Reproduction
General Biology 102
SUNY Orange at
S. S. Seward Institute
Both asexual and sexual reproduction occur in the
animal kingdom
• Asexual reproduction is creation of
offspring whose genes all come from
one parent
• Sexual reproduction is creation of
offspring by fusion of male gametes
(sperm) and female gametes (eggs) to
form zygotes
Mechanisms of Asexual Reproduction
• Many invertebrates reproduce
asexually by fission, separation of a
parent into two or more individuals
of about the same size
• Budding - two new individuals arise from
outgrowths of existing ones
• Fragmentation - breaking of the body
into pieces, some or all of which develop
into adults
– must be accompanied by
regeneration, regrowth of lost body
parts
Reproductive Cycles and Patterns
• Most animals exhibit reproductive cycles
related to changing seasons
• Reproductive cycles are controlled by
hormones and environmental cues
• Animals may reproduce asexually or
sexually, or they may alternate these
methods
• Parthenogenesis is a process in which
an egg develops without being fertilized
• Several genera of fishes, amphibians,
and lizards reproduce only by a complex
form of parthenogenesis
Ovary
size
Ovulation
Hormone
level
Ovulation
Estrogen
Progesterone
Behavior
Time
Femalelike
Malelike
Femalelike
Malelike
• Sexual reproduction is a special problem
for organisms that seldom encounter a
mate
• One solution is hermaphroditism - each
individual has male and female
reproductive systems
• In sequential hermaphroditism, an
individual reverses its sex during its
lifetime
Fertilization depends on mechanisms
that help sperm meet eggs of the
same species
• The mechanisms of fertilization, the
union of egg and sperm, play an
important part in sexual
reproduction
• In external fertilization, eggs shed
by the female are fertilized by sperm
in the external environment
Eggs
• Internal fertilization - sperm are
deposited in or near the female
reproductive tract, and fertilization
occurs within the tract
• - requires critical timing, often mediated
by environmental cues, pheromones,
and/or courtship behavior
• - requires behavioral interactions and
compatible copulatory organs
Ensuring the Survival of Offspring
• All species produce more offspring than
the environment can handle, but the
proportion that survives is quite small
• Embryos of many terrestrial animals
develop in eggs that can withstand harsh
environments
• Many animals retain the embryo, which
develops inside the female
• In many animals, parental care helps
ensure survival of offspring
Gamete Production and Delivery
• Must have systems that produce gametes
• The least complex systems do not contain
distinct gonads
• More complex systems contain many sets
of accessory tubes and glands that carry,
nourish, and protect gametes and
developing embryos
• Many animals with relatively simple body
plans have highly complex reproductive
systems
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)
• Most insects have separate sexes
with complex reproductive systems
Accessory
gland
Testis
Ovary
Ejaculatory
duct
Oviduct
Spermatheca
Vas
deferens
Seminal
vesicle
Male honeybee (drone)
Penis
Vagina
Accessory
gland
Female honeybee (queen)
Reproductive organs produce and
transport gametes: humans
Female Reproductive Anatomy
• The female external reproductive
structures include the clitoris and two
sets of labia
• The internal organs are a pair of gonads
and a system of ducts and chambers that
carry gametes and house the embryo
and fetus
Female Reproductive Anatomy
• Labia majora
pubic bone
• Labia minora
ovary
• Clitoris
oviduct
• Uterus
cervix
• Vagina
endometrium
• Urinary bladder
follicle
• Rectum
corpus luteum
Oviduct
Ovary
Uterus
(Urinary bladder)
(Rectum)
(Pubic bone)
Cervix
Urethra
Shaft
Vagina
Glans
Bartholin’s gland
Clitoris
Prepuce
Labia minora
Vaginal opening
Labia majora
Oviduct
Ovaries
