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The Reproductive System
Part 3
Gamete Formation
 Introduction
 Chromosomes carry genetic information
 In humans, cells contain 46 chromosomes
 Gametes carry only 23 chromosomes
Meiosis
 Special type of cell division in the reproductive tract
 Two cell divisions
 Results in 4 daughter cells
 Genetically unique
 Spermatogenesis or oogenesis
Mother cell
(before chromosome replication)
Chromosome
replication
Chromosome
replication
2n = 4
MITOSIS
MEIOSIS
Replicated
chromosome
Prophase
Metaphase
Chromosomes
align at the
metaphase plate
Sister chromatids
separate during
anaphase
Metaphase I
Daughter cells
of meiosis I
2n
No further chromosomal
replication;
sister chromatids
Meiosis II
separate during
anaphase II
n
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Tetrads align at the
metaphase plate
Homologous chromosomes
separate but sister
chromatids remain together
during anaphase I
Daughter
cells of
mitosis
2n
Tetrad formed by
synapsis of replicated
homologous
chromosomes
Prophase I
n
n
Daughter cells of meiosis II
(usually gametes)
n
Figure 27.5 (1 of 2)
Meiosis
 Non-disjunction
 Chromosomes fail to separate properly
 Trisomy
 Trisomy 13 and 18 = usually fatal
 Trisomy 21= Down syndrome
 Monosomy
 Always fatal
Spermatogenesis
 Begins within testes at puberty
 Spermatogonia
 Stem cells divide mitotically
 Some undergo a growth phase
 Become primary spermatocytes
 Undergo 2 divisions
1. Produces 2 secondary spermatocytes (haploid)
2. Produces 4 spermatids → mature into a spermatozoa
Spermatogenesis
spermatogonia
spermatogonia
growth phase
primary spermatocytes
Division 1
secondary spermatocyte
secondary spermatocyte
Division 2
spermatid
spermatid
spermatid
spermatozoa
spermatid
Spermatogonium
(stem cell)
Cytoplasm of adjacent
sustentacular cells
Basal lamina
Sustentacular
cell nucleus
Type A daughter cell
remains at basal lamina
as a stem cell
Type B daughter cell
Tight junction between
sustentacular cells
Primary
spermatocyte
Secondary
spermatocytes
Early
spermatids
Late spermatids
Cytoplasmic
bridge
Spermatozoa
Lumen of
seminiferous tubule
(c) A portion of the seminiferous tublule wall, showing the spermatogenic cells surrounded by sustentacular cells (colored gold)
Figure 27.7c
Spermatogenesis
 Spermatic cells give rise to sperm
 Mitosis
 Spermatogonia form spermatocytes
 Meiosis
 Spermatocytes form spermatids
 Spermiogenesis
 Spermatids become sperm
Spermatogonium
(stem cell)
Mitosis
Growth
Enters meiosis I
and moves to
adluminal
compartment
Meiosis I
completed
Meiosis II
Basal lamina
Type A daughter cell
remains at basal lamina
as a stem cell
Type B daughter cell
Primary
spermatocyte
Secondary
spermatocytes
Early
spermatids
Late spermatids
Spermatozoa
(b) Events of spermatogenesis,
showing the relative position
of various spermatogenic cells
Figure 27.7b
Spermatogenesis
 Spermatids lose excess cytoplasm and form a tail, becoming
spermatozoa (sperm)
 Major regions
1. Head
 Genetic region
 Nucleus
 Acrosome with hydrolytic enzymes
2. Midpiece
 Metabolic region
 Mitochondria
3. Tail
 Locomotor region
 Flagellum
Approximately 24 days
Golgi
apparatus
Acrosomal
vesicle
Mitochondria
Acrosome
Nucleus
1
(a)
2
Spermatid
nucleus
Centrioles
3
Midpiece Head
Microtubules
Flagellum
Excess
cytoplasm
4
Tail
5
6
7
(b)
Figure 27.8a, b
Spermatogenesis
 Sperm are dependent on sugar in testes and semen
 Do not survive long outside of the body
 Gain motility in epididymis
Oogenesis
 Production of female gametes
 Begins in fetus
 Oogonia multiply by mitosis
 Develop into primary oocytes within follicles
 Eventually produce estrogen
 Primary oocytes begin meiosis but stall in prophase I
 About 400,000 present at birth
Tunica
albuginea
Degenerating corpus
luteum (corpus
albicans)
Germinal
epithelium
Primary
follicles
Ovarian
ligament
Cortex
Oocyte
Granulosa cells
Late secondary follicle
Mesovarium and
blood vessels
Vesicular
(Graafian)
follicle
Antrum
Oocyte
Zona
pellucida
Theca
folliculi
Ovulated
oocyte
Corpus luteum
Corona
Developing
radiata
corpus luteum
(a) Diagrammatic view of an ovary sectioned to reveal the follicles in its interior
Medulla
Copyright © 2010 Pearson Education, Inc.
