Principles of Experimental Embryology

1. Principles of Experimental Embryology Gilbert, Chapter 3

2. Today’s Objectives • Identify the current ways in which developmental biologists approach basic questions • Discuss the fact that Dev. Bio. Has implications and overlap for many areas of science • Define: differentiation, determination, specification • Define Autonomous and Conditional Specification

3. What questions do researchers in Dev. Bio. Ask? • Historically • What happens? • When does it happen? • Where does it happen? • Modern Dev. Biologists: • HOW? • 1894 - Wilhelm Roux - Entwicklungsmechanik - Developmental Mechanics

4. 3 Major Areas of Dev. Bio. Research • 1) How does the outside world influence embryo • Environmental, medical embryology (teratology) • 2) How do forces INSIDE the embryo encourage cells to differentiate? • 3) How do cells organize into tissues and organs?

5. Environmental Developmental Bio. • Environment plays a major role in the development of an organism • Early embryologists: • Alter environment to see if affect phenotype of embryo • Environment could play a role in the ways genes respond and organism develops

6. Example: Sex Determination in a Vertebrate: Alligator (p. 50) • Depends not on chromosomes, but on temperature! (Ferguson and Joanen) • During 2nd and 3rd week of incubation • Eggs incubated at 30ºC or below => Females • Eggs incubated at 34ºC or above => Males • At temps between these, mixture of males and females (see p. 50)

7. Also reflects upon the environment where eggs are laid/incubated • Nests near water - cooler -> females • Nests need levees - warmer -> males • Another effect - NOT a 1:1 sex ratio in nature • 10:1 ratio of females to males! • Why is this beneficial? • Lastly - effects wildlife management & environmental policies

8. What other environmental or ecological factors do you think could affect embryonic development?

10. 2) How do forces inside the embryo influence cell differentiation? (p. 53) • We’ll see MANY examples of this throughout the course • FIRST! Some new terms. . . Differentiation: cell type is specialized and performs specific function • Usually irreversible

11. Process of Cell Commitment • Cells are said to become “committed” to a certain fate • Haven’t yet BECOME that differentiated cell, but are restricted from being other cell types • 2 phases . . .

12. Phases of Cell Commitment (p. 53) • Specification: Cells begin to go toward a differentiated cell type, but this is reversible • Cells are able to differentiate into that cell type if cultured in vitro (and away from other cells) • Determination: Cells are further toward differentiated, but are not functioning as the final cell type.Irreversible • If move cells to NEW area of embryo, they will develop as if in ORIGINAL location.

13. 2 Types of Specification • Autonomous (kind of like Independent) • Cells can differentiate on their own, into their final cell types • Conditional (kind of like Dependent) • Cells must interact with other cells/or their environment to differentiate properly into their final cell type

14. Autonomous Specification (p. 53) • Example • Remove certain cells from a Patella (mollusc) embryo • Those cells grow cilia, just as they would have in the normal embryo http://www.theseashore.org.uk/theseashore/speciespages/Limpets.jpg.html

15. Patella vulgata (common Limpit)

16. Peach colored cells are Autonomously Specified to grow cilia

17. Conditional Specification • Involves interactions from neighboring cells • Each cell still has ability to become more than 1 differentiated cell type • Surrounding or neighboring cells provide signal to restrict the fate of those cells

18. Conditional Specification Example (p. 58-59) • Remove blastomeres from early frog embryo • Transplant from Dorsal (Back) side to Ventral (belly) side • The cells that were transplanted become Belly • What can we conclude about the commitment of those original cells before transplant?

19. Conditional Specification

20. We’ll see many examples of both Autonomous and Conditional of specification as we investigate how embryos develop • Important terms to recall throughout the semester

21. One more Idea . . . • Early embryologists had different views with regard to how early on cells were specified and whether cells were totipotent

22. Mosaic development • Each cell is preprogrammed, and removing 1 cell will result in a defect in the embryo

24. Regulative Development • If you remove cells from an embryo, that embryo will fill in for the missing cells • Fills in the blanks • Won’t be any smaller in size, or missing any pieces • How will the cells know what to replace?

26. 3) How do cells organize into tissues and organs? • Many questions! • How are tissues formed? • How are organs constructed from tissues? • How do organs get to their particular locations? • How do organs grow? • How do organs get polarity?

27. Lots of answers! • We’ll see examples throughout the course as we examine specific organs • Limb -really interesting and well-understood • For now, let’s start with - cells need to stick together to make tissues

28. Differential Cell Affinity • Certain cells have molecules (like proteins, glycoproteins) on their surface that allow them to stick (adhere) to other cells • Important inside the embryo • Cells that are going to coordinate to make a tissue or organ have to be able to stick together!

29. Fig. 3.23 (p. 68)

32. Cells can sort according to their affinity to adhere to other cells (stick) • Here according to Germ layers (endoderm, ectoderm, mesoderm • Again, this is essential in embryonic development and we’ll see it often in our lessons