6
Sexual Development
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
Figure 6.3 Fertilization and production of the conceptus
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
Figure 6.4 Development of the human conceptus
Figure 6.5 Blastocyst in the process of implanting itself
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
Figure 6.6 Stages of prenatal development
Genetic Sex Is Determined at Fertilization
• Our understanding of sex differentiation is based upon
studies with rabbits conducted by the French
embryologist Alfred Jost
Figure 6.7 Alfred Jost’s experiments on rabbits
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 antiMüllerian 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
Figure 6.8 The genetic basis of sex determination
Figure 6.9 Sex determination
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
Figure 6.11 Development of the male and female reproductive tracts
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
Figure 6.12 Development of the male and female external genitalia (Part 1)
Figure 6.12 Development of the male and female external genitalia (Part 2)
Figure 6.12 Development of the male and female external genitalia (Part 3)
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
Box 6.1 Biology of Sex: Atypical Development: Chromosomal Anomalies
Several Types of Atypical Sex Development Exist in Humans
• Gonadal intersexuals, historically called true
hermaphrodites, possess both ovarian and testicular
tissue
Box 6.2 Biology of Sex: Atypical Development: Gonadal Intersexuality
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
Box 6.3 Biology of Sex: Atypical Development: Androgen Insensitivity Syndrome
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)
Box 6.4 Biology of Sex: Atypical Development: Congenital Adrenal Hyperplasia
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
Box 6.5 Biology of Sex: Atypical Development: 5α-Reductase Deficiency
Several Types of Atypical Sex Development Exist in Humans
• Males with hypospadias or a micropenis have a
misplaced urethral opening or small penis,
respectively
Box 6.6 Biology of Sex: Atypical Development: Hypospadias and Micropenis
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
o 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
Figure 6.14 The third interstitial nucleus of the anterior hypothalamus (INAH3)
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
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
Figure 6.17 Hormones and sexual behavior in rats
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
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
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
Figure 6.19 Growth velocity curves for boys and girls
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
Figure 6.24 Hormonal control of puberty
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
Figure 6.21 Typical development of breasts in girls at puberty
Figure 6.22 Puberty is starting earlier
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)
Figure 6.23 Typical development of male external genitalia at puberty
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