Identification of the synaptic pedicles belonging to the different spectral types of photoreceptor in the turtle retina.
In this paper we describe the morphology of the different spectral types of photoreceptor pedicles in the outer plexiform layer (OPL) of the Pseudemys turtle as studied by light (LM) and electron microscopy (EM). Tangential serial thick sections were cut from the oil droplet region to the level where the axons emerge from cell bodies and then serial thin sections through the axons and the entire pedicles were collected and examined. Thus photoreceptor pedicles could be identified by tracing cells from their oil droplets to their synapses in the OPL. Double cone pedicles consisted of closely applied pairs with the principal member's pedicle wrapping around the accessory member's pedicle. Each pedicle was approx. 104 microns 2 in area and contained 12 and 8 synaptic ribbons respectively. Rods comprised 8% of the pedicles in the field, were small (84 microns 2), contained closely packed synaptic vesicles, and on average, 9 long ribbons. Single red and green cone pedicles could not be told apart without following them from their oil droplets, however, both were about the same size (106-127 microns 2) and contained 10-12 ribbons. Blue cone pedicles were small and round (80 microns 2) and arose from short oblique axons giving off from large, greenish, clear oil droplet-containing cell bodies (13% of the cone population). The least common pedicle types (5% of the cone population) were identified tentatively as UV cones because they originated from small, clear oil droplet-containing cell bodies. UV cones had spherical pedicles, elongated in the vertical axis, that arose from extremely long, angled axons. Their very small pedicles (64 microns 2) exhibited characteristic "horns" that projected from the top sides of the pedicle. Both putative UV and blue cone pedicles ended more vitread in the OPL than other pedicles and contained only 5-6 and 8-10 ribbons respectively. Understanding the ultrastructural features that distinguish the different types of photoreceptor pedicle will allow us to begin a study of spectral connections to second order neutrons in the turtle OPL in the future.
Ophthalmology Department, John A. Moran Eye Center, University of Utah, Salt Lake City 84132.
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Goede
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Kolb
D000818:Animals; D001369:Axons / Q000648:ultrastructure; D008854:Microscopy, Electron; D017949:Retinal Cone Photoreceptor Cells / Q000648:ultrastructure*; D017948:Retinal Rod Photoreceptor Cells / Q000648:ultrastructure*; D013569:Synapses / Q000648:ultrastructure*; D014426:Turtles / Q000033:anatomy & histology*