Framework

• Developmental processes are driven by differential gene expression

• Gene expression programs are induced by signals between neighboring tissues of the developing embryo

• Cell movements during embryogenesis place particular tissues into juxtaposition

“Official glossary” (from Wolpert)• Morphogen: Any substance active in pattern

formation whose spatial concentration varies and to which cells respond differently at different levels

• Morphogenesis: The process involved in bringing about changes in form in the developing embryo

• Pattern formation: Process by which cells in a developing embryo acquire identities that lead to a well-ordered spatial pattern of activities

What development accomplishes• Differentiation of all of the required cell-

types from a single fertilized egg (oocyte)

• Morphogenesis: Precise arrangement of these cells into tissues and organs

• Pattern formation: Precise arrangement of tissues and organs to achieve a reproducibly working organism capable of reproduction

• Epigenesis: the de novo formation of an organism from “disordered” egg cytoplasm

Demonstration of nuclear potential in Acetabularia (1930’s)

Figure 2.5page 31Gilbert

The nucleus and Epigenesis

• The nucleus contains the instructions that drive epigenesis/development

• Chromatin is the instructional unit (DNA plus proteins). The state of the chromatin is set by “epigenetic control” mechanisms

• Covert “epigenetic” changes occur during the early cleavages of the fertilized egg into the blastomeres of the embryo

For example: DNA methylation

Embryonic cell division is not the same in all kinds of organisms

So there is a sense that these zygotes “know whatthey are” when the begin to divide. The yolk and its position obviously plays a role in determining the cleavage pattern. But, there are sometimes other products stored in the egg by the mother that act as morphogenetic determinants.

Next slide

Major morphogenetic strategies• Autonomous specification

– Morphogenetic determinants deposited in the egg become segregated by cell division

– Determination of cell fate is early

• Conditional specification– Cell fate determination is later and depends on

the position of the blastomere in the embryo– Removal of cells is compensated for by others– Each cell has the potential to give rise to more

cells than it normally does

Specification of cell fate: Autonomous vs. Conditional

Pages 56-66 of text

Syncitial specification

• Mainly seen in insects (Drosophila)

• Gradients form, over time, of morphogens deposited in the egg by the mother

• These morphogens become segregated by cell membranes which grow into the egg

• Drosophila lectures: End of February

GASTRULATION

• The process that puts cells into position to have their fates determined

• Gastrulation involves a particular repertoire of cell movements which can be classified

• Gastrulation will result in the formation of three “germ layers” from which the organs of the embryo will arise

Movements of Gastrulation

Result of gastrulation: The 3 germ layers

Gastrulation and Conditional Specification

• Newly positioned tissues (germ layers) interact with one another to “induce” organ formation– Cell-cell interactions via receptors (juxtacrine)– Soluble signaling molecules (paracrine)

• Morphogen gradients link position to cell fate

• Signal transduction activates gene expression which leads to specification and lineage commitment

Morphogen gradients: different concentrations of a factor induce different

gene expression

Stages of commitment to cell fate• Specification

– Changes in gene expression which are labile and changeable. The gene expression “allows” that cell to differentiate along a pathway but does not irreversibly commit the cell

• Determination– Further changes in gene expression which seal

the lineage fate of the cell and eliminate alternative choices

Pro-B cells from Pax-5 deficient mice are “specified”

Isyk Oct-25 btk E2AVpreB blk PU.1B29 lyn EBF

“Pro-B cell”

IL-7

Pro-B cells from Pax-5 deficient mice are “specified”but not “determined”

Isyk Oct-25 btk E2AVpreB blk PU.1B29 lyn EBF

“Pro-B cell”

MacrophageM-CSF

Dendritic cellGM-CSF

NK cell

IL-2

Osteoclast

Trance

T cellmouse

Recent data illustrating the concepts of specification vs. determination

Nutt, SL, et. al. (1999) Commitment to the B-lymphoid lineage depends on

the transcription factor Pax-5

Nature, 401:556-562

committed

E2AEBF

specifiedPax-5B lymphocyte

development

Forming a solid organ

• How do cells “stick together” to form tissue?

• Coordination of tissues from multiple germ layers to form a single functional organ– Tissue layers– Organ polarity

• Cell adhesion molecules: Cadherins (p66-74)

Cadherins (Ca2+dependent adhesion molecules)

See alsoWebsite 3.8

Types of cadherins

• E- cadherin: expressed on early embryonic cells in mammals. Later becomes restricted to embryonic and adult epithelial tissue

• P-cadherin: Trophoblast cells (placental)

• N-cadherin: First mesodermal, later CNS

• EP-cadherin: frog blastomere adhesion

• Protocadherins: not connected to catenin

Mechanisms of cadherin based cell “sorting” into tissues

• Differential expression of cadherin type– Neural vs epidermal cells (N vs. E cadherin)

• Different levels of cadherin expression– Oocyte positioning in follicle

• Loss or switch of cadherin expression– Neural crest emigration from the neural tube– Protocadherin switching in frog gastrulation

• See pages 311-312 (Chapter 10 of Gilbert)