androgens (males)Effects of androgens (males) Genital tubercle penis Genital folds scrotum (testes descend later) Absence of androgens (females)Absence of androgens (females) Genital tubercle clitoris Genital folds labia

20 20 Summary of Sexual Differentiation Silverthorn 2009 Table 26-1

21 ANOMALIES OF SEXUALDIFFERENTIATION What can go wrong, and what do the exceptions tell us about the rules?

22 True Hermaphroditism Both ovarian and testicular tissue are present in the same individual – ovary on one side, testis on the other – ovotestes Uncommon Usually 46,XX; sometimes 46,XX/46,XY chimerism or mosaicism Phenotypically variable Some are fertile as females

23 Gonadotropin levels: high in infancy and post-pubertally; normal prepubertal hiatus Immature genital tract & external genitals; short stature Often lethal prenatally Anomalies in Chromosomal Females: Turner Syndrome (XO)

24 Wilson & Foster 1987 Fig Anomalies in Chromosomal Females: Turner Syndrome (XO)

25 Causes masculinization of the external genitalia in females. Can be mild to severe. Can be treated with surgery and exogenous steroids. Anomalies in Chromosomal Females: Congenital Adrenal Hyperplasia (CAH)

26 X X Low aldosterone Low cortisol High androgens Adrenal Cortex Zona reticularisZona fasciculataZona glomerulosa X Anomalies in Chromosomal Females: Congenital Adrenal Hyperplasia (CAH)

27 Money 1987 Anomalies in Chromosomal Females: Congenital Adrenal Hyperplasia (CAH)

28 Wilson & Foster 1987 Fig Anomalies in Chromosomal Females: Congenital Adrenal Hyperplasia (CAH)

29 Testes develop; Wolffian and Müllerian ducts regress; external genitalia appear female. Female body type and gender identity, but sterile. Anomalies in Chromosomal Males: Androgen Insensitivity Syndrome (AIS) (Testicular Feminization (TFM))

30 No 5 -reductase – cant convert T to DHT. Anomalies in Chromosomal Males: 5 -Reductase Deficiency (Guevodoces, Penis at Twelve) 5 Reductase 5 Dihydro- testosterone (DHT) H 5 17-beta Estradiol (E2)Aromatase 17 A Testosterone (T) 19 XY infants have ambiguous genitalia and small, undescended testes.

31 Anomalies in Chromosomal Males: 5 -Reductase Deficiency (Guevodoces, Penis at Twelve)

32 Anomalies in Chromosomal Males: 5 -Reductase Deficiency (Guevodoces, Penis at Twelve)

33 1.XXY (Klinefelter Syndrome) Phenotypically male Small testes, androgen deficiency, low sperm production (sterile) Gynecomastia Low intelligence Nelson 2005 Fig 3.14 XXYXYY 2.XYY Phenotypically male Normal sexual development Extremely tall Low intelligence, Hyperactive Antisocial Anomalies in Chromosomal Males: Trisomies


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