Fine structural analysis of lipofuscin in various tissues of rats of different ages.

Comparative fine structural analysis of intracellular dense inclusions in various tissues of rats of different ages (2, 11 and 29.5 months old) was made. The majority of dense inclusions observed in 2-month-old rats were of the granular type. With advancing age, granular type inclusions gradually decreased in frequency, while compound type inclusions, such as granular-homogeneous and granular-lamellated type increased except for spleen. In the bronchial epithelia, granular-lamellated inclusions appeared at 2 months of age. On the other hand, in the heart, liver, spleen, and skeletal muscle, lamellated or granular-lamellated inclusions were not observed at any of the three age levels. Dense inclusions seemed to have various patterns of distribution, frequency of appearance, and source of organelles in each tissue and cell. They also did not show the maturity of pigment in the process of pigment formation. These findings suggested that these inclusions were not merely age-related granules, but seemed to be influenced by their relationship to the cell metabolism and other functions.