The ultrastructural features of secretory cells of some endocrine glands in aging

Age-related Changes in the Adrenal Cortex: Insights and Implications

Aging is characterized by a gradual decline in physiological function. This process affects all organs including the adrenal cortex, which normally functions to produce essential steroid hormones including mineralocorticoids, glucocorticoids, and androgens. With increasing age, features such as reduced adrenal cortex size, altered zonation, and increased myeloid immune cell infiltration substantially alter the structure and function of the adrenal cortex. Many of these hallmark features of adrenal cortex aging occur both in males and females, yet are more enhanced in males. Hormonally, a substantial reduction in adrenal androgens is a key feature of aging, which is accompanied by modest changes in aldosterone and cortisol. These hormonal changes are associated with various pathological consequences including impaired immune responses, decreased bone health, and accelerated age-related diseases. One of the most notable changes with adrenal aging is the increased incidence of adrenal tumors, which is sex dimorphic with a higher prevalence in females. Increased adrenal tumorigenesis with age is likely driven by both an increase in genetic mutations as well as remodeling of the tissue microenvironment. Novel antiaging strategies offer a promising avenue to mitigate adrenal aging and alleviate age-associated pathologies, including adrenal tumors.

Age-related changes in the inner zone of the adrenal cortex of the rat--a morphologic and biochemical study

In this work, a correlative morphologic and biochemical study on the effects of ageing on the rat adrenal Inner Zone (IZ) was made. Male Wistar rats were studied at 2, 6, 12, 18 and 24 months. Structural data of Zona Fasciculata (ZF) showed age-related increase in cell volume (P < 0.05), decrease in mitochondria (P < 0.01) and smooth endoplasmic reticulum (SER) volumes, and increase in lipid droplets (P < 0.01) and lipofuscin granules (P < 0.01) volumes. In Zona Reticularis, the main change observed was the increase in lipofuscin granules (P < 0.001). Serum corticosterone from unstimulated rats increased until 12 months but decreased thereafter (P < 0.01), to levels below those from 2-month-old rats. Similarly, plasma adrenocorticotrophic hormone (ACTH) presented a maximum at 12 months, followed by a decrease to levels higher than at 2 months (P < 0.05). In rats injected either with only ACTH or dexamethasone, before ACTH stimulation, corticosterone level had a maximum at 12 months. In aged rats, serum high density lipoprotein (HDL) and adrenal cholesterol ester increased significantly (P < 0.05 and P < 0.01, respectively), whereas adrenal corticosterone decreased. Products of lipid peroxidation, assayed with the thiobarbituric acid reaction and fluorimetry showed an age-related increase (P < 0.05). The age-related decrease in mitochondria and SER volumes is consistent with the decrease of serum corticosterone. The increase in lipid droplet and HDL and the reduction of adrenal corticosterone level correlate with the increase of adrenal cholesterol ester content. These suggest a continued uptake of steroid precursor but a reduced steroid synthesis. On the whole, the data provide evidence for an age-related reduced functional ability of IZ and particularly of ZF.

Age-related thickening of basement membrane in stria vascularis capillaries

Ultrastructural examination was undertaken to investigate the pathogenesis of age-related atrophy of the stria vascularis (StV). Basement membrane (BM) thickness was increased in 65-85% of strial capillaries in gerbils aged 33 months or older and often exceeded by several-fold that observed in young controls. In an early stage of thickening the BM expanded slightly around the full capillary profile, after which nodular expansions of BM encircling slender cell processes were often observed at or near one or both poles of the elliptical vessel profile. As widening progressed, the BM consisted of 2-3 layers separated by cell processes in the nodules but fewer strata elsewhere. Association of slender processes of both endothelial cells and pericytes with focal thickening outside the process suggested their participation in genesis of the capillary lesion. In later stages of atrophy, pericytes degenerated and disappeared, while endothelial cells remained intact. Eventually, thick multilayered BM devoid of endothelial cells surrounded a narrow lumen occluded by debris. The age-related change in BM in the inner ear was confined to StV capillaries. Degenerative changes in StV epithelial cells occurred apparently as a secondary consequence of the capillary lesion. The pathologic alterations in marginal cells included extrusion of blebs from the luminal surface, separation and loss of basolateral interfoldings, alteration and depletion of mitochondria and nuclear pyknosis. At the end-stage of degeneration, the StV consisted of a simple or multiple layer of squamous cells lining the scala media.

