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REPRODUCTION AND
DEVELOPMENT
CHAPTER 31
ASEXUAL AND SEXUAL
REPRODUCTION
• Asexual reproduction produces
offspring that are genetically
identical to the parent.
• Mitosis
• Examples are
• fission in which one organism splits in
two.
• budding occurs where part of the
parent’s body becomes separated
from the rest and differentiates into a
new individual.
ASEXUAL AND SEXUAL
REPRODUCTION
• Sexual reproduction occurs when a new
individual is formed by the union of two cells.
• These cells are called gametes, which are formed
by meiosis in the sex organs, or gonads.
• The union of a sperm and an egg produces a
fertilized egg, or zygote.
• In vertebrates, the zygote will develop by mitosis
into a multicellular organism.
ASEXUAL AND SEXUAL
REPRODUCTION
• Parthenogenesis is a special type of
reproduction in which offspring are
produced from unfertilized eggs.
• For example, in honeybees, a queen mates only
once and stores sperm.
• If no sperm are released, the eggs develop into
drones, which are male.
• If sperm are released, the eggs develop into
other queens or workers, which are female.
ASEXUAL AND SEXUAL
REPRODUCTION
• Hermaphroditism is a
reproductive strategy in which
one individual has both testes
and sperm and so can produce
both sperm and eggs.
• Most hermaphroditic organisms
require another individual to
reproduce.
• Some hermaphroditic organisms
change their sex through sequential
hermaphroditism.
• Protogyny is a change from
female to male.
• Protandry is a change from male
to female.
ASEXUAL AND SEXUAL
REPRODUCTION
• In mammals, sex is
determined early in
development.
• If the embryo is XY, it is a male
and will carry a gene on the Y
chromosome whose product
converts gonads into testes.
• The sex-determining gene is SRY
(sex-determining region of the Y
chromosome).
• If the embryo is XX, it is a
female and the gonads will
become ovaries.
Sperm
Ovum
Ovum
X
X
X
Y
XY
Sperm
XX
Zygote
Zygote
SRY
Indifferent
gonads
No SRY
Testes
Ovaries
Develop in early
embryo
Seminiferous
tubules
Leydig cells
(Follicles do not
develop until
third trimester)
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Vertebrate sexual reproduction evolved in
the ocean before vertebrates colonized the
land.
• In external fertilization, gametes are released into
the water.
• Fish, frogs
• In internal fertilization, male gametes are
introduced into the female reproductive tract.
• Some fish, salamanders, reptiles, birds, mammals
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Vertebrates with
internal fertilization
may be classified into
different groupings.
• Oviparity – the eggs are
fertilized internally but
complete development
outside the mother’s body.
• Ovoviviparity – the fertilized eggs complete
development inside the mother and depend on yolk
exclusively for nourishment before being born alive.
• Viviparity – the young develop within the mother and
obtain nourishment from their mother’s blood before being
born alive.
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Most fish and amphibians reproduce by
means of external fertilization.
• In most bony fish, the eggs contain only enough
yolk to sustain the developing embryo for a short
time.
• Fish mature rapidly, but there is high mortality.
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Most cartilaginous fish use internal fertilization
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Amphibians reproduce in the water and have
aquatic larval stages before moving to the land.
• Development is longer than in fish, but the eggs
provide only slightly more yolk.
Red eft
Gilled larva
Eggs
Adults
mating
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Most reptiles are oviparous
• The eggs are surrounded by a leathery shell that is
deposited as the egg passes through the oviduct.
• Most reptiles abandon their eggs after laying
them.
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
Developing ovum
• All birds are oviparous.
Ovary
• Most birds incubate their eggs
after they are laid.
Ovum
Oviduct
Mesentery
Rectum
Uterus
(shell gland)
Opening to cloaca
(a)
(b)
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Some mammals are seasonal breeders,
while others have multiple short
reproductive cycles.
• The females generally undergo the reproductive
cycle, whereas the males are more constant in
their reproductive activity.
• Most females are “in heat,” or sexually receptive
to males, only around the time of ovulation.
• This period of sexual receptivity is called estrus.
• The reproductive cycle in females is called
the estrous cycle.
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• The most primitive mammals, the
monotremes, are oviparous.
• The duck-billed platypus and the echidna
incubate their eggs in a nest and, because they
lack nipples, the young lick milk off their mother’s
skin.
• All other mammals are viviparous.
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Viviparous mammals are
divided into two
subcategories based on
how they nourish their
young.
• Marsupials give birth to
fetuses that are
incompletely developed.
• They complete their
development in a pouch of
their mother’s skin, where
they obtain nourishment from
mammary glands.
EVOLUTION OF VERTEBRATE
SEXUAL REPRODUCTION
• Placental mammals retain their young
within the mother’s uterus.
• The fetus is nourished by a placenta which
allows the fetus to obtain nutrients by
diffusion from the mother’s blood.
