Morphogenesis of T H E Retinal Rods an Electron Microscope Study

In a previous paper the submicroscopic morphology of the outer and inner segments of the rod cell of the adult retina has been described (1). The conclusion was drawn, that the fibrous connection between both segments is constituted by a short cilium, the so called "connecting cilium," which is composed of nine pairs of filaments, a basal body, matrix material, and a surface membrane. The outer or receptor segment of the rod, confirming the previous work of Sj6strand (2), is composed of a pile of numerous flattened sacs or cisternae bound by a thin membrane and constituting close compartments which are probably connected by short tubular stalks to the cilium. The inner segment contains the mitochondria in the distal part (ellipsoid) and, in the proximal part, leading to the rod fiber, the Golgi substance, endoplasmic reticulum, dense particles, and neuroprotofibrils. Both segments of the rod and the connecting cilium are limited by a continuous surface membrane. I t was suggested that this membrane may be the site for the propagation of the nerve impulse originated at the level of the rod sacs by the photochemical reaction. I t was also postulated that the entire outer segment of the rod may be organized as a differentiation of a primitive cilium (1). These observations and interpretations of the submicroscopic morphology of the adult rod cell made it highly interesting to study, with the electron microscope, the morphogenesis and differentiation of the inner and outer segments. For this purpose the retina of young mice was selected since at birth it is completely undifferentiated and the differentiation of the visual cells starts only after the 3rd or 4th day proceeding to completion between the 12th to the 16th day (3, 4). In this material the development of a typical cilium and the differentiation of its apical portion to constitute the outer segment have been demonstrated. The basal portion of the primitive cilium with its basal body remains undifferentiated and constitutes the connecting cilium of the adult rod. This study throws also some light on the macromolecular mechanisms leading to the formation of the complex lamellar structure which constitutes the receptor portion of the rod cell. 209