Download Human Physiology

Chapter 20
Reproduction
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20-1
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
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20-4
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 Xlinked genes
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20-6
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
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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
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20-10
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)
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20-13
Sexual Differentiation:
Internal Embryonic Development
26-3a: or
Sexual
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for reproduction
display.development
in the human embryo
Sexual Differentiation: External Genitalia
Figure 26-3b: Sexual development in the human embryo
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Pathway for Sexual Development:
Review for Genes to Organs
Figure 26-4: Role of the SRY gene in male development
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Formation of Testes and Ovaries continued
Seminiferous
tubules (STs), which produce sperm in adults,
appear within 43-50 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
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20-12
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
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20-14
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
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20-17
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
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20-18
Adrenal Cortex: Steroid Hormone Production
Figure 23-2: Synthesis pathways of steroid hormones
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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
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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
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Regulation of Spermatogenesis
Figure 26-11:
Hormonal
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or display.
control of spermatogenesis
Ovary: Details of Histology & Physiology
Figure 26-12d:
ANATOMY
SUMMARY: Female
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or display.
Reproduction
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
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Endocrine Control of Menstrual Cycle:
Follicular Phase
 FSH
stimulates follicular development
 Estrogen:
+ feedback, limits more follicles
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Endocrine Control of Menstrual Cycle:
Ovulation

Estrogen  LH "surge" & FSH spike  egg release
pushes FSH down ,  new follicle
development
 Inhibin
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Endocrine Control of Menstrual Cycle: Luteal
phase
cells form corpus luteum  progesterone
  progesterone & estrogen maintain endometrium
 Granulosa
 Inhibin
continues to limit new follicular development
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Endocrine Control of Menstrual Cycle:
Late Luteal phase
maintain  progesterone, estrogen & inhibin
 No pregnancy:  progesterone, estrogen & inhibin
 Pregnancy:
Menses,
 FSH & LH  new follicle development
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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
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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
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20-22
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
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20-30
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
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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
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20-75
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
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