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samenvatting tentamen 2 EVO DEVO
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samenvatting van alles wat terug komt bij tentmanen 2 EVO DEVO in P4 van de vu.
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Tentamen 2 EVO-DEVO13 + 14. Cleavage patterns, germ layers & body axis
Founding fathers of EvoDevo, biogenetic law, phylotypic stage, hourglass model:
Karl Ernst: The organs that evolve in an embryo, did evolve in the same manner in evolution. Ernst
Heackel: Development is a short version of evolution. This idea is very important for the research to
evolution and development. His words were: “Ontogeny recapitulates phylogeny”.
Notochord ‘back’ is evolved after vertebrates. The hourglass model means that in the beginning the
early cleavage is due to reproductive differences. In the middle (phylotypic stage) all embryos look
the same. In the late stage, you see again the adaptive differences.
Bilateral body plan, three germ layers and coelom evolutionary very old:
Every first animal starts as a zygote (egg and sperm cell). The first cleavage leads to a
morula, then to blastula. This blastula leads to a blastocoel (hole inside) which
eventually leads to the three germ layers develop to gastrulation. This represents the
gut. Next is nervous & heart. The three germ layers are:
o Ectoderm: upper skin, nervous system, neural cells, brain, nose epithelium,
eyes and inner ear
o Mesoderm: dermis, muscle, somite, skeleton, diaphragm, heart, kidneys,
gonads, oviduct, uterus, testis, vas deferens
o Endoderm: gut lining, oral cavity, esophagus, stomach, liver, pancreas,
trachea, lung epithelium, ureter, prostate and vagina.
Coelom: body cavities. It is compressed out of the blastocoel. It is needed for
the maintain of more complex bodies, like humans. Via this way, organs can
be protected and have the freedom to move a little. First there was no
coelom, it developed later. The formation of body axis: Needed to determine
the positions of organs in an embryo.
Variation in early embryonic development due to reproductive differences:
There are 2 main differences in cleavage patterns: Holoblastic (cell division
divide in whole cell: human, mouse, Xenopus & C. elegans) and meroblastic (cell
divisions are limited due to yolk: zebrafish, chicken, drosophila).
Yolk obstruction is needed because the animal can’t get their nutrients from the
mother, like with an egg. This cleavage pattern depends on
reproductive biology, rather than on evolutionary history.
Evolution of deuterostomes from protostomes large impact:
Protostomes: The cleavage pattern is determinate. This means
that the cell fate is fixed right after cleavage. Via this way you
can track down all the different cells and its function. This can be
done in C. elegans and Drosophila. The mesoderm develops and becomes in designated cells.In
gastrulation, the blastopore becomes the mouth. This is in invertebrates, like fish and amphibians.
Via this way there was an anterior-posterior axis. Neurulation: Inversion of the dorso-ventral axis. An
axochord, long median ventral longitudinal muscle.
Deuterostomes: The cleavage pattern is indeterminate. This means that the cell fate is determined
later. This indetermined cleave allows monozygotic twins and genetic screening because you can
switch any cell without change structure in the end. This is in zebrafish, Xenopus, chicken, mouse and
humans. The mesoderm split and forms the endoderm. In gastrulation, the blastopore becomes the
anus. Neurulation: evolution of notochord by reversal of dorsoventral axis and loss of contractile
function. The notochord is derived from mesoderm, and a tightly packed cell surrounded by
, membrane. It disappears during development. It induces the neural tube and contributes to the
development of the vertebral column.
Neurulation takes place at the dorsal side. The neural cells migrate to contribute to organogenesis
elsewhere: skin, kidney, nervous, heart, skull.
There was a new type of gastrulation: the primitive streak. This is in reptiles, birds and mammals =
amniota. This establishment of bilateral body axis. This is in mammals highly derived due to
placentation.
Great variety of axis-determining mechanisms; unknown why this is:
Body axes are via anterior and posterior (front and rear), dorsal and ventral (belly and back) and left
and right. These axes are determined very early in development. Organs have to form in originated
fashion. In Drosophila have a very quick development, with a quick axis development, with a lot
homologous genes as mammals.
Influence of maternal factors in the egg in some species, but not in mammals:
Developing eggs get maternal mRNA during oogenesis. A drosophila egg has a dorsal-ventral axis
induced by its mother. Nucleus starts to divide, but the cells do not divide with them. Later all the
different nucleus goes to the side of the cell.
Bicoid (bcd) forms an A-P gradient. This is a morphogen for head and thorax development. It
establishes the position of specialized cell types and tissues. Via transplantation: anterior and
posterior become thorax. and head is formed in the middle. Ectopic = away from normal position.
Not a matter of single gene, Bidcoid supress Caudal in anterior, and Nanor suppress Hunchback in
posterior part. This leads to a cascade of events.
Spatzle is used to determine to morphogen of dorso-ventral axis formation. It is concentration
depended on which tissue ends up were.
Also fertilization can trigger D-V axis formation: The sperm entry point is the ventral pole. B-catetin
expression accumulates in the dorsal side. This is experimented with the Speman transplantation.
This induces a cascade for regulatory the formation of ecto, meso and endo (XENOPUS).
Amniotes different from fish and amphibians (primitive streak):
In a chicken, also gravity can define the dorso-ventral axis. This induces the primitive
streak. This streak (Hensen’s node) acts as an organizer in amniotes for gastrulation and
neurulation. Vg-1 induces Nodal > start primitive streak. Epiblast cells migrate inwards
primitive streak, where the establish endoderm and mesoderm starts. The upper
epiblast becomes ectoderm.
In mammals dorso-ventral axis formation relatively late and induced by factors outside the egg
(contact with the uterus):
Formation of A-P axis much the same as other amniotes (primitive streak), but placenta adds further
complication. Epiblasts establish by signals of hypoblast. Conserved regulatory genes involved. In
humans, left identity initiated by nodal excretion from primitive streak node. The Hensen’s node is
the organizer, and from there is the primitive streak. Situs inversus is a symmetric reversal organ
development. They can develop normal, and it is associated with a minor mutation in Nodal.
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