Organizer: a
region of the embryo that is capable of determining the differentiation of
other regions.
The term
“organizer,” or “primary organizer,” was introduced
by the German embryologist
and 1935 Nobel
laureate H. Spemann to designate the material
of the dorsal lip
of the blastopore—the prospective chordamesoderm—in the amphibian gastrula
.
When transplanted
to a remote site, for example, the
ventral side of the
embryo or the blastocoel,
the material of the
dorsal lip not
only differentiates into organs that
would normally arise
had the transplant
not taken place
but also induces
the development of neural and other
structures in areas adjacent to the transplant
site; this is an example
of primary embryonal induction
As a result of the action
of the organizer, a new, more complex
embryo forms in which
the organs are
situated roughly according
to their future distribution.
•
Primary organizer: primitive streak (it induces the development of
notochord)
•
Secondary organizer: notochord (it induces the development of neural
tube)
•
Tertiary organizer: neural tube (it induces the development of
somites of paraxial mesoderm)
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