Lipofuscin refers to heterogeneous mixtures of intracytoplasmic, usually intralysosomal, nondegradable residues of lipid and protein oxidation/polymerization (as well as other moieties, such as advanced glycosylated end products) that accumulate in various somatic cells due to aging or other factors. Excessive and/or temporally inappropriate adrenal lipofuscin pigment aggregations (compared with age-matched controls) can also be a feature of cortical cell degeneration and/or atrophy (due to various causes) or can result from dietary imbalances (e.g., vitamin E deficiency) or administration of certain exogenous chemicals.
In mice, lipofuscin pigment-laden adrenal cortical cells occur most often as late-stage features or sequelae to physiologic regression of the X-zone. Thus, they are common incidental findings in aging mice. The pigment-laden cell aggregates are located primarily in the region adjacent to the corticomedullary junction formerly occupied by the X-zone. Milder, less temporally advanced cases consist of scattered small foci of pigment-laden cells, but as the animal ages the cell clusters often enlarge and can coalesce into circumferential bands surrounding the medulla. Macrophages containing similar lipofuscin and/or hemosiderin can be interspersed among the affected cortical cells.
In rats, cells in the adrenal cortex containing lipofuscin pigment are also a common age-related incidental change. Even though rats do not have an X-zone, the pigment-laden cells tend to be most common in the inner cortex, usually as single cells or very small clusters (Figure 1 and Figure 2), unlike the large, prominent aggregates often seen in mice.
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