The Ventral Photoreceptor Cells of Limulus I. The microanatomy

The ventral photoreceptor cells of Limulus polyphernus 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 mernbranes 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. I N T R O D U C T I O N Physiologists have s tudied the la tera l eyes of Limulus polyphemus with grea t intensi ty and success. A l though it is d e b a t a b l e whe the r this anc ient and a m e n a ble c r ab can see at all, the a n i m a l possesses several o ther s t ructures exquisi te ly sensitive to light. T h e " l a t e r a l o l fac tory nerves" have been shown physiological ly to be a m o n g these l ight receptors (Mil lecchia, B radbury , and M a u r o , 1966). W e have found tha t the la tera l o l fac tory nerves also have morpho log i ca l character is t ics c o m m o n to m a n y o ther pho torecep tors , pa r t i cu la r ly the re t inular cells in the la tera l eye of Limulus. Pa t t en (1893) gave the la tera l o l fac tory nerves their name . H e bel ieved tha t