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Postembryonic development of the posterior lateral line in zebrafish

Laboratoire de Bioinformatique, Université Libre de Bruxelles, 12 Rue des Profs. Jeener et Brachet, 6041 Gosselies, Belgium

View larger version (29K): [in a new window]   Fig. 1. Establishment of the primary PLL (posterior lateral line) system during zebrafish embryogenesis, at 48 (A), 55 (B) and 72 (C,D) hours after fertilisation. The earliest neuromasts belong to the anterior lateral line system (A). The embryonic PLL develops progressively between 55 (B) and 72 hours (C,D). Newly formed neuromasts are arrowed and labelled in black, neuromasts formed previously are marked in grey. ALL, anterior lateral line neuromasts; D1-2, dorsal neuromasts; L1-8, lateral neuromasts; O1, occipital neuromast; MLL, middle lateral line system. Anterior is towards the left and dorsal is upwards.

View larger version (36K): [in a new window]   Fig. 2. The pattern of PLL neuromasts in larvae up to 8 mm. The dashed arrow indicates the progressive completion of the L-PLL by secondary neuromasts. D1-9, dorsal neuromasts; D'1-3, neuromasts of the new dorsal line; L1-8, lateral neuromasts; O1-3, occipital neuromasts; ML, middle lateral line system. Anterior is towards the left and dorsal is upwards.

View larger version (100K): [in a new window]   Fig. 3. Ventral migration of neuromasts during postembryonic growth. The position of the horizontal myoseptum is indicated by the white arrows. (A) Size difference between a primary (left) and a secondary (right) neuromast shortly after the latter has begun to differentiate. (B) Neuromast migration varies along the anteroposterior axis. L1 has not moved, yet the secondary neuromast (NMII) that has formed on the next somitic boundary has migrated a long distance. (C) Onset of neuromast migration: L2 has begun to migrate (and to a lesser extent NMII) but L1 and L3 are still close to the myoseptum. The two axons innervating the neuromast L2 are indicated by an arrowhead. (D) At later stages younger secondary neuromasts (arrowheads) begin to migrate as soon as they differentiate. (E) Two neuromasts are occasionally observed at the same position, and migrate together. (F) The axons of sensory neurones follow the neuromasts in their migration. The tracks of the axons presumably reflect the migratory pathway of the neuromasts. In general the migration follows the intersomitic grooves (arrowheads), but may migrate straight ventralwards (asterisks). (G) As migration proceeds, the neuromasts tend to follow less and less the somitic boundaries (arrowheads). (H) The neuromasts of the dorsal line (D1-4) also migrate ventrally. In this case there is no indication that migration follows intersegmental limits. The neuromasts of the D' line have already formed. Their axons, and the dorsal course of the D axons, are obscured by a stream of melanocytes along the dorsal midline of the fish. Anterior is towards the left and dorsal is upwards.

View larger version (21K): [in a new window]   Fig. 4. Formation of the L'-PLL line. (A,B) The pattern of PLL neuromasts just before and after the L' line has formed. The neuromasts of the L line are not numbered; the dotted arrow (in grey) outlines the distorted geometry of this line due to unequal migration. (C) Each neuromast of the D, D', L, and L' lines later forms a vertical ‘stitch’. The frame corresponds to the boxed area in B, at an older stage. Anterior is towards the left and dorsal is upwards.

View larger version (131K): [in a new window]   Fig. 5. Rearrangement of the terminal part of the L-PLL and structure of the tail. The neuromasts and their axons (green) are visualised in fluorescence and the image is superimposed on a Nomarski view of the same field. The axonal paths are indicated with a dotted line because the fluorescence is too weak to show when superimposed to the diascopic view. Given the thinness of the tail, the neuromasts on the other side of the fish are also seen. Each panel represents a different fish. (A) Terminal neuromasts along the ventral edge of the tail before the onset of the rearrangement. (B) Elongation of the melanocytes (white arrows). (C, D) Formation of a ventral mesenchyme and beginning of the migration of the penultimate neuromast (white arrowhead). (E) Appearance of the caudal fin rays. (F) Rearrangement nearly complete. (G) At a slightly later stage, lower-scale view to illustrate how the homocercal, symmetrical caudal fin of the zebrafish is entirely derived from the ventral fin fold of an intrinsically asymmetrical, heterocercal tail. Anterior is towards the left and dorsal is upwards.

View larger version (74K): [in a new window]   Fig. 6. Formation of stitches. (A) Four consecutive stitches of the L-PLL line. The axons innervating the different neuromasts of one stitch are most likely collaterals of the same axons that innervated the founder neuromast. (B) The first step in the formation of a stitch: a small neuromast comprising only one differentiated hair cell has formed about 50 µm away from the founder neuromast, to which it remains connected by axons (arrowheads). Inset shows a magnified, contrast-enhanced view of the axons connecting the two neuromasts (arrowhead). (C-H) A series of stitches of increasing sizes. As judged from this series, and from the branching pattern of the collaterals, stitching occurs by a succession of budding events, each of which generates one (or at most two) additional neuromast. The arrows point to axonal bifurcations that presumably reflect budding events. The asterisk marks the common root of the innervating fibres. All panels represent stitches at the caudal level of the L-PLL except A,B, which illustrate stitches at the pectoral level; thus, the asterisked axons in B are directed dorsalwards, whereas in C-H they are directed anteriorwards. Scale bar for B-H is shown in E. Anterior is towards the left and dorsal is upwards.

View larger version (24K): [in a new window]   Fig. 7. Distal bent of the fish caudal fin (A) and of the tetrapod limb (B). The main axis of limb and fin are represented by the thick line.