Developmental Biology
Chapter 1 lecture 1
Chicken embryo’s include a big yolk, that is an oocyte, cleaving a single cell would take days. In yolky
cells, the cells only remain on top. In mammals the oocyte is small, there is no yolk, the embryo has
to develop in a womb of a mother. It receives nutrition’s and oxygen from the blood of the mother
and toxic and waist also goes into the blood of the mother. Developmental strategies depend on the
species.
The key point of developmental biology is to find out how a single cell without polarity can become a
grown organism. There are different developmental strategies. One of them is the fruit fly, where
hatching is very important (=uitkomen van dier/larve uit ei). During development, a few post-
embyronic processes are important, which include: metamorphosis, growth and aging.
Regeneration is also a part of developmental biology, meaning that new tissues and skin will form
just like embryogenesis. Changes in your DNA that are given to your children will have affect their
development. Different groups of animals and plants are related and have some similarities and
differences in developmental strategies.
Model systems
Instead of using human embryo’s for research in developmental biology, model systems are used.
There are different kinds of model systems: vertebrates, invertebrates, plants and embryonic stem
cells. As a model system for vertebrates, the frog is often used and so are the chicken and zebrafish.
Oocytes of frogs can be manipulated easily. Zebrafish are often used because the genetics are
known, because regeneration time is fast and because the oocytes and embryos are transparent so
easy to follow. There are some important models for invertebrates: C. elegans, Drosophila, Nassarius,
Arctonoe and spiders. Drosophila is used often because of the fast regeneration time. The C. Elegans
is used because of the fast development. Plants also have embryos, and Arabidopsis is most used as a
model.
Embryonic stem cells (ES) have the advantage that they can renew themselves and therefore can be
cultured in a petri dish. Because ES are pluripotent, they can form whatever cell type when they
receive the right trigger.
!! Understand and know the differences between the model systems and compare them: what are the
advantages and disadvantages.
Preformatism and epigenesis
In the 19th century there were different theories about development. Aristotle already came up with
ideas of preformatism and epigenesis.
Preformatism describes the idea that an embryo is already looking like the adult or very small human
being and only starts growing. No changes occur. A preformed embryo is called a homunculus.
Epigenesis describes the idea that the formation of an embryo looks like constant (re)modeling to
subsequently form an embryo more and more grown and human-like (epigenesis).
Marcello Malpighi saw that blood vessels were formed during chicken embryo forming and
confirmed epigenesis. When the understanding came that all animals and plants are made out of
cells, they realized that there was no preformatism. The development of cells starts with somatic
cells: an egg cell and a sperm cell. Weismann: he said there are somatic cells and germ cells.
Germ cells are n (haploid) and somatic cells are 2n (diploid). Germ cells are pronuclei. Fusion of two
germ cells is needed to form a diploid zygote: 2 x 1n = 2n. The difference between these two
,categories is that a mutation in one of the somatic cells will not affect your children/progeny. If there
is a mutation in the germ cells, the next generation will have this mutation too: either germ and
somatic cells.
Types of development
Mosaic
People thought that development could be mosaic or regulative. Experiments have been: after
cleavage of a two cell stage embryo with four nuclear determinants, the nuclear determinants will
divide over the daughter cells and all daughter cells will have different nuclear determinants: that is
why cells have different developmental properties. (Mosaic or patchwork of cells). Each individual
cell automatically develops in a specific cell type. Researcher Roux fertilized in vitro frog oocyte, a
two cell stage embryo. He heated a needle and he killed the nucleus of one of the two cells. One half
of the embryo developed, not a complete embryo, and this could be evidence for mosaic
development: segregation of nuclear determinants leads to different cell fates.
Mosaic development is also called autonomous specification: each cell develops autonomously
without help of other cells.
Dolly: the cloning of a sheep.
Dolly is created by extraction nuclei of somatic cells, re-fertilization has led to the development of a
new embryo. This means that segregation of nuclear determinants can not be true, because one
single cell can grow a whole new organism. This means that each somatic cell contains all needed
factors for development. Dolly therefore is evidence for the fact that mosaic development does not
occur.
Regulative
To determine what is the type of development of cells, sea urchins (=zee-egels) have been used. Sea
urchin larva are bilateral symmetrical and the two cell stage blastomeres were separated for this
research by Driesch. One cell mostly died, but when he succeeded and the cells remain intact, he got
a normal morula stage embryo and he got a normal larva but smaller. Two separate cells have
both formed a new embryo. That is different from the mosaic development, this is called regulative
development: embryos can regulative the missing parts. Half an embryo can regulate full
development. In the frog experiment of Roux the blastocytes ‘thinks’ they have a sister cell: when
this experiment is repeated with performing separation of the daughter cells, both cells will also
grow into a complete embryo.
Species use both mosaic as regulative development.
Regulative characteristics:
Development fate depends on the conditions of the surrounding: a cells’ fate is not fixed. It
depends on induction.
What is lost could be replaced. Regenerative development is also called conditional
specification.
A human example of regulative development: after fertilization and the cells are separated
you will get a monozygotic twin, they look similar but there are small differences. These
differences are due to development in other conditions or surrounded factors.
,Chapter 1 lecture 2
Context of development
In time, different processes occur:
1. Cleavages
2. Patterning or morphogenesis
3. Cell differentiation
4. Growth
These processes support the formation of an embryo/foetus.
Types of cleavages
There are different types of cleavages: radial (sea urchin), unequal (C. elegans) and spiral cleavage
(Annelids and Molluscs).
