i want 25 multiple choice questions with their model answer below each question that covers all of the following information : General EmbryologyObjectives
By the end of this lecture student should be able to:
Study the 3rd week of development
(Trilaminar germ disc)Contents
Gastrulation:
-Definition
-Time
-Mechanism
Growth of the embryonic disc.GastrulationGastrulation
Ectoderm
Endoderm
Mesoderm
Trilaminar embryonic disc- Definition
- Time
Gastrulation
-Gastrulation is the
process by which the
three germ layers
(ectoderm, mesoderm,
and endoderm) are
formed in the embryo.
-It occurs during the
third week of gestation.
Bilaminar
embryonic discGastrulation
2
3Mechanism
(Primitive streak)
-Gastrulation
begins with
formation of the
primitive streak
(arrow) on the
surface of the
epiblast (E).
E
GastrulationMechanism
(Primitive node)
-The cephalic end
of the primitive
streak is called the
primitive node.
-The primitive
node is an
elevated area with
a central
depression, the
primitive pit.
Primitive
streak
Primitive
node
Primitive
node
Primitive
pit
Gastrulation
Hypoblast
Hypoblast
Epiblast
EpiblastGastrulation
Mechanism
Cephalic end
Primitive
pit
Primitive
node
Primitive
streak
Caudal endInvagination
Mechanism
Gastrulation
Invagination is an inward movement of the epiblast cells toward
the primitive streak.Invagination
Mechanism
Gastrulation
Invagination is
an inward
movement of the
epiblast cells
toward the
primitive streak.Invagination
Mechanism
Gastrulation
- Some of the
cells of the
epiblast migrate
toward the
primitive streak.
-When they arrive
in the region of the
streak:
- They detach from
the epiblast and
slip below it.Invagination
Mechanism
Gastrulation
- Some of the
cells of the
epiblast migrate
toward the
primitive streak.
Primitive
node
Epiblast
Primitive
streak
- When they arrive
in the region of the
streak:
- They detach
from the epiblast
and slip below it.
Invaginated cells
Hypoblast3 Germ layers
Mechanism
Gastrulation
Once the cells have invaginated
Some cells displace the
hypoblast and form
the embryonic
endoderm
The cells which lie
between the epiblast
and the newly formed
endoderm will form
the mesoderm
The remaining
cells in the
epiblast will form
the ectoderm3 Germ layers
Mechanism
GastrulationGastrulation
-The epiblast cells through the process of gastrulation is
the source of all of the 3 germ layers.
-The cells in these layers will form the tissues and organs
in the embryo.-The beginning of the third
week of development, when
gastrulation is initiated, it is
a highly sensitive stage for
teratogens.Gastrulation
-Invagination and cell
migration continues until
the end of the fourth
week when the primitive
streak rapidly shrinks
and disappears.Gastrulation
Cranial
Cloacal membrane:
-It is formed at the
caudal end of the
embryonic disc.
-It is similar in
structure to the
oropharyngeal
membrane, consists
of ectoderm and
endoderm cells with
no intervening
mesoderm.
-It is the future
opening of the anal
canal.
Caudal
Oropharyngeal
(buccopharyngeal)
membrane:
-It is formed at the
cranial end of the
embryonic disc.
-It is consists of
ectoderm and
endoderm cells
with no intervening
mesoderm.
-It is the future
opening of the oral
cavity.Gastrulation
Ectoderm
Mesoderm
Endoderm
Cloacal
Oropharyngeal
membrane
membraneGrowth of the embryonic discGrowth of the
embryonic disc
-The embryonic
disc, initially flat
and rounded.Growth of the
embryonic disc
Cephalic end
-The embryonic disc
gradually becomes
elongated with a broad
cephalic and a narrow
caudal ends.
Caudal endGrowth of the embryonic disc -The embryo develops cephalocaudally