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Mammal
Development
Mammals are Amniotes
An amniote is an embryo with a water sac
The amniote egg has a set of membranes which allow the
embryo to develop away from water
• The amnion lets the embryo float in fluid and keeps it from drying out
• The yolk sac will nourish the developing fetus
• The allantois stores waste products
• The chorion has blood vessels that exchange gases with the outside
In mammals the holoblastic cleavage is modified with a
placenta to exchange gasses and nutrients
Mammalian
Cleavage
• Fertilization is going to happen in the ampulla after the oocyte is
released from the ovary.
• Cleavage starts about a day after fertilization
The Unique Nature of Mammalian Cleavage
• Cleavage first occurs as the eggs are
being pushed towards the ovary
• They divide slowly compared to
other animals
• The first cleavage is done
meridionally (meaning that it splits
side to side)
• In the second cleavage, one split
meridionally and one splits
equatorially (perpendicular)
• This is known as rotational cleavage
Continued
• Unlike the fruit fly, there are zygotic
genes that play a part in cleavage.
• In humans, these are not activated
until around the eight-cell stage
Compaction
Compaction is when cells
change their behavior.
Cell adhesion proteins (like E-
cadherin) are expressed, and
the cells begin to pack in tightly
From the eight-cell embryo,
they divide to become a 16-cell
morula
The Morula
• The Morula consists of inner cells surrounded by
external cells which will become trophoblast cells.
• The internal cells will become the inner cell mass
(ICM)
• The ICM stays to one side of the trophoblast cells
• In other words, the ICM bunches up at one side of
the cell
• The unit is not called a blastocyst
Cavitation
• The cells of the ICM will become the yolk sac, allantois, and amnion
• Around the 64th cell stage, the trophoblast cells secrets a fluid into the
morula to crease a blastocel (imagine an empty spot where the yolk of an
egg would be)
• More precisely, it pumps Na+ into the center. Sodium draws water to it
(remember, water follows solutes).
Mammalian
Gastrulation
• Even through mammals use a placenta to
obtain nutrients directly from the mother,
there are still remnants of a yolk (blastocel)
• The trophoblast stimulates the formation of
the decidua (the maternal portion of the
placenta)
• The ICM gives rise to the epiblast and the
hypoblast
• The epiblast will generate the embryo,
amnion and allantois
• The hypoblast will become yolk sac
cells
Continued
• The epiblast and hypoblast
(primitive endoderm) form the
bilaminar germ disc.
• The hypoblast cells line the
blastocel and give rise to the yolk
sac.
• The hypoblast cells contacting the
epiblast are the visceral endoderm
• The parietal endoderm contacts
the trophoblast.
• A primitive streak forms at the
posterior end of the embryo
• A node forms at the anterior end
of the primitive streak
Axis Formation
• A lot of the HOX genes found in
Drosophila are found in mammals as
well.
• HOX genes can be activated by the
gradient of:
• FGF (fibroblast growth factor) in
the posterior
• Retinoic Acid in the anterior
• HOX genes are numbered 1-13. 1 is the
anterior end and 13 covers the
posterior end
Left/Right Axis
• The bodies of mammals are not symmetrical.
• Various proteins and transcription factors regulate this axis. Some are:
• Nodal protein, Pitx2 transcription factor in the lateral plate mesoderm
are for the left side
• Cerberus is for the right side
• It also inhibits Nodal

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BIO420 Chapter 12

  • 2. Mammals are Amniotes An amniote is an embryo with a water sac The amniote egg has a set of membranes which allow the embryo to develop away from water • The amnion lets the embryo float in fluid and keeps it from drying out • The yolk sac will nourish the developing fetus • The allantois stores waste products • The chorion has blood vessels that exchange gases with the outside In mammals the holoblastic cleavage is modified with a placenta to exchange gasses and nutrients
  • 3. Mammalian Cleavage • Fertilization is going to happen in the ampulla after the oocyte is released from the ovary. • Cleavage starts about a day after fertilization
  • 4. The Unique Nature of Mammalian Cleavage • Cleavage first occurs as the eggs are being pushed towards the ovary • They divide slowly compared to other animals • The first cleavage is done meridionally (meaning that it splits side to side) • In the second cleavage, one split meridionally and one splits equatorially (perpendicular) • This is known as rotational cleavage
  • 5. Continued • Unlike the fruit fly, there are zygotic genes that play a part in cleavage. • In humans, these are not activated until around the eight-cell stage
  • 6. Compaction Compaction is when cells change their behavior. Cell adhesion proteins (like E- cadherin) are expressed, and the cells begin to pack in tightly From the eight-cell embryo, they divide to become a 16-cell morula
  • 7. The Morula • The Morula consists of inner cells surrounded by external cells which will become trophoblast cells. • The internal cells will become the inner cell mass (ICM) • The ICM stays to one side of the trophoblast cells • In other words, the ICM bunches up at one side of the cell • The unit is not called a blastocyst
  • 8. Cavitation • The cells of the ICM will become the yolk sac, allantois, and amnion • Around the 64th cell stage, the trophoblast cells secrets a fluid into the morula to crease a blastocel (imagine an empty spot where the yolk of an egg would be) • More precisely, it pumps Na+ into the center. Sodium draws water to it (remember, water follows solutes).
  • 9. Mammalian Gastrulation • Even through mammals use a placenta to obtain nutrients directly from the mother, there are still remnants of a yolk (blastocel) • The trophoblast stimulates the formation of the decidua (the maternal portion of the placenta) • The ICM gives rise to the epiblast and the hypoblast • The epiblast will generate the embryo, amnion and allantois • The hypoblast will become yolk sac cells
  • 10. Continued • The epiblast and hypoblast (primitive endoderm) form the bilaminar germ disc. • The hypoblast cells line the blastocel and give rise to the yolk sac. • The hypoblast cells contacting the epiblast are the visceral endoderm • The parietal endoderm contacts the trophoblast. • A primitive streak forms at the posterior end of the embryo • A node forms at the anterior end of the primitive streak
  • 11. Axis Formation • A lot of the HOX genes found in Drosophila are found in mammals as well. • HOX genes can be activated by the gradient of: • FGF (fibroblast growth factor) in the posterior • Retinoic Acid in the anterior • HOX genes are numbered 1-13. 1 is the anterior end and 13 covers the posterior end
  • 12. Left/Right Axis • The bodies of mammals are not symmetrical. • Various proteins and transcription factors regulate this axis. Some are: • Nodal protein, Pitx2 transcription factor in the lateral plate mesoderm are for the left side • Cerberus is for the right side • It also inhibits Nodal