Fig. 6

Morphogenesis of the trunk in F. borealis. A Optical sections on the dorsoventral axis, side view and dorsal view of hatched larvae observed during the first step of epidermis retraction. Note the thickened basal lamina stained with phalloidin (magenta), which outlines the epidermal layer. White and blue dashed lines mark the position of developing esophagus and gut, respectively. Blue arrows show area of the gut where epidermis has retracted. The orange asterisk marks a large cell located on the ventral side which is used to orient the larva. B Optical sections (left panel) and movie frame shots (right panel) of larvae observed during the second step of epidermis retraction. At this stage, the epidermal layer (continuous white line in movie frame shots) progressively retracts towards the anterior, leaving the ventral side of the esophagus exposed (white arrows). C Optical sections of a larva observed at the end of the second step of epidermis retraction. At this stage, the retraction of the epidermal layer is visible on the left and right sides of the esophagus and progresses towards the dorsal side (white arrows). D 3D reconstruction and optical section (D’) of a larva during the third step of epidermal retraction. The epidermis has formed a projection over the dorsal side of the esophagus. The gonad primordium is now visible in the posterior trunk, lined by a cuticular material similar to the tail fin. E Juvenile with a pre-house. In this animal collected within 24hpf, it is possible to recognize the major anterior organs and house-producing fields of cells on the trunk epidermis. adf, anterior dorsal field; avf, anterior ventral field; laf, lateral field. F Schematic description of the epidermis retraction during larval development. al, anterior lobe of the trunk; cg, caudal ganglion; de, dorsal epidermis; doe, dorsal side of the esophagus; dor, dorsal oikoplastic row of cells; dp, dorsal projection; ep, epidermis; en, endostyle; fi, surface fibers; fc, fin cell; go, gonad; gu, gut; mo, mouth; no, notochord; oe, esophagus; ph, pharynx; pl, posterior lobe of the trunk; sp, spiracle; uf, upper fin cell; ve, ventral epidermis. Scale bar, 10 microns