Neuronal cell types and connectivity: lessons from the retina

Abstract

We describe recent progress toward defining neuronal cell types in the mouse retina and attempt to extract lessons that may be generally useful in the mammalian brain. Achieving a comprehensive catalog of retinal cell types now appears within reach, because researchers have achieved consensus concerning two fundamental challenges. The first is accuracy-defining pure cell types rather than settling for neuronal classes that are mixtures of types. The second is completeness-developing methods guaranteed to eventually identify all cell types, as well as criteria for determining when all types have been found. Case studies illustrate how these two challenges are handled by combining state-of-the-art molecular, anatomical, and physiological techniques. Progress is also being made in observing and modeling connectivity between cell types. Scaling up to larger brain regions, such as the cortex, will require not only technical advances but also careful consideration of the challenges of accuracy and completeness.

Figure 1. The retina is composed of three nuclear layers, containing cell bodies, and two plexiform layers, containing neurites

The inner and outer plexiform layers are sandwiched between the three layers of somata; the outer nuclear layer, the inner nuclear layer, and the ganglion cell layer. The nuclear layers contain the somata of the five classes of neurons of the retina: photoreceptor, horizontal, bipolar, amacrine, and ganglion cells. Figure adapted from Masland (2012).

Figure 2. SAC dendrites extend roughly radially about the soma and are activated by outward motion

a, Projection of an Off SAC onto the plane tangential to the retina shows the “starburst” arrangement of its dendrites. The preferred directions of individual dendrites are radially outward from the soma. Swellings of distal dendrites are presynaptic boutons (inset), so that SAC dendrites are output elements as well as input elements. b, Projection onto a plane perpendicular to the retina shows the thin stratification of the Off SAC at a characteristic IPL depth, between the INL and GCL. The neuron was reconstructed from a volume imaged by serial EM (Briggman et al., 2011). Scale bar, 50μm. Figure adapted from Kim et al. (2014).

Figure 3. Bipolar cells of the same type tile the retina with little overlap, while ganglion cells of the same type form an overlapping mosaic

Dots represent somata, and the polygons around them represent the hulls of the arbors in an axial view. Somata of cells of the same type are arranged quasi-periodically. a, Cartoon of bipolar cells of a given type covering the area of the retina with little overlap. b, Ganglion cells of a given type cover the retina with substantial overlap, while their somata are arranged as if they repelled each other. The hull and the soma location of one ganglion cell are highlighted.

Figure 4. Anatomical classification of bipolar cells reconstructed via serial EM

Bipolar cells were anatomically classified based mainly on stratification depth (Helmstaedter et al., 2013). The classification mostly agrees with a previous molecular classification (Wässle et al., 2009), except that a new type called XBC was defined, and Type 5 appears to be a mixture of more than two types (not shown). Figure adapted from Helmstaedter et al. (2013) by transposing types 1 and 2, following the classification in Kim et al. (2014).

Figure 5. The arbor density is obtained by blurring the neuronal arbor

RGC arbor reconstructed from LM image (a), and RGC arbor density produced by blurring (b), shown as projections onto the plane tangential to the retina (large), and projections onto two orthogonal planes perpendicular to the retina. The blurring is anisotropic, effective only in the tangential plane. No blurring is applied along the axis perpendicular to the retina, to preserve IPL depth information. The overall shape of the arbor and its IPL depth are preserved, but detailed information about individual dendrites is effectively discarded. Scale bar, 40μm. Figure adapted from Sümbül et al. (2014).

Figure 6. Aligning ganglion cells to a common IPL depth coordinate reveals a remarkable reproducibility of stratification depth within a cell type

a, The arbor traces in the top row were obtained from transgenic lines in which a relatively homogeneous class of cells was labeled (Kim et al., 2008, 2010; Huberman et al., 2008; Osterhout et al., 2011). The ones in the bottom row were obtained by sampling from highly heterogeneous classes labeled in GFP-M, YFP-H and YFP-12 mice (Feng et al., 2000). The dashed lines indicate the positions of the On and Off SAC layers after alignment. b, Stratification properties of all 15 types. Colors of cell types match with the colors of the representative traces in a. Stratification distance refers to the distance of peak stratification plane to the On SAC layer. Scale bars, 40μm. Figure adapted from Sümbül et al. (2014).