Quantitative aspects of synaptic ribbon formation in the outer plexiform layer of the developing cat retina.

The development of synaptic ribbons in rod and cone photoreceptor terminals of the cat retina was studied using quantitative electron microscopy. At the region of the area centralis, synaptic ribbon profiles are initially recognized at PCD (postconception day) 59. Synaptic ribbon density increases rapidly, reaching a peak of 0.55 ribbons/micron 3 at PCD 68 (postnatal day 3) and maintains approximately that value for an additional 8 d. Following PCD 76, ribbon density begins to decrease, to 0.37 ribbons/microns 3 at PCD 82 and 0.25 ribbons/microns 3 at PCD 102. Although ribbon density drops by approximately 50% during this 39-d period, the outer plexiform layer (OPL) volume at the area centralis increases by about 20%. Ribbon density continues to decrease gradually over a protracted period to reach a final adult value of 0.11-0.14 ribbons/microns 3. During the period of high ribbon density, rod spherules with two, or even three ribbon profiles, were routinely observed. In contrast, in the adult, spherules with more than one ribbon profile are only rarely encountered. During development, the length of synaptic ribbon profiles increases from a mean of 0.22 microns at PCD 62 to the 0.47 microns mean length found in the adult.