Figure 27.11a
Oogenesis
 Each month after puberty, a few primary oocytes are activated
 One is selected each month to resume meiosis I
 Result is two haploid cells
 Secondary oocyte  majority of cytoplasm + chromosomes
 First polar body  chromosomes
Follicle development
in ovary
Meiotic events
Before birth
Oogonium (stem cell)
Follicle cells
Oocyte
Mitosis
Primary oocyte
Primordial follicle
Primary oocyte
(arrested in prophase I;
present at birth)
Primordial follicle
Growth
Infancy and
childhood
(ovary inactive)
Each month from
puberty to
menopause
Primary follicle
Primary oocyte (still
arrested in prophase I)
Secondary follicle
Spindle
Meiosis I (completed
by one primary oocyte
each month in response
to LH surge)
First polar body
Meiosis II of polar
body (may or may
not occur)
Polar bodies
(all polar bodies
degenerate)
Vesicular (Graafian)
follicle
Secondary oocyte
(arrested in
metaphase II)
Ovulation
Sperm
Second
Ovum
polar body
Meiosis II
completed
(only if
sperm
penetration
occurs)
Degenating
Ovulated secondary
oocyte
In absence of
fertilization, ruptured
follicle becomes a
corpus luteum and
ultimately degenerates.
corpus luteum
Figure 27.17
Oogenesis
 Secondary oocyte is ovulated
 Sperm penetration of second oocyte completes meiosis II
 Produces
 Ovum (functional gamete)
 Second polar body
The Ovarian Cycle
 Ova prepared and released ≈ 28 days
 Three consecutive phases
1.
Follicular phase
 Period of follicle growth (days 1–14)
2.
Ovulation
 Midcycle
3.
Luteal phase
 Period of corpus luteum activity (days 14–28)
The Ovarian Cycle
 Follicular phase
 Begins with slight increases in FSH
 Stimulates growth of follicle
 Slightly enhances estrogen production
Slide 9
Figure 27.19 Feedback interactions in the regulation of ovarian function.
Hypothalamus
Hypothalamus
5
GnRH
Travels via
portal blood
1
4 Positive
feedback exerted
by large in
estrogen
output.
Anterior pituitary
8
5
1
Progesterone
Estrogen
Inhibin
LH surge
FSH
2
3 Slightly
elevated
estrogen
and rising
inhibin
levels.
LH
Ruptured
follicle
2
6
8
7
Thecal
cells
Granulosa
cells
Androgens
Convert
Inhibin androgens to 2
estrogens
Early and midfollicular phases
Copyright © 2010 Pearson Education, Inc.
Mature follicle
Corpus luteum
Ovulated
secondary
oocyte
Late follicular and
luteal phases
Primary
Vesicular
Corpus
follicle
follicle
luteum Degenerating
Secondary
Ovulation
corpus luteum
follicle
Ovulation Luteal
phase
(Day 14)
(b) Ovarian cycle: Structural changes in the ovarian
follicles during the ovarian cycle are correlated with
(d) changes in the endometrium of the uterus during
the uterine cycle.
Follicular
phase
Figure 27.20b
The Ovarian Cycle
 Hormonal Interactions
 High estrogen levels induce surge of LH
 Effects of LH surge
 Triggers ovulation
 Transforms ruptured follicle into corpus luteum (CL)
 Luteal phase
Figure 27.20a Correlation of anterior pituitary and ovarian hormones with structural changes of the ovary and uterus.
