A Crystalline Body Found in the Rod Inner Segment of the Frog's Eye

The rod portion of the rod cell in the frog's retina is divided into two parts; namely inner and outer segments. The latter is mainly occupied by the pile of so called rod-sacs which are arranged perpendicularly to the long axis of the rod (1-3). The inner segment is connected with the outer segment by a short slender stalk (or connecting cilium) (4, 5), which has a columnar shape and extends from the level of the outer limiting membrane of the retina. The internal organization of the inner segment shows a remarkable polarity. Within the distal portion of the segment are found numerous mitochondria packed very closely and forming a large compact mass. This portion of the segment has been known to light microscopists as the ellips-oid. The mitochondria within this mass show a tendency to be oriented with their long axes parallel to the long axis of the segment, and directed toward the basal body of the connecting cilium. Beyond this portion no mitochondria are seen in the inner segment. Embedded in the mass of mitochondria, a peculiar structure is occasionally found that has been described as "a long straight rod-like body of high electron density" (3). In sections, the body has a sharp angular outline, and is surrounded very closely by numerous mitochondria. Under high magnification (Figs. 2 and 3), highly ordered inner structure can be recognized. Dense lines about 45 A wide separated by lighter interspaces about 50 A wide run parallel to the long axis of the body. Some dense lines may show a beaded appearance, in which case, the beaded granules of adjacent lines are often arranged in phase so as to give the body a closely striated appearance. This may be an effect of the angle of sectioning, which introduces an additional cross-striation to the inner structure of the body but in any case describes the body as a highly ordered structure. The surface of the body is defined by a denser line of about 80 A wide. Its shape and internal structure lead us to consider this structure as some sort of crystalline body, possibly protein or lipoprotein in nature. As described before, the body is surrounded by closely packed mitochondria, so densely packed, in fact, that one can hardly recognize the cyto-plasmic ground substance in this part Of the cell body. There are also some indications that the dense line bounding the …