Entwickslungmechanik

Developmental Mechanisms

Wilhelm Roux:

“We must not hide from ourselves the fact that the causal investigation of organisms is one of the most difficult, if not the most difficult, problem which the human intellect has attempted to solve… since every new cause ascertained only gives rise to fresh questions regarding the cause of this cause.”

Experimental Embryology

Mechanics of Cell Specification

Mechanics of Morphogenesis

Experimental Embryology

Experimental embryology had its beginnings in the testing of hypotheses of cell specification

Experimental Embryology

August Weismann’s Germ Plasm Theory1883First testable hypothesis of cell

specificationDeterminants within zygote were

partitioned into cells as development proceeded

Experimental Embryology

DifferentiatedCells

DeterminedCells

Weismann’s Hypothesis

Experimental Embryology Weismann hypothesis

Egg and sperm provide equal chromosomal contributions to embryo

Chromosomes carried the inherited determinants The chromosomes/determinants were somehow

differentially distributed to embryonic cells Only the germ cells received all determinants

(germ plasm)

Experimental Embryology

Testing Weismann’s HypothesisEach half of a developing frog embryo is

derived from one cell of the two cell staged embryo

Each blastomere are the two cell stage must therefore contain left and right determinants

Experimental Embryology

4 Categories of Experiments

Defect experiment

Isolation experiment

Recombination experiment

Transplantation experiment

Experimental EmbryologyWilhelm Roux (1888) 1st to test Weismann’s hypothesis.Roux’s defect experiment supported mosaic development and partitioning of determinants.

Experimental Embryology

Hans Driesch’s (1892)Isolation Experiment: Dissociation of 4 and 8 cell sea urchin embryos demonstrated regulative development.

Dissociation was accomplished by incubation in Ca free water implies that Ca is important for cell adhesion

Experimental Embryology Roux’s experiment suffered from an experimental

design flaw

By killing one of the two blastomeres, but not removing it, the remaining dead cell prevented rearrangement of the living cells to produce a normal embryo

Driesch’s experiment did not suffer from this flaw since he had separated all the cells of the embryo

McClendon (1910) repeated Driesch’s experiment in frogs with the same result

Experimental Embryology

Major Impact for Embryology

In regulative development, the prospective potency is greater than the prospective fate