Animal Reproduction
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Reproduction in animals: sexual and asexual
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Sexual reproduction: haploid (n) gametes
(egg and sperm) fuse to form a zygote.
Gametes are formed as result of meiotic cell
division
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Asexual reproduction: generation of new
individuals through mitotic cell division.
Asexual reproduction:
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Beneficial in a stable
environment:
Budding: new
individuals arise from
outgrowths of old ones
Fragmentation
(fission): breaking of the
body into several pieces
followed by regeneration
– starfish, sponges,
cnidarians
Parthenogenesis: egg
develops without
fertilization
Sexual reproduction:
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Gametes are formed by meiosis
Results in unique combination of parental genes
Results in offsprings that have a lot of variation.
Results in reproductive success even under
environmental stress
New pathogens
Allows for rapid adaptation
May help get rid of harmful traits more easily
Reproductive cycles

Most animals have cyclical reproductive
activity
Seasonal cycles: only when sufficient
resources are available and when conditions
favor survival of the offspring

Cycles are controlled by hormones and the
hormones production is regulated by
environmental cues like day length and
temperature

Female sheep has a cycle in the late fall early
winter, pregnancy lasts 5 months, lambs are
born in early spring, plenty of food available
Altering between sexual
and asexual cycle:
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Females lay two types
of eggs: eggs that
develop by
parthenogenesis, eggs
that require fertilization
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Often related to
season, sexual
reproduction favored
during times of stress
Hermaphroditism
 Individual has male and female reproductive systems
 Common in animals where animals have to
encounter other animals for sexual reproduction but
are sessile, or burrow in the ground; barnacles,
earthworms
Genital
pore
Male organs:
Seminal
vesicle
(Digestive tract)
Female organs:
Uterus
Yolk gland
Yolk duct
Sperm duct
(vas deferens)
Oviduct
Ovary
Vas efferens
Seminal
receptacle
Testis
(Excretory pore)
Sex reversal
 An individual changes sex during its lifetime
 Blue head wrasse, a type coral reef fish live in
harems with one male and several females. When
male dies the biggest female takes the role of the
male and within a week can start to produce sperm
Fertilization

Process that brings together eggs and sperms of the
same species

Involves pheromones: chemicals released from
one individual influences the physiology and
behavior of other individuals of the same species;
small molecules – volatile or water soluble

Internal and external
External fertilization:
 Female releases eggs into
the environment and the
male releases sperms and
fertilizes them
 More common in aquatic
habitats
 Moist habitat is important to
prevent eggs from drying
out
 Sperm can also swim to the
eggs
 Spawning: environmental
cues cause the whole
population to release
gametes at the same time
Internal fertilization

Sperms are deposited in or near the female’s
reproductive tract

Fertilization occurs within the tract
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Allows sperm to reach the egg efficiently
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Cooperative behavior
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Sophisticated and compatible
Ensuring survival of offspring
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2 strategies:
numerous gametes and zygotes; low
percentage of survival
fewer zygotes higher percentage of survival,
parental care involved, newborns are not
independent
reproductive systems are designed
accordingly
Gamete production and delivery:
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Gametes are produced in the reproductive
system
In simple systems gametes develop from
undifferentiated cells and are shed through
the excretory systems
Elaborate systems have gonads, accessory
tubes and glands that carry , nourish and
protect gametes and even developing
embryos
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Some insect species females have spermatheca to
store sperms for extended periods
In nonmammalian vertebrates digestive, excretory
and reproductive system have a common opening
called cloaca
Accessory
gland
Testis
Ovary
Ejaculatory
duct
Oviduct
Spermatheca
Vas
deferens
Seminal
vesicle
Male honeybee (drone)
Penis
Vagina
Accessory
gland
Female honeybee (queen)
Human female reproductive system
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Ovaries
Oviduct (fallopian tube)
Uterus
Vagina
Vulva
Ovary
(Rectum)
Ovaries
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Packed with follicles containing oocyte which develop
by the process of oogenesis
At birth ovaries together contain 1 to 2 million follicles
500 follicles fully mature between puberty and
menopause
4 week menstrual cycle one follicle matures and expels
its egg – ovulation
Before ovulation follicles also produce estrogen
(estradiol)
After ovulation the remaining follicle is called corpus
luteum – produces estrogen and progesterone
Oviduct (fallopian tube)
 egg is released into the abdominal cavity and
cilia in the oviduct collect the egg
Oviduct
Ovary
(Rectum)
Uterus

thick muscular organ which expand during pregnancy,
endometrium (inner lining) is supplied with blood vessels
Oviduct
Ovary
Uterus
(Rectum)
Vagina

Muscular elastic organ, site of insertion of the penis and also the birth canal
Vulva

