1. Embryonic Development
Involves 3 Components:
1. Cell Division-
The mitotic increase in the number of cells.
2. Differentiation-
The development of specialized cells that are then
organized into tissues & organs.
3. Morphogenesis-
Physical processes that give shape to the organism.
…involves movement & arrangement of cells
and apoptosis (programmed cell death)
All triggered by…..
FERTILIZATION!

2. Fertilization (as studied in Sea Urchins)
1. Acrosomal Reaction-
The discharge of hydrolytic enzymes from the
Acrosome of the sperm cell (via exocytosis)
Proteins at the tip of the sperm attaches to specific receptors on
egg’s “vitelline layer”, digests it, then contacts the plasma membrane.
When the membranes fuse, it depolarizes the membrane and prevents
Any other sperm from entering.
This is often called the “_________ block to polyspermy.”
FAST

3. Fertilization Continued….
2. Cortical Reaction-
Fusion of sperm & egg membranes causes
activation of a signal transduction pathway:
Releases Ca++ ions from the egg’s E.R. which results in
the release of…
“Cortical Granules”
Cortical Granules fuse with plasma membrane and release their
contents into the “perivitelline space”
Osmosis causes it to separate
Eventually hardens to become a “fertilization membrane”
This is often called the “_________ block to polyspermy”.
SLOW

4. Fertilization Finale’
3. Activation of the egg
The release of Ca++ also signals an increase in cellular:
Respiration
and
Protein synthesis
“pre-made” mRNA begins translation immediately.

5. Polarity of the Dividing Egg
Animal Pole-
Has a lower yolk content
Becomes the anterior portion
Cleavage occurs more slowly
Future Ectoderm
Vegetal Pole-
Has the highest concentration of yolk
Becomes the posterior portion of the embryo
Cleavage occurs more rapidly
Future Endoderm & Mesoderm

6. Now for a break from Power Point to old skool white board scribbles….

7. Types of Cleavage Holoblastic- Meroblastic- “Complete” cell division
(Membranes are fully formed before next division)
Usually in cells with little to moderate levels of yolk
Meroblastic-
Cleavage is “incomplete”
(Membranes are not fully formed before next division)
Usually in yolk-rich eggs

8. Is all Cytoplasm the Same??
(of course not!!)
Heterogeneous organization of the cytoplasm already exists
even in the unfertilized egg.
Different:
Pre-made mRNA
Cytoskeleton constructs
Endomembrane products
BLASTOMERES!
…Ultimately producing different kinds of …. ________________
With those blastomeres then having cell to cell interactions initiated
by cytoplasmic differences to begin the morphogenesis.
It’s all part of a phenomenon known as….
Pattern Formation

9. Pattern Formation Controlled by a hierarchy of gene activations:
The development of an organism’s spatial organization.
In other words…..
Putting the right organs in the right places!
Controlled by a hierarchy of gene activations:
1. Egg Polarity Genes-
Maternal genes that set the polar axis of future embryo.
2. Segmentation Genes-
Regulate the origin of segmentation once the anterior
to posterior axis is defined.
Gap Genes 
Pair Rule Genes 
Segment Polarity Genes
3. Homeotic Genes-
Specify for the type of appendages specific for that segment.

10. Homeotic Genes
Products of these genes are regulatory proteins that bind to DNA.
Affect selective gene expression.
“HOMEOBOX” genes-
Specific sequences in homeotic genes
which are very similar in ALL eukaryotes.
…or “HOX”
(Indicates a very conservative evolution)
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11. And now for some…….. Spectacular Animations:
Crash Course!