Embryonic development
Early Embryonic Development
การเจริ ญระยะต้นของเอ็มบริ โอ
July 16, 2010
L2-101, MU Salaya
Dr. Rattanavijit Vijitruth
B404, Biology Dept., Faculty of Science
Mahidol University
peacophile@hotmail.com
http://www.jneuroinflammation.com/content/3/1/6
Embryonic development
1.
Fertilization =
Combination of sperm and
egg nuclei and egg activation
2.
Cleavage
rapid cell divisions without
substantial growth in size
3.
Gastrulation =
4.
Organogenesis = Localized changes in tissue
and cell shape
=
Mass cell movements to
generate three germ layers
The relative size of various eggs
Sea urchin:
Human:
Frogs & fishes, some insect eggs:
Birds & reptiles:
~70 to 150 microns
~100 microns
1-2 mm
many cm
Eggs/ Zygotes have Animal and Vegetal Poles
Eggs store materials needed for development of the
embryo
Yolk
Yolk: lipids, carbohydrates and proteins
organized into granules
Yolk settles to bottom of egg, producing a gradient of
stored material
Top of egg, with little yolk
“animal pole”
Bottom of egg, rich in yolk
“vegetal pole”
Eggs have different amounts of yolk
Large animals developing outside mothers body (birds,
reptiles) have large eggs with lots of yolk
Large animals developing within mother's body
(mammals) have small eggs with very little yolk; they get
their food from the mother through the placenta
Animals which develop into small feeding larvae (sea
urchins, sea stars) also have small, simple eggs
Frogs and fish are intermediate in egg size and yolk
content
Types of egg: yolk distribution in
cytoplasm
Types of egg: amount of yolk
Alecithal:
No yolk present (mammals)
Microlecithal: Little yolk (sea urchin)
Mesolecithal: Moderate yolk (frog)
Polylecithal: lot of yolk (reptiles, birds)
Isolecithal:
two equal hemispheres of yolk
(sea urchin, amphioxus)
Centrolecithal:
Yolk is concentrated in the central region
of the cytoplasm in the egg (insect)
Telocithal:
Yolk is concentrated at one end of the
ovum (frogs, birds)
Cleavage
Cleavage
The series of mitosis division that take the
large zygote to a mass of smaller cells, the
“blastula”
The individual cells during cleavage are
In later stages, a fluid filled cavity, the
“blastocoel”, forms
In most animals, the cell divisions do not
increase the total volume of the cell mass
The volume remains the same because each
generation of daughter cells simply gets smaller
and smaller
called “blastomeres”
In early cleavage, the solid ball of cells
looks like a blackberry and is called a
“morula” (latin for mulberry)
Pattern of cleavage
Blastulation
Holoblastic cleavage
e.g. amphioxus, mammals
Meroblastic cleavage
Morula
(mass of 32 cells)
Blastula
e.g. chick, reptiles
Holoblastic Cleavage
Pattern of cleavage
Cleavage follows different patterns depending upon the
amount of yolk in the egg
Egg with small amount of yolk (sea urchin): entire egg
divides (holoblastic cleavage)
Egg with intermediate amount of yolk (frog egg): entire
egg divides, but animal pole cells divide faster than
vegetal pole cells
Cleavage furrow extends completely
thru the cytoplasm
Cleavage symmetry
Yolk slows cell division
Result:
Result: large number of small cells at animal pole,
pole, small number
of large cells at vegetal pole
Egg with large amount of yolk (bird egg): only the animal
pole divides (meroblastic cleavage)
Produces cells of the same size
Little or no yolk
Produces plate of embryonic cells (blastodisc)
blastodisc) over a large yolk
container
Holoblastic Cleavage
Radial – Echinoderms, amphibians
Spiral -- most molluscs, annelids, flatworms,
roundworms
Bilateral – Ascidians, Urochordates
Rotational -- Mammals
Holoblastic Cleavage
radial cell cleavage
Holoblastic Cleavage
Axes of bilaterally symmetrical animal
spiral cell cleavage
Holoblastic Cleavage
Meroblastic Cleavage
High concentration of yolk in oocyte,
off-center
Cleavage furrow does not extend thru
cytoplasm in region containing yolk
Cleavage symmetry
Radial cleavage
Rotational cleavage
Bilateral -- Cephalopod molluscs
Discoidal -- Reptiles, fishes, birds
Superficial -- Most arthropods
Meroblastic Cleavage
GASTRULATION
Creation of multiple layers of cells for
tissue differentiation
Formation of Gastrula
Fate Mapping
Fate mapping was developed by Walter
Vogt as a means by which to trace the
development of specific regions of the
early embryo.
vital dyes
GASTRULATION
involves a series of cell migrations to
position where they will form the three
primary cell layers
Ectoderm
Forms tissues associated with outer
layers:
skin, hair, sweat glands, epithelium.
The brain and nervous system also
Ectoderm
Endoderm
Mesoderm
–
–
–
forms the outer layer
forms the inner layer
forms the middle layer
develop from the ectoderm
Mesoderm
Endoderm
Forms structures associated with
Forms tissues and organs associated with
movement and support:
Body muscles, cartilage, bone, blood, and all
the other connective tissues.
Reproductive system organs and kidneys
form from mesoderm
the digestive and respiratory systems.
Many endocrine structures, such as the
thyroid and parathyroid glands
The liver, pancreas, and gall bladder arise
from endoderm
Morphogenetic
movements
during
GASTRULATION
Invagination- the infolding of a region of cells
Involution- the inturning of an expanding outer
layer of cells
Delamination- the splitting of one sheet of cells
into two parallel sheets of cells
Ingression- the migration of individual cells
from surface to interior
Epiboly- the movement of epithelial sheets that
spread as a unit to cover the embryo
GASTRULATION
Cleavage: sea urchin
Radial holoblastic cleavage
Cleavage: Sea urchin
Blastula: sea urchin
Gastrulation: sea urchin
Gastrulation: sea urchin
Gastrulation: sea urchin
Ingression of primary mesenchyme cells
Gastrulation: sea urchin
Cell rearrangement during the extension of the archenteron
Gastrulation: sea urchin
Rotational Holoblastic Cleavage in the C. elegans Egg
Gastrulation: C. elegans
• Small/transient blastocoel
• Migration of E cells creates
a tiny blastopore
• P4 cells migrate through
the blastopore
• Followed by mesodermal
cells (MS) and C- and Dderived muscle precusors
creates multiple body layers
Gastrulation: Drosophila
Formation of the cellular blastoderm in Drosophila
Gastrulation: Drosophila
Cleavage: frog
Mesolecithal/displaced radial holoblastic cleavage
Gastrulation: frog
Surface view of an early dorsal blastopore lip
Gastrulation: frog
Discoidal meroblastic cleavage: a chick egg
Cleavage: chick
Gastrulation: chick
Gastrulation: chick
Gastrulation: chick
Cleavage: human
Gastrulation: human
Rotational cleavage
Inner cell Blastocoel
mass
Trophoblast
Implantation
Implantation/Gastrulation
Gastrulation: human
Cleavage: comparing
Gastrulation: human