Medical embryology
Course coordinator: Sharon Kolk
By: Julie van Immerseel, S1000023
Week 1
eModule 1- Recap developmental biology elements previously learned
Just before the neural tube closes, the anterior part expands and forms 3 primary brain vesicles,
called (from anterior to posterior): prosencephalon, mesencephalon and rhombencephalon.
By the end of the embryonic period, the brain has 5 subdivisions, called (from anterior to posterior):
telencephalon, diencephalon, mesencephalon, metencephalon and myelencephalon. (Tel di mesen
met my).
In embryology, cleavage does not equal mitosis.
Cleavage refers to the first process of development that follows fertilization in which a large single
cell zygote is divided into smaller cells known as blastomeres. Mitosis is a process by which cells
divide. The dividing cell is what causes an embryo to become a fetus and have the features of an
average human being.
The notochord is a longitudinal, flexible rod located between the digestive tube and the nerve cord.
It provides skeletal support throughout most of the length of the chordata.
The formation of the neural tube is also known as neurulation.
The first 8 weeks are referred to in humans as the embryonic period and the
subsequent weeks as the fetal period.
By the end of the first period, the nervous system has been divided into the
peripheral and central nervous system.
By the end of the second period, the vast majority of neurons have been produced.
B = ectoderm
C = blastocoel
D = archenteron (cavity formed inside the embryo which eventually forms the gut)
E = mesoderm
F = endoderm
The ectoderm eventually forms the outer covering of the organism and in some phyla also gives rise
to the central nervous system, inner ear, and lens of the eye.
The mesoderm develops into the notochord which runs along the anterior-posterior axis of a
chordata, the lining of the true coelom (body cavity), but also the muscles, skeleton and most of the
circulatory system and the rest of the organs not formed by the endoderm (gonads and kidney).
The endoderm lines the digestive tract (which develops from the archenteron) and gives rise to
organs such as the liver, pancreas and lungs.
The purple structure which has been fully formed on the right is called the: neural tube.
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,After fertilization, the embryo will undergo a series of 5-7 cell divisions, called cleavages/cleavage.
After multiple cell divisions, a hollow ball of cells will develop, the blastula.
At this stage, the embryo folds inwards, creating two germ layers, and outer ectoderm and an inner
endoderm layer.
This stadium is called the gastrula.
After this stage has formed, the third germ layer, the mesoderm will be formed in the middle.
Weblecture 1 + 2
Principles of developmental biology
- The cycle of life
- Animal models to study developmental mechanisms
- Significance of the high conservation of developmental mechanisms
- The early embryonic processes: cleavage, gastrulation and organogenesis
- Fundamental molecular processes in development
- Overview of the important molecules that guide embryonic development
Human embryology
- Cleavage and blastulation: a stage between ovulation and implantation
- Forming of the two-germ-layer stage
- Gastrulation: formation of the three embryonic germ layers
- Neurulation: establishing the neural tube
- Early organogenesis
- The extraembryonic membranes
- The placenta: relation between mother and fetus
- The fetal and postnatal circulation
Animal models to study developmental mechanisms
- Very high conservation of the early developmental steps; the early cleavages, blastulation
steps, cell movement procedures.
- Can be helpful in the quest to therapeutic interventions.
Ontogeny recapitulates phylogeny
= our first, initial building plan is basically the same for every animal model.
Why animal models?
- The principles of the developmental organisms are highly conserved from organism to
organism.
- All models are practical (easy to breed, fast life cycle etc.)
- Animal models for disease can phenocopy human diseases precisely
Developmental stages
Focus on the embryonic stage: fertilization of the oocyte → birth
What is the difference between cleavage and cell division?
There is cleavage after fertilization, however the cells don’t grow in size. The volume stays the same,
the cells just get smaller.
The early cells that are generated are called blastomeres.
Blastulation: the blastomeres will start to reorganize themselves.
The formation of the blastocoel is very important, because we need to create a cavity before we can
go on to the gastrulation.
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,Initial process of gastrulation is the morphogenic movements of blastomeres.
The gastrulation is for one purpose only: the formation of 3 embryonic germ layers → endoderm,
ectoderm and mesoderm.
What are germ layers? Out of these germ layers everything can be formed.
Ectoderm: the outer layer, including the central nervous system.
Mesoderm: everything on the inside except for the digestive system.
Endoderm: digestive system, including our longs.
Cleavage
The cell cycle early in development is pretty short. The cell
cycle will increase as the development proceeds.
Ootypes and patterns of cleavage
It is really important to realize what oocytes you start off
with, in order to say anything about the early cleavage
stages. Because the oocytes are different among animals.
Animal pole = the most terminal part of the egg in which the least yolk is concentrated and the
nucleus resides.
Patterns of cleavage:
Blastos = germ
Main patterns:
- Holoblastic (holos = complete)
- Meroblastic (meros = part)
Specific patterns:
- Radial
- Spiral
- Discoidal
- Superficial
Example: cleavage in sea urchin
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, Cleavage in zebrafish →
The cleavage is on top of the yolk. It will start to grow over the ball of yolk.
<- cleavage in birds.
Cleavage in fish and birds are comparable, because they are
both meroblastic.
Blastula stage
The blastula (blastos = sprout) is a hollow sphere of cells
formed during an early stage of embryonic development in
animals. The blastula is created when the zygote undergoes
the cell division process known as cleavage.
Example: blastula stage in sea urchin
<- Blastula: the final stage of the cleavage process
Why is the cavity
displaced in the
frog? Because the
yolk is in the way.
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