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The Living World
Fifth Edition
George B. Johnson
Jonathan B. Losos
Chapter 36
Reproduction and Development
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
36.1 Asexual and Sexual
Reproduction
• Asexual reproduction produces offspring
that are genetically identical to the parent
 mitosis is the process of cell division that is
involved
 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
Figure 36.1 Asexual reproduction in protists
36.1 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
• the zygote will develop by mitosis into a new
organism
36.1 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
36.1 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
• during mating, each switches roles from producing eggs to
producing sperm
 some hermaphroditic organism change their sex
through sequential hermaphroditism
• protogyny changes from female to male
• protandry changes from male to female
Figure 36.2 Hermaphroditism and
protogyny
36.1 Asexual and Sexual
Reproduction
• In mammals, sex is determined early in
development
 the reproductive systems of human males and
females appear similar for the first 40 days after
conception
 if the embryo is XY, it is a male and will carry a gene
of 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
Figure 36.3 Sex determination
36.2 Evolution of Reproduction
Among the Vertebrates
• Vertebrate sexual reproduction evolved in
the ocean before vertebrates colonized the
land
 in external fertilization, gametes are
released into the water
• this process is common to most fish
 in internal fertilization, male gametes are
introduced into the female reproductive tract
• terrestrial vertebrates and some fish use this
process
36.2 Evolution of Reproduction
Among the Vertebrates
• 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
Figure 36.4 Viviparous vertebrates carry live,
mobile young within their bodies
36.2 Evolution of Reproduction
Among the Vertebrates
• 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
 most cartilaginous fish use internal fertilization
 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 a
little bit additional yolk
Amphibian Reproduction and
Development
Figure 36.5 Life cycle of the redspotted newt
Figure 36.6 The eggs of frogs are
fertilized externally
Figure 36.7 Different ways young
develop in frogs
36.2 Evolution of Reproduction
Among the Vertebrates
• Most reptiles are oviparous
 the eggs are surrounded by a leathery shell that is
deposited as the egg passes through the oviduct
 other species of reptiles are ovoviviparous or
viviparous
• All birds are oviparous
 as the fertilized egg passes along the oviduct, glands
secrete albumin protein (the egg white) and a hard,
calcareous shell
Reproduction in Reptiles and Birds
Figure 36.8 The introduction of
sperm by the male into the female’s
body is called copulation
Figure 36.9 Egg formation and
incubation in birds
36.2 Evolution of Reproduction
Among the Vertebrates
• Some mammals are seasonal breeders
 the females generally undergo the
reproductive cycle, whereas the males are
more constant in their reproductive cycles
 most females are “in heat” or sexually
receptive to males, only around the time of
ovulation
• this period of sexual receptivity is called estrous
– the reproductive cycle in females is called the estrous
cycle
36.2 Evolution of Reproduction
Among the Vertebrates
• In the estrous cycle of most mammals,
changes in the secretion of FSH and LH
by the anterior pituitary causes changes in
egg cell development and hormone
secretion in the ovaries
 rabbits and cats differ from most other
mammals in that they are induced ovulators
• females ovulate only after copulation as a result of
a reflex stimulation of LH
• this makes them extremely fertile
36.2 Evolution of Reproduction
Among the Vertebrates
• 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
36.2 Evolution of Reproduction
Among the Vertebrates
• 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 nipples of her
mammary glands
 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
Figure 36.10 Reproduction in
mammals
36.3 Males
• Sperm is the male gamete that is highly
specialized for its role as a carrier of
genetic information
 sperm do not successfully complete their
development at 37°C
• the sperm-producing organs, called testes, are
found in a sac called the scrotum, which hangs
between the legs and maintains the two testes at a
temperature about 3 cooler than the rest of the
body
36.3 Males
• The testis is composed of several hundred
compartments
 each compartment is 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
Figure 36.12 The testis and
formation of sperm
36.3 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
