1. Chapter 20
Reproduction
20-1

2. Sexual Differentiation
Early embryonic gonads can become testes or ovaries
Y chromosome induces formation of testes
Testosterone (T) from testes induces formation of male sex organs
In absence of T, female sex organs develop
20-4

3. Sex Determination
Each zygote inherits 23 chromosomes from mother and 23 from father
Producing 23 pairs of homologous chromosomes
22 of these are autosomal chromosomes
23rd pair are sex chromosomes
XX in females; XY in males
Y (contains 80 genes) is believed to be derived from X (contains 1098 genes)
Thus, in males ~1000 genes are only represented by a single allele from the X chromosome
Leading to a large # genetic diseases caused by X-linked genes
20-6

4. Genomic Imprinting
The cells of a zygote contain 23 pairs of homologous chromosomes
Thus each gene is represented either by the same or an alternative allele (form) from each homolog
Usually both alleles are expressed
However, in ~80 genes either the maternal or paternal allele is silenced
Accomplished by chemically modifying DNA bases e.g., by methylation
Called epigenetic changes because they don’t change the DNA sequence but they are heritable
Called genomic imprinting
20-9

5. Formation of Testes and Ovaries
1st 40 days after conception, gonads of males and females are similar
Cells that will give rise to sperm and eggs migrate from yolk sac to developing gonads
Gonads have potential to become testes or ovaries until testis-determining factor (TDF) causes conversion to testes
TDF is produced by SRY (sex-determining region of Y) gene
20-10

6. Development of Testes
Leydig cells begin secreting T at 8 weeks of development and peak at 12-14th week
This masculinizes embryonic structures
T levels then decline to very low levels until puberty
Testes descend into scrotum shortly before birth
Because spermatogenesis requires 3oC lower temperature than body
Spermatogenesis doesn't occur in cryptorchidism (undescended testes)
20-13

7. Sexual Differentiation: Internal Embryonic Development
Figure 26-3a: Sexual development in the human embryo

8. Sexual Differentiation: External Genitalia
Figure 26-3b: Sexual development in the human embryo

9. Pathway for Sexual Development: Review for Genes to Organs
Figure 26-4: Role of the SRY gene in male development

10. Formation of Testes and Ovaries continued
Seminiferous tubules (STs), which produce sperm in adults, appear within days following conception
Contain germinal cells which will produce sperm and nongerminal Sertoli cells (appear at day 42)
At ~day 65, Leydig cells (produce T) appear in clusters around STs
In absence of TDF, ovaries develop
Ovarian follicles (functional units of ovary) don't appear until day 105
20-12

11. Development of Accessory Sex Organs
Male accessory sex organs are derived from wolffian ducts
Include epididymis, vas deferens, seminal vesicles, and ejaculatory duct
Female accessory sex organs are derived from Müllerian ducts and include uterus and fallopian tubes
20-14

12. Disorders of Embryonic Sexual Development
Hermaphroditism occurs when individuals have both ovary and testis tissue
Pseudohermaphrodites have either testes or ovaries but have accessory organs and external genitalia that are incompletely developed or inappropriate
Most common cause of female pseudohemaphroditism is congenital adrenal hyperplasia
In males, one cause is testicular feminizing syndrome in which testes are normal but there are no receptors for T
Individual develops very female appearance but is infertile
20-17

13. Gonads During Development and Childhood
Embryonic testes produce lots of T during 1st trimester of pregnancy to masculinize fetus
T production declines during 2nd trimester
Ovaries don’t produce much sex steroid until puberty
Both testes and ovaries remain inactive after birth until puberty
20-18

14. Adrenal Cortex: Steroid Hormone Production
Figure 23-2: Synthesis pathways of steroid hormones

15. Testosterone Production
The most important hormone of the testes
Produced in interstitial cells
Functions of testosterone
Stimulates reproductive organ development
Underlies sex drive
Causes secondary sex characteristics
Deepening of voice
Increased hair growth
Enlargement of skeletal muscles
Thickening of bones

16. Hormone Production by the Ovaries
Estrogens
Produced by follicle cells
Cause secondary sex characteristics
Enlargement of accessory organs
Development of breasts
Appearance of pubic hair
Increase in fat beneath the skin
Widening and lightening of the pelvis
Onset of menses

17. Regulation of Spermatogenesis
Figure 26-11: Hormonal control of spermatogenesis

18. Ovary: Details of Histology & Physiology
Figure 26-12d: ANATOMY SUMMARY: Female Reproduction

19. Menstrual Cycle: Egg Maturation, and Endometrial Growth
Follicular phase
Egg matures
Ovulation
Egg released
Luteal phase
Corpus luteum
Endometrium
Prep for blastocyst
No Pregnancy
Menses
Figure 26-13: The menstrual cycle

20. Endocrine Control of Menstrual Cycle: Follicular Phase
FSH stimulates follicular development
Estrogen: + feedback, limits more follicles

21. Endocrine Control of Menstrual Cycle: Ovulation
 Estrogen  LH "surge" & FSH spike  egg release
Inhibin pushes FSH down ,  new follicle development

22. Endocrine Control of Menstrual Cycle: Luteal phase
Granulosa cells form corpus luteum  progesterone
 progesterone & estrogen maintain endometrium
Inhibin continues to limit new follicular development

23. Endocrine Control of Menstrual Cycle: Late Luteal phase
Pregnancy: maintain  progesterone, estrogen & inhibin
No pregnancy:  progesterone, estrogen & inhibin
Menses,  FSH & LH  new follicle development

24. Onset of Puberty
FSH and LH secretion is high for 1st 6 months of life, but falls to very low levels until puberty
At puberty hypothalamus increases GnRH secretion
This stimulates increased LH and FSH
Which stimulates sex steroid secretion
Which drives changes in secondary sex characteristics and menarche (1st menstrual flow)
Growth of pubic and axiliary hair is due to androgen secretion from adrenal cortex
20-21

25. Onset of Puberty continued
At beginning of female puberty, high E stimulates growth
High pubertal T in boys causes growth spurt that lags that of girls
Age of puberty in girls depends on % body fat and physical activity
Girls with low body fat and high activity levels enter puberty at older age
20-22

26. Testosterone and Age
Secretion of T declines gradually and varyingly in men> 50
Causes are unknown
Not due to low GnRH, LH, or FSH because their levels are elevated
20-30

27. Factors Affecting Menstrual Cycle
Release of GnRH is regulated not only by hormonal feedback but also by input from higher brain centers
Olfactory system can send activity to hypothalamus in response to pheromones
Can cause the “dormitory effect” (Ch 11) in which cycles of roommates become synchronized
20-74

28. Factors Affecting Menstrual Cycle continued
Limbic system (involved in emotions; Ch 8) input to the hypothalamus in times of stress can cause functional amenorrhea (cessation of menstruation)
Also occurs in thin or athletic females with low body weight
Appears to be related to reduced leptin secretion by small adipocytes
20-75

29. Menopause
Is cessation of ovarian activity and menstruation at ~50 years
Ovaries are depleted of follicles and thus produce no E
LH and FSH are high because of no negative feedback
Lack of E from ovaries is most responsible for hot flashes, osteoporosis, and increased risk of atherosclerosis
20-78