Figure 3.

Changes in thymic size, organization, and/or lipid content accompany age-associated thymic involution in humans and mice. (A) In humans, the percentage of the thymus comprised of functional thymic tissue progressively declines with age, and is replaced by adipose tissue, as shown in these hematoxylin and eosin-stained images. The percent of thymus area containing thymic epithelium, representing functional thymic tissue, was calculated via morphometric analysis of cytokeratin immunohistochemical slides. The results for the subjects shown are 91% at 5 days (5d), 55% at 28 years (28y), and 0.5% at 78 years (78y). (B) The mouse thymus grows substantially between postnatal day 1 (1d) (scale bar = 400 µm) and 4 weeks of age (4w) (scale bar = 2 mm), and then declines steadily and is highly involuted by 12 months of age (12m). The small islands of medullary tissue seen at 1d expand and coalesce to form the larger, more organized medullary regions characteristic of adult thymus (4w). Age-associated replacement by adipose tissue is not a prominent characteristic of involution in mice. The corresponding weights (mean ± SD) of murine thymus at the ages shown are 5 ± 0.5 mg at 1d (n = 3), 57 ± 8 mg at 4w (n = 8), and 38 ± 2 mg at 12 m (n = 3).