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Comprehensive summary of Medical Embryology
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Medical EmbryologyLecture 1 & 2 & 3
Everything starts with the cycle of life. There are certain moments when we are growing.
The first thing in the cycle of life is the embryonic stage. In the embryonic stage the fertilization of
the oocyte will take place. Then there is birth. Then there is the juvenile stage, in this the growth
takes place. In some animal models there are larva in this stage. Then there is the adult stage, here
there are fertile individuals that can reproduce. Then we will grow old, until we die.
Animal models are helpful to search for a function of for example a particular gene. You cannot use it
directly in humans, therefore animal models are helpful to see what happens when this gene is
absent. So why animal models?
- The principles of developmental mechanisms are highly conserved from organism to
organism
- All models are practical (easy to breed, fast life cycle)
- Animal models for disease can phenocopy human disease precisely
➔ Focus here on the embryonic stage after fertilization
Embryonic stage
In the embryonic stage there are developmental stages. First there is the cleavage stage (definition of
cleavage is that the cells are getting smaller (2-4-8-16 cells), but the volume stays the same), this is in
the fertilized egg (zygote) and mitosis happens. After cleavage happens there are early cells formed,
these are called blastomeres. Then the second stage is the blastulation stage, in this there is ordening
of the blastomeres. Then there is a formation of the blastocoel, this is a cavity filled with fluid. This
cavity should be there before we can go to the third stage, the gastrulation stage. In the gastrulation
there is morphogenic movements of the blastomeres (this is the initial process of gastrulation).
Gastrulation is for 1 purpose, that is the formation of the 3 embryonic germ layers. These germ layers
are the endoderm, ectoderm and mesoderm. Out of these germ layers, everything can be formed.
Ectoderm is most outside, with the nerve system. Mesoderm is inside, like the muscles and the
endoderm is the digestive system including the lungs. Then the last stage is the organogenesis. In
here there is differentiation of the 3 embryonic germ layers, then there will be the formation of
organs.
Cleavage:
The cleavage is really fast, but takes longer when cleavage is further in development.
There are different types of ootypes and patterns of cleavage. Ootypes can be Isolecithal (equal),
mesolecithal (middle), telolecithal (end) and centrolecithal (center). Lekithos means yolk, so the
different ootypes are saying where the yolk is.
Then the patterns of cleavage: the main patterns are holoblastic (complete) and meroblastic (part).
Blastos means germ, so the holoblastic and meroblastic are based on germ. The specific patterns are
radial, spiral, discoidal, superficial.
Cleavage in sea urchin → first two cleavages are from the animal to the vegetal pole. Then the next
cleavage will be around equator (middle) to divide the 4 blastomeres into 8 blastomeres. Still you
have the animal and vegetal pole. Some of the blastomeres are smaller than others, the biggest ones
are called the macromeres and the smallest ones in the micromeres they are both on the vegetal
pole. Then in between are the mesomeres, these are on the animal pole.
,Amphibians → mesolecithal displaced radial holoblastic cleavage of a mesolecithal ootype. The
cleavage is just above the equator. While the animal-vegetal gradient is determined in the egg prior
to fertilization, the point of entry of the sperm lends the frog its dorsal-ventral axis. The prospective
ventral side of the embryo is on the side of the sperm’s entry, while the prospective dorsal side, the
side at which the blastopore forms, is opposite the sperm’s point of entry.
Cephalopod molluscs → telolecithal bilateral meroblastic cleavage
Fish, reptiles, birds → telolecithal discoidal meroblastic cleavage
Most insects → centrolecithal superficial meroblastic cleavage.
Zebrafish → discoidal meroblastic cleavage, telolecithal ootype → epiboly happens.
- Epiboly is a cell movement in which a cap of about 3000 cells forming an epithelial
monolayer on the top of the animal pole of the egg embryo extends vegetally until it engulfs
the whole thing.
The cleavage of fish and birds is comparable.
