Essentials of Biology
Sylvia S. Mader
Chapter 29
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
29.1 How Animals Reproduce
• Animals generally reproduce sexually (two
parents), but some can also reproduce
asexually (one parent).
– Fragmentation is a mechanism whereby a
new organism can regrow from a fragment of
the original organism.
– Parthenogenesis is a modified form of sexual
reproduction where an unfertilized egg
becomes a whole individual.
Asexual Versus Sexual
Reproduction
• Sexual reproduction involves the gameteproducing gonads of animals.
– The male sperm-producing testes
– The female egg-producing ovaries
• Copulation brings these haploid gametes
together, allowing fertilization to produce a
diploid zygote.
• Some animals are hermaphroditic, having the
reproductive organs of both male and female.
Reproduction in Water Versus on Land
• In most aquatic organisms, external fertilization
occurs and the resulting organism is free-living.
• On land, evolutionary modifications arose to
provide the embryo and nutrients and to prevent
dehydration.
– Extraembryonic membranes are present in the eggs
of birds and reptiles.
– The mammalian placenta, comprised of shared
tissues between the mother and fetus, provides
nutrients, removes wastes, and prevent dehydration.
29.2 Human Reproduction
• The human reproductive tract consists of
two components.
– The gonads.
– The accessory organs that assist reproduction
or, in females, house the embryo.
Male Reproductive System
• There are several components to the male
reproductive system.
– The testes produce the sperm.
– The epididymis is a coiled tubule that receives
sperm produced by the testes.
– The vas deferens stores sperm and conducts
the sperm to the urethra in the penis.
– The penis is a muscular accessory organ that
facilitates transfer of sperm to the female.
Male Reproductive System (cont.)
Male Reproductive System (cont.)
• The sperm is carried in a fluid (semen)
comprised of products from three glands.
– The seminal vesicles produces a fluid that
nourishes the sperm.
– The prostate gland produces a fluid that is
believed to enhance sperm motility.
– The bulbourethral gland produces a
lubricating secretion to facilitate copulation.
Male Reproductive System (cont.)
Male Reproductive System (cont.)
• The release of sperm during copulation
occurs in two phases.
– During emission, the secretions of the three
glands are released as sperm enter the
ejaculatory ducts.
– During expulsion, rhythmic contractions of
muscle expel the semen from the penis.
The Testes
• The testes are
comprised of coiled
seminiferous tubules.
• Sperm production
(spermatogenesis)
occurs continually
within the
seminiferous tubules.
The Testes (cont.)
• The sperm produced
have three distinct
parts.
– The head, carrying the
genetic material
(acrosome)
– The tail, which
provides movement
– The body, which
houses mitochondria
to provide energy for
movement
Hormonal Regulation in Males
• Several glands and hormones regulate sexual
function in males.
– The hypothalamus controls the secretion of luteinizing
hormone (LH) and follicle-stimulating hormone (FSH)
by the anterior pituitary.
– The anterior pituitary also produces FSH to enhance
spermatogenesis.
– The anterior pituitary produces LH to promote
testosterone production.
– Testosterone, produced by interstitial cells, is
essential for normal sexual function.
Female Reproductive System
• There are several components to the
female reproductive system.
– The egg-producing ovaries
– The oviducts (fallopian tubes), which conduct
the egg to the uterus
– The uterus, where development of the embryo
occurs.
– The cervix and vagina provide openings
through which sperm enter during copulation.
Female Reproductive System
(cont.)
The Ovaries
• The ovaries are composed of millions of follicles
that contain oocytes for production of eggs.
• When mature, the egg from a follicle erupts and
is released (ovulation).
• After ovulation, the follicle develops into the
corpus luteum, which degenerates if fertilization
does not occur.
The Ovaries (cont.)
The Ovaries (cont.)
• The ovarian cycle can be divided into two
phases.
– The follicular phase, stimulated by FSH from
the anterior pituitary, results in maturation of
the follicle and release of estrogens
– The luteal phase, stimulated by LH from the
anterior pituitary, results in development of the
corpus luteum and release of progesterone.
The Ovaries (cont.)
The Ovaries (cont.)
• The interactive effects of estrogens and
progesterone on the uterus produce the
menstrual cycle.
• The follicular phase occurs during the first 14
days of the menstrual cycle.
– From days 1-5, hormone levels are low, causing the
endometrium to be sloughed from the uterus
(menstruation).
– During days 6-13, release of estrogen promotes
development of the endometrium (proliferative
phase).
The Ovaries (cont.)
• Ovulation occurs around day 14.
• The luteal phase occurs during the last 14
days of the menstrual cycle.
– During days 15-28, progesterone promotes
development of the endometrium to prepare
for a fertilized embryo (secretory phase).
– If a fertilized embryo does not implant,
endometrium degrades and menstruation
occurs.
The Ovaries (cont.)
Control of Reproduction
• There are several techniques available to
prevent pregnancy.
