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Fig. 2. The two rhabdom types within the retina. Type I (left) is found in mid-band rows 5 and 6 and both hemispheres, whereas type II (right) is only found in MB rows 1-4. Each rhabdom has a distally placed eighth retinula cell (R8) that overlies the main rhabdom composed of microvilli from retinula cells 1-7 (R1-R7). The R1-R7 cells are divided into two groups (Group I and Group II). In type I rhabdoms, R1-R7 contribute to the entire length of the main rhabdom and Group I (R1, R4, R5) and Group II (R2, R3, R6, R7) cells form mutually perpendicular, regular layers of microvilli. (A,B) Longitudinal electron microscopic sections through the main rhabdoms. (A) A row 5 rhabdom, which has very thin layers (5-6 microvilli deep) of orthogonal microvilli. In type II rhabdoms, the R1-R7 form two separate rhabdomal tiers, a distal (D) and a proximal (P) tier. The type II rhabdom is therefore three-tiered. In rows 1, 2P, 3 and 4 the microvilli within the rhabdoms are unordered, because each retinula cell produces microvilli in both orthogonal directions. (B) Rhabdom 2D is an exception. Here the four cells forming the rhabdom are also divided into 2 subgroups (R2, R6 and R3, R7), which form orthogonal layers of microvilli. However, the microvillar layers are about three times thicker (16-17 microvilli deep) than the layers found in rows 5 and 6. Scale bars, 2 µm.

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