Entwickslungmechanik

Entwickslungmechanik

Developmental Mechanisms

Cell SpecificationAutonomous ConditionalSyncytial

Cell SpecificationDifferentiation

The process and the processes associated with a cell becoming specialized

Occurs in multiple steps

Commitment Specification Determination

Terminal differentiation

Cell Specification

CommitmentSpecification

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

Cell Specification

CommitmentDetermination

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

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

Cell Specification

Characteristic of most invertebratesDifferential acquisition of cytoplasmic

determinants (morphogenetic factors) Invariant cleavages and invariant

lineagesGives rise to mosaic developmentSpecification precedes cell migrations

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

Autonomous Cell Specification

Autonomous Cell SpecificationAutonomous Specification in Mollusc Embryos

Autonomous Cell Specification

Autonomous differentiation of dissociated blastomeres of tunicate embryo

Autonomous specification implies localized morphogenetic determinants

Morphogenetic determinants become partitioned into individual blastomeres during cleavage stages

Morphogenetic determinants are both proteins and mRNAs

Autonomous Cell Specification

Characteristic of all deuterostomesSpecification via cell-cell interactionsCleavages are variable – no definite cell

lineagesDevelopment is regulativeCell rearrangements precede &

accompany specification events

Conditional Cell Specification

Conditional Cell Specification

Transplantation experiment Defect experiment

Conditional specification implies the interaction of cells in some manner

Cells must recognize their neighborsCells must influence each others

differentiation

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

Conditional Cell Specification

Conditional Cell SpecificationRegenerationBoth head and tail can be removed from

planariaHead 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

Conditional Cell Specification

Regeneration in flatworms is directed by an A-P morphogen gradient

Conditional Cell SpecificationMorphogen gradientsSource – cell(s) that produce

morphogenSink – site where morphogen is used

(degraded)Cells are either responding to varying

concentrations of morphogen or to varying lengths of exposure times

Conditional Cell SpecificationMorphogen Gradients & Thresholds

Cells in each flag differentiate in response to the morphogen gradient yet retain their original specification as “French” or “US”

Conditional Cell Specification

Amphibian Morphogen Activin

Conditional Cell Specification

Conditional Cell SpecificationMorphogenetic FieldsLimb fields

Region where cells are committed to form a limb Cells within field are not determined to be any

particular part of the limbOther fields

Eye & heart in vertebrates Imaginal discs in Drosophila

Conditional Cell Specification

Limb Field

Conditional Cell SpecificationInvasion and splitting of limb field by nematode parasites in frogs results in duplication of limbs

Syncytial Cell SpecificationSyncytium

Cytoplasm that contains multiple nucleiMany 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

Syncytial Cell

SpecificationLocalized Maternal mRNAs in Drosophila establish protein gradients in the syncytial embryo

Syncytial Cell SpecificationThe 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

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