first concept : What is meant by "regulative embryos" as contrasted with "mosaic embryos".

Or "regulative development" as contrasted with "mosaic development".

The Hans Driesch Experiment (1892)
Sea urchin embryos at the 2 cell and 4 cell stages
can be mechanically separated in to 2 or 4 cells ;
and each isolated cell will continue embryonic development, and will form a half-size or a 1/4th size 'scale model' of a normal pluteus .
(with all the parts, reduced in size in proportion).

This experiment really works; John Allen did his PhD research making half-sized plutei, for the purpose of ecological / biomechanical research. John's office is in this building..

Later, someone did the reverse of Driesch's experiment, and pushed together two urchin eggs at the one-celled stage, with the result that they developed into a double sized pluteus larva .
The goal of these experiments was to understand the mechanisms that control which cells will form which parts of the body, in the right geometry..

Driesch was so surprised by the results that he believed that the control mechanism must be nearly supernatural, not mechanistic, and that each embryo must contain an "entelchy" (a sort of vital spirit that controlled things).

My PhD advisor used to give the equivalent lecture to the one I am now giving, and would say the experiment showed the embryos must contain at least TWO entelchies, one for each cell (or 4!)
{Really, the concept is more subtle, & not so silly}.

Later other researchers separated frog eggs into two cells at the 2-cell stage, and fused others at the one-cell stage; and SOME of them developed..

What is the point?: The mechanisms that control cell fate can operate over at least an 8-fold range of volume in echinoderm embryos, & over a 4-fold range in frogs & salamanders
And over a hundred-fold range in Dictyostelium!.

Therefore, urchins and frogs are said to have "regulative development"
The adjustment of cell fates according to new positions is called "embryonic regulation"..

mammal embryos are even more regulative that echinoderms; many embryos can be fused, or one can be separated into 8 cells, and still develop into normal embryos. We are the champs!.

The embryos of many other species will develop into abnormal "half embryos" or "quarter embryos" if their cells are separated..

Snail and clam embryos do not regulate well.
Their development is said to be "Mosaic" (= non-regulative).

Nematode (round worm) development may be the most mosaic of all kind of animals.
(e.g. Caenorhabditis elegans).

When biologists can't figure out the cause of a certain phenomenon, then they can find kinds of animal in which this phenomenon doesn't occur ! (I am making a joke, here, sort of...).

Embryonic development of Drosophila and other flies is also very mosaic . (but some wasps have very regulative development).

What is the fundamental, causal difference between mosaic as compared with regulative development?.

Some of the explanations that people have proposed.

a) It's just a time difference in how early the embryonic cells become irreversibly committed to differentiate.

Early decision --> mosaic development Later decision --> regulative development .

Special cytoplasmic components in the oocyte-> mosaic
Signals from one cell to other cells --> regulative.

Regulative development is like being able to regenerate
Mosaic development is like NOT being able to regenerate..

The classic case of such a phenomenon is the formation of the "yellow crescent" at the one-celled stage of the embryos of sea squirts (= tunicates = ascidians). Something in this special cytoplasm "turns on" the genes for becoming muscle cells..

Some of Dick Whittaker's research on this is described and shown in pictures on pages 58 and 59 of our textbook.. The other major researcher on this subject is a woman named Billie Swalla, at the U. of Washington. So far, nobody has managed to find out what actual chemical in the yellow cytoplasm selectively turns on the genes for being muscles.
Everyone expects it to be a transcription factor protein..

Four different categories of embryological experiments (surgical ones).

a) Defect experiments - removing, destroying, or damaging some particular part of an (early) embryo, to find out how this changes the development of the rest of the body..

b) Isolation experiments - removing a particular part of an embryo, and then studying the further development of this part that you removed. For example, will it differentiate?.

c) Recombination experiments - replacing part of an embryo with some part taken from another place, or another embryo. For example, grafting the notochord can induce formation of an entire second embryo. Used to study cell-cell signalling..

d) Transplantation experiments - part of the embryo is replaced by the same part of a different embryo, often of another species!.

This method has often been used to prove that a certain differentiated cell type does/does not develop from neural crest, or from any other germ layer or subdivision of a germ layer..

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The still-unsolved mystery of how to explain the results of Wilhelm Roux' "hot needle" or "killed blastomere" experiment
diagrams on top of page 61 of the textbook.

If cells are killed, but left in position, then regulation does NOT occur, and the surviving cells continue to differentiate into the parts of the body that they would normally have formed.

This experiment was actually done 10 years before Driesch's, and the wrong interpretations were drawn form it. People thought different cells received different genes (although this was years before the word "gene" was invented, that was the basic idea)

Driesch's results (that is, the occurance of embryonic regulation) disproved this false interpretation of Roux's "hot needle" experiment. But nobody has ever been able to figure out what is the true explanation of Roux' observations. Why does regulation NOT occur when a damaged cell is left in place. How is regulation prevented by the presence of a moribund (but actually not quite dead) cell? Does this tell us something about the mechanism of regulation?
Does it tell us something about the mechanisms that control cell fate in normal development.

How dead can a cell be, and cause regulation not to occur?
Suppose that you killed one whole embryo at the one-cell stage, and then pushed it together with an undamaged one-cell stage embryo! What would happen? Remember what happens when you push two undamaged one-celled stage embryos together?! They develop into a single pluteus, with half the cells from one original embryo, and half the cells from the other embryo.

So would pushing cell "killed" by a hot needle into the side of a normal one-cell stage cause some kind of (perverse) reverse regulation, in which the normal one-cell stage embryo would be suppressed from forming half the embryo, and would only form the other half.

Nobody has tried this (except for me and an undergraduate in this course; and we failed for non-fundamental reasons, in Dec 1990).

Other interesting experiments would be to try Roux' experiment in mammal embryos at the 2 or 4 cell stage, or in Round Worm embryos at early stages.

Any exam questions on these latter topics could be answerd for full credit just by demonsrating that you understand the concepts.

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