Arm-bearing Microtubules Associated with an Unusual Desmosome-like Junction

Microtubule fine structure is well documented in many diverse cell types (see reviews by Porter, 1966; Schmitt and Samson, 1969) . In most instances a typical 200-250 A diameter structure is described. Occasionally, additional characteristics such as arms or bridges are seen in cells other than typical flagella and cilia . Selected examples of these include : frog neurotubules (Kohno, 1964) ; axostyle microtubules (Grimstone and Cleveland, 1965) ; tentacle microtubules (Rudzinska, 1965) ; spindle microtubules (Krishan and Buck, 1965) ; intranuclear microtubules (Behnke and Forer, 1966) ; sperm microtubules (Robison, 1966) ; axopodial microtubules (MacDonald and Kitching, 1967) ; spermatic, microtubules (McIntosh and Porter, 1967) ; pyramidal cell neurotubules (Palay, 1968) ; groupings of microtubules in several ciliate species (Tucker, 1968 ; Bannister and Tatchell, 1968) ; plant cell plates (Hepler and Jackson, 1968) ; algal spindle fibers (Wilson, 1969) ; dendritic neurotubules (Wuerker and Palay, 1969) ; insect sensory cell microtubules (Smith, 1969) ; heliozoan microtubules (Tilney and Byers, 1969) ; tissue culture cells (Hepler, McIntosh, and Cleland, 1970) ; heliozoan microtubules (Roth, Pihlaja, and Shigenaka, 1970) . Likewise, the fine structure of junctional complexes has been well documented (Farquhar and Palade, 1963, 1965; Kelly, 1966). Generally, these are divided into typical desmosomes (maculae adherentes), zonulae adherentes, and the zonulae occludentes . In addition, the septate desmosome has been described (Locke, 1965 ; Gouranton, 1967 ; Bullivant and Loewenstein, 1968) . However, the number of descriptions of functional complexes associated with microtubules is only a very small proportion of the total reports . In 1966, Noirot-Timothée and Noirot described the insertion of microtubules in the plasma membrane of foregut and hindgut epithelial cells of termites . These junctional areas or "attachment zones" were obscured by dense material adjacent to the internal lamella of the membrane . LaiFook (1967), in describing developing muscle insertions in insects, stated that the microtubules of the tendinous epidermal cells did "not merely approach the membrane but appeared to attach to it at the site of intermediate junctions and hemidesmosomes ." Moulins (1968) also described microtubules inserting on plasma membranes of epithelial cells in Blabera craniifer . The present study provides some additional observations on arm-containing microtubules and their association with and participation in an unusual junctional complex .