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Full summary molecular principles of development
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Full summary for the course molecular principles of development (NWI-BB084B) - Radboud University, Nijmegen
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Molecular Principles of Development7-11-2020 – Basic Concepts of Development
Introduction
Forward genetics is a method where they induce random mutations and then screen for the
phenotypes of interest. They then link the phenotype of interest back to the mutated gene.
The important genes in the genetic model system of drosophila are maternal, gap, pair-rule and
segment polarity genes. The loss of gap gene results in a reduced number of segments; the loss of a
pair-rule gene (e.g. even-skipped) results in only odd-numbered segments to develop; the loss of
segment polarity genes results in segments with similar head and tail ends.
The homeotic genes such as HOX are highly conserved and all have the same function. They play a
role in genomic position and timing of expression. It is expressed along the anterior-posterior (A-P)
axis. Homeotic transformations lead to switched identity, meaning that when these genes are
dysregulated, the cell loses its knowledge on its location and will start differentiating differently.
Overview of Development, Model System Life Cycles
Drosophila start off as a polar egg cell and
form a syncytium during early
development. A syncytium refers to a cell
with many nuclei that share the same
cytoplasm, which allows for free diffusion
of proteins. The nuclei will migrate to the
periphery of the cytoplasm where they will
form the syncytial blastoderm. These nuclei
will form cells, resulting in a cellular
blastoderm.
Gastrulation and segmentation happen in order to generate the shape of the body. During the
gastrulation phase, the dorsal tube and the ventral neural tube form. In order for segmentation to
take place, the cells need to know their location (which they know from the blastoderm stage
onwards). Imaginal discs are epithelial tissue that grows in the larva and differentiate into the
external parts of the adult during metamorphosis.
A blastula has the least number cells, followed by gastrula and neurula.
Drosophila versus vertebrate development
Drosophila Zebrafish Xenopus Mouse/Mammals
Maternal axes Anterior- Animal vegetal Animal vegetal No polarity
and symmetry posterior (A-P) (radial symmetry) (radial (point-symmetry)
and dorsal- symmetry)
ventral (D-V)
Syncytium and Nuclear Meroblastic division Holoblastic Holoblastic
cleavages divisions, with a yolk syncytial division (yolk is division (yolk is
syncytium layer (cleaves are cleaved as well) cleaved as well)
superficial)
Early embryonic 9 minutes 15 minutes 25 minutes Mouse
cell division time
,Body Axes, Patterning
Embryonic patterning is the process of establishing
positional information at the molecular level amongst
similar cells. The patterning establishes the body axes
(e.g. D-V, A-P, medial-lateral / left-right). It starts with
polarity and symmetry breaking which is caused by
asymmetric cell divisions of molecular gradients.
Patterning is not the same as differential gene
expression. Patterning is the process that establishes
differential gene expression among similar cells and is
directly related to the position within the embryo.
Patterning happens well before differentiation.
The maternal factors set up the body axes: bicoid in the
anterior part of the embryo inhibits caudal translation in
the posterior part, leading to polarity. It causes a
cascade of gene expression (e.g. bicoid -> gap genes ->
pair-rule genes -> segmentation genes -> selector
genes).
Germ Layers, Induction, Fate, Determination
There are three germ layers: endoderm (yellow), mesoderm (red), ectoderm (blue).
Vertebrates Insects
Endoderm Stomach, colon, liver, pancreas, urinary bladder, Gut
part of urethra, epithelial parts of trachea, lungs,
part of pharynx, thyroid, parathyroid, intestines
Mesoderm Muscle, bone, cartilage, connective tissue, adipose Muscle, heart, blood
tissue, circulatory system, lymphatic system,
dermis, genito-urinary system serous membranes,
notochord
Ectoderm Surface epidermis: hair, nails, lens of eye, Cuticle, nervous system
sebaceous glands, cornea, tooth enamel, anterior
pituitary, epithelium of mouth and nose
Neural crest: peripheral nervous system, adrenal
medulla, melanocytes, facial cartilage, dentin of
teeth
Neural tube: brain, spinal cord, posterior pituitary,
motor neurons, retina
,Fate maps tell us what the progeny of the cells will be.
Fate is the predicted outcome based on the position in
the embryo. Specified cells have received a signal steering
them into a certain lineage and determined cells will
become what they will be without being able to reverse it.
When a not yet determined cell (e.g. from a gastrula) is
transplanted into a host embryo, it will be regulative,
meaning that it will structures that are typical in that region. When a determined cell (e.g. from a
neurula) gets transplanted into another neurula, it will be mosaic, meaning that it will still form the
structure it was supposed to form instead of adapt (e.g. eye-like structure in a weird place).
, Molecular Principles of Development
08-11-2022 – Axes Germ Layers part 1
Very Early Development
Maternal to Zygotic Transition (MZT)
The sperm only delivers the DNA, meaning that the oocyte delivers most of the starting “material”.
This results in maternal control taking place initially, followed by zygotic control, in which the embryo
starts taking control. Cleavage is a type of embryonic cell division in which the number of cells
increases without an increase of total cytoplasmic volume.
The maternal to zygotic transition (MZT) is also known as mid-blastula transition (MBT) for flies, fish
and frogs. A hallmark for the MZT is the expression of embryonic genes (zygotic genome activation)
and the degradation of maternal mRNA. The MBT is slightly different: it contains zygotic genome
activation, loss of cell cycle synchrony (meaning that cells divide at different times now) and the cells
become more motile, which is needed for gastrulation.
MBT is regulated by multiple layers. One of them is the titration of a repressor by increasing genomic
DNA. It assumes that the oocyte carries a repressor, which inhibits ZGA. By increasing the amount of
genomic DNA, it will titrate the amount of repressor, thereby allowing ZGA. The nucleus-to-
cytoplasm ratio changes quite a bit during early development. When you compartmentalize a cell
(one side with nucleus, one without), the side with the nucleus will divide and therefore have more
nuclei. This same side will also lose its cell cycle synchrony faster than the other side. Polyspermic
embryos (embryos fertilized with multiple sperm cells) also transcribe zygotic mRNA faster than
normal embryos. Limiting DNA replication factors slow the cell cycle. Overexpression of these
replication factors will cause additional rapid cell divisions and delay zygotic gene expression. This
indicates that the aforementioned repressors that are titrated by genomic DNA are replication
factors.
Dorsal-Ventral Body Axis
In frogs, the ectoderm, endoderm and animal-vegetal axis are maternally determined. After ZGA but
before gastrulation, the border between the endoderm and ectoderm becomes the mesoderm. After
ZGA, the dorsal-ventral and anterior-posterior pattern also takes form. mRNA generally prefers to be
in the animal pole since there is more cytoplasm/room for them. However, some specific mRNAs are
transported to the vegetal half. This breaks the radial symmetry and contributes to mesoderm and
endoderm specification. Examples of these mRNAs are gdf1, wnt11b and vegt.
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