• The young are born in a more developed
state
• Drink milk after birth.
MALES
• Sperm is the male gamete
and is highly specialized for its
role as a carrier of genetic
information.
• Sperm do not successfully
complete their
Flagellum
Body
Mitochondrion
development at 37°C.
Nucleus
• The sperm-producing
Centriole
Acrosome
Head
organs, called testes, are
found in a sac called the
scrotum, which maintains
the testes at a
temperature 3° cooler
than the rest of the body.
Tail
(top): © David M. Phillips/Photo Researchers
MALES
• The testis is
composed of several
hundred
compartments
packed with large
numbers of tightly
coiled tubes called
seminiferous tubules.
• These are the sites for spermatogenesis
• The process of spermatogenesis begins in germinal cells
toward the outside of the tubule.
• As the cells undergo meiosis, they move toward the
lumen of the tubule.
MALES
• After a sperm cell is manufactured within
the testis, it is delivered to a long, coiled
tube called the epididymis.
• The sperm cell is not motile when it first arrives at
the epididymis and must remain there for at least
18 hours before motility develops.
• From the epididymis, the sperm is delivered to
another long tube, the vas deferens.
• When sperm is ejaculated, it travels from the vas
deferens to the urethra.
FEMALES
• In females, eggs
develop from cells
called oocytes.
• These are located in
the outer layer of
female gonads
called ovaries.
• During each reproductive cycle, one or a few of
these oocytes are initiated to continue their
development.
• This process is called ovulation.
FEMALES
• The fallopian tubes (also known as uterine
tubes, or oviducts) transport eggs from the
ovaries to the uterus.
• The uterus is lined with a stratified epithelial
membrane called the endometrium.
• The surface of the endometrium is shed during
menstruation.
FEMALES
• To fertilize an egg successfully, the sperm
must make its way far up the fallopian tube.
• The egg is moved down the fallopian tube by
contractions of smooth muscle lining the tube.
• Sperm swim against the current created by
these contractions.
• An egg loses its capacity to develop within 24
hours of ovulation.
• Sperm can remain viable for up to 6 days.
FEMALES
Oviducts
• When the first sperm cell
penetrates the oocyte’s
protective layers the oocyte
blocks the entry of other sperm.
• The oocyte completes meiosis II,
forming a haploid ovum.
• When the female haploid
nucleus joins with the male
haploid nucleus, the egg is
fertilized and becomes a
zygote.
• A fertilized egg attaches itself to
the endometrial lining to continue
development.
Uterus
Ovary
Cervix
Vagina
(a)
Sperm
First polar body
Egg
3
2
Nucleus
1
Sperm nucleus
fertilizing egg
Acrosome
Sperm
Zona pellucida
Extracellular space
Egg membrane
(b)
Granulosa cells
HORMONES COORDINATE THE
REPRODUCTIVE CYCLE
• The female reproductive cycle, called a
menstrual cycle, is composed of two distinct
phases.
• The follicular phase in which an egg reaches
maturation and is ovulated.
• The luteal phase where the body prepares for
pregnancy.
• These phases are coordinated by a family of
hormones whose production is controlled by the
hypothalamus.
HORMONES COORDINATE THE
REPRODUCTIVE CYCLE
• The follicular phase
corresponds to days 0 to 14
of the reproductive cycle.
• The anterior pituitary starts
the phase by secreting FSH
and LH in small amounts.
• Initially, several follicles (a
follicle is an oocyte and its
surrounding tissue) are
stimulated to grow.
• The follicle begins to
secrete the female
hormone estrogen.
HORMONES COORDINATE THE
REPRODUCTIVE CYCLE
• The low but rising levels of estrogen in the
bloodstream act as negative feedback.
• The output of FSH and LH are reduced.
• This ensures that only one oocyte matures at a
time.
• A rise in estrogen signals the end of the
follicular phase.
HORMONES COORDINATE THE
REPRODUCTIVE CYCLE
• The luteal phase occurs
during days 14 through 28 of
the reproductive cycle.
• The higher levels of estrogen
begin to have a positivefeedback effect on FSH and
LH secretion.
• The surge in LH causes
ovulation and the wall of the
follicle bursts.
• The follicle is released into one
of the fallopian tubes.
• LH directs the repair of the
ruptured follicle so that it fills in
to become the corpus luteum.
HORMONES COORDINATE THE
REPRODUCTIVE CYCLE
• The corpus luteum secretes progesterone which
also inhibits FSH and LH secretion.
• Progesterone completes the body’s preparation of the
uterus for fertilization, including the thickening of the
endometrium.
• If fertilization does not occur, production of progesterone
slows and eventually stops.
• The decreasing levels of progesterone cause the thickened
layer of the endometrium to be sloughed off.
• This process, menstruation, usually occurs about midway
between successive ovulations.
