Chapter 10
Animal Growth and Development
Fertilization
• The beginning of new animals begins with
fertilization.
• 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
Fertilized Egg
Growth, Differentiation and
Form
• Differentiation is the process in which cells
become specialized or different from other
cells.
• Morphogenesis is when the specialized cells
form organs and tissues.
Development in Animals
• Cleavage is the first step in development of
ALL multi-celled organisms. Cleavage
converts a single-celled zygote into a multicelled embryo by mitosis.
• Frog embryos divide to produce 37,000
cells in a little over 40 hours.
• By the end of cleavage the embryo consists
of many cells and is called a blastula.
Blastula
Gastrula
• As cells differentiate, they begin to migrate.
• This forms three layers of cells and the
embryo is now called a gastrula.
• These three layers are called germ layers
and will form the major structures of the
body.
• Gastrulation forms the ectoderm, endoderm
and mesoderm.
• Ectoderm forms tissues associated with
outer layers: skin, hair, sweat glands,
epithelium. The brain and nervous system
also develop from the ectoderm.
• 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.
• 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.
Gastrulation
Pattern Formation
• Blastulation and gastrulation establish the
main body axis. Organ formation occurs in
the next stage of the development of the
embryo. During organ formation, cell
division is accomplished by migration and
aggregation.
• Pattern formation is the result of cells
"sensing" their position in the embryo
relative to other cells and to form structures
appropriate to that position.
• Homeobox genes are pattern genes; they
coordinate with gradients of information
molecules to establish the body plan and
development of organs.
Homeobox Genes
Induction
• Induction is the process in which one cell or
tissue type affects the developmental fate of
another cell or tissue. As a cell begins to
form certain structures, certain genes are
turned on, others are turned off. Induction
affects patterns of gene expression through
physical contact or chemical signals.
Formation of the vertebrate eye is a well
known example.
Human Development
• The period of time from fertilization to birth
(usually 9 months) is divided into
trimesters, each about three months long.
During pregnancy the zygote undergoes 40
to 44 rounds of mitosis, producing an infant
containing trillions of specialized cells
organized into tissues and organs.
• After fertilization in the oviduct, the embryo
begins to cleave as it moved down the
oviduct.
• After 5 days it is called a blastocyst. It looks
like a blastula.
• The blastocyst sinks into the uterine lining
once it reaches the uterus. This is known as
implantation.
Useful Terms
• Some of the blastocyst forms the new baby and
some forms the assessory structures that provide
nourishment.
• The amnion is a membrane that surrounds the
embryo.
• The chorion is just to the outside of the amnion.
• As gastrulation begins, the chorion extends villi
into the lining of the uterus. The villi and uterine
lining form the placenta.
First Trimester
• The three embryonic tissue layers form. Cellular
differentiation begins to form organs during the
third week. After one month the embryo is 5 mm
long and composed mostly of paired somite
segments. During the second month most of the
major organ systems form, limb buds develop. The
embryo becomes a fetus by the seventh week.
Beginning the eighth week, the sexually neutral
fetus activates gene pathways for sex
determination, forming testes in XY fetuses and
ovaries in XX fetuses. External genitalia develop.
Second Trimester
• The fetus increases in size during this
trimester, and bony parts of the skeleton
begin to form. Fetal movements can be felt
by the mother.
Third Trimester
• During this trimester the fetus increases in
size. Circulatory and respiratory systems
mature in preparation for air breathing.
Fetal growth during this time uses large
parts of its mother's protein and calcium
intake. Maternal antibodies pass to the fetus
during the last month, conferring temporary
immunity.