148 SUBCELLULAR PARTICLES 



become detectable in fixed tissue or in homogenates only at some later stage could 

 be explained on the basis of the observations made by Holter and Marshall (51) 

 and by Chapman-Andresen and Holter ( 19) in their elegant studies of pinocytosis 

 in the amoeba Chaos chaos. According to these authors, the vacuoles are initially 

 of low density; they become progressively denser owing to either loss of water, 

 gain of matter, or both, and later show signs of loss of engulfed material, pre- 

 sumably due to digestion. It is possible that the vacuoles are very fragile in their 

 early stages and are destroyed by fixation or homogenization. 



The appearance of the lysosomal enzymes within the vacuoles presumably oc- 

 curs after the latter have first been formed. This could happen either by a process 

 of de novo synthesis within the vacuole, or by secretion from the outside. Rose { 81 ) 

 has recently published a study which appears to support the latter concept. This 

 author has described in HeLa cultures a special cell variant called VP cell ( variant 

 pinocytic cell) and characterized by a lack of shrinking of the pinocytic vacuoles, 

 which accumulate in the juxtanuclear zone as large spheres of 4-25 microns 

 (VP granules). Associated with the latter are often seen smaller particles of 1-8 

 microns, to which Rose has given the name VP satellite. In addition both normal 

 and VP cells contain minute particles 0.3 to 0.6 micron in diameter and termed 

 microkinetospheres. These appear to be very active and to establish repeated con- 

 tacts with pinocytic vacuoles, causing changes in the size, refractive index and 

 staining properties of the latter. The VP satellites, on the other hand, seem to 

 arise in VP cells from coalescing microkinetospheres. To explain these observa- 

 tions, Rose (81) has advanced the hypothesis that microkinetospheres secrete 

 digestive enzymes into the vacuoles, and suggested that these granules as well as 

 the VP satellites may be related to the liver lysosomes and the kindney droplets. 



Other relevant information is provided in a recent paper by Straus (94), who 

 has described some changes occurring in the kidneys of rats injected with egg- 

 white 18 hours before killing. In comparison with untreated controls, these 

 preparations showed a decrease in the number of small droplets, an increase in 

 the number of large ones associated with a corresponding fall in the enzyme con- 

 tent per droplet, an increase in the proportion of the enzymic activities found in 

 the final supernatant, and an absolute increase in total cathepsin and nucleases 

 but not in acid phosphatase nor /^-glucuronidase. These results were interpreted 

 as an indication that after injection of egg-white the small droplets are trans- 

 formed into large ones, with a concomitant release of part of their enzyme con- 

 tent into the surrounding cytoplasm. In conformity with previous remarks, the 

 alternative possibility that the excess free activities may have originated not from 

 the surrounding cytoplasm but from fragile pinocytic vacuoles, would deserve con- 

 sideration. New synthesis was invoked to account for the net increase in catheptic 

 and nucleolytic activities. 



It is obvious that considerable additional work will be required before the rela- 



