Fig. 5. Early expression of Hro-hh in gut tissues, but not in germinal plate, prior to the establishment of segmental boundaries. Photomicrographs of embryos processed by in situ hybridization for Hro-hh. In these and all subsequent illustrations, embryos are shown in lateral views, with anterior to left and ventral down, unless otherwise stated. (A) Anteroventral view of a stage 8 embryo (78 hours AZD) showing the partially formed germinal plate (dotted outline); transcripts (arrow) occur at the anterior, micromere-derived end of the germinal plate, from which prostomial tissues and proboscis arise, but not in the more posterior, teloblast-derived region that will form segmental ectoderm and mesoderm. (B) Lateral view of the same embryo, at higher magnification. (C,D) An embryo at early stage 9 (100 hours AZD), showing the presence of two distinct groups of cells expressing Hro-hh at the anterior end of the germinal plate; there are still no transcripts visible within the segmental portion of the germinal plate. (E) By early stage 10 (135 hours AZD), the proboscis is starting to differentiate in the everted position. Hro-hh transcripts are present in the central core of the proboscis (extent indicated by double-headed arrow), in a ring of cells defining the future oral opening (white arrow) and in transverse bands of cells at the posterior end of the SYC, corresponding to posterior midgut (black arrows). Relatively weak expression is also observed in the prospective esophagus, between the circumoral ring and the yolk cell. (F) A higher magnification view of the embryo shown in E, looking down along the longitudinal axis of the foregut and focussed at the level of the circumoral ring. In this view, Hro-hh-expressing longitudinally oriented fibers (arrows) appear as dots surrounding the core of the proboscis. (G) At mid stage 10 (145 hours AZD), transcripts are clearly present throughout the extent of esophagus and proboscis (doubleheaded arrow), in the circumoral ring (white arrow), and in the rectum (black arrows), which is becoming morphologically distinct from the crop. Patches of Hro-hh expression are also visible at the surface of the anterior portion of the crop. (H) An embryo at the same age as that in E, in which the color reaction was allowed to proceed longer. This image is focused on the lateral edge of the germinal plate. Hro-hh transcripts are visible along the edge of the germinal plate (black arrowhead) and in transverse segmentally iterated bands (white arrowheads). (I) Parasagittal section (ventral is down, posterior to right) through the posterior segments of an embryo at a similar stage to that shown in H (paired arrows in H). In three segments, precursors of the rings of stained rectal muscle (arrows) lie at the dorsal edge of the intersegmental septa. Staining is also present in the body wall, with a clear boundary at the boundary between the prospective midbody and caudal sucker (black arrowhead). Scale bar: A, 150 µm; B 75 µm; C,E,G,H, 100 µm; D, 50 µm; F, 25 µm; I, 20 µm.

Return to article