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normal development and under experimental conditions led to their designation as
the “organization center” or “organizer” (Organisationszentrum/Organisator)
(Spemann 1921 ; Spemann and Mangold 1924 ). During experiments to test the
extent of cell fate determination in the gastrula, Spemann and Mangold showed that
the dorsal (upper) lip of the newt gastrula could retain its fate when transplanted to
the ventral side, typically forming a normally patterned “secondary embryo” that
matched the axial organization of the host. By grafting dorsal lips from albino
embryos into pigmented hosts (Fig. 6.10), they assessed the contribution of donor
cells, and strikingly demonstrated that the secondary dorsal tissues including the
neural tube were formed from induced host cells, with donor cells forming noto-
chord and somites (Spemann 1921 ; Spemann and Mangold 1924 ). The term orga-
nizer was thus coined by Spemann to reflect the ability of the dorsal (upper)
blastopore lip to direct the development of the characteristic embryonic structures
in vertebrates (notochord, somites, dorsal neural tube).
Spemann’s characterization of the organizer was highly influential and rapidly
led to the identification of homologous regions in other vertebrates: birds (chick and
duck, Hunt 1929 ; Waddington 1930 ), mammals (chick into rabbit, Waddington
1934 ; rabbit into rabbit/chick, Waddington 1936 , 1937 ; mouse into frog, Blum et al.
1992 ; and mouse into mouse, Beddington 1994 ), lamprey (Bytinski-Salz 1937 ;
Yamada 1938 ), and teleost fish (minnow, perch, and trout, Oppenheimer 1934a, b;
Luther 1935 ). Similar dorsalizing and neuralizing activities have also been found
for the prospective notochordal cells of invertebrate cephalochordates (Tung et al.
1962 ), suggesting the organizer is a basal feature of the chordates.
In bird and mammalian embryos, organizer activity is limited to the anterior
primitive streak, including the discrete anterior tip (Hensen’s node, “Knoten,”
Viebahn 2001 ; Blum et al. 2007 ).^2 In teleosts, the organizer corresponded to a thick-
ened area of the blastoderm on the dorsal side, termed the embryonic shield. The
initial characterization of the organizer identified several embryological properties
of the organizer that are central to its functions and are still studied in the context of
axial development. These are: the tissue-autonomous involution and convergent
extension of the dorsal lip and the differentiation into axial mesoderm (notochord),
the induction and regionalization of the neural plate, and self-organization/develop-
mental plasticity (i.e., regulation) (Spemann 1938 ).
As intriguing as these observations were, the organizer remained largely a phe-
nomenon until the application of molecular biological methods to developmental
biology in the 1980s–1990s. These were first used to characterize the neural induc-
ing activity of the organizer. This aspect was the historical readout of organizer
(^2) In this chapter, Hensen’s node is used to refer to the anterior tip of the primitive streak in all birds
and mammals, and is considered equivalent to the dorsal lip/organizer. Often the mouse organizer
is referred to as the “node” without the eponym. However, the node can also refer to the posterior
notochord “node” involved in left-right patterning, which lacks organizer activity. This terminol-
ogy can cause confusion since the latter structure is embryologically distinct from Hensen’s node.
In human embryology, Hensen’s node is referred to as the primitive node/knot or Hensens’ knot
(Gray 1918 ; Larsen et al. 2009 ).
D.W. Houston
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