Organizer

organizer

See PIM and PDA.

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organizer [′ȯr·gə‚niz·ər]

(embryology)

Any part of the embryo which exerts a morphogenetic stimulus on an adjacent part or parts, as in the induction of the medullary plate by the dorsal lip of the blastopore.

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Organizer 

(embryology), a region in the embryo of chordate animals that exerts an inductive influence on adjacent areas.

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 (seeGASTRULA). 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 (seeINDUCTION). 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.

Two organizers are distinguished. The cephalic organizer consists of the anterior section of the notochord and the material of the prechordal plate and induces formation of the anterior sections of the brain. The trunk organizer, consisting of the remaining material of the notochord and the somites, induces formation of the posterior sections of the brain and the trunk and tail structures. Organizers analogous to amphibian organizers have been found in all classes of chordates; these include Hensen’s node in birds and the posterior section of the embryonal disc in teleosts.

The term “organizer” is also applied to other embryonic organ rudiments that exert an inductive effect on adjacent areas; these are the secondary and tertiary organizers, as distinct from the chordamesoderm, which is called the primary organizer. Thus, the rudiment of the eye that originates as a result of primary embryonal induction is a secondary organizer; it induces formation of the iens in the ectoderm. The lens, in turn, is a tertiary organizer that induces formation of the cornea.

The term “organizer” stresses the concept that it is the rudiments of organs of living embryos that act as inductors and not substances excreted from the tissues of embryos or adults.

REFERENCES

Saxen, L., and S. Toivonen. Pervichnaia embrional’naia induktsiia. Moscow, 1963. Pages 21–26. (Translated from English.)
Tokin, B. P. Obshchaia embriologiia. Moscow, 1970. Pages 262–80.
Bodemer, C. Sovremennaia embriologiia. Moscow, 1971. Pages 155–57. (Translated from English.)

G. M. IGNAT’EVA