3:4.0 SOME OTHER 'POINTS TO PONDER' 3:4.1 Morphological features

As the camera lucida drawings of the cells were made from two-

dimensional microscopic views, the somata appeared different according to the arrangement of the primary dendrites. Thus some cells had

circular somata while others showed more irregular outlines. The orientation of a non-spherical soma with respect to the plane of the retina also affected size measurement. In some cases, two or more

primary dendrites emerged from an elongated somal extension, rather than from the 'main' soma itself.

It appeared that the primary dendrites that gave rise to more profuse arbors or longer and/or thicker branches had a thicker calibre than the others.

The best possible camera lucida drawings could usually be made from isolated cells, rather than from areas where many neighbouring ganglion cells with considerable overlap were labelled. In the Rana pipiens specimens, more isolated cells could be found because the labelling was done from partial optic nerve cuts instead of complete ones. Consequently, the overall morphological studies of these isolated cells were more rewarding.

In general, the cells had more irregular dendritic tree shapes and branching patterns than the cells. Circular trees were more common among the a^b cells. The cell trees were irregular as well, and, in most cases, elongated. This difference in the regularity of dendritic tree shape between the three cell types was reflected in the difference in their mosaic regularities. This is where 'dendritic tessellation' warrants consideration (see Section 3:5*2).

The axons of some well-labelled, isolated Rana pipiens cells could be traced to the optic disc area because of the high contrast between the fibres and the background. In this area, the axons of the cells looked thicker than those of the a^b cells. However, the proximal segments of these axons were not distinguishable, being very much

thinner in both types close to the soma, as in some other species (fish: Kock and Reuter, 1978b; Cook, 1982; urodeles: Carras, Coleman and

Miller, 1992; rabbit: Carras et al. 1992 and R.F. Miller, personal communication). The cell axons appeared similar to the cell axons . However, no measurements were made.

3:4.2 Population features

As described for individual cell types, the density gradient patterns of different types of a-cell generally followed the overall pattern of ganglion cell density across the retina. Thus the a-cells were more numerous in the visual streak area than away from it. The visual streak in Xenopus being very weak (Graydon and Giorgi, 1984), the cell density gradient across the retina was also weak. Consequently, gradients in soma size, dendritic tree size and soma spacing were not very

pronounced. On the other hand, a stronger visual streak was apparent in the retinae of Bufo mavinus, Rana esculenta, Rana pipiens and Hyla

moovei. Larger soma and dendritic tree sizes and longer nearest

neighbour distances away from the visual streak were prominent in these retinae.

Unquantified observations across the species suggest that, among the three types of a-cell, the cells had the largest numbers of intersecting dendrites (in other words, the highest coverage factors; see Discussion, Section 4:5*3)•

After preliminary plotting of different types of a cell, ’holes' could often be located in an otherwise regular mosaic. A subsequent search could identify cells belonging to the mosaic in the region of those holes, which had been considered difficult to type or missed altogether in the preliminary search.

When nearest neighbour distance (NND) was measured from a large area with strong cell density gradients, the estimated degree of regularity was affected by the wide range of NND values present.

'Crisper' results (that is, higher mosaic ratios and narrower

distribution curves with shorter tails) could be obtained in such cases by confining the NND analysis to areas where the cell density gradient was not prominent, or by analysing areas of differing density

separately. Table 2 shows such a phenomenon in a Rana esculenta retina: analyses from a smaller area (shown in Fig. 28) may be compared there with those from the whole retina.

3:5-0 SPECIFIC FINDINGS : Xenopus laevis ', a \ '

---^ -> • '*7 "r

I V ' 1X1 ^ v’i. y \-^

3:5-1 General observations

CLC- and HRP-labelled retinae from Xenopus laevis were examined. All the three a-cell types (0%, a^b and a^) were found. The distribution of these cells confirmed the existence of a weak visual streak. The a^b cells were more numerous than the and a^ cells.

Retrograde HR? labelling of the ganglion cells from ipsilateral and contralateral optic tracts as well as from the optic chiasm labelled

all three types of a-cell in the retina. However, the incomplete nature of the labelling made it impossible to study whether a 'hole' in the distribution of a particular type of a-cell in the contralateral retina represented a labelled a-cell of the same type in the ipsilateral

retina.

As a rough estimate of cell density, 143 cells and 460 a^b

cells could be identified in a retinal area of 16.153 (95% of the Wic3* whole retina), while II5 Oq cells were found in an area of 10.7I mm^ of \ the same retina in which their labelling was adequate for plotting.

Thus the average densities of the a^, a^b and a^ cells in this Xenopus

retina (of approximate diameter 6 mm in the flat-mount) were 8.9, 28.5 and 10.7 per m m ^ , respectively, giving estimated total numbers of I5I Oa cells, 485 a^b cells and I83 a^ cells for the whole retina.

