Embryology part 2
Hello again
Embryogenesis of the birds
Example: Gallus domesticus. The egg is of the telolecithal type .
Note: When can I said embryo?
When the zygote start its 1st cleavage
Structure of the newly laid egg:
In birds, there is one ovary and one oviduct in the female (the deft side). The immature
ova in the ovary are large, but they are different in size. The immature ovum, which is
in the 1ry oocyte stage, becomes ready to leave the ovary in the Process of ovulation.
Soon, the Try oocyte passes through the wide funnel- shaped opening of the oviduct
and it divides into the 2ry oocyte and 1ry polar body. Fertilization takes place internally
and the sperms introduced by the male pass to reach the anterior part of the oviduct
where they can remain active for two to three days. The penetration of the 1st sperm
into the 2ry oocyte activates it to complete its maturation and divide into the mature
ovum and 2ry polar body. The nucleus of the mature ovum unites with that of the
sperm, and thus fertilization is completed. It is possible that more than two sperms
can penetrate the ovum and this is known as polyspermy, but only one sperm is
concerned with the fertilization while the 5thers disappear.
The ovum:
It is of a diameter of about 3 cm. it consists of a small germinal disc (about 2 mm)
formed of tiny cytoplasm and nucleus (germinal vesicle), and great mass of yolk. This
yolk mass is formed of white and yellow yolk arranged in concentric layers that
alternate each other. The central part of yolk is of the white yolk and is known as
latebra from which a stalk of white yolk known as the neck of latebra extends upwards
reaching the germinal disc.
The ovum is surrounded by the vitelline membrane, which arises from special
follicular cells in the ovary!
1
,Structures surrounding the ovum:
The ovum ( fertilized or non- fertilized) continues its passage through the oviduct, the
glandular cells of the wall of the oviduct secrete the albumen, The albumen secreted
firstly is dense in its consistency and is known as dense albumen. The ovum passes in
a spiral fashion or movement and these results in the twisting of the dense albumen
at the two ends in the form of two cords. Each is known as chalaza the distal part of
the oviduct secretes another type of then albumen the watery albumen that consists
of about 25% water. Then in the basal part of the oviduct, the cells of the shell gland
in the oviduct form the inner and outer shell membranes, which are closely touching
each other but are not fused together. Between them and at the blunt end of the egg
there is an air chamber.
The shell gland then deposit a hard calcareous shell. Composed mainly of calcium,
which is, and is porous.
During egg laying the blunt end leave the cloacal aperture firstly and so the air
penetrates through the shell reaching the air chamber. The duration needed for the
passage of the ovum in the oviduct until egg laying is about 16-24 hours .
Development of the chick embryo .
Cleavage leading to blastulation in the germinal disc
The zygote cleaves according to the meroblastic or discoidal type, since not all the
ovum divides but only its fertilized germinal disc. The cleavage furrows follow the
normal pattern as indicated before in the case of amphibians.
The 1st cleavage plane is meridional giving rise to 2 blastomeres which are
incompletely separated from each other, their cytoplasm is continuous ventrally and
peripherly.
The 2nd cleavage plane is also meridional but it passes at the right angle of the 1st
one. resulting in the formation of blastomeres which are still continuous with each
other both ventrally and peripherly.
2
, After that, cleavage planes become irregular. Circular cleavages take place, correspond
to the latitudinal planes of lower vertebrates. Tangential cleavages are also occurred,
separating cells in various directions. The central blastomeres touch each other but the
peripheral ones rest directly on the yolk. Due to the increase in number of blastomeres,
they separate from the underlying yolk mass in the central area.
Therefore, when seen from above in a surface view, the blastomeres of the germinal
disc show two main areas :
1- Area pellucida :
It is transparent, since a subgerminal cavity separates it from the yolk mass .
2- Area opaca :
It is the peripheral area and its cells are touching the yolk mass and therefore
this area appears darker in color. At this stage, which is an early blastodisc
(blastoderm) stage, the final blastula stage is not yet formed.
The cells of the area pellucida are arranged into two layers. The outer layer is
the epiblast, the inner is the hypoblast, and between them, there is the
blastocoel
Theories of formation of the hypoblast :
1- By delamination :
The future cells of the hypoblast arrange themselves in a layer just underneath the
cells of the future epiblast. Then the cells of the hypoblast gradually separate as one
layer or lamina which passes to the inside forming the final hypoblast and a new cavity
appears which is the blastocoel, while the subgerminal cavity gradually disappears .
2- By involution :
At the future posterior region of the area peliucida, there is a zone of active cell
division and the new cells of the future hypoblast undergo involution in this area and
finally the hypoblast is formed .
3- By infiltration :
The future hypoblast cells which are originally mixed with the cells of the epiblast,
migrate downwards either singly or in groups and then interconnected close to the
yolk material until a complete hypoblast is formed .
