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Rhabdoms are transparent rods, found in the center of each ommatidium in the compound eye of arthropods. These rods are constructed from the seven photoreceptor cells in the ommatidium. Each photocell is long and thin. They pack their area of the ommatidium completely filling the space. The part of each photocell nearest the central axis of the ommatidium fuses with the other photocells and collectively they form the rhabdom. The rhabdom acts as a light guide, to enhance the detection of the photons entering the ommatidium.
The rhabdom is the photoreceptor light-sensing organelle and is the functional equivalent of the outer segment of vertebrate rod and cone cells. Each R cell contributes one rhabdomere to the composite rhadom. One rhabdomere consists of 60,000 tightly-packed microvilli, each 50 nm in diameter and 1-2 µm in length. This results in a tremendous increase in surface area to house the tens of millions of rhodopsin molecules and associated signaling molecules that are responsible for the detection of light. (From ]Greek] word rhabdma, rhabdos, rod).
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