Follicles
Corpus luteum
Uterus
Uterine wall
Endometrium
Cervix
Vagina
Ovaries
• The female gonads, the ovaries, lie in the
abdominal cavity
• Each ovary
– Has tough protective capsule
– contains many follicles
• Follicle consists of one egg surrounded
by one or more layers of follicle cells
• Ovulation expels an egg cell from the
follicle
• Remaining follicular tissue grows within
the ovary, forming a mass called the
corpus luteum (body, yellow)
• The corpus luteum secretes hormones,
depending on whether pregnancy occurs
Oviducts and Uterus
• Egg cell is released into the abdominal
cavity near the opening of the oviduct, or
fallopian tube
• Cilia in the tube convey the egg to the
uterus
Vagina and Vulva
• Vagina - thin-walled chamber that is
the repository for sperm during
copulation and serves as the birth
canal
• opens to the outside at the vulva,
Mammary Glands
• Mammary glands are not part of the
reproductive system but are important to
mammalian reproduction
• Within the glands, small sacs of
epithelial tissue secrete milk
Male Reproductive Anatomy
• Urinary bladder
vas deferens
• Urethra
prostate gland
• Epididymis
testis
• Erectile tissue
scrotum
• Seminal vesicle
glans penis
• Blulbourethral gland pubic bone
Male Reproductive Anatomy
• In most mammals, the male’s external
reproductive organs are the scrotum and
penis
• Internal organs are the gonads, which
produce sperm and hormones, and
accessory glands
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
Seminal
vesicle
(behind
bladder)
(Urinary
bladder)
Prostate gland
Bulbourethral
gland
Urethra
Erectile tissue
of penis
Scrotum
Vas deferens
Epididymis
Glans penis
Testis
• The male gonads, or testes, consist of
highly coiled tubes surrounded by
connective tissue
• Sperm form in these seminiferous tubules
• Production of normal sperm cannot occur
at the body temperatures of most
mammals
• The testes of many mammals are held
outside the abdominal cavity in the
scrotum, where the temperature is lower
than in the abdominal cavity
Ducts
• From the seminiferous tubules of a
testis, sperm pass into the coiled tubules
of the epididymis
• During ejaculation, sperm are propelled
through the muscular vas deferens and
the ejaculatory duct, and then exit the
penis through the urethra
Glands
• Three sets of accessory glands add
secretions to the semen, the fluid
that is ejaculated
• The two seminal vesicles contribute
about 60% of the total volume of
semen
• The prostate gland secretes its products
directly into the urethra through several
small ducts
• The bulbourethral gland secretes a clear
mucus before ejaculation that neutralizes
acidic urine remaining in the urethra
Penis
• The human penis is composed of three
cylinders of spongy erectile tissue
• During sexual arousal, the erectile tissue
fills with blood from the arteries, causing
an erection
Human Sexual Response
• Two reactions predominate in both
sexes:
– Vasocongestion, the filling of tissue
with blood
– Myotonia, increased muscle tension
• The sexual response cycle has four
phases: excitement, plateau, orgasm,
and resolution
In humans and other mammals, a complex
interplay of hormones regulates gametogenesis
• The process of gametogenesis is based
on meiosis, but differs in females and
males
• Oogenesis is development of mature ova
• Spermatogenesis is production of
mature sperm
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
Epididymis
Seminiferous tubule
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
• Oogenesis differs from spermatogenesis
– 3 major ways:
– During meiotic divisions, cytokinesis
is unequal, with almost all of the
cytoplasm in one daughter cell, the
secondary oocyte
– Unlike sperm, eggs are not produced
continuously throughout life
– Oogenesis has long “resting” periods,
whereas sperm are produced in
uninterrupted sequence
Reproduction
• Where do babies really come
from?