Figure 36.11 The male reproductive
system
Figure 36.13 Human sperm cells
36.3 Males
• The penis is an external tube containing
two long cylinders of spongy tissue
 below and between them is a third cylinder of
spongy tissue that contains in its center a
tube called the urethra
 the penis is designed to inflate
• when nerve impulses from the CNS cause the
arterioles leading into this tissue to expand, blood
collects within these spaces
• continued stimulation by the CNS is required for
erection to continue
Figure 36.14 Structure of the penis
36.3 Males
• While erection can be achieved without any
physical stimulation of the penis, physical
stimulation is required for semen to be delivered
 stimulation, such as by repeated thrusts into the
vagina of a female, leads first to the mobilization of
sperm
• muscles encircling the vas deferens contract, moving the
sperm along the vas deferens to the urethra
 the stimulation finally results to the strong contraction
of muscles at the base of the penis
• ejaculation is the forceful ejection of 2 to 5 ml of semen
36.3 Males
• Semen contain sperm and a collection of
secretions from glands
 these secretions, such as from the prostate
gland, provide metabolic energy sources for
the sperm
 there are several hundred million sperm in the
small volume of semen ejaculated
• males with fewer than 20 million sperm per ml are
considered sterile
36.4 Females
• In females, eggs develop from cells called
oocytes
 these are located in the outer layer of
compact masses of cells called ovaries
 all the oocytes needed for a lifetime are
already present at birth
 during each reproductive cycle, one or a few
of these oocytes are initiated to continue their
development
• this process is called ovulation
36.4 Females
• At birth, a female’s ovaries contain some 2
million oocytes, all of which have begun
the first meiotic division
 at this stage, they are referred to as primary
oocytes
 each primary oocytes waits to receive the
proper developmental signal to continue on
with meiosis
• the signal is FSH and very few of the oocytes will
receive it
36.4 Females
• With the onset of puberty, females mature
sexually
 at this time, the release of FSH initiates the
resumption of meiosis in a few oocytes
• In humans, only a single oocyte continues to
mature to become an ovum (plural, ova) or egg
– the other oocytes will regress
 only about 400 of the approximately 2 million
oocytes a woman is born with are actually
ovulated
Figure 36.16 The ovary and
formation of an ovum
36.4 Females
• The fallopian tubes (also known as
uterine tubes, or oviducts) transport ova
from the ovaries to the uterus
 in humans, the uterus narrows to a muscular
ring called the cervix
 the uterus is lined with a stratified epithelial
membrane called the endometrium
• the surface of the endometrium is shed during
menstruation
Figure 36.15 The female
reproductive system
36.4 Females
• Mammals other than primates have more
complex female reproductive tracts
 part of the uterus divides to form uterine “horns”
Figure 36.17 The mammalian uterus—several examples
36.4 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 a
few days
 a fertilized egg attaches itself to the
endometrial lining to continue development
36.5 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
36.5 Hormones Coordinate the
Reproductive Cycle
• The follicular phase corresponds to days
0 to 14 of the reproductive cycle
 it is when the egg develops through the ovary
 the anterior pituitary starts the phase by
secreting FSH which binds to receptors on the
surface of cells surrounding the egg (called a
follicle) and triggers resumption of meiosis
 several follicles are stimulated by FSH but
only one grows to maturity
36.5 Hormones Coordinate the
Reproductive Cycle
• FSH levels fall at the end of the follicular
phase due to negative feedback in the
form of estrogen produced by the ovary
 a rise in estrogen signals the end of the
follicular phase
36.5 Hormones Coordinate the
Reproductive Cycle
• The luteal phase occurs during days 14
through 28 of the reproductive cycle
 after shutting down FSH production, the
hypothalamus causes the anterior pituitary to
secrete LH
• this hormone causes ovulation because LH is
carried in the bloodstream and causes the wall of
the follicle to burst
• after the egg’s departure, LH directs the repair of
the ruptured follicle so that it fills in to become the
corpus luteum
36.5 Hormones Coordinate the
Reproductive Cycle
• The corpus luteum secretes progesterone
which also inhibits FSH
 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 caused the
thickened layer of the endometrium to be sloughed off
• this process, menstruation, usually occurs about midway