External female genitalia consisting of skin and muscle folds – labia majora, labia
minora; erectile tissue – clitoris; hymen- piece of tissue covering the vaginal
opening
Oviduct
Ovary
Uterus
(Urinary bladder)
(Rectum)
Cervix
Urethra
Shaft
Vagina
Glans
Clitoris
Prepuce
Labia minora
Vaginal opening
Labia majora
Oviduct
Ovaries
Follicles
Corpus luteum
Uterus
Uterine wall
Endometrium
Cervix
Vagina
Mammary glands
 not part of the reproductive system but very
important for reproduction
 produces milk only in females
Human male reproductive system:
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Testes (singular testis)
Scrotum
Epididymis
Ducts
Accessory glands
Penis
Testes (singular testis)
 Male gonads
 Consists of highly coiled seminiferous tubules where
sperm form
 Ledig cells between the tubules produce
testosterone
(Urinary
bladder)
(Rectum)
Testis
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Scrotum
folds of skin keeps the testes temperature
2oC below abdominal cavity – needed for
sperm production
(Urinary
bladder)
(Rectum)
Testis
Scrotum
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Epididymis
6m long coiled tubes – 3 weeks for the sperm
to pass
sperm maturation is completed here
(Urinary
bladder)
(Rectum)
Epididymis
Testis
Scrotum
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Ducts
During ejaculation sperm is ejaculated through vas deferens
Ejaculatory duct from the seminal vesicles join with vas deferens
Urethra – outlet for reproductive and excretory system
(Urinary
bladder)
(Rectum)
Vas deferens
Ejaculatory duct
Urethra
Vas deferens
Epididymis
Testis
Scrotum
Accessory glands
 Seminal vesicles produces secretions that combine
with sperm to form semen (60% of the volume)
 Prostrate glands: secretes anticoagulants and
nutrients into the urethra; enlarged prostrate
common in men over 40 and cancer in men over 65
 Bulbourethral gland: secretes mucus that
neutralizes acid from any urine that is left over, also
carries sperm that is released before ejaculation
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Penis
Contains urethra
3 cylinders of spongy erectile tissue which is derived from blood
vessels
During erection the veins are sealed off and pressure builds
Seminal vesicle
(Urinary
bladder)
(Rectum)
(Public bone)
Vas deferens
Erectile
tissue of
penis
Ejaculatory duct
Prostate gland
Bulbourethral gland
Urethra
Vas deferens
Glans penis
Epididymis
Testis
Scrotum
Prepuce
Gamete formation and the role of hormones

Coordinated action of hormones from hypothalamus, anterior
pituitary and gonads
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Hypothalamus releases gonadotropin releasing hormone (GnRH)
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GnRH directs pituitary to release gonadotropins, Follicle
Stimulating Hormones (FSH) in females and luteinizing
hormones (LH) in males - target tissue are gonads
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Gonads release sex hormones in males – testosterone in males,
estrogen and progesterone in females; influence gametogenesis
directly and indirectly
Role of sex hormones
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Gamete formation
Development of primary sexual characteristics in
males (seminal vesicles, external anatomy)
Development of secondary sexual characteristics in
males (voice, facial hair, muscles)
Development of secondary sexual characteristics in
females (physical changes)
Epididymis

Seminiferous tubule
Spermatogenesis
Testis
Cross section
of seminiferous
tubule
Spermatogonium
Sertoli cell
nucleus
Mitotic division, producing
large numbers of spermatogonia
Differentiation and onset of meiosis I
Primary spermatocyte
(in prophase of meiosis I)
Meiosis I completed
Secondary spermatocyte
Meiosis II
Early
spermatids
Lumen of
Seminiferous tubule
Spermatids
(at two stages of
differentiation)
Differentiation
Neck
Sperm cells
Head
Midpiece
Tail
Plasma membrane
Acrosome
Nucleus
Mitochondria
Figure 26.00b
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Oogenesis
Ovary
Primary germ cell
in embryo
Differentiation
Oogonium
Oogonium
in ovary
Mitotic
division
Primary
oocyte
within
follicle
Primary
oocyte
Completion of meiosis I
and onset of meiosis II
First polar body
Growing
follicle
Secondary
oocyte
Ovulation
Entry of sperm
triggers
completion
of meiosis II
Second polar body
Mature follicle
Ruptured
follicle
Ovum
Ovulated
secondary oocyte
Corpus luteum
Degenerating corpus luteum
Hormonal control of
male reproductive
system
 GnRH released by
hypothalamus
 GnRH stimulates
FSH and LH release
from anterior pituitary GnRH
 FSH acts on sertoli
cells in seminiferous
tubules promoting
spermatogenesis
 LH stimulates Leidig
cells to make
testosterone which
FSH
stimulates
spermatogenesis and
develops secondary
characteristics
 -ve feedback by rising
levels of testosterone
controls the levels of
FSH, LH and GnRH
Stimuli from other
areas in the brain
Hypothalamus
Anterior
pituitary
Negative
feedback
LH
Leydig cells
make
testosterone
Sertoli cells
Spermatogenesis
Primary and
secondary sex
characteristics
Testis
Control by hypothalamus
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Hormonal control of
female reproductive
system
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Integrated cycle
involving ovaries and
uterus (menstrual
cycle/ uterine cycle/
ovarian cycle)
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GnRH is released and
stimulates production
of FSH and LH (1,2)
Hypothalamus
Stimulated by high levels
of estrogen
GnRH
Anterior pituitary
FSH
Pituitary gonadotropins
in blood
Inhibited by combination of
estrogen and progesterone
Inhibited by low levels of
estrogen
LH
Control by hypothalamus
Inhibited by combination of
estrogen and progesterone
Hypothalamus
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FSH and LH stimulate
follicle growth (3)
Growing follicle produces
estrogen (4)
Steep rise in estrogen (5)
Increasing FSH
concentrations trigger
increased LH
concentrations (6)
Anterior pituitary
FSH
Inhibited by low levels of
estrogen
LH
Pituitary gonadotropins
in blood
LH
FSH
FSH and LH stimulate
follicle to grow
LH surge triggers
ovulation
Ovarian cycle
Growing follicle
Corpus
luteum
Mature
follicle
Follicular phase
Ovulation
Degenerating
corpus luteum
Luteal phase
Progesterone and
estrogen secreted
by corpus luteum
Estrogen secreted
by growing follicle in
increasing amounts
Ovarian hormones
in blood
Peak causes
LH surge
Progesterone
Estrogen
Estrogen level
very low
Progesterone and estrogen promote thickening
of endometrium
Uterine (menstrual) cycle
Endometrium
Menstrual flow phase Proliferative phase
Days
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Stimulated by high levels
of estrogen
GnRH
0
5
10
Secretory phase
14 15
20
25
28
Control by hypothalamus
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Mature follicle develops and
ovulation occurs after LH surge (7)
Stimulated by high levels
of estrogen
GnRH
Anterior pituitary
FSH

