"The New York Times"
August 27, 2001

What Only the Embryo Knows

By STEPHEN JAY GOULD

Thomas Henry Huxley designated three men as the finest intellects of
19th century natural history: his dear friend Charles Darwin; his
most worthy opponent Georges Cuvier; and Karl Ernst von Baer, who
discovered the mammalian egg cell in 1827 and wrote the founding
treatise of modern embryology in 1828. Of these three, posterity
has largely forgotten von Baer, who suffered a severe mental
breakdown in the 1830's, but then recovered and moved to Russia
(not uncommon for a German-speaking Estonian national), where he
enjoyed a distinguished second university career, largely in
anthropology and lasting well into the 1870's.

 In 1828, von Baer enunciated the central principle of
embryological development, later known as "von Baer's law" and now
regarded as the correct interpretation of Ernst Haeckel's famous
(and erroneous) claim that "ontogeny recapitulates phylogeny," or
that the successive forms of embryology repeat the adult stages of
a lineage's evolution with the gill slits of an early human
embryo representing an ancestral fish and the later tail an
ancestral reptile, for example.

 By contrast, von Baer proposed a principle of progressive
specification and differentiation: One can first tell that an
embryo will become a vertebrate and not some sort of invertebrate,
then a mammal and not another kind of vertebrate, then a carnivore
and not a rodent or ruminant, then a dog and not a cat, and finally
Buster the Beagle and not another breed.

 Von Baer summarized his principle in an epigram: "The development
of the organism is the history of growing individuality in every
respect." In other words, successive narrowing and determination of
parts as complexity coagulates. No turning back after the blueprint
becomes finalized from a broad mass of initial potential. For an
appropriate literary metaphor, think of Lot's wife or Omar
Khayyam's lines: "The moving finger writes; and having writ, moves
on."

 Von Baer's law epitomizes the central issue, unfortunately rarely
discussed and little understood, in our current debate over
embryonic stem cells. The very structure of material reality
imposes a principle of trade- offs in both nature and human
affairs: One always gives something in order to gain. In
particular, we usually pay for complexity by surrendering
flexibility and von Baer's law encapsulates the embryological
version of this structural generality.

 In genetic terms that von Baer could not know, each cell of our
body contains a full set of genes. But embryological
differentiation into a specialized adult role as a brain cell,
liver cell or heart cell, for example leads to a "freezing" or
"turning off" of most of this potential apparatus, leaving active
only those few components regulating the specialized adult form and
function. The cells of the earliest, undifferentiated embryo
(little more than a clump of identical units in appearance)
maintain full capacity to develop in any direction; that is, all
their genes remain potentially active and recruitable.

 The irony of the trade-off, explicitly recognized by von Baer
nearly 200 years ago, inheres in the evolved surrender of this
embryonic flexibility as development proceeds toward our maximal
complexity. Cut a planarian flatworm in two, and the tail end
regenerates a head while the head end regrows a tail. For in this
simplest of bilaterally symmetrical invertebrates, with minimal
differentiation of internal organs, all cells retain the embryonic
potential to build any part of the body. This capacity for
regeneration the ability of cells at a wound site to
"dedifferentiate," or return to a state of early embryonic
flexibility becomes progressively lost in animals that evolve
greater adult complexity by von Baer's universal process of
"locking in," with increasing specialization of parts. We have, in
short, traded regenerative capacity for the undeniable evolutionary
advantages of maximal complexity.
 

 For this reason, we must use embryonic stem cells if we wish to
pursue a large body of enormously important, highly promising and
deeply humane research in how specific tissues and organs grow from
the broad potential of early cells derived from the fertilized
ovum. Speaking personally, I do not grant the status of a human
life to a clump of cells in a dish, produced by fertilization in
vitro and explicitly destined for discard by the free decision of
the man and woman who contributed the components. But I also have
no desire to offend the sensibilities of those who disagree. Thus,
if I could derive cells of similar flexibility in a different way,
I would gladly do so, even at considerable extra time and expense.
(By analogy, I did not mean to mock or flout our laws in using
marijuana to stave off severe and continuous nausea during some
particularly nasty and lengthy chemotherapy 20 years ago. But I
tried all the available anti- emetics, and they just didn't work. I
continue to regard my decision as fair, humane and, believe me,
importantly sustaining and life-affirming.)

 Unfortunately, von Baer's law, and nature's broader structural
rules of trade-off between complexity and flexibility, give us no
alternative to embryonic stem cells for now  and the research is
important and far more than merely theoretically lifesaving.
(Moreover, if we hope to find ways to dedifferentiate adult cells
and therefore learn to recover the requisite flexibility from cells
derived without offense to anyone then we must experiment with
embryonic cells in order to understand and control the mechanism of
their broad potentiality).

 As an old man, from his Russian periphery, von Baer made the
famous and rueful remark that all new and truly important ideas
must pass through three stages: first dismissed as nonsense, then
rejected as against religion, and finally acknowledged as true,
with the proviso from initial opponents that they knew it all
along. Genetic technology has brought us through the first stage.
Our current debate on stem cells resides in von Baer's second
stage, with the religious views of a clear, if powerful, minority
setting an unfortunate opposition to one of the most vital avenues
of beneficial research in our time. The third stage will arrive,
and we will marvel that we ever rejected a pathway toward knowledge
so imbued with life-saving capacity. May this third stage come
soon, as our understanding differentiates further into a true and
humane grasp of the virtues of flexibility.
Stephen Jay Gould, a professor of zoology at Harvard, is the author
of ``Questioning the Millennium.''

Oryginal:
http://www.nytimes.com/2001/08/27/opinion/27GOUL.html?ex=3D999929357&ei=3D=1&en=3Dfffaa9e7a30bc561
 

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