Ma. Ros et al., FORMATION OF EXTRA DIGITS IN THE INTERDIGITAL SPACES OF THE CHICK LEGBUD IS NOT PRECEDED BY CHANGES IN THE EXPRESSION OF THE MSX AND HOXD GENES, Anatomy and embryology, 190(4), 1994, pp. 375-382
By in situ hybridization we studied the expression patterns of Msx and
Hoxd genes during the late development of the chick leg autopodium (f
oot) and compared them to patterns during the experimental development
of interdigital extra digits. Extra digits are induced in the third i
nterdigital space after various experimental manipulations, such as tr
ansient isolation of the interdigit, or removal of the interdigital ma
rginal ectoderm and mesoderm. Msx1 and Msx2 are normally expressed in
the interdigital tissue programmed to die. Our experiment changes the
fate of the interdigital tissue from cell death to chondrogenesis and
provides a good model for studying Msx involvement in defining areas o
f programmed cell death. Among the proposed roles for Herd genes is th
eir involvement in the specification of digit identity early in develo
pment. The induction of extra digits allows us to examine whether this
new morphogenetic commitment of the interdigital tissue involves chan
ges in the domains of expression of Herd genes. Our results show that
extra digits develop without a previous modification of the normal pat
tern of expression of Msx or Hoxd genes. This observation does not sup
port the correlation between the expression of Msx genes and programme
d cell death and suggests a role for these genes in maintaining the in
terdigital tissue in an undifferentiated state. Our results show that
an increased number of digits can be formed without modifications in t
he pattern of expression of the 5'-located Hoxd genes and suggest the
existence of latent or residual digit organization mechanisms past the
time when digits are normally determined, independent of Hoxd gene ex
pression.