Transcript Developmental Biology

The Early Development of
Birds & Mammals
Gilbert - Chapter 11
Goals
• Become familiar with the cleavage and
gastrulation patterns in birds and
mammals
• Examine the adaptations specific to the
amniote egg
• Understand the specialized structures
that develop in mammals to allow for
embryonic implantation
Early Avian Development:
Chick
• Why Chick embryos as a model
system?
– Easily obtained
– Control developmental stage needed by
incubating when ready
– Relatively large embryo
• Good for surgical manipulation
• Many similarities to human development
– Inexpensive
Early Avian Development:
Chick
• Fertilization occurs internally prior to
secretion of shell
• Egg is telolecithal
– VERY yolky!
• Cleavage is discoidal
Cleavage in Chick embryo
• 1st cleavages occur in a small area of
cytoplasm called the blastodisc
– These do not penetrate through whole cell,
so cells are continuous with each other
• As cleavage continues, a small disc of
cells about 4-6 layers deep is created in
the center of the blastodisc
• In the center of that disc
– Cells in the deeper layers die off, forming a single
celled layer of blastoderm called the AREA
PELLUCIDA
– This will form the actual embryo - these cells are
also called the EPIBLAST
• On the margins of that disc
– Deeper cells do not die off, creating a thicker layer
called the AREA OPACA
• Underneath this disc is a fluid-filled cavity
called the subgerminal space
Occurs while inside the
hen!
Gastrulation in Chick Embryo
• First step: form a 2 layered blastoderm
– EPIBLAST (embryo proper) & HYPOBLAST
(extraembryonic membranes)
– 2 mechanisms
• 1) Cells delaminate from the epiblast and
migrate as “islands” into the subgerminal
cavity
– Primary hypoblast
• 2) Cells near the posterior end migrate as a
sheet toward the anterior end (inside the
subgerminal cavity)
– Secondary hypoblast
Avian Gastrulation
• Second step:
– Formation of germ layers
• Avian embryos form a structure called the
primitive streak
• This is where the formation of germ layers
begins
• Thickening of the epiblast near the posterior
end
• As the primitive streak extends, a
depression forms at the midline, called
the primitive groove
• It is through the primitive groove that
cells will migrate into the blastocoel
– Analogous to the amphibian blastopore
• At the anterior end, a thickening called
Hensen’s node is a funnel shaped
depression where cells begin to migrate
by ingression into the blastocoel
– Analogous to the organizer (dorsal
blastopore lip)
The first cells that
migrate through
Hensen’s node
are endoderm,
then
chordamesoderm
• Creation of the primitive streak
establishes the anterior-posterior axis of
the embryo
– Begins at the tail end, moves toward the
head end
• Cells continue to ingress through the
primitive streak, laying down the
mesoderm layer
• At this point, the primitive streak
regresses
Regression of the Primitive
Streak
• During regression, the dorsal axis of the
embryo is laid down
– Notochord, and neural precursors become
established
– Head ectoderm begins to develop
• Hensen’s node regresses to the
posterior end, becomes anus
• In this case, the result is actually a flat
embryo with 3 germ layers, rather than
a spherical embryo
• The avian embryo is still atop the yolky
mass
– Endoderm separates the embryo from the
yolk
The Chick Organizer
• Hensen’s node!
• Transplant to an ectopic location, form a
secondary axis
Avian Gastrulation: Summary
•
•
•
•
Complex!
Initiated by Hensen’s node
Sets up the body axes
Establishes the germ layers
Mammalian Development
• Difficult to study
– Internal fertilization and development
• Fertilization occurs IN the oviduct!
– Few embryos produced
– Lengthy development periods
– Mice: mother must be sacrificed to harvest
embryos
– Some early development can be viewed in in vitro
culture systems
• Fertlization, cleavage, some of gastrulation (mice)
Mammalian Cleavage
• Very tiny egg!
• Meiosis not completed until fertilization
• Different from other types of cleavage
–
–
–
–
–
Holoblastic, rotational
Slower
Asynchronous
1st cleavage: normal, meridional cleavage plane
2nd cleavage:
• One blastomere meridional
• One blastomere equitorial
– Embryonic gene expression occurs right away
Cleavage and Compaction
• Another major difference = Compaction
– After the 3rd cleavage (8 cells),
blastomeres express large amounts of
adhesion molecules, like cadherins
– Cells become a tight, compact ball of cells
• Inner cells sealed off from outside environment
– At the 16 cell stage, embryo becomes a
morula
• Small group of cells inside surrounded by layer
of external cells
Morula
• Cells on the outside are called
trophoblast and become extraembryonic
membranes and embryonic portion of
the placenta
• Cells on the inside are called Inner Cell
Mass (ICM) and produce the embryo
proper
This is the first differentiation event in
mammalian cells
The Blastocyst
• As the morula develops, a fluid filled
cavity forms (blastocoel)
• In this case the ICM is positioned on
one side of the trophoblast, creating not
a blastula, but a blastocyst
• The trophoblast will develop into
specialized tissues that will allow for the
embryo to implant into the placenta
• The inner cell mass
– Develops into two layers: Epiblast and
Hypoblast
– Gastrulation will occur here
– Similar to chick embryo
• ICM is the source of pleuripotent stem
cells!
Early Mammalian
Development: Summary
• Very different cleavage pattern
• Form a blastocyst
– ICM, trophoblast
• Gastrulation in humans is very similar to
chick
• The node is the organizer
• Mouse - very unusual gastrulation
pattern!