Cell Differentiation in the Early Embryo ___________________________________ • Shortly after fertilization, the cleavage of the egg ___________________________________ into cells allows different fates to begin to develop in different cell populations ___________________________________ • Different species use different strategies to determine the fate of individual cells and groups of cells during cleavage ___________________________________ • Remember: differentiation means choosing a different set of proteins to express from the DNA ___________________________________ that you (and everyone else) inherited ___________________________________ ___________________________________ III. Early Cell Differentiation ___________________________________ A. Differentiation in individual cells is all ___________________________________ about choosing which DNA to Express B. Levels of Commitment to Differentiation ___________________________________ A. Staged Commitment B. Specification C. Determination ___________________________________ C. Three Major Strategies of Specification A. Autonomous B. Conditional ___________________________________ C. Synctitial ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ B. Levels of commitment to any certain fate... ___________________________________ • Much the same idea as competence.... ___________________________________ – A cell may be capable of differentiating to a certain cell fate: “competent” ___________________________________ – It may have gone far enough to be able to adopt that fate in a neutral part of the embryo: “specified” – It may have gone far enough to be able to adopt that ___________________________________ fate in an antagonistic part of the embryo: “determined” ___________________________________ – It may express all of the known characteristics of that fate: “terminally differentiated” ___________________________________ ___________________________________ Compare these terms to terms of “potency” ___________________________________ • Many overlaps between competence and potency.... ___________________________________ – A cell capable of differentiating to all fates: “totipotent” = complete competence ___________________________________ – A cell capable of differentiating to fewer fates: “multipotent” = partial competence = specified? ___________________________________ – A cell capable of differentiating to a single fate: “single potency” = determined – A cell expressing that single fate: ___________________________________ “terminally differentiated” ___________________________________ ___________________________________ C. Specification of Limited Potential in the Early Embryo ___________________________________ • Three Basic Strategies ___________________________________ – Autonomous Specification: you become the cell types dictated by the egg cytosol you get ___________________________________ – Conditional Specification: you become the cell types dictated by the signals you receive ___________________________________ – Synctitial Specification: autonomous specification without the membranes ___________________________________ • Most organisms use a combination of strategies one and two ___________________________________ ___________________________________ Autonomous (mosaic) specification ___________________________________ Sea Squirts ___________________________________ ___________________________________ ___________________________________ ___________________________________ A cell doesn’t need contact with other cells to fulfill its fate ___________________________________ ___________________________________ Conditional specification ___________________________________ Sea Urchins ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Conditional specification ___________________________________ What matters is the signals in any given location: if cells are transplanted they adopt the fate of the new position ___________________________________ ___________________________________ ___________________________________ If cells are removed, others adopt their fate ___________________________________ ___________________________________ ___________________________________ ___________________________________ • Most signals in the cleavage stage embryo reach most or all of the cells ___________________________________ • They tend to form gradients from high to ___________________________________ low from the point of their release ___________________________________ • In a pure “Conditional Specification” different cell fates would result from ___________________________________ different concentrations along the gradient ___________________________________ ___________________________________ ___________________________________ • In the “Combination Specification” a ___________________________________ greater complexity would form as signal gradients interacted with cells that got different starting material from the egg ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Gilbert’s Flag deal.... ___________________________________ Pure “Conditional Specification” ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Gilbert’s Flag Deal... ___________________________________ The “Combo Specification” The gradient is ___________________________________ specifying to the cells but they each have their own history ___________________________________ ___________________________________ If you transplant them to a new place, they will become those ___________________________________ cell types but will display their historical nature ___________________________________ ___________________________________ Part Figure II.10 Syncytial specification in Drosophila melanogaster ___________________________________ Insects reproduce ___________________________________ their nuclei during cleavage without making new cell membranes ___________________________________ Their nuclei then express ___________________________________ proteins into the common cytosol based on what TF’s ___________________________________ that region inherited from the egg cytosol. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ IV. Gastrulation ___________________________________ A. Background Information ___________________________________ B. Invertebrates 1. Sea Urchins 2. Snails ___________________________________ 3. Tunicates 4. C. Elegans 5. Drosophila melanogaster ___________________________________ C. Vertebrates 1. The Frog ___________________________________ 2. Zebrafish 3. The Chick Embryo 4. Mammals ___________________________________ ___________________________________ ___________________________________ ___________________________________ Gastrulation is a developmental process that takes the organism from the blastula stage through the gastrula stage ___________________________________ ___________________________________ Structure Æ Process Æ Structure ___________________________________ ___________________________________ ___________________________________ Gastrulation is the formation of the three germ layers ___________________________________ Terminology of cell movements in gastrulation: ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Gastrulation is the formation of the three germ layers ___________________________________ Terminology of cell movements in gastrulation: ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Gastrulation in the sea urchin ___________________________________ blastula ___________________________________ ___________________________________ ___________________________________ ___________________________________ gastrula (pluteus larva) ___________________________________ ___________________________________ Gastrulation in the sea urchin ___________________________________ ___________________________________ Skeletogenic Mesenchyme ___________________________________ ___________________________________ Invaginating ___________________________________ Endoderm ___________________________________ ___________________________________