Electron microscopy of retinal tapetum (Caiman crocodilus).

The distribution and ultrastructure of the retinal tapetum lucidum in Caiman crocodilus is described. In the light adapted eye the tapetum is recognized in the superior half of the fundus. It consists of guanine containing crystalline platelets which are spread almost over the entire retinal pigment epithelial cells which can be divided into different layers: 1. The basal surface facing the choriocapillary vessels is differentiated into numerous densely packed true microfolds which are commonly described as "basal infoldings". 2. A remarkable high content of mitochondria as well as a great number of lipid droplet-like spherical bodies is observed. 3. In the nuclear zone a Golgi apparatus and all stages of phagocytosis are found. 4. Within the level of intercellular junctions, crystalline platelets are arranged mainly perpendicularly to the long axis of the neighbouring receptor cells, whereas 5. platelets in large pigment epithelial cell processes extending from the apical cell surface between the photoreceptors are oriented parallel to the receptor axis. 6. Heavy pigmentation of the apical tips of pigment cell processes by melanosomes is observed, but only within the lower half of the fundus. The functional significance of the tapetum of Caiman crocodilus is discussed.