331induce a full body axis, like the Organizer. Unlike the organizer, however, these cells
did not contribute any cells to the axis itself and differentiated into gut endoderm.
This region of the embryo was named the Nieuwkoop Center, in honor of the man
who discovered the mesoderm-inducing capacity of cells in the vegetal pole (Gerhart
et al. 1989 ). The Nieuwkoop center does not form in embryos treated with UV light
during the first cell cycle, and all cells in resulting embryos adopt ventral fates
(Gimlich and Gerhart 1984 ). Thus, the Nieuwkoop center is both necessary and suf-
ficient to induce overlying cells in the embryo margin to become the Organizer.
7.5.1.4 The Three-Signal Model
A third inducing activity was uncovered by experiments that recombined the dorsal
marginal zone (DMZ) with the ventral marginal zone (VMZ). In these experiments,
the tissue was explanted after mesoderm had been specified, just before the onset of
gastrulation. When explanted at this stage and grown in isolation, the VMZ gener-
ates ventral mesodermal cell types, such as blood and mesothelium, but does not
produce notochord and generates only a little muscle. By contrast, when combined
with the DMZ, cells from the VMZ differentiate into muscle at a high frequency and
aMesoderm inducing factorbFig. 7.6 Experiments to find the mesoderm inducing signals in Xenopus. (a) Animal cap explants
differentiate into atypical epidermis when cultured alone, but form mesoderm in combination with
vegetal cells, or with mesoderm-inducing factors. (b) The Three Signal Model hypothesizes that
there are three mesoderm-inducing activities in frogs: The first signal induces dorsal mesoderm
(black arrow) in animal caps, the second signal induces ventral and ventrolateral mesoderm (yel-
low arrows) in animal caps, while the third signal dorsalizes ventral mesoderm in Ventral Marginal
Zone (VMZ) explants (white arrows) (Dale and Slack 1987b)
7 Establishment of the Vertebrate Germ Layers