The effects of ageing on the morphology and function of the zonae fasciculata and reticularis of the rat adrenal cortex

The morphological counterpart of the well-known age-dependent marked impairment of glucocorticoid secretion of rat adrenals was investigated by use of morphometric techniques. For this purpose 4-, 8-, 16- and 24-month-old rats were studied. Despite the notable lowering of both basal and ACTH-stimulated production of corticosterone by collagenase-dispersed inner adrenocortical cells, ACTH and corticosterone plasma concentrations displayed significant increases with ageing. Zona fasciculata (ZF) and zona reticularis (ZR) showed a notable hypertrophy in aged rats, which was due to rises in both the average volume and number of their parenchymal cells. The hypertrophy of ZF and ZR cells was in turn associated with increase in the volume of the mitochondrial compartment and proliferation of smooth endoplasmic reticulum, i.e., the two organelles involved in steroid-hormone synthesis. All these morphologic changes, conceivably due to the chronic exposure to high levels of circulating ACTH, are interpreted as a response enabling ZF and ZR to compensate for their age-dependent lowering in glucocorticoid secretion. Stereology also demonstrated that ZF and ZR cells underwent a striking age-related lipid-droplet repletion. Lipid droplets are the intracellular stores of cholesterol esters, the obligate precursors of steroid hormones in rats. This finding is in keeping with the contention that the mechanism underlying the age-dependent decline in rat-adrenal glucocorticoid secretion mainly involves impairments of the utilization of intracellular cholesterol previous to its intramitochondrial transformation to pregnenolone.

Age-related changes in the morphology and function of the zona glomerulosa of the rat adrenal cortex

The age-related changes in the morphology and function of rat adrenal zona glomerulosa (ZG) were investigated by coupled stereological and radioimmunological techniques. For this purpose 4-, 8-, 16- and 24-month-old rats were studied. Aging caused a notable lowering in the plasma aldosterone concentration and a marked decrease in both basal and ACTH- or angiotensin II (ANG-II)-stimulated secretion of collagenase-dispersed ZG cells. Plasma renin activity (PRA) underwent an age-dependent decrease, while the plasma level of ACTH displayed a significant rise. ZG and its parenchymal cells did not evidence any age-related morphologically demonstrable alteration in their growth, nor ZG cells showed any marked ultrastructural change, with the exception of a severe lipid-droplet repletion. This last finding is in keeping with the aging-induced decrease in the secretory activity of ZG cells, inasmuch as lipid droplets are the intra-cellular stores of cholesterol esters, the obligate precursors of steroid hormones in rat adrenals. ACTH and ANG-II are well known to be involved in the maintenance of the growth of rat ZG; thus, the combined impairment of ANG-II production (as evidenced by PRA lowering) and increase in ACTH secretion may maintain unchanged ZG growth during aging.

Rat adrenal zona fasciculata cell electrophysiological responses to ACTH and gamma-MSH vary with age

A comparison was made of the resting membrane potential (MP) as well as the MP responses to K+, ACTH and gamma-MSH of superfused adrenocortical, zona fasciculata cells from young (Y) and old (O) male rats. The resting MP of these cells did not vary with age. However, the MP responses to altered extracellular [K+] varied significantly with age. A prolonged biphasic depolarization was observed following ACTH administration; these responses were significantly reduced in O cells. In contrast, stimulation with gamma-MSH did not consistently elicit depolarization in the Y but caused a consistent and prominent depolarization in O cells. These findings suggest that aging is associated with changes in primary membrane events which could explain the reduced ACTH-stimulated steroidogenesis associated with age. Elevated cellular responses to gamma-MSH may contribute to a maintenance of, or increase in, circulating corticosterone levels in aged rats.