92 Don W. Fawcett and Mario H. Burgos 



coarse, as well as fine, acidophilic granules, golden-brown 

 lipochrome pigment and lipid droplets of various sizes. In 

 addition, many of these cells contain large angular inclusions 

 known as the crystalloids of Reinke (Reinke, 1896). These are 

 visible in the cytoplasm of unfixed interstitial cells as highly 

 refractile, pale yellow bodies which are usually rod-like, 

 rectangular or trapezoidal in shape, but are often rounded 

 at the ends. They are peculiar to the human testis, and their 

 origin, chemical composition and physiological significance are 

 unknown. 



Appearance with the electron microscope 



The electron microscope reveals an even greater variation 

 in the fine structure of the interstitial cells than was appre- 

 ciated with the light microscope, and provides additional 

 evidence of a transition from spindle cells to Leydig cells. The 

 spindle cells in the lamina propria of the seminiferous tubules 

 have elongated nuclei and a homogeneous cytoplasmic matrix 

 of low density to electrons. Their cytoplasm is traversed 

 by a few canalicular strands of ergastoplasm which show 

 vesicular dilations along their length. Mitochondria are 

 few in number and simple in their internal structure. The 

 only cytoplasmic inclusions observed are a few small lipid 

 droplets. 



The larger fusiform cells that are found along the blood 

 vessels and interspersed with the Leydig cells have round or 

 indented nuclei and their cytoplasm is characterized by the 

 presence of large numbers of extremely thin (50 a) filaments of 

 indefinite length (Fig. 10). These are most abundant in the 

 interior of the tapering cell processes where they generally 

 run parallel to the long axis of the cell. The mitochondria and 

 endoplasmic reticulum tend to be near the periphery of the 

 cell. Many small vesicles (300-500 a) are found just beneath 

 the plasma membrane and these sometimes appear to open at 

 the cell surface. Some of these cells contain sizeable agglomera- 

 tions of dense osmiophilic granules clustered around vacuoles 

 of lower density. 



