Chapter 28
Reproductive System
28-1
Sperm Cell Development
• Spermatozoa produced in seminiferous tubules. At puberty,
GnRH (Gonadotropin-releasing hormone) secretion increases leading
to increased LH and FSH release from anterior pituitary.
(Note: GnRH is synthesized and released from neurons within the
hypothalamus)
FSH levels promote sperm formation, FSH levels promote
interstitial cells to produce large amounts of testosterone.
– Spermatogonia (germ cell) divide (mitosis) to form primary
spermatocytes and daughter spermatagonia
– Primary spermatocytes divide (first division of meiosis) to
form secondary spermatocytes
– Secondary spermatocytes divide (second division of meiosis)
to form spermatids
28-2
Sperm Cell Development
– Spermatids develop an acrosome (helps sperm penetrate ovum) and
flagellum
– Sustentacular (Sertoli, nurse) cells nourish sperm cells and form
a blood-testis barrier (isolates sperm from immune system) and produce
hormones (androgens, estrogens, & inhibins)
– Interstitial cells produce testosterone. Sustentacular cells convert
it to dihydrotestosterone (DHT) and estrogen. These are active
28-3
hormones that promote sperm formation
Spermatogenesis
28-4
Regulation of Reproductive
Hormones in Males
28-5
Puberty
• Before birth, placenta secretes human chorionic
gonadotropin hormone which stimulates secretion of
testosterone by fetal testes (testosterone causes the enlargement
& differentiation of male genitals & is required for descent of testes
near end of fetal development)
• From birth to puberty, no stimulation of secretion of
testosterone
• Puberty: age at which individuals become capable of
sexual reproduction
– Previous to puberty: small amounts of testosterone from adrenal
gland inhibits GnRH
– At puberty, pituitary becomes less sensitive to testosterone
inhibition. Amount of GnRH increases, amount of LH and FSH
increases
– Elevated FSH causes sperm cell formation
– Elevated LH causes interstitial cells to secrete larger amounts of
testosterone
28-6
Maturation and Fertilization of Oocyte
• Oogenesis is the production of a secondary
oocyte in ovaries
• Oogonia (stem cell) are cells from which
oocytes develop. The oogonia divide by
mitosis to produce other oogonia and
primary oocytes.
• Five million oocytes produced by the 4th
month of prenatal life. At birth, about 2
million begin first meiotic division but stop at
prophase (at this stage called primary oocyte). All
remain at this state until puberty.
• Note: oogonia can form after birth from stem
cells but unclear.
• Primary oocytes are surrounded by granulosa
cells and called a primordial follicle
• Primordial follicle becomes a primary
follicle when oocyte and granular cells
enlarge (usually occurs at puberty)
• Primary follicle becomes secondary follicle
(which is released during ovulation) and enlarges
to form mature or Graafian follicle
– Usually only one is ovulated, others
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degenerate
Maturation and Fertilization of Oocyte
28-8
Ovulation, Fertilization, Follicle Fate
• Ovulation: release of a secondary oocyte from an
ovary. Unlike spermatogenesis, division of
cytoplasm during meiosis is uneven and polar
bodies are very small, oocyte very large
• Graafian follicle become corpus luteum (endocrine
body that secretes progesterone & estrogen)
• Fertilization: begins when a sperm cell binds the
plasma membrane of secondary oocytes and
penetrates into cytoplasm.
• Secondary oocyte completes meiosis II forming
one polar body. Fertilized egg now a zygote
• Fate of corpus luteum
– If fertilization occurs, corpus luteum persists
– If no fertilization, becomes corpus albicans (atrophied
corpus luteum)
28-9
Menstrual Cycle
• Puberty
– Begins with menarche (first
episode of menstrual bleeding)
– Begins when GnRH levels increase
• Menstrual Cycle
– About 28 days long
– Phases
• Menses
• Proliferative or follicular
phase (functional layer
proliferates; follicle matures)
(time between the ending of menses
& ovulation)
• Secretory or luteal phase
(maturation and secretion of
uterine glands; presence of
corpus luteum) (period after
ovulation & before next menses)
• Menses
– Amenorrhea: absence of a
menstrual cycle
– Menopause: cessation of menstrual
cycles
28-10
Hormone Regulation during Menstrual Cycle
28-11
Uterine Cycle
28-12
• Female fertility
– Sperm ejaculated into vagina during
copulation and transported through
cervix and uterine tubes to ampulla
– Sperm cells undergo capacitation,
enabling them to release acrosomal
enzymes to digest away follicular cells
• Pregnancy
– Oocyte can be fertilized up to 24 hours
after ovulation
– Sperm cells can be viable for up to 6
days in female tract
– Ectopic pregnancy: Implantation
occurs anywhere other than uterine
cavity
• Fertilization. Occurs in uterine tube.
Multiple mitoses occur after union of
oocyte and sperm nuclei, (pronuclei)
forming an embryo. Outer layer of
embryonic mass is the trophoblast.
Implantation. Trophoblast also secretes
HCG (keeps corpus luteum from degenerating;
blood levels of estrogen & progesterone do not
decrease & menses does not occur).
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Fertilization
29-14
Blastocyst Formation
• Zygote divides to form 2 cells
about 18-39 hours after
fertilization
– 2 cells divide to form 4, 8, and so
on
– Pluripotent: Ability to develop
into wide range of tissues
• Morula: solid ball of 12 or
more cells
• Blastocyst or hollow sphere of
cells
– Implantation: burrowing into
uterine wall
– Placenta develops from
trophoblast cells
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Blastocyst
29-16
Formation of Placenta
A. Frontal section of the
uterus and uterine tube
showing development 7
days after fertilization
B. Implantation of the
blastocyst with
syncytiotrophoblast
beginning to invade the
uterine wall at 8-12 days
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Placenta Formation
C. Intermediate stage of
placental formation at
about 14-20 days. As
maternal blood vessels
are encountered by the
syncytiotrophoblast,
lacunae are formed and
filled with maternal
blood
D. Cytotrophoblast cords
surround the
syncytiotrophoblast and
lacunae, and embryonic
blood vessels enter the
cord at about 1 month
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Mature Placenta and Fetus
( fetal blood vessels & maternal blood vessels are in close contact,
& nutrients are exchanged between fetal & maternal blood, but
fetal & maternal blood do not mix)
29-19
Formation of the Germ Layers
• Amniotic cavity: forms
inside inner cell mass and
surrounded by layer of cells
called the amnion or amniotic
sac. Forms after implantation
– Embryonic disk: composed of
ectoderm and endoderm and is
part of the inner cell mass
– Amniotic cavity eventually
surrounds the developing
embryo providing a protective
fluid bag
• Yolk sac: forms inside
blastocele from endoderm &
provides nutrition to the
developing embryo.
29-20
Germ Layers
29-21