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S.A. Kohan, G.A. Zampighi; Structure of Ribbon Synapses in Mouse Photoreceptors Revealed by Conical Tomography . Invest. Ophthalmol. Vis. Sci. 2006;47(13):148.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the three–dimensional organization of photoreceptor ribbon synapses in an effort to understand the mechanisms that underlie the fast and continuous supply of vesicles for fusion at release sites.
Mouse retinas were prepared for thin sectioning electron microscopy. In the EM, thin sections were tilted at a 55o angle and images were collected at 5o rotating increments until completing a 360o turn (conical tomography). The projections of the conical series were aligned to a common reference system and then reconstructed using the weighted back projection algorithm. The maps were studied by examining individual planes and volume rendering techniques.
We reconstructed five ribbon synapses of photoreceptors (cones and rods) and their second order neurons (bipolar cells) in the outer plexiform layer. They are characterized by the 30–50 nm wide electron–dense ribbons of variable length that extend perpendicular to the plasma membrane. The majority of vesicles (66 ± 12 nm in diameter) are free in the cytoplasm. Some vesicles are tethered to the ribbon by 4–6 nm diameter filaments of variable length. A separate vesicle population is docked to the plasma membrane but no longer tethered to the ribbon. The ratio between the number of vesicles tethered to the ribbon and the vesicles docked to the presynaptic membrane is ∼ 5 to 1.
A cytoskeletal matrix assembled at active zones is the central organizer that provides the structural scaffold to ensure an efficient mechanism for docking and fusion of synaptic vesicles to the plasma membrane.
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