Collage A is expressed in non-myotome somite cells throughout development. In each panel, black arrowheads indicate sclerotome mesothelium and white arrowheads indicate the external cell layer mesothelium. (A, B) During embryonic stages, non-myotome cells express ColA. Expression does not overlap with MLC, which marks myotome, stained brown in the double-labeled embryo in panel (B). During development, the position of ColA expressing cells tracks the position of sclerotome and external cell layer derivatives observed by TEM (see text). (C) and (D) show overviews, (E) shows a detail of (D) in brightfield (left) and DAPI nuclear stain (right). Arrowheads mark the same nuclei in (D-E); arrow marks the fin box mesothelium. (F) In later larvae, the sclerotome mesothelium extends to the neural tube level and continues to express ColA (brightfield, left panel; DAPI, right panel), as does the external cell layer and fin box mesothelium (arrow). (G) A frontal section shows no variation in morphology or gene expression along the anterior-posterior axis of segments (black arrows mark the medial ends of the myosepta). (H-I) In late larvae, non-muscle cells are seen within the myosepta. These are often adjacent to the mesothelium of the external cell layer or the sclerotome (H), but were sometimes observed in the middle of myosepta (I). Brightfield (left) and DAPI (right) panels are shown; arrows mark the medial (black) and lateral (white) ends of the myosepta. (J) Overview of an early metamorphic stage. The mesothelia derived from sclerotome and external cell layer, the fin box and myosepta are all ColA positive. Black arrows mark the medial ends of myosepta; labeled myoseptal cells are connected to the sclerotome mesothelium similar to (H). All sections are transverse except (G), which is frontal. Micrographs were taken at 1,200× (A-G) or 1,000× (J) magnification.