The Reproductive Cycles of Females
• In females, the secretion of hormones
and the reproductive events they
regulate are cyclic
Menstrual Versus Estrous Cycles
• Two different types of cycles occur in
females
• Humans and other primates have
menstrual cycles; other mammals have
estrous cycles
• In both cases, ovulation occurs after the
endometrium has started to thicken in
preparation for implantation
• In menstrual cycles:
– The endometrium is shed from the
uterus in a bleeding called menstruation
– Sexual receptivity is not limited to a
timeframe
• In estrous cycles:
– The endometrium is reabsorbed by the
uterus
– Sexual receptivity is limited to a “heat”
period
The Human Female Reproductive Cycle: A Closer Look
• The female reproductive cycle is one
integrated cycle involving two organs:
the uterus and ovaries
• Secretion of GnRH from the
hypothalamus and of FSH and LH from
the anterior pituitary orchestrates the
female reproductive cycle
• Five kinds of hormones participate in an
elaborate scheme involving positive and
negative feedback
Control by hypothalamus
Inhibited by combination of
estrogen and progesterone
Hypothalamus
Stimulated by high levels
of estrogen
GnRH
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
Days
Menstrual flow phase Proliferative phase
0
5
10
Secretory phase
14 15
20
25
28
The Ovarian Cycle
• In the ovarian cycle, hormones stimulate
follicle growth, which results in ovulation
• Following ovulation, the follicular tissue
left behind transforms into the corpus
luteum
The Uterine (Menstrual) Cycle
• Cycle after cycle, the maturation and
release of egg cells from the ovary are
integrated with changes in the uterus
• If an embryo has not implanted in the
endometrium by the end of the secretory
phase, a new menstrual flow commences
Menopause
• After about 450 cycles, human females
undergo menopause, the cessation of
ovulation and menstruation
Hormonal Control of the Male Reproductive
System
• Testosterone and other androgens are
directly responsible for the primary and
secondary sex characteristics of the
male
• Androgen secretion and sperm
production are controlled by
hypothalamic and pituitary hormones
LE 46-14
Stimuli from other
areas in the brain
Hypothalamus
GnRH
Anterior
pituitary
Negative
feedback
FSH
LH
Leydig cells
make
testosterone
Primary and
secondary sex
characteristics
Sertoli cells
Spermatogenesis
Testis
In humans and other placental mammals, an
embryo grows into a newborn in the mother’s
uterus
• Pregnancy, or gestation, is the condition
of carrying one or more embryos in the
uterus
• Duration of pregnancy in other species
correlates with body size and maturity of
the young at birth
Conception, Embryonic Development, and Birth
• Conception, fertilization of an egg by a
sperm, occurs in the oviduct
• After fertilization, the zygote undergoes
cleavage and develops into a blastocyst
before implantation in the endometrium
Cleavage starts
Cleavage continues
Ovary
Fertilization occurs
The blastocyst implants
Uterus
Ovulation
Endometrium
From ovulation to implantation
Endometrium
Inner cell mass
Cavity
Blastocyst
Implantation of blastocyst
Trophoblast
First Trimester
• Human gestation can be divided into
three trimesters of about three months
each
• The first trimester is the time of most
radical change for both the mother and
the embryo
• During its first 2 to 4 weeks, the embryo
obtains nutrients directly from the
endometrium
• Meanwhile, the outer layer of the
blastocyst mingles with the endometrium
and eventually forms the placenta
• Blood from the embryo travels to the
placenta through arteries of the umbilical
cord and returns via the umbilical vein
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
• The first trimester is the main period of
organogenesis, development of the body
organs
5 weeks.
14 weeks.
20 weeks.
Second Trimester
• During the 2nd trimester
– The fetus grows and is very
active
– The mother may feel fetal
movements
– The uterus grows enough for the
pregnancy to become obvious
Third Trimester
• During the 3rd trimester, the fetus grows
and fills the space within the embryonic
membranes
• A complex interplay of local regulators
and hormones induces and regulates
labor, the process by which childbirth
occurs
from
ovaries
Oxytocin
from fetus
and mother’s
posterior pituitary
Induces oxytocin
receptors on uterus
Stimulates uterus
to contract
Stimulates
placenta to make
Prostaglandins
Stimulate more
contractions
of uterus
Positive feedback
Estrogen
• Birth, or parturition, is brought
about by a series of strong,
rhythmic uterine contractions
Placenta
Umbilical
cord
Uterus
Placenta
(detaching)
Uterus
Cervix
Umbilical
cord
Dilation of the cervix
Expulsion: delivery of the infant
Delivery of the placenta
The Mother’s Immune Tolerance of
the Embryo and Fetus
• A woman’s acceptance of her
“foreign” offspring is not fully
understood
• It may be due to suppression of the
immune response in her uterus
Modern Reproductive Technology
• Amniocentesis and chorionic villus
sampling are invasive techniques in
which amniotic fluid or fetal cells are
obtained for genetic analysis
• Noninvasive procedures usually use
ultrasound imaging to detect fetal
condition
• Modern technology can help infertile
couples by in vitro fertilization
Head
Head
Body
Body