||Fig. 2. Developmental expression of Xenopus Mad. (A) A
developmental northern blot of total embryonic RNA shows that
XMad is maternal and expressed at all stages of early development.
Three transcripts are detected with the sizes (in kb) indicated on the
left. The most abundant mRNA is 3.4 kb, and minor transcripts of
4.3 kb and 3.0 kb are also detected. It is not known whether these
transcripts derive from closely related genes or splicing variants.
Lane numbers correspond to developmental stages (Nieuwkoop and
Faber, 1967): (7, 9) blastula, (11) gastrula, (15, 18) neurula, (26)
tailbud tadpole and (38) swimming tadpole. (B) In situ hybridization
shows that XMad transcripts are uniformly distributed in the early
gastrula (stage 10). Dense staining covers the entire prospective
ectoderm and marginal zone, but vegetal cells (the lighter region) do
not stain efficiently by this procedure (Harland, 1991). A northern
blot on isolated dorsal, ventral, animal and vegetal regions confirmed
the uniformity of XMad expression at early stages (not shown).
(C) A mid-gastrula embryo (stage 12) split sagitally reveals that
XMad is expressed in the ectoderm and neurectoderm (arrows mark
the ectodermal-mesodermal boundary), and expression in the
underlying mesoderm is greater in the posterior, adjacent to the yolk
plug (YP). This embryo is lightly pigmented and the brown line
anterior to the yolk corresponds to involuted bottle cells. The embryo
is positioned with the anterior to the left and dorsal at the top. (D) At
tailbud tadpole stage 26 XMad expression is high in the central
nervous system and head. (E) A close-up of the head of the embryo
in D, highlighting XMad expression in the brain (b), eye (e) and head
neural crest derivatives (mc, mandibular crest; hc, hyoid crest; abc,
anterior branchial crest; pbc, posterior branchial crest). Expression in
the otic vesicle, between the hyoid crest and anterior branchial crest,
is also visible. Scale bars, 0.1 mm.