A quantitative ultrastructural study of the inner plexiform layer of the rat retina

Abstract

The inner plexiform layer (IPL) and adjacent regions of the rat retina were studied by electron microscopy. The IPL contains neural processes of amacrine, bipolar, and ganglion cells, and glial (hluller) processes. Type 1 amacrine processes (maximum diameter 3.1 P ) traced from their somata structurally resemble amacrine perikarya. These processes rarely form synapses. Type 2 amacrine processes (maximum diameter 0.7 p) are radially oriented, forming synaptic expansions e n passant, giving origin to collaterals, also with expansions. The type 2 amacrine processes form conventional synapses onto bipolar, ganglionic and other amacrine processes, onto amacrine and ganglionic somata, and onto dendritic spines. Amacrine perikarya in rare cases contain synaptic vesicles and form somatic synapses onto surrounding processes.Bipolar processes traverse the IPL, forming terminal expansions up to 6 P in diameter near the ganglion somata, which they contact without synaptic vesicle clustering. Bipolar processes form synapses onto dyads. In 73% of dyads, both processes are vesiculated. One process of the dyad pair could usually be identified as an amacrine by its reciprocal synapse back onto the presynaptic bipolar process. The other dyad process, when vesiculated, contains vesicles which are significantly larger and less concentrated than in the adjacent amacrine, and is presumably a ganglion cell dendrite.The ratio of the incidence of amacrine synapses:bipolar synapses is about 8 : l . A subdivision of the IPL into thirds shows that the highest incidence of amacrine synapses is in the middle third. Amacrine somata are significantly larger than bipolar somata. Synaptic vesicles in receptor terminals of the outer plexiform layer are significantly larger and more concentrated than those in axonal processes of the IPL. The functional aspects of these findings are discussed in relation to physiological investigations and interspecies differences described in the literature.Most ultrastructural studies on the retina have been concerned with the outer retinal regions: the rods and cones, and the synapses in the outer plexiform layer (Reviews: Cohen, '63, '69). Mounting interest in the ultrastructural organization of the inner plexiform layer (IPL) has followed the first quantitative studies of Dowling and Boycott ('65, '66) on the primate retina. The recordings from the ganglion cells have shown that processing of visual information occurs between the photoreceptor input and ganglion cell output (De Valois, '66). The activity of the photoreceptor cells is coded for movement, J, COMP. NEUR., 140: 439-478.shape, intensity, and colour before impulses from ganglion cells are relayed to higher centres.The ratio of receptor: bipolar : ganglion cells in the rat retina is approximately 3 6 : 14: 1 (Lashley, '32). The value of the "bipolar cells" in this ratio is probably less than 14, since Lashley ( ' 3 2 ) did not differentiate between amacrine, bipolar, and horizontal cells in the inner nuclear layer. The bipolar cells, however...

Quantitative Morphometry of Perifoveal Capillary Networks in the Human Retina

“…Detailed histologic studies have revealed that the vertebrate IPL is a non-uniform, layered structure with significant dissimilarities in the density and complexity of synaptic contacts between IPL strata. [55][56][57] Furthermore, there is significant heterogeneity in the ratio of amacrine to bipolar to ganglion cell synapses between IPL strata. The axon terminals of ON-and OFF-bipolar cells ramify in distinct IPL strata with terminal arborisations of OFF-type and ON-type cells synapsing in sublamina a and b of the IPL, respectively.…”

Glycogen Accumulation in Retinal Neurons and Glial Cells of Streptozotocin-diabetic Rats: Quantitative Electron Microscopy

“…Neural processes in the IPL were identified by synaptic vesicles or synapses. 7 ' 8 The mean diameter of the synaptic vesicles was 39 m/i-(SE = 0.64; N = 100). The synaptic vesicles were usually rounder and considerably smaller than the vacuoles in the Muller processes and endfeet, as can be seen by comparing the above means.…”

The primary visual system of the rat: A primer of its anatomy

“…The third type of synaptic elements in the outer plexiform layer are the dendritic processes of ganglion cells which are exclusively postsynaptic. The synaptic arrangements that have been described (Dubin, 1970;Leure-dupree, 1974;Sosula & Glow, 1970) are:…”

“…Numerically, the incidence of amacrine synapses to bipolar synapses is, according to Sosula and Glow (1970), approximately 8: I, with the highest incidence of amacrine synapses in the middle third of the inner plexiform layer. This is a somewhat lower ratio than that set by Lashley (1932) of 36:14:1 with respect to receptor:bipolar:ganglion cells; however, Lashley did not distinguish between amacrine and bipolar cells in the inner plexiform layer.…”