Principles of Experimental Embryology Gilbert, Chapter 3

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

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

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?

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

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)

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

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

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

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 . . .

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.

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

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

Patella vulgata (common Limpit)

Peach colored cells are Autonomously Specified to grow cilia

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

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?

Conditional Specification

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

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

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

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?

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?

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

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!

Fig. 3.23 (p. 68)

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