The doublet microtubules of rods of the rabbit retina.

The connecting cilium of the rabbit photoreceptor rod is composed of nine outer doublets, lacking dynein side arms. The central singlet microtubules are absent. In cross section, there is an inner dense ring situated between the doublets and the center core of the cilium. As the doublet microtubules progress from the connecting region into outer segments, the cylindrical array of the nine pairs of doublets spreads out as a brush-like arrangement into the incisure cavity of the outer segment. The microtubules continue as doublets for much of the length of the outer segment. The B-tubules terminate first; the A-tubules extend as single tubules into the apical region of the photoreceptor. Before the B-tubules end, they open up, forming hook-shaped projections from the A-tubules. The gradual reduction in length of these hook-shaped structures suggests that near their distal ends each B-tubule opens because of the separation of protofilament 1 of the B-tubule from protofilament 1 of the adjacent A-tubule. Subsequently, the B-tubule protofilaments terminate individually.