The Fine Structure of the Retina. V. Abnormal Retinal Rods and their Morphogenesis

Electron microscope studies of vertebrate retinas (1-4) have established that the outer and inner segments of each rod are connected through the so called "connecting cilium" (2). Examination of a large number of sections of vertebrate retinas occasionally reveals however, abnormal rods whose outer and inner segments are in direct continuity. Morphogenetic studies of such abnormal rods can serve to check the earlier supposition concerning the capacity of the ciliary vesicles or tubules to induce the formation of rod sac precursors (5). This brief note describes the results obtained to date from observations on adult mouse, rabbit, and human retinal rods, and on morphogenesis in kitten retinal rods. The procedures employed in preparing tissue samples were essentially those described in a previous paper (5). In any given region of mouse or rabbit retina, abnormal rods are seen intermixed with much more numerous normal rods. An example is shown in Figs. 1 and 2. In these abnormal rods, the distal region (outer segment), where piles of rod sacs are contained, is directly connected with the proximal region (inner segment), in which mito-chondria are observed, and often a row of rod sacs descends into the proximal region (Figs. 1 and 2). Except for these abnormalities, the arrangement of rod sacs in the distal region appears as regular as in normal rods, and neither the width nor the length of the region differs much from that of normal rods (Fig. 1). In the section shown in Figs. 1 and 2, profiles of the basal body or ciliary tubules are not recognizable. In man, abnormal rods are found seldom in the central region of the retina but with comparative frequency in the peripheral region. The human abnormal rod usually appears to have a short distal region (outer segment) (Figs. 3 and 4) in contrast to the long region of the mouse or rabbit. The width of this distal region is sometimes almost equal to that of the proximal region, that is, several times as wide as that of the normal outer segment (Fig. 4). In some abnormal rods the arrangement of rod sacs is as regular as in normal rods (Fig. 3); in others, however, rod sacs are divided into distinct groups with various orienta-tions, and in some cases, arrays of rod sacs are even parallel to the plasma membrane of the rod (Fig. 4). One or two centrioles can be seen in Figs. …