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Chapter 25:
Animal Reproduction
And
Development
There are two types of reproduction in
animals – sexual and asexual.
• Asexual reproduction is
more efficient than
sexual reproduction.
• There is no waste of
gametes that fail to unite
to form an offspring.
• Many animals reproduce
asexually, at least some
of the time.
• Sexual reproduction has
the advantage of genetic
recombination, which can
lead to more adaptation of
the species.
• Fertilization may be
internal, as with humans,
or external, as with many
fish species.
Asexual Reproduction
• In asexual reproduction, there is no fusion of
sperm and egg
• A common form of asexual reproduction is
budding, where a miniature version of the animal
grows directly on the body of the adult, drawing
nourishment from the parent. When it is large
enough, it breaks off, becoming independent.
• The females of some species can reproduce by a
process in which haploid egg cells develop into
adults without being fertilized. Males are not
necessary to the survival of the species.
Sexual Reproduction
• Spermatogenesis, or the production of sperm, does
not begin until males reach puberty. At that time,
cells in the testes begin producing testosterone, the
hormone that initiates spermatogenesis and
stimulates the development of secondary sexual
characteristics.
• Oogenesis, the formation of egg cells, actually
begins in the developing ovaries of the female
fetus. By the time a girl is born, she has all the
oocytes she will ever produce. By puberty, only
about 400,000 oocytes remain of the original 2
million, and they mature into eggs.
Steps in Human Reproduction
• Copulation
• Fertilization
• Implantation
• Gestation
• Birth
• An end to life as you knew it!
Fertilization has three functions:
1. transmission of genes from both parents to
offspring
2. restoration of the diploid number of
chromosomes reduced during meiosis
3. initiation of development in offspring
Steps in Fertilization
• Contact between sperm and egg
• Entry of sperm into the egg
• Fusion of egg and sperm nuclei
• Activation of development
Cleavage is the first step in development of ALL multi-celled
organisms. Cleavage converts a single-celled zygote into a
multi-celled embryo by mitosis. Usually, the zygotic
cytoplasm is divided among the newly formed cells. Frog
embryos divide to produce 37,000 cells in a little over 40
hours.
The blastula is produced by mitosis of the zygote, and is a ball
of cells surrounding a fluid-filled cavity (the blastocoel). The
decreasing size of cells increases their surface to volume ratio,
allowing for more efficient oxygen exchange between cells
and their environment. RNA and information carrying
molecules are distributed to various parts of the blastula, and
this molecular differentiation sets the stage for the layering of
the body in the next phases of development.
Cleavage of the zygote begins while it is still in
the oviduct, producing a solid ball of cells
(morula). The morula enters the uterus,
continuing to divide and becomes a blastocyst
Implantation
The uterine lining becomes enlarged and prepared for
implantation of the embryo in the trophoblast layer.
Twelve days after fertilization, the trophoblast has
formed a two-layered chorion. Human chorionic
gonadotropin (hCG) is secreted by the chorion, and
prolongs the life of the corpus luteum until the placenta
begins to secrete estrogen and progesterone. Home
pregnancy tests work by detecting elevated hCG levels
in the woman's urine.
Gastrulation involves a series of cell
migrations to positions where they will
form the three primary cell layers.
•The ectoderm forms the outer layer.
•The endoderm forms the inner layer.
•The mesoderm forms the middle layer.
Ectoderm
The ectoderm forms tissues associated with outer
layers: skin, hair, sweat glands, epithelium. The brain
and nervous system also develop from the ectoderm.
Mesoderm
The mesoderm forms structures associated with movement
and support: body muscles, cartilage, bone, blood, and all
other connective tissues. Reproductive system organs and
kidneys form from mesoderm.
Endoderm
The endoderm forms tissues and organs
associated with the digestive and respiratory
systems. Many endocrine structures, such as
the thyroid and parathyroid glands, are
formed by the endoderm. The liver, pancreas,
and gall bladder arise from endoderm.
So what happens when there is no fertilization of the egg?
1. follicle stage (10-14 days average duration) - production of
ova/eggs occurs in tiny cavities in the ovary called follicles;
enlarging follicle produces estrogen causing the uterus to get
ready for embryo implantation (uterus thickens its lining)
2. ovulation (1 day) - follicle enlarges and ruptures ovary
wall; egg is released to the oviduct (usually only 1 is released
at a time)
3. corpus luteum stage (10 -14 days average duration) yellow tissue fills the follicle after ovulation called the
corpus luteum; "yellow body" secretes progesterone which
maintains the thickness of the uterus in case a pregnancy
occurs
4. menstruation (3-5 days average duration) - periodic
shedding of the thickened lining of the uterus
Women’s Reproductive Health Concerns
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Endometriosis
Breast Cancer
Ovarian Cancer
Cervical Cancer
Premenstrual Syndrome
Premenstrual
Dysphoric Disorder
• Dysmennorhea
• Polycystic Ovarian
Syndrome
• Fibrocystic Disease
• Abnormal Bleeding
• Unplanned Pregnancy
• Miscarriage
• Multiple Births
• Sexually Transmitted
Diseases
Sex, pregnancy, and childbirth
can all be wonderful, beautiful
experiences. The trick is
approaching all of them with your
eyes wide open, and having as
much knowledge as possible.
Chapter 25 Credits/Resources
The Reproductive System
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookREPRO
D.html#Fertilization%20and%20Cleavage
The Visible Embryo
http://www.visembryo.com/baby/index.html
Regents Prep: The Human Reproductive Cycle
http://regentsprep.org/Regents/biology/units/reproduction/human.cfm
Washington State Midwives Reproductive Health Care Resources and
Websites
http://www.midwivesofwa.org//reprodhc.htm
Webshots
www.webshots.com