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

2

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 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 What kinds of PROCESSES are required?

5 Differentiation is 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 1.Cell Biology 2.Genetics 3.Molecular Biology

9 Let’s Review the Basics 1.The body is made of millions to billions of cells. 2.Cellular machinery is largely made up of proteins 3.Because of their different tasks, different cells contain different proteins 4.Proteins are made up of chains of amino acids, and these amino acids are "encoded" in the cell's DNA 1.Information flows from DNA to RNA to Protein 2.When one gene is mutated, one protein is affected (usually disabled). 5.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) 1.Mosaic development 2.Regulative development

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

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

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

17 C. elegans

18 How do cells know which genes to activate as they go through development? Most organisms use 2 sources of info 1.parents 2.neighbors

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

20 Mother cell Unequal localization of "determinants" Cell division transfers determinants to a single daughter cell Cells are now different. Segregation of determinants Cell type ACell type B 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. Information from parents:

21 Mother cell Cell interactions Cell type A 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 type B Cell division Information from neighbors:

22 Cells don’t have to be inside an animal to communicate with each other Examples 1.Yeast 2.Slime mold (Dictyostelium)

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

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 cAMP signal Slug/Grex 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 Invertebrates Vertebrates Why use model organisms? What features do they have in common?