Changes in Mitochondrial Appearance 101 



in addition, in occasional parenchymal cells. In the liver and 

 pancreas, mitochondria, identifiable by their internal folds, 

 were observed to contain silver granules. Further deposi- 

 tion of silver caused disruption of the internal folds and the 

 formation of intramitochondrial vacuoles. Similar changes in 

 mitochondrial appearance have been noted in our laboratory 

 by Dr. Jules M. Weiss after intravital administration of neutral 

 red. We have also seen occasional macrophages from normal 

 animals in which granular aggregates were segregated within 

 mitochondria. 



The endodermal epithelium lining the yolk-sac cavity is a 

 transient tissue in that it, like the rest of the placenta, lives its 

 life only during the period of gestation. Moreover, in many 

 animals, the yolk-sac is a flourishing organ early in pregnancy 

 but becomes reduced in size and relative importance after 

 the establishment of the chorioallantoic placenta. At mid- 

 gestation in the guinea pig, mitochondria in the yolk-sac 

 epithelium are normal in appearance and have well-defined 

 internal membraneous folds. Near term, however, the mito- 

 chondria are swollen, their internal folds have vanished and 

 many seem to have been converted into pigmented structures. 

 In the mare's yolk-sac, which becomes reduced in size during 

 the middle span of pregnancy, numerous cells become de- 

 tached from the basement membrane. In these effete cells, 

 swollen and disorganized mitochondria are the rule. These 

 changes are not ascribable to poor or delayed fixation, since 

 adjacent cells may contain mitochondria of normal size and 

 structure. 



The adrenal cortex can be regarded as a transient tissue in 

 that moribund and dead cells are frequent in the inner, juxta- 

 medullary zone, whereas mitoses are ordinarily encountered 

 only in the outer zones. These and other cytological con- 

 siderations lead to the assumption that cells in the outer 

 portion of the zona fasciculata are in an active secretory 

 phase; those in the inner fasciculata are becoming exhausted 

 and those in the zona reticularis are dying. The electron 

 microscopical appearance of these zones has recently been 



