Medical embryology
2nd year Biology
Lecture 1 + 2: Human embryology and developmental biology
The cycle of life:
Animal models to study developmental mechanisms.
Initial building plan is basically the same for every animal
model. → Ontogeny recapitulates phylogeny
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 etc.)
- Animal models for disease can phenocopy human
diseases precisely
Cleavage: division of cells. Blastomeres are generated and have the same combined volume
of the fertilized egg. The cell cycle will increase (take longer), so it takes longer for the
cleavages to occur, as development proceeds.
Blastocoel: cavity with fluid
Gastrulation: formation of 3 germ layers → endoderm, ectoderm, mesoderm
Important: what oocytes you start off with, because they are different among animal groups.
Different locations of the yolk (dooier; lekythos)
Patterns of cleavage: depends on where the yolk is positioned.
- Holoblastic: complete cleavage
- Meroblastic: partial cleavage
Specific patterns: radial, spiral, discoidal, superficial
Cleavage in Sea Urchin→
Blastomeres with different sizes: smaller: micromeres;
bigger: macromeres; in between: mesomeres.
1
, Medical embryology
2nd year Biology
Cleavage in zebrafish: Cleavage in birds:
when the blastomeres grow over the bole of yolk it is called: epiboly (in zebrafish)
Cleavage in fish and birds are comparable: both meroblastic.
Blastula stage: final stage of the cleavage process
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 known as cleavage.
2
, Medical embryology
2nd year Biology
Effect of ootype on localization blastocoel→
Gastrulation: a phase early in the embryonic
development of most animals, during which the
single layered blastula reorganized into a
trilaminar (three-layered) structure known as the
gastrula. These three germ layers are known as
the ectoderm, mesoderm and endoderm.
Cells on the inside are now able to communicate with the orange cells on the outside
Some of the ectoderm cells (orange) move inward forming the endoderm (red).
First, 2 layers are formed: ectoderm and endoderm.
Gastrulation: types of cell movement:
- Invagination: infolding of a region of cells
- Involution: inward movement of an expanding outer layer
- Ingression: migration of individual cells from the surface layer into the interior of the
embryo.
- Delamination: splitting of one cellular sheet into two more or less parallel sheets
- Epiboly: movement of epithelial sheets (usually of ectodermal cells) that spread as a
unit to enclose deeper layers of the embryo.
Involution example in Xenopus laevis. (IMZ= involuting marginal zone).
Bottle cells will move inward and dive underneath the ectodermal layer.
Archenteron is the future gut.
Notochord is important for the positioning of the body axis, part of
mesodermal germ layer.
Induction process= process in which the mesoderm will form because of molecular
interaction with ectodermal cells and endodermal cells.
3
, Medical embryology
2nd year Biology
So, first involution and later on epiboly in Xenopus to form the third layer: mesoderm.
Early gastrulation:
Mid to end gastrula:
Neurulation is the process after gastrulation in which the neural tube is formed. Neurulation
only occurs when mesoderm induces the overlying ectoderm to become neuro-ectoderm and
that is only on the dorsal side.
Gastrulation in zebrafish:
- Like in human, you have an epiblast and hypoblast.
- Epiblast will form the ectodermal structures
- Hypoblast will be replaced, first for definitive endoderm and then mesoderm will be
formed.
4
2nd year Biology
Lecture 1 + 2: Human embryology and developmental biology
The cycle of life:
Animal models to study developmental mechanisms.
Initial building plan is basically the same for every animal
model. → Ontogeny recapitulates phylogeny
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 etc.)
- Animal models for disease can phenocopy human
diseases precisely
Cleavage: division of cells. Blastomeres are generated and have the same combined volume
of the fertilized egg. The cell cycle will increase (take longer), so it takes longer for the
cleavages to occur, as development proceeds.
Blastocoel: cavity with fluid
Gastrulation: formation of 3 germ layers → endoderm, ectoderm, mesoderm
Important: what oocytes you start off with, because they are different among animal groups.
Different locations of the yolk (dooier; lekythos)
Patterns of cleavage: depends on where the yolk is positioned.
- Holoblastic: complete cleavage
- Meroblastic: partial cleavage
Specific patterns: radial, spiral, discoidal, superficial
Cleavage in Sea Urchin→
Blastomeres with different sizes: smaller: micromeres;
bigger: macromeres; in between: mesomeres.
1
, Medical embryology
2nd year Biology
Cleavage in zebrafish: Cleavage in birds:
when the blastomeres grow over the bole of yolk it is called: epiboly (in zebrafish)
Cleavage in fish and birds are comparable: both meroblastic.
Blastula stage: final stage of the cleavage process
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 known as cleavage.
2
, Medical embryology
2nd year Biology
Effect of ootype on localization blastocoel→
Gastrulation: a phase early in the embryonic
development of most animals, during which the
single layered blastula reorganized into a
trilaminar (three-layered) structure known as the
gastrula. These three germ layers are known as
the ectoderm, mesoderm and endoderm.
Cells on the inside are now able to communicate with the orange cells on the outside
Some of the ectoderm cells (orange) move inward forming the endoderm (red).
First, 2 layers are formed: ectoderm and endoderm.
Gastrulation: types of cell movement:
- Invagination: infolding of a region of cells
- Involution: inward movement of an expanding outer layer
- Ingression: migration of individual cells from the surface layer into the interior of the
embryo.
- Delamination: splitting of one cellular sheet into two more or less parallel sheets
- Epiboly: movement of epithelial sheets (usually of ectodermal cells) that spread as a
unit to enclose deeper layers of the embryo.
Involution example in Xenopus laevis. (IMZ= involuting marginal zone).
Bottle cells will move inward and dive underneath the ectodermal layer.
Archenteron is the future gut.
Notochord is important for the positioning of the body axis, part of
mesodermal germ layer.
Induction process= process in which the mesoderm will form because of molecular
interaction with ectodermal cells and endodermal cells.
3
, Medical embryology
2nd year Biology
So, first involution and later on epiboly in Xenopus to form the third layer: mesoderm.
Early gastrulation:
Mid to end gastrula:
Neurulation is the process after gastrulation in which the neural tube is formed. Neurulation
only occurs when mesoderm induces the overlying ectoderm to become neuro-ectoderm and
that is only on the dorsal side.
Gastrulation in zebrafish:
- Like in human, you have an epiblast and hypoblast.
- Epiblast will form the ectodermal structures
- Hypoblast will be replaced, first for definitive endoderm and then mesoderm will be
formed.
4
