Framework

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. Pages 56-66 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. determination Nutt, SL, et. al. (1999) Commitment to the B-lymphoid lineage depends on the transcription factor Pax-5 Nature, 401:556-562 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 311-312 (Chapter 10 of Gilbert)