1. 6
Sexual Development

2. Fertilization Is the Fusion of One Sperm with One Ovum
Fertilization, or conception, occurs when a single sperm enters a single ovum to produce a zygote
Prior to fertilization, sperm must undergo capacitation and the acrosome reaction
After the sperm fuses with the plasma membrane, a chemical reaction blocks the entry of other sperm, and the ovum completes its second meiotic division

3. Figure 6.3 Fertilization and production of the conceptus
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4. Fertilization Is the Fusion of One Sperm with One Ovum
A conceptus is the entire collection of cells derived from the fertilized ovum from the 2-cell stage onward
By the 32-cell stage, the conceptus is called a blastocyst—it develops a fluid-filled cavity and implants itself in the wall of the uterus
The inner cell mass becomes the embryo and the outer layer of cells forms part of the placenta
Around 6 days after fertilization, the blastocyst secretes the hormone human chorionic gonadotropin (hCG), which signals mother’s body that implantation has occurred

5. Figure 6.4 Development of the human conceptus
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6. Figure 6.5 Blastocyst in the process of implanting itself
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7. During Embryonic Life the Body Plan and Organ Systems Develop
By 2 weeks after fertilization, the embryo consists of three layers of cells: ectoderm, mesoderm, and endoderm
During the embryonic phase of (2–6 weeks postconception) all major organ systems have begun to develop
After this point, the embryo is referred to as a fetus; subsequent fetal development involves growth and the functional maturation of body systems

8. Figure 6.6 Stages of prenatal development
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9. Genetic Sex Is Determined at Fertilization
Our understanding of sex differentiation is based upon studies with rabbits conducted by the French embryologist Alfred Jost

10. Figure 6.7 Alfred Jost’s experiments on rabbits
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11. Genetic Sex Is Determined at Fertilization
Six week old embryos possess undifferentiated structures called “genital ridges” that will develop into either testes or ovaries
Male development depends upon the presence of the SRY gene, which causes the fetus to develop testes, which secrete testosterone and anti- Müllerian hormone (AMH)
Female development (with the exception of ovaries) proceeds in the absence of specific genetic instructions, although several genes are involved in both stimulating ovarian development and inhibiting testis development

12. Figure 6.8 The genetic basis of sex determination
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13. Figure 6.9 Sex determination
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14. Sexual Development Involves Growth or Breakdown of Precursor Structures
At 6 weeks postconception, both male and female embryos possess two sets of ducts that run from each gonad to the future site of the external genitalia
In male fetuses, testosterone stimulates the Wolffian ducts to develop into the epididymis, vas deferens, ejaculatory ducts, and seminal vesicles, and AMH causes the Müllerian to regress and disappear
In female fetuses, in the absence of AMH, the Müllerian ducts develop into the oviducts, uterus, and the deeper part of the vagina, while the Wolffian ducts regress and disappear in the absence of testosterone

15. Figure 6.11 Development of the male and female reproductive tracts
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16. Sexual Development Involves Growth or Breakdown of Precursor Structures
At this time, the external genitals of male and female embryos are identical and consist of the genital tubercle (forms glans of penis or clitoris), urethral folds (forms shaft of penis or labia minora), and urethral swellings (forms scrotum or labia majora)
In male fetuses, testosterone must be converted to 5-dihydrotestosterone (DHT) for masculinization of the external genitalia to occur
In female fetuses, feminization of genitals occurs in the absence of hormonal signals

17. Figure 6.12 Development of the male and female external genitalia (Part 1)
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18. Figure 6.12 Development of the male and female external genitalia (Part 2)
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19. Figure 6.12 Development of the male and female external genitalia (Part 3)
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20. Several Types of Atypical Sex Development Exist in Humans
Chromosomal anomalies can occur when there are fewer or more than two sex chromosomes
Turner syndrome (XO) individuals are phenotypic females who do not enter puberty and are infertile
Klinefelter syndrome (XXY) individuals are phenotypic males who are generally tall with somewhat feminized secondary sex characteristics and low testosterone levels

21. Box 6.1 Biology of Sex: Atypical Development: Chromosomal Anomalies
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22. Several Types of Atypical Sex Development Exist in Humans
Gonadal intersexuals, historically called true hermaphrodites, possess both ovarian and testicular tissue

23. Box 6.2 Biology of Sex: Atypical Development: Gonadal Intersexuality
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24. Several Types of Atypical Sex Development Exist in Humans
In androgen insensitivity syndrome (AIS), genetic males who are unresponsive to androgens develop as phenotypic females but with no internal reproductive structures

