Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 165

Spinal cord signal, Wnts
(Wnt-1, Wnt-3a, Wnt-4 and Wnt-7b
expressed by the spinal cord)

Wnt - 4

Wnt - 4

Induced
mesenchyme

Wnt - 11
or X
Ureter tip

Pax - 8

Wnt - 4

Stroma

Collecting
duct

Figure 3.37 A model showing how Wnt-4 operates as a second tubule inductive signal. A
ureter-derived signal, in the form of Wnt-11 or X, induces the expression of Wnt-4, which
autoregulates itself and triggers tubule morphogenesis. Wnt-4 signaling involves Pax 8. The
spinal cord acts as an inducer tissue as it expresses a panel of Wnts, including Wnt-4, that
triggers the autoregulated Wnt-4 gene expression to induce tubules.
Source : From Vainio et al. (1999) .

the embryonic brain and spinal cord can induce their nephrogenic differentiation
( Sariola et al., 1989 ).
Embryonic neurons, whose axons establish contacts with mesenchymal cells,
are the most effective inducers of differentiation of mesenchymal cells in vitro , and
maybe in vivo as well ( Sariola et al., 1989 ).

Neural Control of Primary Sex Determination

For more than a century, it has been established that sex in animals is determined by
sex chromosomes. The idea originates in an observation made in 1891 by the zoolo-
gist Hermann Henking (1858-1942) that a chromosome is present only in a part of
the sperm cells of the European bug, Pyrrhocoris apterus L . He named the chro-
mosome the “X-Faktor” and expressed the idea that it could play a role in insect
sex determination. Later, the American biologist Clarence Erwin McClung (1870-
1946) expanded the research, identifying the “X-element” as a possible factor of
sex determination in other insects and animals; he also observed that only half of