50 



Cellular Structure and Activity 



the results support the view that cytoplasm 

 contains submicroscopic fibrous particles. 

 The thixotropy sometimes displayed by cy- 

 toplasm favors the same conclusion. When 

 cells at rest or in division are subjected to 

 high hydrostatic pressure the phenomena ob- 

 served (decrease in viscosity, reduction of 

 amoeboid movement and cell division) re- 

 semble those observed in certain fibrovis sys- 

 tems (Marsland, '51) and are interpreted as 

 due to the breaking of interparticle bonds 

 faster than the rate of their reformation 

 (Eyring, Johnson, and Gensler, '46). When 

 mineral oil is injected into the axoplasm of 

 a squid giant fiber the oil takes on an elon- 

 gated, rather than a globular, shape (Cham- 

 bers and Kao, '52), indicating the presence 

 in the axoplasm (a specialized form of cyto- 

 plasm) of an oriented submicroscopic fibrous 

 lattice. From stvidies involving magnetic con- 

 trol of artifically introduced intracellular 

 metallic particles, Crick and Hughes ('49) 

 suggested the presence in cytoplasm of sub- 

 microscopic particidates in fibrous arrays. 



It is supposed by some that this fibrous 

 submicroscopic lattice — sometimes called the 

 "cytoskeleton" — is an internal molecular 

 framework characteristic of all cells. 



Polarized Light Evidence. Tissue cells as 

 such have received relatively little careful 

 study in polarized light as compared with 

 muscle and nerve fibers and other specialized 

 structures which lend themselves well to 

 such study. Fresh nerve cells show bire- 

 fringence of the type expected if they pos- 

 sessed submicroscopic fibrous arravs with the 

 same orientation as the neurofibrils of fixed 

 and stained preparations (Chinn, '38). Hil- 

 larp and Olivecrona ('46) investigated many 

 types of epithelial cells and observed form 

 birefringence positive with respect to the 

 base-apex axis, together with intrinsic bire- 

 fringence negative with respect to this axis. 

 This was interpreted to indicate the presence 

 of protein (or possibly nvicleoprotein) sub- 

 microscopic filaments oriented parallel to the 

 base-apex axis with lipid molecules oriented 

 with their paraffin chains perpendicular to 

 the protein filaments. In the fresh marine 

 egg it is possible to demonstrate regions in 

 the cytoplasm which show positive birefrin- 

 gence (Inoue and Dan, '51; Swann, '51a, b; 

 Hughes, '52). In certain instances it has been 

 possible to show that such regions do, in 

 fact, possess a fibrous structure as seen in 

 fixed cells with the electron microscope (Mc- 

 Culloch, '52). In the case of nerve axoplasm 

 a semiqviantitative analysis has been made 

 of the form birefringence, which is positive 



with respect to the long axis of the fiber 

 (Bear, Schmitt and Young, '37). The con- 

 clusion that an oriented fibrous submicro- 

 scopic array exists has been confirmed by 

 subsequent EM study (Schmitt, '50b; Fernan- 

 dez-Moran, '52). 



Electron Microscope Evidence. In most of 

 the cell types thus far studied, including 

 fibroblasts, blood, tumor, nerve and liver 

 cells, examination of thin sections has re- 

 vealed a reticular fibrous system in the cyto- 

 plasm. The appearance of the fibrils depends 

 importantly on the fixative vised. Acid fixa- 

 tives usually produce coarse, somewhat ir- 

 regularly contoured, fibrils probably result- 

 ing from the aggregation of finer filaments. 

 Lehmann and Biss ('49) identify the rather 

 coarse fibrous structures which they observed 

 in Tubifex eggs with the chromidial strands 

 of Monne, though they may also be inter- 

 preted as cytoplasmic filaments upon which 

 microsomes are attached. In well fixed prepa- 

 rations the fibrous structures may appear as 

 thin filaments or as the system which has 

 been called by Porter "endoplasmic reticu- 

 lum." The latter may appear tubular or as 

 a linear series of elongate, bladder-like struc- 

 tures. While this structure appears to be 

 rather characteristic of cells generally, the 

 structure in unfixed cytoplasm which corre- 

 sponds to this picture in fixed cells is diffi- 

 cult to assess. 



In a carefvil EM studv of the unfertilized 

 sea urchin egg, McCulloch ('52) observed 

 bundles of nodose fibrils, about 1100 A in 

 thickness, oriented in directions consistent 

 with the positive birefringence which he ob- 

 served in the living egg cells. McCulloch's 

 failure to observe fibrous structures generally 

 distributed in the cvtoplasm may suggest 

 that such fibrous molecviles are either too 

 thin to be resolved by the EM imder the 

 conditions or that the fixation caused reac- 

 tions which obscured them. 



While the EM evidence to date is not in- 

 consistent with the view that the ground 

 substance of cytoplasm contains a fibrous 

 component, it must be realized that the sys- 

 tem is highly complex and that much must 

 be done before the high resolution of the 

 EM will reveal the true molecular basis of 

 the fibrous system in cytoplasm. 



Chemical Identification of the Fibrous Con- 

 stituents. To gain an understanding of the 

 role of the fibrous cytoplasmic constituents 

 in cell functioning it is necessary eventually 

 to isolate these constituents by fractionation 

 procedures and to determine their composi- 

 tion and physical and chemical properties. 



