||Figure 3. Expression in the Lateral Plate Mesoderm of Early Embryos
(a) Diagram showing the location of the forelimb field in the lateral mesoderm of Amblystoma maculatum tailbud-stage embryo (redrawn from Harrison, 1918). Note that the disc-shaped forelimb anlage is just posterior and ventral to the pronephros (Pn). (b) Transverse section through a stage 41 (3 day) Xenopus embryo. The left side of this slightly oblique section corresponds to the prospective forelimb field region (section immediately posterior 10 the Pn; note Pn at right side this section) and is shown at higher magnification in (d). Arrows in (d) indicate nuclei in the somatopleure (the most external layer of the lateral plate mesoderm) that express XlHbox 1. Arrowheads indicate positive mesodermal nuclei in the splanchnopleure that surrounds the endoderm (En). (c)shows a more anterior section (through the pronephric region) and (e) a more posterior section of the embryo shown in (d); note that the somatopleure nuclei do not stain with anti-XIHbox 1 antibodies in either region. In this serially sectioned tadpole the distance between sections (c) and (d), and (d) and (e). is 200 F in each case. All sections were stained with an&Xenopus antibody B (Oliver et al., 1988). The yolk platelets in the En display background staining, which is nonspecific. The central nervous system (CNS) and lung bud mesoderm (Lu) also express high levels of XlHbox 1 protein. A small proportion of myotome (My) nuclei ([d] and [e]) are stained, which we believe to be neural
crest derivatives and not muscle cell nuclei. Abbreviations: D, dorsal; V, ventral: A, anterior: P, posterior: EC. ectoderm: No, notochord.