1. Framework
Developmental processes are driven by differential gene expression
Gene expression programs are induced by signals between neighboring tissues of the developing embryo
Cell movements during embryogenesis place particular tissues into juxtaposition

2. “Official glossary” (from Wolpert)
Morphogen: Any substance active in pattern formation whose spatial concentration varies and to which cells respond differently at different levels
Morphogenesis: The process involved in bringing about changes in form in the developing embryo
Pattern formation: Process by which cells in a developing embryo acquire identities that lead to a well-ordered spatial pattern of activities

3. What development accomplishes
Differentiation of all of the required cell-types from a single fertilized egg (oocyte)
Morphogenesis: Precise arrangement of these cells into tissues and organs
Pattern formation: Precise arrangement of tissues and organs to achieve a reproducibly working organism capable of reproduction
Epigenesis: the de novo formation of an organism from “disordered” egg cytoplasm

4. Demonstration of nuclear potential in Acetabularia (1930’s)
Figure 2.5
page 31
Gilbert

5. The nucleus and Epigenesis
The nucleus contains the instructions that drive epigenesis/development
Chromatin is the instructional unit (DNA plus proteins). The state of the chromatin is set by “epigenetic control” mechanisms
Covert “epigenetic” changes occur during the early cleavages of the fertilized egg into the blastomeres of the embryo

6. For example: DNA methylation

7. Embryonic cell division is not the same in all kinds of organisms
Next slide
So there is a sense that these zygotes “know what
they are” when the begin to divide. The yolk and its position obviously plays a role in determining the cleavage pattern. But, there are sometimes other products stored in the egg by the mother that act as morphogenetic determinants.

9. Major morphogenetic strategies
Autonomous specification
Morphogenetic determinants deposited in the egg become segregated by cell division
Determination of cell fate is early
Conditional specification
Cell fate determination is later and depends on the position of the blastomere in the embryo
Removal of cells is compensated for by others
Each cell has the potential to give rise to more cells than it normally does

10. Specification of cell fate: Autonomous vs. Conditional
Pages of text
Specification of cell fate: Autonomous vs. Conditional

11. Syncitial specification
Mainly seen in insects (Drosophila)
Gradients form, over time, of morphogens deposited in the egg by the mother
These morphogens become segregated by cell membranes which grow into the egg
Drosophila lectures: End of February

12. GASTRULATION
The process that puts cells into position to have their fates determined
Gastrulation involves a particular repertoire of cell movements which can be classified
Gastrulation will result in the formation of three “germ layers” from which the organs of the embryo will arise

13. Movements of Gastrulation

14. Result of gastrulation: The 3 germ layers

15. Gastrulation and Conditional Specification
Newly positioned tissues (germ layers) interact with one another to “induce” organ formation
Cell-cell interactions via receptors (juxtacrine)
Soluble signaling molecules (paracrine)
Morphogen gradients link position to cell fate
Signal transduction activates gene expression which leads to specification and lineage commitment

16. Morphogen gradients: different concentrations of a factor induce different gene expression

17. Stages of commitment to cell fate
Specification
Changes in gene expression which are labile and changeable. The gene expression “allows” that cell to differentiate along a pathway but does not irreversibly commit the cell
Determination
Further changes in gene expression which seal the lineage fate of the cell and eliminate alternative choices

18. Pro-B cells from Pax-5 deficient mice are “specified”
IL-7
Im syk Oct-2
l5 btk E2A
VpreB blk PU.1
B29 lyn EBF
“Pro-B cell”

19. Pro-B cells from Pax-5 deficient mice are “specified” but not “determined”
T cell
mouse
Im syk Oct-2
l5 btk E2A
VpreB blk PU.1
B29 lyn EBF
Macrophage
M-CSF
“Pro-B cell”
Trance
Dendritic
cell
Osteoclast
IL-2
GM-CSF
NK cell

20. Recent data illustrating the concepts of specification vs
Recent data illustrating the concepts of specification vs. determination
Nutt, SL, et. al. (1999) Commitment to the B-lymphoid lineage depends on the transcription factor Pax-5
Nature, 401:
E2A
EBF
Pax-5
B lymphocyte
development
committed
specified

21. Forming a solid organ How do cells “stick together” to form tissue?
Coordination of tissues from multiple germ layers to form a single functional organ
Tissue layers
Organ polarity
Cell adhesion molecules: Cadherins (p66-74)

22. Cadherins (Ca2+dependent adhesion molecules)
See also
Website 3.8

23. Types of cadherins
E- cadherin: expressed on early embryonic cells in mammals. Later becomes restricted to embryonic and adult epithelial tissue
P-cadherin: Trophoblast cells (placental)
N-cadherin: First mesodermal, later CNS
EP-cadherin: frog blastomere adhesion
Protocadherins: not connected to catenin

24. Mechanisms of cadherin based cell “sorting” into tissues
Differential expression of cadherin type
Neural vs epidermal cells (N vs. E cadherin)
Different levels of cadherin expression
Oocyte positioning in follicle
Loss or switch of cadherin expression
Neural crest emigration from the neural tube
Protocadherin switching in frog gastrulation
See pages (Chapter 10 of Gilbert)