Microtubules and extracellular microtubule-like structures in the retina of the rainbow trout.

In the retina of the rainbow trout (Salmo gairdneri, Richardson) two types of microtubular structures are demonstrated. Besides the normal type of microtubules (about 200 A in diameter), occurring in all cell types of the retina, a second type is described which is termed microtubule-like structure (MLS) because of its extracellular localization. These MLS have a diameter of about 250 A under the same preparative conditions in which the normal microtubules appear 180-200 A thick. The interspace between the tubules is smaller than between the microtubules. Specific MLS to membrane associations exist, which are analyzed by serial sectioning and tilting procedures. It is suggested that the MLS have their origin at small membranous extrusions of the plasmalemma. These extrusions could contain nucleation sites for the MLS-formation within the extracellular space. It remains unknown which cell type produces the MLS proteins and which factors are responsible for the aggregation of the subunits to intact MLS.