How does a fertilized egg become an animal?

1. How does a fertilized egg become an animal? Clam egg and sperm

3. Developmental Biology is the study of a PROCESS whereby a single cell (the fertilized egg) divides and selectively activates expression of genes to produce a complex organism composed of many cell types. Ex ovo omnia!

4. What kinds of PROCESSES are required? • To form an embryo, the following (and more!) must occur: • Gametes form and fuse (Reproduction) • Cells multiply (Growth) • Generation of Asymmetry • Axis Determination (Positional information) • - Anterior/Posterior (Head-Tail) • - Dorsal/Ventral (Back-Front) • - Left/Right • Cells differentiate • Structures are built from cells (Morphogenesis) • Animal cells organize into sheets and move • Plant cells form structures without moving

5. Differentiationis a central idea of development: All cells have the same DNA, but DIFFERENT CELLS express DIFFERENT GENES

6. Nature supports an incredible diversity of plant and animal body plans

7. Yet all of these organisms share conserved developmental mechanisms that are evidence of their evolution from a common ancestor. Our challenge is to understand both this diversity and this unity.

8. Developmental Biology is studied using the following TOOLS • Cell Biology • Genetics • Molecular Biology

9. Let’s Review the Basics • The body is made of millions to billions of cells. • Cellular machinery is largely made up of proteins • Because of their different tasks, different cells contain different proteins • Proteins are made up of chains of amino acids, and these amino acids are "encoded" in the cell's DNA • Information flows from DNA to RNA to Protein • When one gene is mutated, one protein is affected (usually disabled). • All cells have the same DNA but different cells express different genes

10. Development Occurs at an Unfamiliar Scale • If a cell was the size of a basketball (8 inches) • a mouse would be the size of Chapel Hill (10 miles) • a gene would be about an inch long. =

11. Development Occurs at an Unfamiliar Scale • If a protein was the size of a Volvo (10 feet) • a cell would be the size of Chapel Hill (10 miles) • a gene would be about 1.5 miles long but the strand of DNA would only be a few feet wide. =

12. TESTS • Exam #3 March 31 (covers March 3-29) • FINAL May 5 (covers April 5-26) • NO make-up exams • Regrade requests must be submitted to your TA within one week of exam

13. Two Extreme Models for Differentiation from the late 1800’s (neither is correct) Mosaic development Regulative development

14. The Mosaic Development model proposes that cells become progressively committed to specific cell fates • Roux’s landmark experiments with frog embryos: • do cells have fixed identities that they can maintain without influence from their neighbors? Kill 2 cells with a hot needle “YES”! and allow the remaining 2 cells to develop 4-cell stage Differential segregation of genetic potential? Only half an embryo develops

15. Roux’s landmark experiments Figure 3.16. Destroying (but not removing) one cell of a 2-cell frog embryo results in the development of only half the embryo.

16. The Regulative Development model proposes that cells retain the ability to adjust their fates in response to their cellular environment • Driesch’s experiments with sea urchin embryos: • do cells have fixed identities that they can maintain without influence from their neighbors? “NO”! Each cell regulated its development to produce an entire embryo (No differential segregation of genetic potential)

17. C. elegans

18. How do cells know which genes to activate as they go through development? • Most organisms use 2 sources of info • parents • neighbors

19. Information from parents: The Cell lineage Mother cell But what makes “red” different from “blue” in the first place?

20. Information from parents: Segregation of determinants Mother cell • mechanism to generate asymmetry and subsequent cellular differentiation • determinants are usually proteins or mRNA. • information (proteins/ RNA) can be passed on uniformly, or can be segregated to one of the progeny cells. Unequal localization of "determinants" Cell division transfers determinants to a single daughter cell Cells are now different. Cell type A Cell type B

21. Information from neighbors: Cell interactions Mother cell • an alternative mechanism to generate asymmetry and subsequent cellular differentiation • cell division places daughter cells in different environments • different environments lead to different cell fates Cell division Cell type A Cell type B

22. Cells don’t have to be inside an animal to • communicate with each other • Examples • Yeast • Slime mold (Dictyostelium)

23. Single yeast cells talk to each other when they want sex! • 2 yeast cell types: "A" and "alpha" "A" "alpha" cell cell • only cells of different mating types can mate HOW? "alpha" factor "alpha" factor receptor "A" "alpha" cell cell "A" factor receptor each cell type makes a specific signal (factor) and has receptors only for the opposite signal "A" factor

24. These cell-cell signals lead the yeast cells that receive them to move together, change shape and ultimately fuse, producing a diploid cell

25. The slime mold develops into an animal only when it (they?) gets hungry!

26. The remarkable life cycle of a slime mold Slug/Grex cAMP signal Figure 2.10

27. Dictyostelium discoideum(slime mold) slug stage

28. The Cells of the Grex Differentiate

29. Conclusion: Even cells of the most simple eukaryotic organisms sense their environment, migrate, adhere to each other, differentiate, and interact Now, on to more complicated ones!

30. Breakthroughs in Modern Biology 1. All organisms share similar cellular machinery 2. All animals use this machinery in similar ways to direct embryonic development

31. Model Organisms in Developmental Biology Plants InvertebratesVertebrates Why use model organisms? What features do they have in common?