Entwickslungmechanik

1. Entwickslungmechanik Developmental Mechanisms

2. Cell Specification • Autonomous • Conditional • Syncytial

3. Cell Specification • Differentiation • The process and the processes associated with a cell becoming specialized • Occurs in multiple steps

4. Cell Specification • Commitment • Specification • Determination • Terminal differentiation

5. Cell Specification Commitment • Specification • A cell is said to be specified when: • Cells differentiate autonomously when removed from normal environment (embryo) and placed in a neutral environment (culture medium) • Placing cells into a non-neutral environment (a different place in the embryo) will cause the cells to follow the fate of other cells the new location rather than their original fate

6. Cell Specification Commitment • Determination • A cell is said to be determined when: • Cells differentiate autonomously even when placed in a non-neutral environment • When moved to a different location within the embryo, the transplanted cells differentiate according to their original fate

7. Cell Specification Terminal Differentiation • When a cell can no longer change or be changed into anything other than the cell type it is • Can be associated with permanent changes in DNA • Methylation is a prominent factor • B-cells (plasma cells) rearrange the immunoglobulin (Ig) genes so that they can now only form a single type of Ig

8. Autonomous Cell Specification • Characteristic of most invertebrates • Differential acquisition of cytoplasmic determinants (morphogenetic factors) • Invariant cleavages and invariant lineages • Gives rise to mosaic development • Specification precedes cell migrations

9. Autonomous Cell Specification • Isolated blastomeres will differentiate into the same cell type they would have differentiated into if left in the embryo • Removal of cells from the embryo results in an embryo missing the cell types the removed cells would have differentiated into • Embryo is in essence a mosaic of cells. Removal of any cell from the early embryo results in a missing region of the embryo

10. Autonomous Cell Specification Autonomous Specification in Mollusc Embryos

11. Autonomous Cell Specification • Autonomous differentiation of dissociated blastomeres of tunicate embryo

12. Autonomous Cell Specification • Autonomous specification implies localized morphogenetic determinants • Morphogenetic determinants become partitioned into individual blastomeres during cleavage stages • Morphogenetic determinants are both proteins and mRNAs

13. Conditional Cell Specification • Characteristic of all deuterostomes • Specification via cell-cell interactions • Cleavages are variable – no definite cell lineages • Development is regulative • Cell rearrangements precede & accompany specification events

14. Conditional Cell Specification Transplantation experiment Defect experiment

15. Conditional Cell Specification • Conditional specification implies the interaction of cells in some manner • Cells must recognize their neighbors • Cells must influence each others differentiation

16. Conditional Cell Specification • Cell-Cell interactions • Restriction of fates • Inhibition by neighboring cells • Inductions • Neighboring cells initiate induction events • Morphogen gradients • Soluble molecules which diffuse from source cells and affect the differentiation of cells at a distance

17. Conditional Cell Specification Regeneration • Both head and tail can be removed from planaria • Head and tail will regenerate from anterior and posterior ends respectively of trunk • An anterior – posterior gradient allows middle piece to “know” what parts to regenerate on each end

18. Conditional Cell Specification Regeneration in flatworms is directed by an A-P morphogen gradient

19. Conditional Cell Specification Morphogen gradients • Source – cell(s) that produce morphogen • Sink – site where morphogen is used (degraded) • Cells are either responding to varying concentrations of morphogen or to varying lengths of exposure times

20. Conditional Cell Specification Morphogen Gradients & Thresholds Cells in each flag differentiate in response to the morphogen gradient yet retain their original specification as “French” or “US”

21. Conditional Cell Specification Amphibian Morphogen Activin

22. Conditional Cell Specification

23. Conditional Cell Specification Morphogenetic Fields • Limb fields • Region where cells are committed to form a limb • Cells within field are not determined to be any particular part of the limb • Other fields • Eye & heart in vertebrates • Imaginal discs in Drosophila

24. Conditional Cell Specification Limb Field

25. Conditional Cell Specification Invasion and splitting of limb field by nematode parasites in frogs results in duplication of limbs

26. Syncytial Cell Specification • Syncytium • Cytoplasm that contains multiple nuclei • Many insect eggs begin development as a syncytium – multiple nuclear divisions occur before cellularization • The syncytial cytoplasm contains gradients of localized proteins and mRNAs • Upon cellularization, unequal amounts of morphogens becomes localized within cells

27. Syncytial Cell Specification Localized Maternal mRNAs in Drosophila establish protein gradients in the syncytial embryo

28. Syncytial Cell Specification • The morphogens typically functioning in syncytial specification are transcription factors • The unequal distribution of these factors into cells of the embryo allow for the specific activation or repression of genes within the nuclei of the newly formed cells

29. Cell Specification • No embryo uses only one of the types specification • Regulative embryos use autonomous specification frequently later in development • Embryos that use autonomous specification early use conditional specification for the formation of some organs • Syncytial specification can only be used for very early events, both autonomous and conditional specification occur later