3
Hello again
Embryogenesis of the birds
Example: Gallus domesticus. The egg is of the telolecithal type .
Note: When can I said embryo?
When the zygote start its 1st cleavage
Structure of the newly laid egg:
In birds, there is one ovary and one oviduct in the female (the deft side). The immature
ova in the ovary are large, but they are different in size. The immature ovum, which is
in the 1ry oocyte stage, becomes ready to leave the ovary in the Process of ovulation.
Soon, the Try oocyte passes through the wide funnel- shaped opening of the oviduct
and it divides into the 2ry oocyte and 1ry polar body. Fertilization takes place internally
and the sperms introduced by the male pass to reach the anterior part of the oviduct
where they can remain active for two to three days. The penetration of the 1st sperm
into the 2ry oocyte activates it to complete its maturation and divide into the mature
ovum and 2ry polar body. The nucleus of the mature ovum unites with that of the
sperm, and thus fertilization is completed. It is possible that more than two sperms
can penetrate the ovum and this is known as polyspermy, but only one sperm is
concerned with the fertilization while the 5thers disappear.
The ovum:
It is of a diameter of about 3 cm. it consists of a small germinal disc (about 2 mm)
formed of tiny cytoplasm and nucleus (germinal vesicle), and great mass of yolk. This
yolk mass is formed of white and yellow yolk arranged in concentric layers that
alternate each other. The central part of yolk is of the white yolk and is known as
latebra from which a stalk of white yolk known as the neck of latebra extends upwards
reaching the germinal disc.
The ovum is surrounded by the vitelline membrane, which arises from special
follicular cells in the ovary!
1
,Structures surrounding the ovum:
The ovum ( fertilized or non- fertilized) continues its passage through the oviduct, the
glandular cells of the wall of the oviduct secrete the albumen, The albumen secreted
firstly is dense in its consistency and is known as dense albumen. The ovum passes in
a spiral fashion or movement and these results in the twisting of the dense albumen
at the two ends in the form of two cords. Each is known as chalaza the distal part of
the oviduct secretes another type of then albumen the watery albumen that consists
of about 25% water. Then in the basal part of the oviduct, the cells of the shell gland
in the oviduct form the inner and outer shell membranes, which are closely touching
each other but are not fused together. Between them and at the blunt end of the egg
there is an air chamber.
The shell gland then deposit a hard calcareous shell. Composed mainly of calcium,
which is, and is porous.
During egg laying the blunt end leave the cloacal aperture firstly and so the air
penetrates through the shell reaching the air chamber. The duration needed for the
passage of the ovum in the oviduct until egg laying is about 16-24 hours .
Development of the chick embryo .
Cleavage leading to blastulation in the germinal disc
The zygote cleaves according to the meroblastic or discoidal type, since not all the
ovum divides but only its fertilized germinal disc. The cleavage furrows follow the
normal pattern as indicated before in the case of amphibians.
The 1st cleavage plane is meridional giving rise to 2 blastomeres which are
incompletely separated from each other, their cytoplasm is continuous ventrally and
peripherly.
The 2nd cleavage plane is also meridional but it passes at the right angle of the 1st
one. resulting in the formation of blastomeres which are still continuous with each
other both ventrally and peripherly.
2
, After that, cleavage planes become irregular. Circular cleavages take place, correspond
to the latitudinal planes of lower vertebrates. Tangential cleavages are also occurred,
separating cells in various directions. The central blastomeres touch each other but the
peripheral ones rest directly on the yolk. Due to the increase in number of blastomeres,
they separate from the underlying yolk mass in the central area.
Therefore, when seen from above in a surface view, the blastomeres of the germinal
disc show two main areas :
1- Area pellucida :
It is transparent, since a subgerminal cavity separates it from the yolk mass .
2- Area opaca :
It is the peripheral area and its cells are touching the yolk mass and therefore
this area appears darker in color. At this stage, which is an early blastodisc
(blastoderm) stage, the final blastula stage is not yet formed.
The cells of the area pellucida are arranged into two layers. The outer layer is
the epiblast, the inner is the hypoblast, and between them, there is the
blastocoel
Theories of formation of the hypoblast :
1- By delamination :
The future cells of the hypoblast arrange themselves in a layer just underneath the
cells of the future epiblast. Then the cells of the hypoblast gradually separate as one
layer or lamina which passes to the inside forming the final hypoblast and a new cavity
appears which is the blastocoel, while the subgerminal cavity gradually disappears .
2- By involution :
At the future posterior region of the area peliucida, there is a zone of active cell
division and the new cells of the future hypoblast undergo involution in this area and
finally the hypoblast is formed .
3- By infiltration :
The future hypoblast cells which are originally mixed with the cells of the epiblast,
migrate downwards either singly or in groups and then interconnected close to the
yolk material until a complete hypoblast is formed .
3
