Principles of Experimental Embryology Gilbert Chapter 3 Todays

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 2 nd and 3 rd 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. 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

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