The cellular architecture of the larval zebrafish tectum, as revealed by gal4 enhancer trap lines

Abstract

We have carried out a Gal4 enhancer trap screen in zebrafish, and have generated 184 stable transgenic lines with interesting expression patterns throughout the nervous system. Of these, three display clear expression in the tectum, each with a distinguishable and stereotyped distribution of Gal4 expressing cells. Detailed morphological analysis of single cells, using a genetic "Golgi-like" labelling method, revealed four common cell types (superficial, periventricular, shallow periventricular, and radial glial), along with a range of other less common neurons. The shallow periventricular (PV) and a subset of the PV neurons are tectal efferent neurons that target various parts of the reticular formation. We find that it is specifically PV neurons with dendrites in the deep tectal neuropil that target the reticular formation. This indicates that these neurons receive the tectum's highly processed visual information (which is fed from the superficial retinorecipient layers), and relay it to premotor regions. Our results show that the larval tectum, both broadly and at the single cell level, strongly resembles a miniature version of its adult counterpart, and that it has all of the necessary anatomical characteristics to inform motor responses based on sensory input. We also demonstrate that mosaic expression of GFP in Gal4 enhancer trap lines can be used to describe the types and abundance of cells in an expression pattern, including the architectures of individual neurons. Such detailed anatomical descriptions will be an important part of future efforts to describe the functions of discrete tectal circuits in the generation of behavior.

Keywords: Gal4/UAS; anatomy; neuron; periventricular; single-cell morphology; tectum; transgenics; zebrafish.

Figure 1

Gal4 expression patterns from the ET screen. Twenty-four examples are shown of expression patterns from the ET screen, with the line number indicated. All are dorsal images of 5- or 6-dpf larvae. The scale bar (bottom right) indicates roughly 200 μm. All larvae carry the indicated Gal4 insertion and UAS:Kaede.

Figure 2

Three tectal lines arising from the ET screen. (A), (C), and (E) show dorsal images of 5-dpf larvae, anterior at the top. A dotted line indicates the approximate boundary of the animal. (B), (D) and (F) contain corresponding high-magnification images of the left tectum in the same ET line. (A) Shows the s1013t expression pattern. Expression is seen primarily in the tectum (arrowhead), but also in the pallium (arrow), the hindbrain (small arrowhead) and the medulla oblongata (small arrow). Expression in the tectum is strong and spread evenly across the A-P and M-L axes (B). The midline (arrowhead) and boundary between the tectal cell body layer and neuropil (dotted line) are indicated in (B). s1038t (C) has strong expression in the tectum (arrowhead) and pineal (arrow). Expression in s1038t is strongest in the posterior tectum (D), and axons from s1038t-positive neurons are seen extending beyond the tectum (arrowhead). (E) Shows s1156t expression in the tectum (arrowhead), forebrain (arrow), and ocular muscles (small arrowheads). As compared to the other lines, s1156t is expressed in a smaller subset of tectal neurons (F), and is prominent in cell bodies located at the superficial surface of the tectal neuropil (arrowhead). Scale bars in all panels indicate 100 μm. Animals are heterozygous both for the indicated Gal4 insertion and UAS:Kaede.

Figure 3

Axonal targets of s1038t-positive PV neurons. (A) Schematic of a 5-dpf larva with the approximate positions of the slices shown in (B–E). Kaede fluorescence (green) shows s1038t PV neurons and cell bodies (DAPI, false colored red) provide landmarks throughout the brain. Axons of PV neurons [cell bodies indicated by arrowhead, (B)] extend ventrally [arrow, (B)] and out of the tectum. They can be seen to course medially [arrowhead, (C)] before terminating in the superior Raphe nucleus [small arrowheads, (B)]. Other axons from these neurons terminate in the superior reticular formation [arrow, (D)] or extend caudally [arrowhead, (D)], terminating in the medulla oblongata [arrowhead, (E)]. Asterisks indicate non-neural staining. Scale bars indicate 100 μm. Larvae are 6-dpf, and are heterozygous for the s1038t Gal4 insertion and UAS:Kaede.

Figure 4

Variegated UAS:mGFP reveals single cell types in the larval tectum. Panels show Gal4 expression patterns in red (photoconverted Kaede), and individual neurons in green (mGFP). Each pair of panels [e.g. (A) and (B)] comprises a dorsal view showing the structure in the A/P and M/L axes (A) and a rotated image that illustrates the D/V extent of the neuron (B). Superficial neurons have neurites spreading away from the cell body [arrowhead, (A)] in the A/P and M/L axes. The arbor is highly stratified to a dorsal layer of the neuropil, probably the SO (B), with a single extension, possibly an axon, crossing into a deeper layer, probably the SFGS [arrow, (B)]. An unrelated neurite is also visible [asterisk in (A) and (B)]. Cells with structures typical of radial glia have cell bodies in the ventral periventricular layer [arrow, (C)], and a bushy dorsal extension that passes through the neuropil [arrowhead, (C)]. These cells invariably extend from their cell body at the ventricular boundary [arrow, (D)] to the extreme dorsal edge of the neuropil [arrowhead, (D)]. Periventricular neurons have their cell bodies in the periventricular layer [arrow, (E)], and extend dendrites into the tectal neuropil [arrowhead, (E)]. These dendrites are typically but not always stratified in one or more D/V layers of the neuropil [arrowheads, (F)]. A distinct category of periventricular neuron, the shallow periventricular neuron, is shown in (G) and (H). Shallow periventricular neurons have cell bodies at the boundary of the cell body layer and the neuropil [arrows, (G) and (H)], send dendrites exclusively into deeper layers of the neuropil [arrowheads, (G) and (H)], and have axons that extend ventrally, rostrally, [small arrowheads, (G) and (H)] and contralaterally [small arrow, (G)] to targets beyond the tectum. All scale bars represent 50 μm, and apply to both panels in a pair. Orientations are approximate. Animals are 5 or 6 dpf, and are heterozygous for the indicated Gal4 insertion, UAS:Kaede, and BGUG.

Figure 5

Cell types present in each line. For each tectal ET line, the types and frequencies of cells observed are shown. The number of times that each cell type was observed in repeated single cell analyses is indicated for each line and cell type.

Figure 6

Laminar profiles of PV neurons with and without extra-tectal axons. Panels show the individual s1038t PV neurons (green) in the context of the entire s1038t tectal expression pattern (red). Each pair of panels [e.g. (A) and (B)] comprises a dorsal image and a rotation of that image. PV neurons without extra-tectal axons typically have abundant laminated neurites in the superficial [arrowhead, (B)] layers of the tectal neuropil, and a smaller laminated arbor in intermediate layers [arrow, (B)]. Some of these PV neurons (C,D) have an exclusively superficial arbor [arrowhead, (D)]. PV neurons with axons exiting the tectum [arrowheads in (E) and (G)] usually have neurites in the deep [arrowheads, (F)] and intermediate [arrow, (F)] layers of the neuropil, or exclusively in the deep layers [arrowheads, (H)]. (I) shows a summary of all analyzed s1038t PV neurons, indicating the regions of tectal neuropil where they have neurites and whether or not they have axons extending beyond the tectum. The number of neurons observed with dendrites in given regions or combination of regions is shown on the right side of (I). All scale bars represent 50 μm, and apply to both panels in a pair. Orientations are approximate. All animals are 5 or 6 dpf, and are heterozygous for the s1038t Gal4 insertion, UAS Kaede, and BGUG. Supplementary movies 1 and 2 show animated z-series of (E) and (G), and more clearly show the extratectal axons belonging to these neurons.