CELL ULTKASTRUCTURE L\ MAMMALS 



Nucleus ranged in a layer outside the proteins. Also 

 As a rule, there is only one nucleus in each ^ mosaic arrangement of tlic lipid and pro- 

 cell. The striated muscle cell is an exception ^^"^ molecules has been suggested. The ap- 

 and may contain several nuclei. A double- P'^'"^'"^ uncertainty is explained by the fact 

 contoured membrane surrounds the nucleo- ^^'^^ ^'^^y ^^^^^^ ^^ known about what struc- 

 plasm with a total thickness of about 250A ^^"'^ ^^ stained most intensely with osmium 

 (Fig. 3). Discontinuities have been demon- tetroxido the proteins or the lipids, 

 strated in the nuclear membrane, reminiscent ^*'*^^ Surface. The plasma membrane on 

 of pores. There does not seem to be a free ^^^ ^^'^® surface of cells shows four types of 

 communication between the nucleoplasm differentiation— microvillus, brush border 

 and the cell cytoplasm, however, because extension, stereocilium, and cilimu. 

 the "pores" appear to be plugged by a dense Microvilli. The microvillus is essentially a 

 substance of unknown nature. The structure ^^^/'^ ^"^^ ^'^"^ projection of the cytoplasm 

 of the nucleoplasm or chromatin is finely '^^'hich is covered by the plasma membrane, 

 granulated. The granules have a diameter of ^^^ micro\'illus does not contain any pecu- 

 about 250A and are clustered in a zone ^^^^' structures but a slightly dense ground 

 adjacent to the nuclear membrane but can, substance. The free surface of most epithelial 

 in addition, be seen distributed evenly cells does display a varying number of micro- 

 throughout the nucleoplasm. The nucleolus ^^^ ^^'^^^ ^ g^eat variation in length and 

 represents a heavy aggregation of these thickness (Fig. 4). Supposedly, the microvilli 

 granules (Fig. 1). Very little has been done ^^^^ resorptive functions and certainly 

 so far on the ultrastructure of the chromo- contribute to the increase of the cell surface, 

 somes. Brush border extensions. The brush border 



extensions are longer than the microvilli, 

 Centriole mostly thicker and occur in greater abun- 

 The cell center or the centriole (centro- dance. They all are of the same length and 

 .some) is located in the neighborhood of the are found on the surface of the intestinal 

 nucleus, mostly within the Golgi zone of the epithelial cells (Fig. 5) and on the proximal 

 cell. The centriole is a round or slightly convoluted tubule cells of the nephron (cross 

 elongated body with size and structure es- reference: kidney ultrastructure). Similar 

 sentially similar to the basal body of the structures (Fig. 8) are also seen in the ef- 

 cilium (cross reference: ciliated epithelia) ferent ducts of the testis (cross reference: 

 which represents a dense cortex and a lighter ciliated epithelia ultrastructure). Although 

 core. The cortex is composed of nine paired essentially representing extensions of the 

 filaments and a matrix which embeds the apical cell cytoplasm, the brush border ex- 

 filaments. The core is structureless (Fig. 2). tensions do contain some fine and dense 



_, ^w , striations oriented longitudinally, sometimes 



Flasnia Membrane t. a- i ^^i "^i ^i 



extendnig down nito the apical cytoplasm 



The plasma membrane is the outermost below the level of their bases. Histochemical 



limit of the cell. It has an average thickness tests seem to prove that the enzyme alkaline 



of 70-100 A and stands out as a single dense phosphatase is associated with the brush 



line in the electron micrograph. It is com- border extensions. 



posed of lipid and protein molecules, their Stereocilia. Stereocilia are longer and 



mutual arrangement remaining unknown. It narrower than the brush border extensions, 



has been assumed that the dense line seen in but the lumibcr of stereocilia on each cell is 



the electron micrograph may represent the about identical with that of the brush border 



protein layer. The lipids would then be ar- extensions. Each stereocilium contains three 



93 



