A known function of macrophages is to remove dead cells. In the adrenal cortex (AC), macrophages are mainly observed in the zona reticularis (ZR), situated deep in the gland. This finding is in agreement with the theory that AC parenchyma cells originate in the outer layers and migrate to the ZR where they die and are eliminated.
When adrenocorticotropic hormone (ACTH) secretion is suppressed by hypophysectomy, or corticosteroid administration, AC atrophy follows, apoptosis is more frequent and more macrophages are likely to be present to remove dead cells. To ascertain this point and verify if changes may be observed along ageing, we performed a quantitative study.
Male Wistar rats aged 2, 6, 12, 18 and 24 months, were divided into 2 groups of 4 animals each. The rats of the first group were injected with 4 mg/Kg of dexamethasone phosphate (DEX) i.m. for three consecutive days; the animals of the second group (controls), received a similar volume of saline. At the fourth day, the animals were sacrificed and the adrenals prepared for light and electron microscopy following usual procedures.
Epon sections 1 mm thick were stained and observed. At all ages, the ZR structure of DEX injected animals did not differ substantially from the controls. However, shrunken parenchyma cells and nuclei were more frequent in the aged and DEX treated animals. Among these, macrophages with elongated nuclei and cytoplasm with heterogeneous content were observed. In random areas of ZR from three different ages, a quantitative study was made using conventional stereological methods for the determination of the relative volume (Vv) and numerical density (Nv) of macrophages.
The results for Vv (%) of control/DEX-treated (n=3) rats are the following: 1.25/2.26 at 2 months, 2.55/3.08 at 12 months and 5.01/8.78 at 24 months; as for Nv (x 103/mm3) of control /DEX-treated animals the results were as follows: 13.2/28.9 at 2 months, 30.2/39.1 at 12 months and 31.4/47.5 at 24 months.
These findings confirm an age-related increase in ZR macrophages, which is enhanced when ACTH secretion is suppressed.