LH
FSH
(a) Fluctuation of gonadotropin levels: Fluctuating
levels of pituitary gonadotropins (follicle-stimulating
hormone and luteinizing hormone) in the blood
regulate the events of the ovarian cycle.
Copyright © 2010 Pearson Education, Inc.
The Ovarian Cycle
 Luteal Phase
 Corpus luteum remains functional only if pregnancy occurs
 Functions
 Produces estrogen and progesterone
 Inhibit pituitary release of LH and FSH
 Maintain uterine lining
The Ovarian Cycle
 Luteal Phase
 No fertilization  corpus luteum degenerates  ovarian
hormone levels drop sharply
 Birth control pills
 Mimic hormones produced by corpus luteum
 Prevent ovulation
The Menstrual Cycle
 Definition
 Cyclic changes in endometrium in response to ovarian hormones
 Three phases
1. Days 1–5: Menstrual phase
2. Days 6–14: Proliferative (preovulatory) phase
3. Days 15–28: Secretory (postovulatory) phase
Endometrial
glands
Blood vessels
Menstrual
flow
Functional layer
Basal layer
Days
Menstrual
phase
Proliferative
phase
Secretory
phase
(d) The three phases of the uterine cycle:
• Menstrual: Shedding of the functional layer of the
endometrium.
• Proliferative: Rebuilding of the functional layer of
the endometrium.
• Secretory: Begins immediately after ovulation.
Enrichment of the blood supply and glandular secretion of
nutrients prepare the endometrium to receive an embryo.
Both the menstrual and proliferative phases occur before ovulation, and
together they correspond to the follicular phase of the ovarian cycle. The
secretory phase corresponds in time to the luteal phase of the ovarian cycle.
Figure 27.20d
The Menstrual Cycle
 Menstrual phase (days 1-5)
 Follicles growing within ovary during this time
 Functional layer of endometrium sloughs
 Menstrual flow occurs
 Endometrial tissue, fluid, and mucus pass through vagina
The Menstrual Cycle

Proliferative phase (days 6-14)
 Estrogen from follicle stimulates growth of endometrium
 Preparation for pregnancy
 Ovulation usually occurs at the end of this cycle (day 14)
The Menstrual Cycle
 Secretory phase
 Corresponds with luteal phase of ovarian cycle
 Progesterone and estrogen produced after ovulation
 Further development of endometrium
The Menstrual Cycle
 Secretory phase
 In the absence of fertilization
 Estrogen and progesterone levels fall
 Endometrium deteriorates
 CL degenerates
 Another menstrual cycle begins
 If fertilization occurs
 Human chorionic gonadotropin (HCG) produced


Maintains CL
Home pregnancy tests
Estrogens
Progesterone
(c) Fluctuation of ovarian hormone levels:
Fluctuating levels of ovarian hormones (estrogens
and progesterone) cause the endometrial changes
of the uterine cycle. The high estrogen levels are
also responsible for the LH/FSH surge in (a).
Figure 27.20c
Endometrial
glands
Blood vessels
Menstrual
flow
Functional layer
Basal layer
Days
Menstrual
phase
Proliferative
phase
Secretory
phase
(d) The three phases of the uterine cycle:
• Menstrual: Shedding of the functional layer of the
endometrium.
• Proliferative: Rebuilding of the functional layer of
the endometrium.
• Secretory: Begins immediately after ovulation.
Enrichment of the blood supply and glandular secretion of
nutrients prepare the endometrium to receive an embryo.
Both the menstrual and proliferative phases occur before ovulation, and
together they correspond to the follicular phase of the ovarian cycle. The
secretory phase corresponds in time to the luteal phase of the ovarian cycle.
Figure 27.20d
Menopause
 Cessation of reproductive cycles
 By age 45-50 ovarian follicles cease to respond to FSH and LH
 Follicles stop producing estrogen
 No inhibition of pituitary
 No LH surge
 No ovulation, no corpus luteum, no progesterone
Menopause
 Symptoms associated with high FSH and LH levels
Vaginal dryness
b) Irritability/depression
c) Vasodilation of skin blood vessels  hot flashes & night sweats
d) Thinning of skin, breast atrophy
a)
 Clinical findings
 Increased cholesterol
 Loss of bone mass (osteoporosis)