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 Terminal differentiation 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