Inhibited by combination of
estrogen and progesterone
Hypothalamus
Inhibited by low levels of
estrogen
LH
Pituitary gonadotropins
in blood
After ovulation follicular tissue is
converted to corpus luteum (8)
LH

Estrogen and progesterone
secreted by corpus leutum helps
build and maintain endometrium (9)
FSH
FSH and LH stimulate
follicle to grow
Ovarian cycle
Growing follicle

Drop in estrogen and progesterone
levels cause disintegration of the
upper third of the endometrium (10)
Corpus
luteum
Mature
follicle
Follicular phase
Ovulation
Progesterone and
estrogen secreted
by corpus luteum
Ovarian hormones
in blood
Peak causes
LH surge
Progesterone
Estrogen
Synchronized changes in ovary and
uterus ensures preparation of
uterine lining for possible
implantation
Degenerating
corpus luteum
Luteal phase
Estrogen secreted
by growing follicle in
increasing amounts
Estrogen level
very low
Progesterone and estrogen promote thickening
of endometrium
Uterine (menstrual) cycle
Endometrium
Menstrual flow phase Proliferative phase
Days

LH surge triggers
ovulation
0
5
10
Secretory phase
14 15
20
25
28
Conception, embryonic development and birth
 Secondary oocyte is released from the ovary and enters the oviduct
 Fertilization occurs: sperm enters oocyte, meiosis of oocyte finishes,
produces a zygote.
 Conception: fertilization of sperm by egg
 Cell division (cleavage) begins in the oviduct, moves towards the uterus
 Cleavage continues and forms a blastocyst
 Blastocyst implants in endometrium (1week after conception)
Cleavage starts
Cleavage continues
Ovary
Fertilization occurs
The blastocyst implants
Uterus
Ovulation
Endometrium
From ovulation to implantation
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Blastocyst implants in endometrium (1week
after conception)
Endometrium
Inner cell mass
Cavity
Blastocyst
Implantation of blastocyst
Trophoblast
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Human chorionic gonadotropin (HCG) is
secreted by the embryo, maintains the
endometrium. HCG tapers off after the 1st
trimester
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Human gestation is divided into trimesters
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1st trimester:
blastocyst forms
trophoblast, which
develops into
placenta
main period of
organogenesis
(very vulnerable)
at 8 weeks it is
fetus
about 5 cm
5 weeks.
LE 46-17
2nd trimester
 HCG declines
 Movements start
14 weeks.
LE 46-17
3rd trimester
 fetus grows to a
final weight of 34kg, 50cm length
 labor starts at the
end of 3rd
trimester
20 weeks.
Maternal
arteries
Maternal
veins
Placenta
Maternal portion
of placenta
Umbilical cord
Chorionic villus
containing fetal
capillaries
Fetal portion of
placenta (chorion)
Maternal blood
pools
Uterus
Fetal arteriole
Fetal venule
Umbilical cord
Umbilical arteries
Umbilical vein
from
ovaries
Oxytocin
from fetus
and mother’s
posterior pituitary
Induces oxytocin
receptors on uterus
Stimulates uterus
to contract
Stimulates
placenta to make
Prostaglandins

Induction
of labor
Stimulate more
contractions
of uterus
Positive feedback
Estrogen
Stages of labor
Placenta
Umbilical
cord
Uterus
Placenta
(detaching)
Uterus
Cervix
Umbilical
cord
Dilation of the cervix
Expulsion: delivery of the infant
Delivery of the placenta