2. Embryology at a glance
Watch this
Then this

3. C. Cell Differentiation • By day 4, the embryo forms a morula
–a ball containing 16-
cells
–the cells have now
differentitated into
• an inner cell mass
– gives rise to the
morula
embryo proper (namely YOU)
(means “mulberry”)
•an outer cell mass
embryonic contribution
to the placenta

4. D. Cavitationand the Blastocyst
• Next a fluid-filled
outer cell mass
cavity
“blastocyst”
and the
“cavitation”
forms
inner cell mass
between the inner cell mass
• This is called
• This gives rise to
the blastocyst
“blastocyst”= “germ-bag”

5. Week #2: BilaminarDisk Formation (1)
•The
inner cell mass
now differentiates into 2 distinct cell layers
epiblast
hypoblast
–the epiblast
–the hypoblast
• This is called the
bilaminar embryonic disk

6. Week #2: BilaminarDisk Formation (2a)
• A cavity forms next to
the epiblast
–It is called the amniotic
cavity
–it becomes filled with
amniotic cavity
amniotic fluid
–this will eventually
surround and suspend the embryo and provide
• a shock-free environment
• a constant temperature
–and allow free movement

7. Week #2: BilaminarDisk Formation (2b)
• The cells lining the
amniotic cavity are shed on occasion
• These cells, along with
proteins, carbohydrates, fat, enzymes, hormones and embryonic excretions can be extracted and examined
amniotic cavity
• This procedure is called an
amniocentesis

8. Week #2: BilaminarDisk Formation (3)
amniotic cavity
yolk sac

9. Week #3: Gastrulation (1)…
• With further cell differentiation, a 3rdcell layer
forms between
the epiblastand hypoblast
• The embryo is now a trilaminardisk
• This process is called gastrulation and forms the mesoderm around day 16
Forms lining of GI tract

10. Week #3: Gastrulation(2)…
• The epiblastis now
called the ectoderm
• The hypoblast is now
called the endoderm
• The middle layer,
composed of cells that migrated from the epiblast, becomes the mesoderm
• These 3 cell layers give
rise to all the cells of your body!

11. Week #3: Gastrulation(3a)…
•The
ectoderm
will give rise to
–organs that maintain contact with the outside
world •
epidermis (epithelial lining of the skin)
And the nervous system
skin

12. Week #3: Gastrulation(3b)…
•The
endoderm
will give rise to
–the epithelial lining of the digestive and
respiratory tracts
–plus outgrowths of the digestive tract that give
rise to the: •
liver
pancreas

13. Week #3: Gastrulation(3c)…
•The
mesoderm
will give rise to
–connective tissues •
connective tissue proper
That is, basically everything between the inner and outer
linings of your body!
bone
cartilage
blood
–muscle

14. While the external “look” of a human is called bilateral symmetry because if you cut us in half on the median plane, both sides look roughly exact. However, our internal design is called tube within a tube as seen here.

17. Four Weeks
Figure 29–7a, b

18. Eight Weeks the 8thweek the fingers and toes have • The face
Figure 29–7c, d
• The face
• By the end of
formed
now begins to look human-like
• (The fetus
remains at very high risk of malformations through the 8th week)

19. Twelve Weeks • This is the
Figure 29–7c, d
• This is the
period of functional maturation of all organ systems
• (By this age, the
fetus is at lower risk of malformations)

20. HOMEWORK
What are the three germ layers and what mature features do they form?