CHAPTER II 



CELL DIFFERENTIATION AND THE TYPICAL STRUCTURE OF 

 THE ELEMENTARY TISSUES 



In the preceding discussion a general view of the type of cell activity and 

 the structural basis therefor has been briefly presented. Emphasis has been 

 laid on the fact that the complicated phenomena of life are manifested through 

 the agency of the tissues and cells. The histological cells, alone or in com- 

 bination, are capable of all the activities manifested by the living body. 

 Throughout the different phases of the physiological discussions which follow 

 it will be assumed that the reader has some knowledge of this structural basis. 

 However, for the purpose of reference there is presented in this chapter a 

 brief but elementary review of the characteristic cytological structure of the 

 tissues and cells of the animal body. 



THE ESSENTIAL STRUCTURE OF THE TYPICAL CELL 



The typical cell is a spherical or ovoid mass of protoplasm. It is of 

 microscopic size and varies from 6 or 7 micra in diameter for the lymphocytes 

 and erythrocytes to 150 to 200 micra for the diameters of the larger cell 

 bodies of the neurones. Its structure is quite complex, but the most general 

 differentiation is into the cell-mass or cytoplasm, and its contained nucleus. 

 The cytoplasm is sometimes bounded by a definite cell membrane, but in 

 differentiated animal tissues this membrane is usually not present. 



The Cell Body. The cell body or ctyoplasm is a complex semi-fluid mass, 

 the determination of the detailed relations of which has presented many 

 difficulties. The cell cytoplasm is usually described as having a framework 

 of spongioplasm supporting a homogeneous hyaloplasm. In some cells there 

 are formed materials resulting from the cellular activity called metaplasm, 

 figure 12. 



Cell protoplasm includes several kinds of stainable granules and fibrils, 

 some are essential constituents while others are formed by the reactions of the 

 protoplasm and are in a sense extraneous material. These structural 

 features are made more evident by their selective affinity for certain staining 

 reagents. 



The exact form of the spongioplasm or reticulum varies greatly in different 

 types of cells, and even in different parts of the same cell. Its affinity for 

 stains discloses a fine network, the reticulum, which increases in amount 

 and also in constancy in the type of arrangement in the older cells. 



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