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???displayArticle.abstract??? XMyoD is the earliest marker of muscle development in Xenopus embryos and is expressed in presumptive somites in the late gastrula. In the early gastrula, in situ hybridization showed XMyoD transcripts in precursors of both muscle and non-musclemesoderm. Embryos ventralized by UV irradiation made no muscle, but expressed XMyoD transiently. Embryo explants that differentiated ventral mesoderm also expressed XMyoD transiently. These results show that the initiation of XMyoD expression is not sufficient to convert cells to muscle and suggest that XMyoD is expressed in response to a general mesodermalizing signal; expression is stabilized and enhanced only in muscle precursors that have received a dorsalizing signal.
Fig. 1. Albino embryos were examined by whole-mount hybridization using digoxigenin-labeled XMyoD antisense and
sense probes as described in the methods. (A) Hybridization to a family of developing Xenopus embryos. From top to
bottom: stage 13, stage 24, stage 32, stage 40, and stage 40 (sense control). (B) Negative control hybridization to a stage 11
gastrula with a sense probe. (C) View from the vegetal pole of a stage 10.25 gastrula. Arrows point to the limit of the
dorsal lip. Note the grainy signal above background levels throughout the lateral and ventral marginal zones. (D-F) Stage
11 gastrula. (D) Dorsal view, (E) Lateral view, (F) Ventral view. Figures D to I are oriented with the blastopore at the
bottom of the picture. The blastopore lip is arrowed in D-I. (G,H) In situ hybridization to stage 13 neurulae. (G) Dorsal
view, (H) Ventral view, (I) Stage 15 neurula.
Fig. 2. XMyoD mRNA is expressed in ventralized
gastrulae. Total RNA from two pools of five control and
UV ventralized embryos were isolated at stages 9 (lanes 1,
2), 10+ (lanes 3, 4), 10.75 (lanes 5, 6), 11.5 (lanes 7, 8),
12.5 (lanes 9, 10) and stage 22 (lanes 11, 12). One embryo
equivalent of RNA was electrophoresed on a formaldehyde
gel and blotted onto a nylon membrane. The filter was
hybridized sequentially with complementary DNA probes
of XMyoD and EFlor as described in the methods.
Fig. 3. (A) Transient expression of XMyoD mRNA in
individual embryos. Total RNA was isolated from
individual control and UV ventralized embryos at stages
11, 12 and 14. RNA isolated from one individual (st.
11) or 1/2 embryo (st. 12, 14) was loaded on each well.
Control embryos were loaded in lanes 1-5 and UVtreated
embryos were loaded in lanes 6-15 (st. 11, 12).
In the stage 14 samples, nine UV-treated individuals
were loaded. Filters were sequentially hybridized with complementary DNA probes for the XMyoD, EFla, and muscle specific
cardiac actin (m-act) genes as described in the methods. The arrow indicates the cardiac actin band (m-act), the
band above it results from cross-hybridization of the probe to cytoskeletal actin, which is expressed ubiquitously.
(B) Expression of XMyoD in ventralized neurula stage embryos. For examination of graded embryos at stage 20, two
individual controls were loaded (lanes 1, 2) in addition to one individual at each DAI level ranging from 0-4 (lanes 3-7).
Filters were sequentially hybridized as described in the methods.
Fig. 4. Transient expression of XMyoD mRNA in the ventral marginal zone of gastrula explants. (A) Normal gastrulating
embryos were dissected at stage 10+ (see materials and methods) as viewed schematically from the vegetal pole.
(B) Ventral (VMZ), dorsal (DMZ), and lateral (LMZ) marginal zone regions of dissected explants were incubated in 1/3 x
MR until control embryos reached stage 11 and 20. At stage 11, two and a half explants were loaded per well for Northern
analysis; at stage 20, one explant was loaded per well.