– Abstinence (avoid copulation)
– Oral contraceptives (inhibition of FSH and
LSH synthesis)
– Contraceptive implants (disrupt ovarian cycle)
– Intrauterine devices (alter uterine conditions)
– Barrier methods (block sperm)
– Spermicides (kill sperm)
Control of Reproduction (cont.)
Infertility
• Infertility is a failure to achieve pregnancy.
• In the US, about 15% of couples experience
infertility problems.
– 40% of cases are attributed to the male.
– 40% of cases are attributed to the female.
– 20% of cases are attributed to both.
• One means of overcoming infertility is through
assisted reproductive technologies (ART).
Assisted Reproductive
Technologies
• Artificial insemination by donor involves insertion of
harvested sperm into the vagina.
• For in vitro fertilization, an egg is fertilized outside
the woman’s body and implanted later.
• Gamete intrafallopian transfer is the same as in vitro
fertilization except that the egg and sperm are placed
into the fallopian tubes immediately.
• A single sperm is injected directly into an egg in
intracytoplasmic sperm injection.
Sexually Transmitted Diseases
• There are several causes of sexually
transmitted diseases (STDs).
– Viruses are the cause of AIDS, genital warts,
genital herpes, and a form of hepatitis.
– Bacteria can cause chlamydia, pelvic
inflammatory disease, gonorrhea, and
syphilis.
– Yeast and protozoa can cause infections.
29.3 Human Development
• Human development includes all of the
events from fertilization to the full
formation of a child.
• The stages of development in humans are
similar to those in other land animals,
including the presence of nourishing
extraembryonic membranes.
29.3 Human Development
(cont.)
• There are four extraembryonic
membranes.
– The chorion is part of the placenta and
facilitates exchange of gases, nutrients, and
wastes.
– Blood cells form in the yolk sac.
– The blood vessels of the allantois are part of
the umbilical cord.
– The amnion contains fluid that cushions and
protects the embryo.
29.3 Human Development
(cont.)
Fertilization
• Fertilization of an egg by sperm forms a
diploid zygote.
– The acrosome of a sperm secretes enzymes
that digest the zona pellucida of the egg.
– The sperm then binds to the plasma
membrane of the egg and enters the cell.
– The genetic information of the sperm fuses
with the egg nucleus to complete fertilization.
Early Embryonic Development
• The embryonic development involves
developmental events prior to
implantation.
– Ovulation releases a mature egg.
– Fertilization occurs to form a zygote.
– The zygote undergoes cleavage (cell division)
to form a morula.
– The morula develops into a fluid-filled ball of
cells called a blastocyst.
– The blastocyst implants in the uterus.
Early Embryonic Development
(cont.)
Later Embryonic Development
• The next step in development is
gastrulation.
– Cells of the embryo begin to invaginate.
– The amnion begins to develop.
– After the initial development of the outer
ectoderm and inner endoderm, the mesoderm
develops.
– Gastrulation is complete when the three germ
layers are present.
Later Embryonic Development
(cont.)
Later Embryonic Development
(cont.)
Neurulation
• The nervous system forms during the
neurulation by induction.
– The mesoderm cells form a dorsal supporting
rod called the notochord.
– The neural plate forms dorsal to the
notochord.
– The neural tube forms as the neural plates
begin to fold.
– Other mesoderm cells consolidate into
somites to give rise to vertebrae and muscles.
Neurulation (cont.)
Organ Formation Continues
• Human embryos at five weeks have flippers
called limb buds which give rise to arms and
legs.
• At the same time the head and sensory organs
develop.
• The umbilical cord has formed, connecting the
embryo to the chorion.
• The allantois forms and the connection to the
placenta is completed.
Organ Formation Continues
(cont.)
Placenta
• The placenta is comprised of the embryonic fetus
and the uterine wall.
• The chorionic villi of the chorion project into the
uterine wall to facilitate exchange between fetus and
mother.
• The umbilical cord carries fetal blood to and from the
placenta.
• While fetal and maternal blood do not mix, a variety
of chemicals can cross the placenta into the fetus.
Placenta (cont.)
Fetal Development and Birth
• Human fetal development occurs during
the last six months of gestation.
– The fetus increases in size and weight.
– The genitalia develop during the third month.
– Hair develops after the third month.
– The heartbeat can be detected after 2.5
months.
– Fetal movement begins.
The Stages of Birth
• Birth is triggered when the fetal hypothalamus
triggers the pituitary to stimulate the release of
androgens into the bloodstream.
• The placenta uses the androgens to form
estrogens, which stimulate formation of oxytocin
and prostaglandin.
• The estrogens, oxytocin, and prostaglandin
cause the uterus to contract.
The Stages of Birth (cont.)
• The movement of the fetus out of the
uterus is called parturition.
– During the first stage of parturition, the cervix
dilates and the amnion bursts.
– The baby is born and the umbilical is severed
during the second stage.
– The placenta (afterbirth) is expelled from the
uterus in the third stage.
The Stages of Birth (cont.)