Animal Reproduction and Development: A Summary
AP Biology
Spring 2011
Sexual vs. Asexual Reproduction

Asexual reproduction
 Useful strategy in stable
environment
 Asexual reproduction by
fragmentation, budding
(ex. Sponge) or
parthenogenesis results
in offspring identical to
parents
Costs of Sexual Reproduction
Sexual reproduction permits adaptation
through variations but is biologically costly
because sexes are separate
 Animals must produce gametes and find
each other (usually) for fertilization to occur
 Through sexual reproduction, offspring are
more likely to have a gene combination
that is suitable to their new environment

Costs of Sexual Reproduction
Variations on Sexual
Reproduction
Some animals, tapeworms and
roundworms, are hermaphrodites
producing both eggs and sperm at the
same time
 More typically, vertebrates have
separate sexes that are fixed for lifeeither male or female

Variations on Sexual
Reproduction
Most land animals have internal fertilization, while
most marine animals use external fertilization
 Offspring may develop inside mother or outside of
maternal body
 Most female mothers invest in yolk to help nourish
developing young

Stages of Reproduction and
Development

Gamete formation
 Eggs or sperm form and mature within the
parents
Stages of Reproduction and
Development

Fertilization
 Begins when a
sperm penetrates
an egg and is
completed when
the sperm nucleus
fuses with the egg
nucleus, resulting
in formation of
zygote
Stages of Reproduction and
Development

Cleavage
 Repeated mitotic divisions
 Convert zygote to a blastula
 Cell numbers increase but not cell size
 Cleavage is over when blastula forms
 Blastula’s cells is blastomeres, encloses a
fluid filled cavity the blastocoel
Stages of Reproduction and
Development

Gastrulation
 Blastula enters gastrulation
 Results in 3 germ layers or tissues
 Ectoderm: outer layer, gives rise to nervous
system and outer layers of the integument
 Endoderm: inner layer, gives rise to gut and
organs derived from it
 Mesoderm: middle layer, muscle, organs of
circulation, reproduction, excretion, and
skeleton derived from it
Zygote  morula  blastula
 gastrula
Stages of Reproduction and
Development

Organ formation
 Organ formation begins as germ layers
subdivide into populations of cells destined
to become unique in structure and function
Stages of Reproduction and
Development

Growth, tissue specialization
 During growth and tissue specialization,
organs acquire specialized chemical and
physical properties
Early Marching Order- Information
in the Egg
Sperm contributes little more than the
paternal DNA
 The oocyte contains the majority of
materials that will affect early
development
 Penetration of egg by sperm triggers
structural reorganization in the egg
cytoplasm

Early Marching Order- Information
in the Egg
In a frog egg, microtubules move
granules from the animal pole to form a
gray crescent near the equator opposite
penetration site
 Near the crescent, body axis of frog
embryo will become established and
gastrulation will begin

Early Marching OrderInformation in the Egg
Early Marching Order- Cleavage
Cleavage divides up maternal cytoplasm
 After fertilization, the zygote begins a
series of divisions in which each cell is
pinched into two cells (blastomeres)
 This process is NOT random and
different blastomeres will end up with
different genetic messages in a process
known as cytoplasmic localization

Early Marching Order- Cuts

Orientation:
 Radial cleavage starts with cuts
perpendicular to mitotic spindle, producing
cell of similar size but with different parts of
cytoplasm
 In other cells (frogs) cuts do not go all of the
way through, so the cells produced are
smaller at the animal pole
Early Marching Order- Cuts

Orientation
 In mammals, rotational cleavage results in
an inner cell mass (future embryo), which
forms on the inside of a hollow sphere
Early Marching Order- Cuts

Complete and Incomplete:



The amount of yolk stored inside the egg also
affects cleavage patterns
Incomplete: when an abundance of yolk
impedes the cytoplasmic division, like in the
insects, reptiles, birds, and most fishes
Complete: when little yolk is present, the first
cut divides all the cytoplasm in complete
cleavage (amphibians and mammals)
Structure of Blastula
Blastula structure varies with species cleavage
patterns
 In sea urchins, complete cleavage
results in blastula that is a hollow
ball
 In highly yolky eggs (birds and
some fish), a blastodisk forms on
top of the blastula
 In mammals, early embryo is
called blastocyte

Zygote
Blastula
Morula
Gastrulation
From Grastrula to Blastula
A hundred to thousands of cells may
form at cleavage- depending on species
 Starting with grastrulation, cells migrate
about and rearrange themselves

 In most animals, the small ball of cells
formed at cleavage develops into a grastrula
with three distinct germ layers; ectoderm,
mesoderm, endoderm
 Specific patterns of cell migration occur
within the gastrulation process
Grastulation
ectoderm
mesoderm
endoderm
From Grastrula to Blastula

Gastrulation proceeds through
embryonic induction which is the
process in which developmental fates of
embryonic cell lineages change when
exposed to signals (gene products) from
adjacent tissues
Cell Differentiation

From grastrulation onward, cell lineages
also engage in selective gene
expression, which is the start of cell
differentiation
Morphogenesis

Morphogens are signaling molecules
produced by master genes
 They diffuse out and form a concentration
gradient in the embryo
 A morphogen’s effect on target cells is
proportional to its concentration
Morphogenesis and Pattern
Formation
Morphogenesis is the progression of
differentiated cells into tissues and
organs; it is the result of several events
 Sheets of cells expand and fold as cells
change shape forming organs such as
the neural tube

Morphogenesis and Pattern
Formation
Programmed cell death helps sculpt
body parts
 Controlled cell death, called apoptosis,
is genetically programmed elimination of
tissues and cells that are used for only
short periods in the embryo or adult
 Pattern formation is the process by
which certain body parts form in a
specific place

Pattern Formation
Embryonic induction: developmental
fates of embryonic cell lineages change
when exposed to signals- gene
products- from adjacent tissues
 Pattern Formation: a sculpting of
specialized tissues and organs from
clumps of cells in the proper places in
the embryo, in the proper order

Pattern Formation

Theory of pattern formation
 The formation of tissues and organs in
ordered, spatial patterns
 Morphanogens and other inducer
molecules diffuse through embryonic
tissues, activate master genes
 Products of homeotic genes and other
master genes interact with control elements
to map out the overall body plan
Pattern Formation

Evolutionary constraints on development
 The basic body plans of the major animal
groups have not changed due a limited number
of master genes
 These genes have imposed phyletic constraints
in addition to the more well-known physical and
architectural constraints
○ Physical constraint: surface area to volume ratio
○ Architectural constraint: imposed by body axes
○ Phyletic constraint: imposed by interactions
among genes that regulate development in a
lineage
Age and Death
Aging may be partly a result of time
running out of internal biological clocks,
which are genetically preset
 Aging also may be partly an outcome of
cumulative assaults on DNA and other
biological molecules during the life cycle
