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Development of the nervous system in the amphibian embryo is initiated during gastrulation by an inductive interaction between chordamesoderm and dorsal ectoderm. The induced ectoderm forms the neural plate while uninduced ectoderm generates epidermis. We screened for genes activated during gastrulation and expressed specifically in the nervous system of Xenopus laevis in the expectation that clones representing such genes will constitute useful markers for the study of early neurogenesis. Probes were prepared from adult brain RNA by subtraction with RNA from ovary and from different combinations of adult kidney, muscle, and skin; cDNA libraries prepared from early to late neurulaembryo RNA were screened with these probes. Six clones were chosen for further study. Three of these clones are not represented in the maternal RNA population but are activated at the late gastrula stage; the other three increase from a maternal base. Expression of five of the genes is restricted to the neural plate during embryogenesis, and all six are restricted to the central nervous system in premetamorphic tadpoles and adults. One of the clones encodes an apparently neurospecific isoform of beta-tubulin; the identity of the other clones is unknown. Expression of all six genes is suppressed in axis-deficient embryos that lack dorsal structures including the brain.
FIG. 1. Expression patterns of six genes. cDNA clones were selected as described in the text, and inserts were used in RNA gel blots. The
name of each clone is given at the left and refers to -all gels in A-D. The position of 18S and 28S rRNA is indicated. (A) Comparison of RNA
preparations from different adult tissues. B. brain; K, kidney; L, liver; M, skeletal muscle; S, skin. Exposures: 17-5, 5 days; 24-15, 5 hr; all
others, 1 day. (B) Developmental profile. Stages (22) are given at the top of each lane. Exposures: 24-39, 2 days; -lI others, 1 day. (C) RNA
distribution in neurula embryos. Embryos were dissected as shown in Fig. 2 and described in the text. Embryos dissected at stage 12 and cultured
until siblings reached stage 17 were used, except in the experiment with clone 17-30, which used embryos dissected at stage 17. Endo, endoderm
plus mesoderm; ecto, ventralectoderm; neur, neural plate or closing neural tube. Exposures: 17-5, 24-39, and 13-6, 5 days; 17-30, 10 days; 24-10
and 24-15, 2 days. (D) RNA distribution in stage-50 tadpoles. Exposures: 17-5 and 24-39, 5 days; all others, 1 day. In A, B, and D, 2 ttg ofRNA
selected once on oligo(dT)-cellulose was applied on each lane; in C, RNA isolated from fragments of 12 embryos was used in each lane.
FIG. 2. Dissection of embryos. The dashed lines indicate the incisions made with glass needles. At stage 12 to 121/2, the neural plate was
dissected without underlying mesoderm (meso); ventral ectoderm (ecto) and endoderm together with mesoderm (labeled endo) were also
collected and cultured for 5 hr at 240C in modified Barth's solution (88 mM NaCI/1 mM KCI/0.8 mM MgCl2/0.8 mM CaCl2/2.4 mM NaHCO3/7.5
mM Tris, pH 7.6) before RNA extraction. At stage 17-19, the closing neural tube was dissected together with the underlying chordamesoderm
(neur). Ectoderm (ecto) was peeled off; the lateral plate mesoderm was collected together with the endoderm (labeled endo).
FIG. 3. Gene expression in axis-deficient embryos. Embryos
treated with ultraviolet irradiation before first cleavage were cultured
until untreated siblings reached stage 25, and grade-5 (29) embryos
were selected. RNA was analyzed by gel blotting. Each lane was
loaded with 2 ,g of RNA enriched as described in the legend to Fig.
1. In each pair, the left lane contains RNA from grade-5 embryos and
the right lane contains RNA from normal embryos. Arrowheads
indicate the position of 18S rRNA.
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