25. Box 6.3 Biology of Sex: Atypical Development: Androgen Insensitivity Syndrome
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26. Several Types of Atypical Sex Development Exist in Humans
Congenital adrenal hyperplasia (CAH) leads to increased androgen production by the adrenal gland
Genetic females with CAH may have masculinized genitals (e.g., elongated clitoris, fused labia)

27. Box 6.4 Biology of Sex: Atypical Development: Congenital Adrenal Hyperplasia
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28. Several Types of Atypical Sex Development Exist in Humans
Individuals with 5-reductase deficiency cannot convert testosterone to DHT
Genetic males have genitals that are not fully masculinized at birth

29. Box 6.5 Biology of Sex: Atypical Development: 5α-Reductase Deficiency
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30. Several Types of Atypical Sex Development Exist in Humans
Males with hypospadias or a micropenis have a misplaced urethral opening or small penis, respectively

31. Box 6.6 Biology of Sex: Atypical Development: Hypospadias and Micropenis
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32. Hormones Influence Sexual Differentiation of the Central Nervous System
The CNS contains sexually dimorphic structures and cell groups:
Onuf’s nucleus innervates motor neurons of the pelvic floor, including the base of the penis
It is larger and contains more neurons in men than in women
Medial preoptic area of the anterior hypothalamus is associated with male sexual behavior in rodents
Within this region in humans, the third interstitial nucleus of the anterior hypothalamus (INAH3) is larger in males than in females

33. Figure 6.14 The third interstitial nucleus of the anterior hypothalamus (INAH3)
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34. Hormones Influence Sexual Differentiation of the Central Nervous System
Numerous structural, functional, and chemical differences have been identified throughout the brains of men and women, although the significance of these differences is unclear
These differences may arise as a consequence of hormone action during sensitive periods of development

35. Early Hormonal Exposure Influences Later Sexual Behavior
Experiments in rodents reveal both organizational and activational actions of hormones
Organizational effects of hormones can affect brain circuitry and generally occur during sensitive periods of development
Activational effects occur when the presence of a hormone has a direct effect on a behavior or response
Primates exhibit multiple sensitive periods during which hormonal effects can impact subsequent behavior

36. Figure 6.17 Hormones and sexual behavior in rats
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37. Other Y-Linked Genes Besides SRY Influence Development
Several Y-linked genes are necessary for normal spermatogenesis
There is a gene on the Y chromosome that increases stature
In mice, some brain and behavioral differences are influenced by genetic mechanisms not involving SRY and testicular hormones

38. External Factors Influence Prenatal Sexual Development
Environmental factors such as maternal stress or exposure to hormones and related chemicals and drugs can affect sexual development
Prenatal hormone exposure creates a predisposition that can be modified by social and environmental circumstances, leading to differences in sexual behavior and brain anatomy

39. Puberty Marks Sexual Maturation
Puberty is the biological transition to sexual maturity
Pubertal growth spurts both begin and end earlier in girls than in boys, leading to an increase in height as well as changes in skeletal structure and body composition

40. Figure 6.19 Growth velocity curves for boys and girls
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41. Puberty Marks Sexual Maturation
Puberty is initiated by the pulsatile secretion of GnRH, which stimulates gonadotropin and gonadal hormone secretion
Body weight or the accumulation of a critical amount of body fat may use hormonal signals to trigger the hypothalamus to initiate pubertal maturation

42. Figure 6.24 Hormonal control of puberty
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43. Puberty Marks Sexual Maturation
In girls, changes during puberty include breast development, growth of axillary (armpit) and pubic hair, and growth and maturation of the internal reproductive tract and external genitalia
The onset of menstruation, or menarche, is a dramatic event in female pubertal development and has been starting at progressively younger ages in Western countries

44. Figure 6.21 Typical development of breasts in girls at puberty
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45. Figure 6.22 Puberty is starting earlier
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46. Puberty Marks Sexual Maturation
In boys, growth of the penis and growth of pubic, body, and axillary hair, as well as deepening of the voice usually follow enlargement of the testes
Ejaculation may occur following masturbation or during sleep (nocturnal emissions)

47. Figure 6.23 Typical development of male external genitalia at puberty
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48. Intersexuality Raises Complex Social and Ethical Issues
Intersex conditions may affect an individuals psychological well-being as well as anatomy and sexual function
The Accord Alliance is an organization devoted to promoting the medical and psychological health of people with intersexed conditions, in part by increasing social awareness, understanding, and acceptance of people with these conditions