HORMONES COORDINATE THE
REPRODUCTIVE CYCLE
• If fertilization does occur high in the fallopian tube, the zygote
undergoes a series of cell divisions, called cleavage, while
traveling toward the uterus.
• At the blastocyst stage, it implants in the lining of the uterus.
• The embryo secretes human chorionic gonadotropin (hCG).
• This maintains the corpus luteum and prevents
menstruation.
Uterus
Fallopian tube
3
Cleavage
2
Morula
Egg
Fertilization
Developing follicles
Blastocyst
Implantation
1
Corpus
luteum
Ovary
Ovulation
4
EMBRYONIC DEVELOPMENT
• During cleavage, the mammalian zygote
divides rapidly into a larger and larger
number of smaller and smaller cells.
• The resulting tightly packed mass of about 36 cells
is called the morula.
• Each individual cell in the morula is called a
blastomere.
• Further divisions of the blastomeres in the morula
lead to a hollow ball of 500–2,000 cells
• This is called the blastocyst.
EMBRYONIC DEVELOPMENT
• The blastocyst contains a fluid-filled cavity
called the blastocoel.
• Within the blastocyst is an inner cell mass.
concentrated at one pole that goes on to form
the developing embryo.
• The outer sphere of cells, called the trophoblast,
releases hCG.
EMBRYONIC DEVELOPMENT
• The implantation of
the blastocyst in the
uterine lining
initiates the
formation of
membranes.
• Amnion encloses the
developing embryo.
• Chorion forms from the trophoblast and interacts
with uterine tissue to form the placenta.
• The placenta connects the developing embryo to the
blood supply of the mother.
EMBRYONIC DEVELOPMENT
• Gastrulation occurs 10
to 11 days after
fertilization and
involves certain
groups of cells moving
inward from the
surface of the inner
cell mass.
• The lower cell layer of the inner cell mass becomes the
endoderm and the upper layer becomes the
ectoderm.
• The moving cells differentiate into mesoderm
• They grow inward along a furrow called the primitive
streak.
EMBRYONIC DEVELOPMENT
• Neurulation is the
stage of
development that
begins in the third
week of embryonic
development.
• The three primary
germ layers begin
their development
into the tissues and
organs of the body.
EMBRYONIC DEVELOPMENT
• The notochord forms first from mesoderm.
• The neural tube forms from ectoderm.
• Somites form along the side of the notochord and
will become muscles, vertebrae, and connective
tissue.
• Between two layers of mesoderm, the coelom
forms.
FETAL DEVELOPMENT
• Organogenesis,
the process of
forming body
organs, begins
in the fourth
week of human
pregnancy.
Future lens
Pharyngeal
arches
Upper limb
bud
Lower limb
bud
Umbilical
cord
Neural tube
forming
Tail
Somites
Head
(growth accelerated)
Future
external
ear
Developing
heart
Forebrain
(a)
Retinal
pigment
(b)
Foot
plate
Hand
plate
FETAL DEVELOPMENT
• During the second month of pregnancy,
great changes in morphology occur as the
embryo takes shape.
• It begins to look distinctly human
• Development is essentially complete at the
end of the third month.
• Only the lungs and brain need to develop more.
• The developing human is now referred to as a
fetus instead of an embryo.
FETAL DEVELOPMENT
• The second
trimester is a time
of growth.
• By the end of the
sixth month, the
fetus still cannot
survive outside the
uterus without
special medical
intervention.
Development is essentially complete.
Bones actively enlarge.
The developing human is now referred
to as a fetus.
Facial expressions and primitive
reflexes are carried out.
All of the major body organs
have been established.
Arms and legs begin
to move.
Mother can feel baby kicking.
(c)
Following a period of rapid growth,
the fetus is born.
Neurological growth
continues after birth.
(d)
FETAL DEVELOPMENT
• The third trimester is a
period of rapid
growth.
• All of the growth is fueled
by the mother’s
bloodstream, passing into
the fetal blood supply
within the placenta.
• The placenta contains
blood vessels that extend
from the umbilical cord
into tissues that line the
uterus.
Chorion
Amnion
Umbilical
cord
Placenta
Maternal
artery
Maternal
vein
Uterine
wall
FETAL DEVELOPMENT
• At approximately 40 weeks
from the last menstrual
cycle, the process of birth
begins, including labor
and delivery.
• Oxytocin and prostaglandins
work by positive feedback to
stimulate uterine
contractions
Placenta
Umbilical
cord
Uterus
Vagina
Cervix
(a)
(b)
(c)
Uterus
Placenta
(detaching)
Umbilical
cord
(d)
FETAL DEVELOPMENT
• The mammary glands are prepared to
nourish the baby after birth.
• Prolactin stimulates milk production usually by the
third day after delivery.
• Oxytocin is released following the infant suckling
at the breast and initiates milk release.
• Growth continues rapidly after birth.
• Different organs grow at different rates.
• Neurological growth of the baby continues
long after birth.