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Part 3
The Reproductive System
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
Copyright © 2010 Pearson Education, Inc. Figure 27.5 (1 of 2)
Mother cell
(before chromosome replication)
Chromosome
replication
Chromosome
replication
2n = 4
MITOSIS
Replicated
chromosome Prophase
Chromosomes
align at the
metaphase plate
Sister chromatids
separate during
anaphase
2n 2n
Metaphase
Daughter
cells of
mitosis
Tetrad formed by
synapsis of replicated
homologous
chromosomes
Tetrads align at the
metaphase plate
Homologous chromosomes
separate but sister
chromatids remain together
during anaphase I
No further chromosomal
replication; sister chromatids
separate during
anaphase II
Daughter cells of meiosis II
(usually gametes)
n n n n
Prophase I
Metaphase I
Daughter cells
of meiosis I
Meiosis II
MEIOSIS
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
primary spermatocytes
secondary spermatocyte secondary spermatocyte
spermatid spermatid spermatid spermatid
spermatozoa
growth phase
Division 1
Division 2
Figure 27.7c
Basal lamina
Spermatogonium
(stem cell)
Cytoplasm of adjacent
sustentacular cells Sustentacular
cell nucleus
Tight junction between
sustentacular cells
Lumen of
seminifer- ous tubule
Late spermatids
Early
spermatids
Secondary
spermatocytes
Cytoplasmic
bridge
Primary
spermatocyte
Spermatozoa
Type B daughter cell
Type A daughter cell
remains at basal lamina
as a stem cell
(c) A portion of the seminiferous tublule wall, showing the spermato-
genic cells surrounded by sustentacular cells (colored gold)
Spermatogenesis
Spermatic cells give rise to sperm
Mitosis
Spermatogonia form spermatocytes
Meiosis
Spermatocytes form spermatids
Spermiogenesis
Spermatids become sperm
Figure 27.7b
Basal lamina
Spermatogonium
(stem cell)
Mitosis
Growth
Late spermatids
Early
spermatids
Secondary
spermatocytes
Primary
spermatocyte
Spermatozoa
Type B daughter cell
Enters meiosis I
and moves to
adluminal
compartment Meiosis I
completed
Meiosis II
Type A daughter cell
remains at basal lamina
as a stem cell
(b) Events of spermatogenesis,
showing the relative position
of various spermatogenic cells
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
Figure 27.8a, b
Centrioles Spermatid
nucleus
Golgi
apparatus Acrosomal
vesicle Mitochondria
Approximately 24 days
Excess
cytoplasm
Nucleus
Acrosome
Microtubules
Flagellum
Tail
Midpiece Head (a)
(b)
1 2
3
4
5
6 7
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
Copyright © 2010 Pearson Education, Inc. Figure 27.11a
Medulla
Tunica
albuginea
Germinal
epithelium
Cortex
Oocyte Granulosa cells
Late secondary follicle
Antrum
Primary
follicles
Oocyte
Zona
pellucida Theca
folliculi
Ovulated
oocyte
Mesovarium and
blood vessels
Vesicular
(Graafian)
follicle
Corona
radiata
Developing
corpus luteum
Corpus luteum
Ovarian
ligament
Degenerating corpus
luteum (corpus
albicans)
(a) Diagrammatic view of an ovary sectioned to reveal the follicles in its interior
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
Figure 27.17
Meiotic events Follicle development
in ovary Before birth
Infancy and
childhood
(ovary inactive)
Primary oocyte
Primary oocyte (still
arrested in prophase I)
Vesicular (Graafian)
follicle
Primary follicle
Primordial follicle
Primordial follicle
Oocyte
Ovulated secondary
oocyte
In absence of
fertilization, ruptured
follicle becomes a
corpus luteum and
ultimately degenerates. Degenating
corpus luteum
Secondary follicle
Primary oocyte
(arrested in prophase I;
present at birth)
Oogonium (stem cell)
Each month from
puberty to
menopause
Meiosis I (completed
by one primary oocyte
each month in response
to LH surge)
First polar body
Mitosis
Growth
Meiosis II of polar
body (may or may
not occur)
Polar bodies
(all polar bodies
degenerate)
Ovum Second
polar body
Meiosis II
completed (only if sperm penetration occurs)
Sperm
Ovulation
Secondary oocyte
(arrested in
metaphase II)
Follicle cells
Spindle
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
Copyright © 2010 Pearson Education, Inc.
Hypothalamus
Late follicular and
luteal phases
Slightly
elevated
estrogen
and rising
inhibin
levels.
Positive feedback exerted by large in estrogen output.
Mature follicle Corpus luteum
Ovulated secondary oocyte
Ruptured
follicle
LH surge
Progesterone Estrogen Inhibin
Hypothalamus
Early and midfollicular phases
Travels via
portal blood
Granulosa cells
Inhibin
Androgens
Convert androgens to estrogens
Thecal cells
Anterior pituitary
GnRH
FSH LH
Figure 27.19 Feedback interactions in the regulation of ovarian function. Slide 9
1
1
2 2
2
5
5
4
8
6
8
7
3
Figure 27.20b
(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.
Primary
follicle Secondary
follicle
Vesicular
follicle Ovulation
Corpus
luteum Degenerating
corpus luteum
Follicular
phase Ovulation
(Day 14)
Luteal
phase
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
Copyright © 2010 Pearson Education, Inc.
Figure 27.20a Correlation of anterior pituitary and ovarian hormones with structural changes of the ovary and uterus.
(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.
FSH
LH
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
Figure 27.20d
(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.
Menstrual
phase
Menstrual
flow
Endometrial
glands
Blood vessels
Functional layer
Basal layer
Proliferative
phase
Secretory
phase
Days
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
Figure 27.20c
(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).
Progesterone
Estrogens
Figure 27.20d
(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.
Menstrual
phase
Menstrual
flow
Endometrial
glands
Blood vessels
Functional layer
Basal layer
Proliferative
phase
Secretory
phase
Days
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
a) Vaginal dryness
b) Irritability/depression
c) Vasodilation of skin blood vessels hot flashes & night sweats
d) Thinning of skin, breast atrophy
Clinical findings
Increased cholesterol
Loss of bone mass (osteoporosis)