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Summary Human Embryology – Lecture 5
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Human Embryology – Lecture 5
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Human Embryology – Lecture 5 (Lecture 9 + 10)22-02-2018
Aantekeningen gemaakt a.d.h.v. hoorcollege + aantekeningen
*Notochord is very important for induction of overlying ectoderm to become nervous
tissue.
Prenatal period:
- Embryonic period first 8 weeks
- Fetal period Remaining 30 weeks.
Within first 8 weeks all the major organ systems are already developed, except for
parts of the lungs and urogenital systems.
Pre-embryonic period: the first 2 weeks of development, when the zygote becomes
a spherical, multicellular structure.
Blastocyst implants itself into the uterum wall this can be seen as the first stage of
pregnancy.
Embryonic period: all major organ systems appear.
First organ system that will be developed: neural system
Trimesters:
1st: week 1-13
2nd: week 14-26
3rd: weeks 27 and on (until birth 38-40 weeks)
Week 1: Cleavage and initiation of implantation
Week 2: The bilaminar embryo and fully implantation
Week 3: The trilaminar embryo and gastrulation
Blastocyst:
- Inner cell mass + blastocoel inside
o Inner cell mass will become the embryo
- On the outside Trophoblast: protective for the embryo and provide lots of
nutrients. Interacts with mothers uterum wall.
Around day 9: bilaminar embryo, with hypo- and epiblast. + trophoblast and
syncythiotrophoblast.
Epiblast will split and another cavity will be formed within epiblast layer this will
become the amnionic sac.
Uterine epithelium + uterine blood vessels + uterine glands will interact with
syncythiotrophoblast, in order to generate the placenta which will provide nutrients
and is important for the removal of waste products.
, Cavity underneath the hypoblast = yolk sac.
Part of trophoblast + syncythiotrophoblast will become the chorion.
Epiblast = on top
Hypoblast = underneath
By day 16, gastrulation has already kicked in.
Allantois is very important for the initial storage of waste products.
Extra-embryonic mesoderm has formed around the yolk sac and the amniotic cavity
is split into extra-embryonic coloem.
Amniotic cavity is expanding and will fill up the uterine cavity.
Body stalk will be the umbilical cord.
Placenta = really highly vesseled.
Formation of germ layers:
Just like in chicken: as soon as primitive streak is formed:
Epiblast cells will move in and go to lateral and cranial sites, but first:
1. hypoblast is replaced by definitive endoderm.
2. Formation of mesoderm, in between definitive endoderm and the overlying
epiblast. (sometimes this epiblast is now already called the ectoderm, but in
fact they are all epiblast cells).
3. Remaining epiblast cells will become ectoderm. this will end up in the
trilaminar embryo.
Epiblast cells will form all 3 germ layers.
The three germlayers will differentiate into specialized cell types, depending on the
signals they will get within the embryo.
Most of the ectoderm: skin, nerval system
Endoderm: lining of all organ systems (e.g. bone/muscles/blood cells)
Mesoderm: gut system + lungs (grow out of future gut)
Mesoderm has the highest variety of cell types.
Ectoderm is being induced by the underlying Notochord (mesodermal structure)
Neurulation will kick in.
Nerval system: first organ that will start to appear.
Proliferation + accumulation of epiblast cells.
Primitive streak + chorda formation neurulation.
Mesoderm will need to go everywhere inside the embryo.
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