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?
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
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

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 (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