2. Between
fertilization
and birth, we
are an
embryo
We build ourselves from a single cell
This process is called development
When an egg first gets fertilized, it is called a
zygote
Cells then rearrange in a process called
gastrulation
They also begin to develop organs
(organogenesis)
Our cells also go through differentiation
3. The Cycle of Life
• The following cycle is a generalized life
cycle. Various organisms have modifications
• However, most incorporate this process
1) Fertilization: the joining of the pronuclei
of the sperm and egg (gametes)
2) Cleavage: after fertilization, the zygote is
divided into smaller blastomeres. It
starts to cleave in other words. The
resulting sphere is called a blastula look
at day 1 vs. day 2 on the next slide)
4.
5. Cycle of Life
(continued)
3) Gastrulation occurs when the
blastomeres reposition themselves and
form Germ Layers. (look at day 14 on
the previous slide)
4) Organogenesis: The germ layers
(usually three) start to form organs.
5) In some organisms who are not born
sexually mature (without their sex
organs), metamorphosis is necessary. A
common example is in a butterfly
6. Cycle of Life
(continued)
6) Germ Cell (don’t confuse with
Germ layer) Formation: Also known
as (gametogenesis), some cells are
set aside from the rest of the
somatic cells. Germ cells will move
to the gonads.
7. Example
of a Frog
• Gametogenesis and Fertilization
• These processes are controlled by
various environmental factors such as:
• Photoperiod (the length of
daylight)
• Temperature
• Rainfall
• These environmental factors cause certain
hormones to be released from the frog’s
pituitary gland. So, for frogs, and any other
organisms, this is a seasonal process.
• Like humans, the two haploid nuclei join to
make a diploid zygote nucleus
8. Example of a Frog (continued)
• Cleavage and Gastrulation
• The volume of the egg stays the same during
cleavage. In other words, smaller cells are being
produced
• During gastrulation, a blastopore forms opposite
the point where the sperm entered.
• Cells moving through this blastopore become the
internal germ layers (endoderm and mesoderm)
• The remaining cells on the outside become the
ectoderm
9. Example of
the Frog
(continued)
• Organogenesis
• Cells on the dorsal part of the
mesoderm (middle germ layer) form
a rod which is called the notochord.
• The notochord will influence cells
above it in the ectoderm to become
nervous tissue. This will form the
neural tube
• The neural tube formation will, in
turn, influence other organ
formation.
10. Example of the Frog
(continued)
• Metamorphosis and Gametogenesis
• The change from tadpole to frog requires changes in most all parts of the organism and is therefor a great
example:
• Paddle tail disappears
• Limbs form
• Mouth changes from plant to fly eating (as well as tongue development)
• Gills disappear and lungs form
• This is not just about sexually mature organs, but the tadpole will not survive winter and will need to
burrow in the mud. Therefore, the speed of this change depends on the environment.
11. Other organism
examples used
in
developmental
biology
The frog is one of the organisms often used in providing
examples of development. There are others as well.
Arabidopsis thaliana is a common weed which
developmental biologists know a lot about. Its
reproductive process is similar to most other plants. We
use this species as a base (other plants might have
modified versions
Drosophila melanogaster is the common fruit fly. Its
genome is well known and reproduces very quickly
12. Sanjay Acharya
Marco Roepers
The photo above is of the fruit fly D.
melanogaster. The photo to the right is the
Thale Cress (A. thaliana)
13. Overview of
Early Animal
Development
• Patterns of Cleavage- Often, blastomeres form in
the zygote without an increase in the cytoplasm.
• In other words, the cells get smaller as they
exponentially grown in number
• At some point, the cells will finally grow in size.
This is limited by a couple of factors:
• The inherited patterns of cell division
(what’s the instruction in your genes)
• The amount of yolk in the cytoplasm and its
location.
• This determines where cleavage can
occur. The yolk impedes cell division.
Cells tend to grown better on the
opposite side.
14. Gastrulation-
• Germ layers are formed in this process. Think of your body in three layers:
• The most outer part of your body is your skin
• There is an inner part which is all the tubes (like stomach, intestine, trachea, etc)
• The part in between these is the middle layer.
• Endoderm- innermost layer forms the body’s tubes and lungs
• Mesoderm- heart, kidney, muscles, etc.
• Ectoderm- Skin, nervous system, brain
So early on in gastrulation, we can understand which organs are going to
form in organogenesis due to the germ layer developed in gastrulation.
15. Michaels, Chinami, "Some of the Cells that Arise from Animal Gastrulas with Three Germ Layers". Embryo Project Encyclopedia (2014-08-21). ISSN: 1940-5030 http://embryo.asu.edu/handle/10776/8153.
18. Evolutionary Embryology
• Embryos of varying animals have similar forms. There is a law which summarizes this called Von Baer’s Law:
• The general features of a large group of animals appear earlier in development than do specialized features of a
smaller group.
• All vertebrates look similar after gastrulation
• Less general characteristics develop from the more general
• As we get further along in the developmental process, greater differences appear
• The embryo of a given species, instead of passing through the adult stages of lower animals, departs more and
more from them
• We start looking different before we are born
• Therefore, the early embryo of a higher animal is never like a lower animal, but only like its early embryo
• We are only like each other in the early stages
20. Evolutionary Embryology (continued)
• To Summarize Von Baer: there is a phylotypic stage in which various vertebrates
have a similar structure and the least amount of genetic variation.
• Could this be a basic vertebrate body plan?
• Is this evidence of evolution as Darwin and others thought?
• There are also homologies. This is when we can find similar structures in different
animals. The classic example is of the forearm. Similar bone layout can be found in
organisms like bats, whales, and humans.
• This is not to be confused with analogous similarities where similar environmental
pressures lead to similar adaptations.
22. Medical
Embryology
• Genetics
• Malformations are abnormalities caused by
genetics. If there is more than one
abnormality, we call it a syndrome
• Disruptions and Teratogens- a disruption is
caused by an outside agent
• Chemicals, radiation, etc.
• The agent that causes it is called a teratogen