between successive ovulations
Figure 36.18 The human menstrual cycle
36.5 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
• because hCG comes from the embryo and not from the
mother, it is hCG that is tested in all pregnancy tests
Figure 36.19 The journey of an
ovum
36.5 Hormones Coordinate the
Reproductive Cycle
• Two additional hormones, secreted by the
pituitary, are important in the female
reproductive system
 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
• it also initiates labor and delivery
36.6 Embryonic Development
• During cleavage, the 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
36.6 Embryonic Development
• The blastocyst contains a fluid-filled
cavity called the blastocoel
 within the ball 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 the hCG
36.6 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 the uterine tissue to form the
placenta
• the placenta connects the developing embryo to
the blood supply of the mother
36.6 Embryonic Development
• Gastrulation occurs 10 to 11 days after
fertilization and involves certain groups of
moving inward from the surface of the
inner cell mass
 the lower cell layer of the blastocyst becomes
endoderm
 the moving cells differentiate into mesoderm
and endoderm
• they grow inward along a furrow called the
primitive streak
36.6 Embryonic Development
• Neurulation is the stage of development that
beings in the third week of embryonic
development
 it involves the three primary germ layers beginning
their development into the tissues and organs of the
body
• 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
Table 36.1 Stages of Mammalian
Development
36.7 Fetal Development
• Organogenesis, the process of forming body
organs, begins in the fourth week of pregnancy
 this is a crucial time during development because the
proper course of events can be interrupted easily
• alcohol use during pregnancy is one of the leading causes of
birth defects, producing fetal alcohol syndrome
 most spontaneous abortions (i.e., miscarriages) occur
during this period
36.7 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 embryo
Figure 36.20 The developing human
Figure 36.20 The developing human
36.7 Fetal Development
• The second trimester is a time of growth
 during the fifth month, the head and body
become covered with fine hair, called lanugo,
which will later be lost
 by the end of the six month, the fetus cannot
survive outside the uterus without special
medical intervention
36.7 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
• these tissues are called the decidua basalis
Figure 36.21 Structure of the placenta
36.7 Fetal Development
• Growth continues rapidly after birth
 different organs grow at different rates
• allometric growth refers to the fact that different
parts of the body grow or cease growing at
different times
• for example, the lower part of the human jaw
grows at a faster rate than the rest of the skull
36.8 Contraception and Sexually
Transmitted Diseases
• Birth control is a way to avoid reproduction
without avoiding sexual intercourse
 contraception methods differ from one another in their
effectiveness and their acceptability to different
couples
•
•
•
•
•
•
abstinence
prevention of egg maturation
prevention of embryo implantation
sperm blockage
sperm destruction
sterilization
Figure 36.23 Three common birth
control methods
36.8 Contraception and Sexually
Transmitted Diseases
• Sexually transmitted diseases (STDs) are
diseases that spread from one person to another
through sexual contact
 gonorrhea
• is caused by a bacterium, Neisseria gonorrhoeae
• produces symptoms of discharge from the penis or vagina
• in women, if left untreated, could lead to pelvic
inflammatory disease (PID), which could lead to sterility
36.8 Contraception and Sexually
Transmitted Diseases
• Chlamydia
 caused by the bacterium Chlamydia trachomatis
 women usually experience no symptoms until the
infection is established
 can also lead to PID
• syphilis
 caused by the bacterium Treponema pallidum
 the disease progresses in four stages following the
appearance of a small, painless legion, called a
chancre, on the penis or hidden in the vagina
36.8 Contraception and Sexually
Transmitted Diseases
• Genital herpes
 causes by the herpes simplex virus type 2
(HSV-2)
 the most common STD in the US
 causes red blisters on the penis or on the
labia, vagina, or cervix
• the blisters rupture and scab over
• the lesions heal but the virus travels to the dorsal
root ganglion by way of the sensory neurons where
they become dormant
Inquiry & Analysis
• How might
heightened public
awareness explain
why the trend in
levels of gonorrhea
differs from that of
chlamydia?
Graph of Trends in STDs in the
United States