Blastula stage
There is first cleavage after cleavage after cleavage. This gives lots of cells, this is called the morula,
because there are lots of cells packed. At a certain moment these blastomeres will organize a bit, and
then the blastocoel is being created. So there is a cavity created within the ball of cells. This is then
called the blastula. This have the blastocoel on the inside and the blastoderm on the outside, with
the individuals blastomeres.
- The blastula 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.
←Blastula stage in sea urchin
Displaced cavity in the frog, because this has to do with the ootype. There is easier cleavage on the
animal pole because there the yolk is not in the way. The animal pole can so form easier a blastocoel.
Sea urchin → equal radial holoblastic cleavage
- Yolk sparse and evenly distributed
- Isolecithal egg
- Holoblastic cleavage
- Radial cleavage pattern
Frog → unequal radial holoblastic cleavage → displaced blastocoel
- Moderate vegetal yolk disposition
- Mesolecithal egg
- Holoblastic cleavage
- Radial cleavage pattern
,Gastrulation
First the cells of the animal and vegetal pole cannot communicate with each other. Then there is cell
movement, whereby the communication is possible. Then there is induction, this process makes sure
that there will be 3 germ layers.
So cells on the outside are able to communicate with cells on the inside. So some of the ectoderm
cells move inward forming the endoderm.
- Gastrulation is a phase early in the embryonic development of most animals, during which
the single-layered blastula is reorganized into a trilaminar structure known as the gastrula.
These 3 germ layers are known as the ectoderm, mesoderm and endoderm
Types of cell movement;
- Invagination → infolding of a region of cells. (sea urchin)
- Involution → inward movement of an expanding outer layer (xenopus)
- Ingression → migration of individual cells from the surface layer into the interior of the
embryo (sea urchin)
- Delamination → splitting of one cellular sheet into two more or less parallel sheets (chicken)
- Epiboly → movement of epithelial sheets (usually of ectodermal cells) that spread as a unit
to enclose the deeper layers of the embryo. (xenopus / zebra fish)
Gastrulation of sea urchin is first ingression and later invagination
Xenopus → involution
Notochord → mesodermal nature and is important for the body axis.
Early gastrulation Xenopus: There are deep cells, these are moving inward. Then there is forming of
the dorsal blastopore lip. Later in gastrulation epiboly will happen. Then a dorsal, lateral and ventral
blastopore lip will be formed. The end gastrula will have just a dorsal blastopore lip.
After the gastrulation process, neurulation occurs. This only happens when mesoderm induces the
overlying ectoderm to become neuro-ectoderm, this is only on the dorsal side. A neural tube will be
formed in the neurulation process. Neurulation is the first evidence of organ building.
Gastrulation of zebrafish;
- Blastoderm formation by epiboly
- Hypoblast formation by involution/delamination
- Completion of gastrulation by epiboly
Gastrulation in chicken
- Early gastrulation → formation of the hypoblast by delamination and ingression of epiblast
cells
- Late gastrulation → formation of the primitive streak by involution of epiblast cells
, Neurulation:
Neural crest cells → mainly peripheral nervous system (ectoderm)
- Neural crest cells are a temporary group of cells unique to vertebrates that arise from the
embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including
melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons
and glia.
- The differentiation of neural crest cells is locally influenced by different paracrine factors
o Neurons and glia of cranial ganglia
o Cartilage and bone
o Connective tissue
o Sympatho-adrenal cells
o Sensory neurons and glia
o Pigment cells
Neurulation is the stage of organogenesis in vertebrate embryos,
during which the neural tube is transformed into the primitive
structures that will later develop into the central nervous system.
Neurulation process: → see this image ---------------------------→
1. Neural plate
2. Neural fold
3. Neural groove
4. Neural folds apposing
5. Neural tube
6. Migration neural crest cells
Mesodermal development;
Frog= holoblastic
Chicken= meroblastic
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