Distinct lobes of Limulus ventral photoreceptors. II. Structure and ultrastructure.

Abstract

The structure of Limulus ventral photoreceptors fixed in situ has been investigated using light and electron microscopy and computer-assisted reconstruction and planimetry. Photoreceptors occur singly and in clusters. All photoreceptors have two types of lobes. The rhabdomeral lobe (R lobe) appears to be specialized for light sensitivity, containing the rhabdomere, which completely covers its external surface and forms infoldings into the lobe. The structure of the external rhabdom differs from that within infoldings. The other main structures of the R lobe are the palisades along the rhabdom, multivesicular bodies, lamellar bodies, and mitochondria. The arhabdomeral lobe (A lobe) bears the axon and contains the nucleus, clusters of residual bodies, lamellar arrays of endoplasmic reticulum, masses of glycogen, lipid droplets, and Golgi complexes. The R lobe and A lobe are analogous to the outer and inner segments of vertebrate photoreceptors. In single photoreceptors A and R lobes are separated by an indentation filled with glial processes. Computer reconstructions of cell clusters reveal that each cell has both types of lobes and an axon. Most of the rhabdom is formed from abutting arrays of external rhabdom from the R lobes of different members of the cluster. Efferent fibers containing characteristic angular granules penetrate single cells and clusters in glial invaginations. The main, if not exclusive, target of the efferent fibers is the internal rhabdom.

Relationship between light sensitivity and intracellular free Ca concentration in Limulus ventral photoreceptors. A quantitative study using Ca-selective microelectrodes.

“…Even so, the light-induced Ca; increase was comparatively large and fast. This is not surprising since the R lobe has been shown to extend over tens of microns (Stern et al, 1982 ;Calman et al, 1982 ;Payne and Fein, 1983; see also Fig. 1).…”

“…It might be argued that what looks like two lobes could very well be a two-cell cluster . However, the sensitivity profile of the cell does not support that possibility : clusters of photoreceptors have been shown to bejoined at their rhabdomeral lobes (Calman and Chamberlain, 1982). In this case, the maximum sensitivity should be at the junction and not localized at one end of the cell, as shown by the sensitivity profile .…”

“…Sometimes, by carefully orienting the photoreceptors on the side of the nerve, it was possible to better focus the microspot and to see what appeared to be two lobes (Calman and Chamberlain, 1982;Stern et al, 1982) . Fig.…”

“…Since the Ca microelectrode measures a very localized volume of the cell, one possible explanation for these variations could be that the Ca electrode was not always near the site where the increase in Cai occurred (Levy and Fein, 1983). Recently, it was shown that ventral photoreceptors have two distinct functional lobes (Stern et al, 1982;Calman and Chamberlain, 1982). Stern et al .…”

“…Sometimes a cell would have an almost uniform sensitivity from one end to the other, a situation that might occur if the R lobe occupied a position all along the length of the cell (Calman and Chamberlain, 1982). The light-induced Ca; increases in such cells were either very large or very small (see Levy, 1983).…”