- In embryos of the sea urchin, all the blastomeres are of the same size and at the 8 cell stage
all cells lie exactly on top of each other. This is called radial cleavage.
- In the 4 cell stage in C. elegans, one cell is larger than the other, small cells lying on top of
large cells, small cells micromeres and big cells are macromeres, unequal cleavage. This can
already be seen at the 2 cell stage, where a macromere and micromere cell develop. By
staining the DNA in C. elegans the cleavages can be followed.
- In cleavage where small cells lie twisted to one side between the big cells, the twisting is left
right left right etc, this is called spiral cleavage (annelids and molluscs.)
- Unequal cleavage thus gives to two cells unequal in size. There can also be asymmetric
cleavage, this means that it gives rise to two cells with different developmental properties.
- Another type of cleavage is rotational cleavage: first cleavage is equal but in the left cell the
cleavage plane is ventral and in the other cell is horizontal. This is holoblastic (complete)
cleavage. Rotational cleavages occur in mammals!!
The type of cleavage is determined by the amount of yolk. In amphibia, there is not so much yolk,
resulting in complete and also unequal cleavages. This is called: holoblastic cleavages. don’t have a
huge amount of yolk, occurs in amphibia, complete cleavages which are unequal. In fishes or chicken
the amount of yolk is very high, and therefore it is impossible to cleave through the whole oocyt: so
only the cells on top of the yolk are being cleaved. Cleavages are incomplete, this is called
meroblastic cleavages.
Patterning or morphogenesis
This explains how from 1 cell stage embryo you get body axis with anterior side (head) and the
posterior side (tail) and there is a back side (dorsal) and a belly side (ventral). You also have a left
right axis. When gastrulation (movement of cells) occurs, the outside of the embryo will be covered
by ectoderm and the inside forms a digestive tube which exists of endoderm. The mesoderm layer is
in between of these two. Because of programmed cell death, an organism has their fingers more
attached to each other than the other. Apoptosis can lead to the development of fingers: this is for
example a difference between a duck and a chicken.
Also, migration of neural crest cells (neurale lijst cellen) from the neural tube to the face is a form of
patterning. These cells start to form the muscles and bones of the face.
Transcription factors can regulate the expression of genes. Some of these proteins can influence
DNA. On different levels gene expression can be regulated: DNA, (pre)mRNA, protein. These different
levels can also change a cells’ fate.
Genetic feedback loops can be positive or negative.
- An activator allows transcription of gene 1, which activates gene 2 and this makes a protein
that can stimulate activation of gene 1.
- In a negative feedback loop, it is the same story but the protein made by gene 2 now silences
the gene activity of gene 1 and the proteins aren’t produced anymore.
, Expression of genes can also change: there are changes in time (temporal gene expression), changes
dependent on position (spatial gene expression) and changes in time and position (spatio-temporal
gene expression). Spatial gene expression means that at one side different genes are active than in
the other side. When study the change in time and space, this is called spatio-temporal gene
expression: and after the cells are located it’s only a matter of growth. Also growth can be regulated.
At certain times, certain parts of the body grow faster than other. For example, the head starts to
grow very fast first.
Progressive determination of cell fate
From an undifferentiated state to a differentiated state, transcription factors are important. A cell
doesn’t get his destination in one discussion, it is a gradual process. At first the undifferentiated cell
has to decide that is has to be a muscle cell, this is dependent of a lot of factors in time that
determine what the cell will become . This is called a progressive determination of cell fate.
Specification is autonomous differentiation in a neutral environment. When adding oxygen and
nutrients, the development of a cell into its purpose can be seen. If you extract a cell of its tissue and
place it another tissue type of the embryo (change of surroundings), it will form what it had decided
to be: an irreversible commitment has taken place. A change of surroundings will not affect its
purpose anymore. This is called determination. If the cell gets final ‘visible’ changes, it has reached
the state of differentiation.
Overt: specification, determination. Overt means that something is clearly apparent or obvious.
Covert: differentiation. Covert describes a secret or not openly acknowledged process.
In time, overt processes will change to covert processes and this will lead to progressive
determination.
Region A will form different cells than region B. When you put a cell of region B in region A, you can
see whether a cell has been committed or not yet. If the cell adjusts into the environment in A, the
cell was not determined yet. If the cell remains what it would become in region B, then we say the
cell already decided and the cell is determent.
If you transplant the region that will form the eye to another place, then you see that it adjusts to the
environment. If you transplant the part of cells in the neurula stage cells that later form the eye and
you transplant it to other place it will develop an eye because it is already determined.
Positional information: mechanisms
Positional information describes how cells know where they are. Spatio-temporal expression of
genes along an axis in the embryo is important. There are 4 known mechanisms of positional
information:
1. Gradient model
There is a source that produces a molecule/protein that has a high concentration on the anterior side
and a low concentration in the posterior side. This called the gradient of morphogen. If the
concentration is higher than the threshold it will become cells with the blue color. Then there is an
second threshold, if it is lower than this one it will become red and everything in-between will
become white. It depends on setting a gradient, this can be established by diffusion, direct contact or
a gap junction. The gradient is also responsible for cell signaling and can be passed from one cell to
its adjacent cells.
2. Asymmetry of cytoplasmic determinants
The dots in the figure are represent molecules outside the nucleus that are divided asymmetrically.
These cytoplasmic determinants control cell division and this leads to different daughter cells. When
the oocyte of the xenopus is stained immediately after fertilization for maternal factors, which are
