Effects Ofaging Stress And Hormones

The histopathology of immunocompetent organs can be modified by many exogenous and endogenous factors. Clearly, the age of the animal is reflected in lymphoid histology. At sexual maturity, the thymus begins to involute with major effects on the cortex and minor effects on the medulla (Schuurman etal., 1991). Because of the decrements in cortex function, the host has a lower production of virgin T cells necessary for primary immunological responses. In addition, the parenchyma is replaced by adipose and connective tissue. Lymph nodes may be smaller in older adult animals but are capable of antigenic stimulation and expansion of germinal centers (Schuurman et al., 1994).

Psychological stress induces the secretion of corticosteroids via the hypothalamus-pituitary-adrenal axis. In some species, the major target organ is the thymus, Within a few days, the lymphocytes within the cortex disappear and the lobules decrease in size. In contrast to the normal thymus, the medulla has a higher density of lymphocytes (Van Baarlen et al., 1988). Similar histology is observed in acute debilitating diseases and severe systemic toxicity.

An altered balance between estrogens and androgens can also affect the histopathology of the thymus. Depending on the stage of pregnancy, thymic weights increase or decrease with involution of the cortex (Clarke and Kendall, 1989). Normal thymic architecture is restored following birth. Treatment with luteinizing hormone-releasing hormone (LHRH) also produces a similar phenomenon in rats and mice (Greenstein et al., 1987).

Nutritional status can result in thymic atrophy. Poor diet and malnutrition often result in thymic atrophy and depressed immune responsiveness (Mittal et al., 1988). In part, this response may reflect increased levels of glucocor-ticosteroids in the sera of malnourished or undernourished animals (Adlard and Smart, 1972). Reduced immune responses can be induced by decreased amino acid uptake, vitamin B„ or zinc deficiencies (Good and Lorenz, 1992).

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