Microtubules are a class of proteinaceous, intracellular fibers found in a wide variety of organisms. They were originally defined by their characteristic morphology as seen in the electron microscope where they appear as tubes of 250 a outside diameter with a 150 A hole. They are unbranched and may run for many micrometers. Since the widespread use of glutaraldehyde as a fmative for electron microscopy, microtubules have been found in all eucaryotic cells examined. For example, they are the principal fibrous constituent of the mitotic spindle (Figs. l a and b); they are present in cilia and flagella (Figs. and); they are incorporated into highly ordered arrays in numerous unicellular organisms (Figs. and); and they are a common component in the cytoplasm of many elongate cells (Figs. and).
The functions of microtubules have been investigated in a variety of systems. Comparative and experimental studies have implicated microtubules as causal agents in the development and maintenance of anisometric cell form. During differentiation of the lens of the chick eye, the epithelial cells elongate by a factor of about 4. At this time the cytoplasm of each cell contains about 100 microtubules running parallel t o the developing long axis (1). When neural cells are cultured under appropriate conditions, they elongate and extend their processes for many micrometers. These slender protrusions contain numerous microtubules (2). The nuclei of many spermatids are elongate, and during spermiogenesis, nuclear shape change is associated with bundles of microtubules (3). The literature abounds with reports of microtubules found within cells changing their shapes, and several excellent reviews are available (4-6). There are experimental conditions that block microtubule formation or disrupt existing tubules: the plant alkaloids colchicine, podophyllotoxin, and vinblastin seem t o be rather specific in their capacity t o act reversibly at low concentrations to dissassemble microtubules . High hydrostatic pressure (6,000 psi) or reduced temperature (about 2ยฐC) have a similar, though less specific effect (9). These treatments have been applied t o many elongate and elongating cells t o provide a wealth of circumstantial evidence that assembling and assembled microtubules contribute to the definition of the cell shape.