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Developmental Patterns: What’s a
Protostome and a Deuterostome?
Early Development
• Fertilized Egg
• Blastula
What Characterizes Cleavage?
• Outcome
– large egg divided into typical small cells
– large increase in number of cells, chromatin,
surface membrane
• Characteristics
no growth
cell cycle: rapid S, little G1
shape the same except for cavity (blastocoele)
use of storage reserves (from oogenesis)
embryonic gene activity relatively unimportant
Cleavage Patterns I
• Holoblastic
– complete cleavage
• Isolecithal
– evenly distributed
• Mesolecithal
– slight polarization
of yolk
Cleavage Patterns II
• Meroblastic
– incomplete
• Telolecithal
– more yolk at one
• Centrolecithal
– yolk in the center
What Characterizes Gastrulation?
• Outcome
– increase in overt differentiation
– morphogenetic changes
• Characteristics
slowing of cell division
little if any growth
increasing importance of embryo gene activity
cell movements and rearrangements
establishment of basic body plan
In What Ways Do Cells Move?
• Common Processes
changes in cell contacts
cell-cell communication
interactions of cells with extracellular matrix
all precise and orderly
The Body Plan
• Gastrula establishes basic pattern
– ectoderm, endoderm and mesoderm
– primary germ layers
primordial organ
– germ layers bring cells to new positions to
• self-differentiate in correct spatial relationships
• be induced by new neighbors
Early Development of Selected
• Sea Urchin
– echinoderm, deuterostome
• Snail
– spiralian, protostome
• Tunicate
– chordate, deuterostome
• Nematode
– C. elegans, protostome
What’s A Sea Urchin?
Adult has 5-fold symmetry
Small radially and completely cleaving egg
Moderate yolk
Easy to work with in lab
Lots of gametes
Patterns of Sea Urchin Cleavage
• Radial, holoblastic
• First three cleavage
planes perpendicular
• Divisions are equal
• Fourth division is
unequal 4 micro, 4
macro and 8
SU Fate Map I
Where Do Layers Originate?
SU Fate Map II
Where Do Layers Originate?
Fate Map of 64-Cell Stage and