The Ventral Photoreceptor Cells of Limulus

Abstract

The ventral photoreceptor cells of Limulus polyphemus resemble the retinular cells of the lateral eyes both in electrical behavior and in morphology. Because of the great size of the ventral photoreceptor cells they are easy to impale with glass capillary micropipettes. Their location along the length of the ventral eye nerve makes them easy to dissect out and fix for electron microscopy. Each cell has a large, ellipsoidal soma that tapers into an axon whose length depends upon the distance of the cell from the brain. The cell body contains a rich variety of cytoplasmic organelles with an especially abundant endoplasmic reticulum. The most prominent structural feature is the microvillous rhabdomere, a highly modified infolding of the plasmalemma. The microvilli are tightly packed together within the rhabdomere, and quintuple-layered junctions are encountered wherever microvillar membranes touch each other. Glial cells cover the surface of the photoreceptor cell and send long, sheet-like projections of their cytoplasm into the cell body of the photoreceptor cell. Some of these projections penetrate the rhabdomere deep within the cell and form quintuple-layered junctions with the microvilli. Junctions between glial cells and the photoreceptor cell and between adjacent glial cells are rarely encountered elsewhere, indicating that there is an open pathway between the intermicrovillous space and the extracellular medium. The axon has a normal morphology but it is electrically inexcitable.

Effects of intracellular injection of calcium buffers on light adaptation in Limulus ventral photoreceptors.

“…The approximate concentration of EGTA buffer in the cytoplasm after pressure injection of buffer can be computed from data in Tables I and II and a value for the cell volume of 5 X 10 -a° liters (Clark et al, 1969;Stell and Ravitz, 1970). The computed final concentration of intracellular EGTA (no calcium added) ranges between 1.6 and 45 raM.…”

“…The ventral photoreceptors of Limulus have large axons, up to 20 #M in diameter (Clark et al, 1969). Diffusion of EGTA down the axon would lower the EGTA concentration in the soma and this might account for at least part of the recovery of the transient-to-plateau ratio observed after injection.…”

“…Table III summarizes the results of experiments on 15 cells. The volume of ventral photoreceptors is as large as about 5 X 10 -l° liters (Clark et al, 1969;Stell and Ravitz, 1970). When the injection volume was a large fraction of the photoreceptor volume (about 2 X 10 -l° liters), the injection of mannitol solutions sometimes produced effects similar to those seen after smaller injections of EGTA solutions: the slope of the responseintensity curve became closer to unity for bright stimuli.…”

Light-Evoked and Spontaneous Discrete Waves in the Ventral Nerve Photoreceptor of Limulus

“…A few years ago, peculiar photoreceptors were discovered in a nerve on the ventral side of Limulus (Clark et al, 1969). These receptors are not organized into an eye, and no propagated nerve impulses have been observed originating from them upon light stimulation.…”

Isopotentiality and an optical determination of series resistance in Limulus ventral photoreceptors.

“…Ventral rudimentary eyes (Demoll, 1914;Clark et al 1969) were dissected, pinned onto Sylgard 184 (Dow Corning, Midland, MI) and bathed in artificial sea water containing 425 mM-NaCl, 23 mM-MgCl2, 2515 mM-MgSO4, 10 mM-KCl, 10 mM-CaCl2 and 10 mM-Tris-HCl adjusted to pH 7*8. Before micro-electrode penetration, the tissue was bathed in Pronase (Calbiochem, LaJolla, Ca., grade B) 20 mg/ml.…”