April 11th lectureEmbryology Biology 441 Spring 2008 Albert Harris
Asexual Development by Budding, Regeneration of Organs, and Sorting Out of Dissociated CellsThese phenomena have in common that bodies or parts of bodies get be formed in other ways than by embryonic development of an egg.If scientists ever learn how to use embryonic stem cells to form replacement body parts, It may be essential to form these phenomena. ********************************************************* Development of unfertilized oocytes, as occurs male ants & bees, etc. and many other insects, and in some species of fish and lizards:
Development of unfertilized eggs IS
In male bees and ants, the resulting animals are haploid;
but in many cases they become diploid
by resorbing the second polar body.
*********************************************************
In some species, buds of somatic tissues form whole new animals.
Occurs in sponges, hydra, Corals, Bryozoa, & Tunicates
Although many of these are primitive, Tunicates (= Sea Squirts)
are members of the same phylum as we are: Chordates
All of these can develop sometimes from fertilized eggs,
but other times from buds of tissues within which cells rearrange
(And/or cells re-differentiate) to form all the cells of their bodies).
For example, Hydra form eggs and sperm, which develop
through embryonic stages of gastrulation, etc.
But mostly develop by asexual budding,
in which differentiated cells rearrange.
About half the species of sea squirts are NOT colonial,
and develop only from fertilized eggs.
Budding of colonial sea squirts:
John Allen, who recently earned his PhD in this department,
observed that then ends of arms of sea urchin plutei
sometimes bud off to form new blastula stage embryos,
which can themselves develop into plutei.
I don't think much research has been done on this!
*********************************************************
Another phenomenon that can be regarded as asexual is
"sorting out" of dissociated and randomly inter-mixed cells.
This has been reported in some species of colonial sea squirts
(and also sponges, hydra, other cnideria, and also larval sea urchins)
as H.V.Wilson was the first to discover this phenomenon (in sponges)
and later Wilson also discovered sorting-out in corals:
Randomly mixed cells rearrange to form
normally-functioning individuals.
*********************************************************
AS WE ALREADY LEARNED EARLIER IN THIS COURSE
Dissociated amphibian embryo cells sort out by germ layer,
and by subdivision of germ layers,
but can't form frog or salamander bodies,
just tissues and some organs, in abnormal geometrical patterns.
These phenomena were discovered and described in many research
papers by Johannes Holtfreter.
In the 1950s, Moscona and Trinkaus (separately)
proved that embryonic mouse and chicken tissues can be dissociated
Into random mixtures of cells, and then sort out by differentiated cell type
(but can't re-form functional animals, but only tissues and organs)
This led to the discovery of selective cell-cell adhesion proteins,
including N-CAM ("Neural Cell Adhesion Molecule")
and several kinds of Cadherins. ("Cadherin E, Cadherin P", many others)
*********************************************************
Debates on whether sorting out of dissociated cells is because
of qualitative differences (in kind of adhesion molecules)
or because of quantitative differences (in amount of adhesion):
Steinberg's "Differential Adhesion Hypothesis"
Evidence: transitive hierarchy of internal versus external position of sorting out
Alternative hypothetical explanations by Harris (1976) and Brodland (2005)
" differential cell surface contraction hypothesis"
This link is to computer simulations of different scientists' explanations of cell sorting. This is the web site of Dr. Wayne Brodland, at the University of Waterloo in Ontario.
*********************************************************
And if one human kidney is removed,
then the other enlarges to almost double size.
Such enlargement is called "hypertrophy"
*********************************************************
A FUNDAMENTAL QUESTION THAT USUALLY ISN'T ASKED:
Do these regeneration & sorting out phenomena imply or require
some mechanism that normally inhibits growth
of liver and kidney cells when these organs are normal size.
In other words, should we think of regeneration
As some kind of de-inhibition?
Or should we think of regeneration as some kind of stimulation?
Our own epidermis is constantly replaced
(and this replacement will speed up when and where
many epidermal cells are artificially removed).
An old hypothesis was that "chalone"
(a hypothetical substance) constantly inhibits skin growth.
When some of epidermis is removed, less total chalone would be produced,
de-inhibiting epidermal cell growth and division.
A bioassay was needed to identify the chemical used for this self-inhibition.
Whether chalone really exists, or not, has never been proven.
*********************************************************
This one of a few known examples in which cells switch
from one differentiated cell type to another
This phenomenon is called "Wolffian Lens Regeneration"
In contrast, Xenopus (frog) tadpoles regenerate lenses
by budding of cells from the inner surface of the cornea,
which is closely analogous to its normal embryonic development.
[should we look for de-inhibition mechanisms?]
*********************************************************
Regeneration of salamander legs:
Cartilage, muscle and other cell types (seem to) de-differentiate,
& form a "blastema" of rapidly dividing undifferentiated cells,
which then differentiate to form normal geometry of
cartilages and muscle.
But radioactive labeling of chondrocytes and myoblasts
indicates little or no switching from one cell type to another.
Does that mean organ shapes are caused by cell rearrangements?
If bone and other organ shapes are formed by rearrangement of
already differentiated cells during leg regeneration,
then is that evidence:
* That replacement legs could be made artificially out of stem cells?
* That normal embryonic formation of bones, muscles etc.
is caused by rearrangement of already-differentiated cells?
* Or do you think that salamanders have two completely
different mechanisms for making legs?
A completely different mechanism used if their leg gets cut off?
|