The total number of labelled non-a-cells for this retina

(estimated from a sample obtained as in Methods, Section 2:3-1) was 1 . 39.110, giving the following percentages of the total ganglion cell

number for the three a-cell types: a^ cells: 0.38%; a&b: 1-21%; a^:

0.45%- However, these may be overestimates, since there appeared to be some preferential labelling of the a-cells in this material.

3:5-2 Morphological features of the a-cells in Xenopus laevis

Morphologically, the Xenopus a-cells conformed, in general, to the

descriptions in Section 3:3-0. Some details and special characteristics are presented here. The soma size frequency distributions of the a- cells are presented in Figure 11 along with the size distribution for the non-a-cells.

Neither the soma size nor the tree size varied very much with

distance from the weak visual streak in any of the three cell types. ^ However, they were largest in the ventral retina, along with larger cell spacing. This was probably because, the ventral retina being larger than the dorsal part, the absolute distances of the cells from the visual streak were larger in the former.

The Qa cells in Xenopus laevis: The general morphological features of

C u c. ,

this type of cell are described in Section Soma area, measured m 1 from all l43 Qo cells in a CLC-labelled Xenopus retina (see Fig. I5A) ^ ^

'T 1:3

showed a range between II8 and 479 (mean: 270.0; SD: 64.7). Figure 8 shows a typical Xenopus cell.

Some cells were symmetrical, having primary dendrites projecting from all sides of the soma. There were also very asymmetrical cells arborising in one direction from the soma only. Others were in between. The dendritic tree was usually irregular in shape. The orientation of the asymmetrical dendritic trees did not seem to be strictly related to the radial location of the cells as in the goldfish, where more central cells had more symmetrical trees and more peripheral cells had trees biased towards the margin (Hitchcock and Easter, I986).

The (%a cells in Xenopus did not show a crystalline regularity in their somal distribution. The irregularities in their dendritic tree shape and the varied orientations seem to be related to this fact.

Differently orientated asymmetric trees probably compensated for the \ relative irregularities in somal distribution by tessellating with the

neighbouring trees, and thereby ensuring more or less uniform dendritic

n

coverage over the retina. This business of tessellation was

L f' _

particularly obvious in places where two cells came close. The ^ ^ trees, in these cases, showed a tendency to 'flee away' from each other,

A - v v L r o .pa

probably avoiding extensive overlapping and forming their own 'personal territories' (see Figs. I3A; 65). However, some very marginal cells did show predictable dendritic orientation parallel to the margin. In these cells, two thick primary dendrites emerged from two sides of the soma giving rise to an elongated tree almost perpendicular in its orientation to the radially arranged nerve fibre bundles (see Fig. 40C) .

Although the dendrites did vary in depth along their often long courses and from one to another, this variation was confined to a

dep t h range in sublamina a ( a r o u n d 7 5 % of the distance from the GCL to the I N L ) .

The Qab cells in Xenopus laevis: The general morphological features of this type of cell are described in Section 3*3*2. For the measurement of soma area, 100 Oab somata were chosen, using random number tables, from those in the same CLC-labelled retina (Fig. 15B) as was used for the Qa cells. The soma area ranged from 117 to 270 pm^ (mean: 195-8; SD: 33-7). A typical Xenopus o^b cell is shown in Figure 9-

In soma size, dendritic tree size and primary dendritic calibre, these cells were usually smaller than the cells. The a^b cells in

Xenopus did not show any strict segregation of the dendrites into outer and inner subtrees. Measurement of depth for all dendritic orders up to the most distal, where the dendrites were very fine, revealed a more diffuse arborisation between their outermost and innermost limits. Nevertheless, these cells still had two clearly demonstrable major sets of dendrites, in terms of both length and calibre, in two different IPL depth planes at about 40% and 70% of the distance from the GCL to the INL. Under low power objectives, these two conspicuous sets of

dendrites gave these cells almost the appearance of typical bistratified c e l l s .

The cells in Xenopus laevis: The general morphological features of this type of cell are described in Section 3:3-3- A typical Xenopus

cell is shown in Figure 10. Using a random number table, ll4 cells were chosen from those in the same CLC-labelled retina (Fig. I5C) as was used for Qa and a^b somal area measurement. The somata in Xenopus

measured from 94 to 3^7 (mean: 174.4; SD: 46.5)- However, much of this wide variation and, therefore, high standard deviation is probably attributable to the inadequacy of labelling in many of these cells. In the Xenopus retina, the well-labelled clq cells usually had a very large soma, often larger than the and a^b somata at equivalent retinal

locations: these large-bodied cells had a very large dendritic tree. In ") o the case of poorly labelled cells, on the other hand, the soma looked ) H o — ^ smaller and the primary dendrites thinner. In some of these cases, it ^ ' was not possible to discern the whole dendritic tree.