![]() Biology of Reproduction Summers |
text: Human Reproductive Biology2nd Edition - Richard E. Jones & Kristin H. Lopez,: Read pages 213-219; 463-486 for this lecture acronyms end |
XIII. Development of Sexual Behavior back to XII. Sexual Differentiation A. T (from testes) masculinizes the brain (hypothalamus) 1. converted to E2 intracellularly a. aromatase in mPOA & VMH 2. E2 binding to E-Ra or b produces male sexual behavior a. E-Ra promotes increased & clustered A-R & P-R i. A-R also important for male sexual behavior b. but... DA via D1 receptors can also activate E-Ra c. female rats have a-feto-protein ® binds to E2 and keeps it from entering the brain 3. Critical Period = Window of Effectiveness a. time during development when "T" has an effect masculinizing the brain producing male behavior
MAMMAL | GESTATION LENGTH IN DAYS |
CRITICAL PERIOD DAYS (time after fertilization) |
DAYS AFTER TESTES APPEAR |
Hamster | 16 | 16-21 | 4.5 |
Mouse | 19-20 | 20 | 7.5 |
Rat | 21-22 | 18-28 | 5.0 |
Guinea Pig | 68 | 30-35 | 5.0 |
Human | 270 | T goes up 84-126 | 21 |
b. fetal hormone organize sexual behavior in males i. postpubertal hormones maintain behavior in males and females c. Evidence for critical period in humans i. Guevedoces (1) raised as females, plenty of T, male behavior as adults ii. Adrenogenital Syndrome (1) high androgens - more male-like behavior iii. Progestogens (given to prevent loss of pregnancy) via P-R or A-R, females born to women with this treatment have more male-like behavior iv. Estrogen +feedback in homosexuals (1) other studies have refuted this finding, and even suggested positive feedback in heterosexual males XIV. Sexual Orientation A. Genes associated with homosexuality in males 1. X chromosome region Xq28 (tip of long arm) shared by some (up to 67%) homosexual brothers a. X-linked recessive i.also autosomal markers at 7q36, 8p12 and 10q26 b. heritability: 53% of the variance in homosexuality is genetic i. identical twin studies c. adopted brothers of homosexuals have a higher than normal incidence of homosexuality 2. genes code for proteins (mostly enzymes) a. genetic background represents a predisposition i. triggered or supressed by external factors B. Genes and homosexuality in females 1. 52% heritability associated with same-sex orientation in women C. Testicular Feminization Syndrome 1. no androgen receptors (Xq11) a. normal testis, female external genitalia 2. XY heterosexual female D. neuroanatomical sexual dimorphism: 3rd interstitial nucleus of the mPOA (=INAH3) 1. smaller in women 2. smaller in homosexual men E. multimorphisms and sexual orientation: SCN 1. shape (not size) of SCN differ in heterosexual males and females a. AVP secreting cells 2. SCNAVP 2x larger in homosexual men 3. SCN = biological clock ® homosexual men arise and retire earlier a. SCN also regulates reproductive function + rhythmicity F. Differences in hypothalamus suggest hormonal variability 1. fetal environment can influence androgen and estrogen levels a. may influence masculinization during critical period i. maternal stress (increased CRH, ACTH and F) decreases androgen and estrogen levels (1) advance fetal adrenal conversion of DHEA to F (2) increased homosexuality in males and females ii. however, Human male without aromatase was heterosexual (N = 1) (1) DA via D1 may be able to organize morphological and functional sex differences in the abscence of steroid iii. masculinization limits E provoked LH surge (1) 50% of homosexual men had LH surge (2) 33% of heterosexual men had LH surge (a) those with lowest [T] (both groups) (i) no masculinization of human brain? (ii) E will never provoke LH surge in male rat 2. male homosexuality may appear as a mosaic of traits (some sex-typical, others sex-atypical and yet others that are sex-exaggerated) a. differences in the timing and/or concentration of T (A-R, ERa, PR, D1) exposure i. homosexual men may be exposed to particularly high androgen levels very early in development G. Fetal environment can affect non-steroidal events 1. increased # of older brothers increased chance of homosexuality a. each older brother increases chance by 33% i. multiply by chance of homosexuality (1-2%) to get overall increase 2. progressive immunization of mothers to minor Y-linked histocompatibility antigens a. non-self immune action against H-Y antigen, protocadherin or neuroligin by maternal antibodies impedes male sexual differentiation of the brain i. male mice whose mothers are immunized to H–Y prior to pregnancy show reduced sexual behaviour towards receptive females ii. H-Y antigen was once thought to determine gonadal sex (1) before discovery of TDF