From the mists of early electron microscopy (EM) images there gradually emerged, during the 1950s and 1960s, long, rod-shaped structures. Various investigators missed them, dismissed them, or called them canaliculi, endoplasmic reticulum, or filamentous elements. Finally Slautterback (1963) and Ledbetter and Porter (1963) gave them a full description, recognized their ubiquity, and bestowed their final name, for now still in quotation marks, as “microtubules”.
“Fibrillae” had been noted in flagella and mitotic spindles as early as 1900, but their existence and relationship to one another were disputed. EM provided concrete evidence for their existence, although in one of the earliest EM images of dendritic microtubules the structures were described as “long tubular elements of the endoplasmic reticulum, about 180 Å wide and remarkably straight” (Palay, 1956). Better images of spindle microtubules came from many workers including Roth and Daniels (1962), and microtubules turned up more often and at better resolution once glutaraldehyde was added to the standard osmium EM fixation procedure (Sabatini et al., 1963).
The unification, in which different fibrils, filaments, and tubules were all classified as one structure, was an extension of the better EM resolution. As Ledbetter and Porter stated, “on the basis of size and structure there is reason to regard these [spindle] tubules as essentially identical with those in the interphase cortex.” Slautterback (1963) saw similar arrays of tubules in protozoa, as had others. In his extensive survey of other's work he correctly pulled together many disparate sightings of tubule-like structures to create a unified concept of “microtubules”.
But his subsequent discussion departed into more doubtful territory. “It seems reasonable to assume,” he wrote, “that the membrane bounding the microtubules has properties similar to those of other complex phospholipid–protein membranes with which it is continuous. One of the best established properties of such membranes is their ability to concentrate ions at their surfaces. Such a situation would greatly favor the ability to transport ions in the tubule parallel to its long dimension.”
Slautterback's idea of a plumbing system for the cell was based on the observed association between microtubules and membranous organelles involved in secretion. Although this idea was not borne out by subsequent experiments, his concept of microtubules as a widespread and consistent structure was confirmed when several groups described the 13-protofilament structure of microtubules (Ledbetter and Porter, 1964; Phillips, 1966; Tilney et al., 1973).