Some contributions to the cell biology of photoreceptors. Proctor lecture.

If one takes on the task of studying the biochemistry of defined regions of photoreceptors, the first problem is how to gain access to all distinctive regions of these cells since, as shown in the human retina of Figure 1, substantial parts of these cells are buried in the retina vitread to the external limiting membrane. When a retina is detached from the pigment epithelium, access is obtained to the photoreceptor outer and inner segments, the regions protruding into the retinal ventricle (ie, the clinician's sub-retinal space), but such important regions as the cell bodies and synaptic terminals remain inaccessible. Moreover, mass isolations of inner segments have not been accomplished. However, some thirty years ago, the biochemist Oliver Lowry described" a means of sampling distinctive levels of the brain or retina for biochemistry. For eyes, this system consisted of rapidly removing and rapidly freezing an eye in liquid nitrogen near its freezing point, and then cutting 6-Aim frozen tangential sections from the eye, serially cutting through the retina. Such sections could be stained for orientation or freeze-dried. Because the sections are tangential and the eye spherical, the distinctive retinal layers are spread out in the sections like broad bands of a bulls-eye target, with the composition and width of particular bands varying with the depth within the retina represented by the center of the section (Fig. 2). As the banding remains discernable and their identity known in freeze-dried sections with guidance from occasional stained sections, one can cut pieces from known retinal layers in freeze-dried sections, weigh these on quartz-fiber, fish-pole balances, and carry out assays for substrates or enzymes on these pieces using the elegant cycling techniques originally introduced by Dr. Lowry.