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Organisme Dier
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EmbryologieEmbryonic development is very similar in the vertebrates. Animals have a
very common development in body-plan.
Fertilization and cleavage initiate embryonic development.
1. Contact
2. Acrosomal reaction – after contact the hydrolytic enzymes will break
down the jelly-coat.
Now the Actin filament can get to the sperm-binding receptors. This
gives a signal to the
rest of the coat, that no other sperm-cells can enter the egg-cell.
3. Contact and fusion of the sperm and egg-cell membranes.
4. The Cortical Reaction – the cortical granule gets released into a
fertilization envelope, this
makes it impossible for other sperm-cells to enter. It activated the
cleavage.
5. Entry of the sperm nucleus.
Egg activation
- The rise of Ca2+ in the cytosol increases the rates of cellular respiration
and protein
synthesis by the egg cell.
- these rapid changes in metabolism activated the egg.
- proteins and mRNAs are needed for activation, these are present in the
egg.
- sperm nucleus merges with the egg nucleus.
The difference to humans is the thick layer of follicle cells that surrounds
the egg’s envelope.
Cleavage
Zygote is one cell on top of yolk (ei geel) these will divide into a Gray
crescent (2 cell forming stage). There is another division to a 4 cell stage
and then to a 8 cell stage.
Many cell divisions later the blastula is formed with a blastocoel inside. A
blastula has at least 128 cells.
Holoblastic cleavage – complete division of the egg, little yolk by frogs
Meroblastic cleavage – incomplete division of the egg, much yolk by
reptiles and birds
Morphogenesis
The process by which cells occupy their appropriate locations.
- Gastrulation: the movement of cells from the blastula surface to the
interior of the embryo.
The three-layered embryo it created is called a gastrula.
- Organogenesis: the formation of organs.
1
,Ectoderm (outer embryo Mesoderm (middle embryo
layer) layer)
- epidermis of skin and its - skeletal and muscular systems
derivatives - circulatory and lymphatic
- nervous and sensory systems systems
- pituitary gland, adrenal medulla - excretory and reproductive
- jaws and teeth systems (except germ cells)
- germ cells - dermis of skin
- adrenal cortex
Endoderm (inner embryo layer)
- epithelial lining of the digestive tract and associated organs
- epithelial lining of respiratory, excretory, and reproductive tracts and
ducts
- thymus, thyroid and parathyroid glands
Gastrulation in frogs
The blastocoel shrinks and an archenteron appears in the endoderm. The
mesoderm starts to surround the endoderm, but leaves a spots open. This
is the yolkplug, the ectoderm isn’t surrounding fully and on this spot you
can see the endoderm.
Gastrulation in chicks
- prior to gastrulation, embryo is composed of an upper layer (epiblast)
and a lower layer
(hypoblast)
- during gastrulation, epiblast cells move toward the midline of the
blastoderm and then into
the embryo toward the yolk.
All cells that form the embryo are from the epiblast!
Developmental adaptations of amniotes
Creating an eggshell: birds, reptiles and monotremes
Protecting the egg in the uterus: marsupian mammals (buidel) and
eutherian mammals
In both adaptations there is a fluid around the egg in a sac (amnion). This
protects the egg in both cases from drying-out and allows dry land
reproduction.
Mammals and reptiles incl. birds are called amniotes.
Organogenesis
Various regions of germ layers develop into rudimentary organs.
Neurulation
- ectoderm forms the neural plate and neural fold.
- mesoderm forms the notochord.
- endoderm forms a coelom, the Archenteron.
2
,The neural plate gets folded to the inside and forms a tube, during the
neural folds are being pushed towards each other and combine so the tube
closes. Then the ectoderm combines at the closing point and there are
neural crest cells. These neural crest cells lay loose between the ectoderm
and the formed neural tube.
Now you have an ectoderm -> crest cells -> neural tube -> (mesoderm)
notochord -> somite with coelom next to the notochord -> endoderm with
archenteron.
Morphogenesis in animals involves cell shape, -position and -survival.
It’s not in plants cause it only appears when there is no thick cell-wall.
Convergent extension movements shape a sheet of cells.
The microtubules stretch the cell and the actin filaments lay at the small
end. This causes a ring form.
Programmed cell death (apoptosis) is very important for normal
embryologic development.
2e + 3e college
Embryo drawing
Slices (CT scan):
- coronal = front to back
- transverse = through the middle – liver on the right, looking up from
feet, back down.
- sagittal = left to right
Heart lays on top of the liver.
Uterus drawing
Fimbriae fingers hold the ovary and take the ovum into the uterine tube.
Ampulla is the place of fertilization. The ovum gets pulled to the uterine
cavity by peristaltic muscle movement and cilia.
The ovum dies if it isn’t fertilized within 24 hours and the sperm-cells die
if they don’t fertilize.
Morulla stage has 8-16 cells
Inner cell mass are stem-cells and are used for cloning. These cells are
pluripotent and can become every type of cell. They form the embryo.
Gastrulation forms 3 layers out of the two layers.
3
, The blood cells are attracted to
from the placenta.
Ventral cavity is the yolk sac
Dorsal cavity is the Amnion sac
Allantois collects the excretions of the embryo.
Chorion collects oxygen through the shell (chicken egg) so the embryo
can breathe.
Chorio-allantoic membrane is the amnion and chorion combined.
There is a high death rate in undeveloped embryo’s.
Human children are born in a very juvenile state, so that the head can get
through the small birth canal. Due to this the parents have to look after
the child for a long time before it’s grown up.
Histology
Epithelial tissue: a sheet of cells on the outer part of a organ. The cells
are touching each other and are polarized (basal and epical end).
Types:
- simple epithelium = single layered, good for diffusion (liver and lungs).
- simple squamous = flat
- simple cuboidal = tube from cube like cells (kidney-duct)
- simple columnar = long cells (guts)
- pseudostratified ciliated columnar = cilia die speeksel opbrengen
(longen).
- stratified squamous = layer of more cells, tough (huid en slokdarm)
4
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