The narrow evaginations or finger-like projections distributed at ran- 

 dom along most cell surfaces and in continuity with the cytoplasm of 

 the cell, are called "microvilli." It has been suggested that these may 

 represent the fixed, static profiles of active cell surfaces such as those 

 seen in phase-contrast movies of living cells. In the striated border of 

 intestinal epithelium, the microvilli are constant in diameter and length 

 and are closely packed to form a continuous surface structure. The 

 brush border in the proximal convoluted tubule of the kidney is made up 

 of a series of closely spaced membrane extensions arranged in a hex- 

 agonal pattern. 



Recent electron microscopy studies have demonstrated that cilia such 

 as those of epithelial cells have a characteristic, highly organized internal 

 structure. In their simplest form, they are slender extensions of the cell 

 surface, usually about 0.2 microns in diameter and of variable lengths 

 up to many microns. The individual cilium is made up of a ring of nine 

 peripheral and two centrally located filaments, all of which run in the 

 direction of the long axis (Figure 3-32). The protoplasm in which the 

 entire system of peripheral and central filaments is embedded is con- 

 tinuous with the cytoplasm and enclosed by the cell membrane. In longi- 

 tudinal sections, the filaments can be seen to terminate in a dense, 

 transverse plate, called the basal body, or kinetosome, located at the 

 base of each cilium in the cortex of the cell. In some cell types, striated 

 fibers appear to originate from the basal bodies and to extend for some 

 distance into the interior of the cell. The basal body itself is cylindrical 

 and is also composed of a peripheral ring of nine loci continuous with 

 the ciliary filaments. So far, most evidence indicates that the architecture 

 of cilia is basically the same in all forms from protozoans to primates. 

 There is a rather striking similarity of structure between the centrioles, 

 cilia, and basal bodies. The centriole, like the cilium and basal body, 

 is essentially cylindrical in form and shows in cross-section a ring of 

 nine peripheral filaments. 



Localized thickenings of closely apposed cell membranes have been 

 reported in a variety of cell types. The thickened portions of each mem- 

 brane are generally so close together that they appear at low magnifica- 

 tion as a single structure, which has been called the desmosome. These 

 contiguous regions constitute a sort of bridge between adjacent cells. 

 This bridge is not continuous as is presumed to be the case with the 

 plasmadesmata between adjacent cells in many plant cells or the inter- 

 cellular bridges between groups of developing mammalian germ cells. 



Extensive infoldings of the cell membrane into the interior of the cell 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 65 



