Jd. Furlow et al., A SET OF NOVEL TADPOLE SPECIFIC GENES EXPRESSED ONLY IN THE EPIDERMISARE DOWN-REGULATED BY THYROID-HORMONE DURING XENOPUS-LAEVIS METAMORPHOSIS, Developmental biology, 182(2), 1997, pp. 284-298
Four genes were identified in a screen for thyroid hormone-induced dow
n-regulation of gene expression in Xenopus laevis tadpole tails. All f
our encode extracellular glycoproteins that are expressed exclusively
in the apical cell layer of the entire tadpole epidermis, which is the
equivalent of the mammalian fetal periderm. The onset of the four nov
el genes' expression late in embryogenesis, their activity throughout
the life of the tadpole, their repression by exogenously added thyroid
hormone, and the spontaneous cessation of their expression at the end
of tadpole life are closely coordinated. These facts suggest that the
protein products of these genes form a novel albeit temporary barrier
or other structure in the tadpole epidermis that functions in lieu of
the cornified, stratified epithelium of the adult epidermis. We have
exploited the cloning of these genes for use as cell-specific markers
to follow the appearance and loss of apical cells during development.
We were able to demonstrate directly that the apical cells are derived
from a stratification of the embryonic ectoderm at the onset of the f
ormation of a true epidermis. The apical cells uniformly cover the sur
face of the tadpole until metamorphosis, when the expression of the fo
ur larval epidermis-specific genes is lost coordinately over the entir
e tadpole. In contrast, the adult epidermis develops with a distinct r
egional specificity: adult keratin is first expressed up to a line sep
arating the body and tail epidermis and finally appears in the tail on
ly at metamorphic climax. Finally, our analysis reveals that the TH-in
duced down-regulated gene expression program during metamorphosis is v
ery different from the previously described up-regulated program which
involves multiple cell types and several waves of gene expression cha
nges. The down-regulated program only consists of the repression of a
small number of genes which are expressed in larval cells preprogramme
d to die during the larval to adult transition at metamorphosis. (C) 1
997 Academic Press.