Mammalian development begins with fertilization and cleavage of the egg. The egg develops membranes that allow development outside of water. In mammals, the placenta exchanges gases and nutrients between the embryo and mother. Cleavage is rotational, with zygotic genes activating later than other animals. Cells compact and the morula forms an inner cell mass and trophoblast cells. The trophoblast secretes fluid to form a blastocyst cavity. The inner cell mass forms the epiblast and hypoblast, which generate the embryo and extraembryonic tissues through gastrulation. Axis formation is guided by gradients of genes like HOX and left/right asymmetries are regulated by proteins including Nodal.

1. Mammal
Development

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