Thymus and aging: morphological, radiological, and functional overview

Abstract

Aging is a continuous process that induces many alterations in the cytoarchitecture of different organs and systems both in humans and animals. Moreover, it is associated with increased susceptibility to infectious, autoimmune, and neoplastic processes. The thymus is a primary lymphoid organ responsible for the production of immunocompetent T cells and, with aging, it atrophies and declines in functions. Universality of thymic involution in all species possessing thymus, including human, indicates it as a long-standing evolutionary event. Although it is accepted that many factors contribute to age-associated thymic involution, little is known about the mechanisms involved in the process. The exact time point of the initiation is not well defined. To address the issue, we report the exact age of thymus throughout the review so that readers can have a nicely pictured synoptic view of the process. Focusing our attention on the different stages of the development of the thymus gland (natal, postnatal, adult, and old), we describe chronologically the morphological changes of the gland. We report that the thymic morphology and cell types are evolutionarily preserved in several vertebrate species. This finding is important in understanding the similar problems caused by senescence and other diseases. Another point that we considered very important is to indicate the assessment of the thymus through radiological images to highlight its variability in shape, size, and anatomical conformation.

Characterization of an age-response relationship to GSK's recombinant hepatitis B vaccine in healthy adults: An integrated analysis

“…By 50 y of age around 80% of functional thymus tissue has been lost. 10 Our model predicts that the hepatitis B seroprotection rate remains 90% up to 49 y of age, and above 80% until 60 y of age in adults who are otherwise healthy. While we did not investigate interactions between gender and immune senescence, our results are comparable to results of a modeling study of anti-HBs concentrations in more than 11,000 healthy adults in the Netherlands.…”

“…[5][6][7][8][9] Immune senescence is characterized by altered composition of bone marrow with reduced capacity to produce and nurture stem cells, and atrophy of the thymus gland with reduced output of Tcells. 10 The response to vaccination is impaired in the elderly due to functional defects at multiple levels in innate and adaptive immune responses. These include reduced capacity of antigen presenting cells to take up and present antigen, lower numbers of T-cells capable of responding to new antigen, reduced magnitude and quantity of the B-cell antibody response, and poor effector T-cell activation and signaling due to reduced numbers and loss of receptor diversity.…”

“…10,29 Thymic involution is one of the most important events that contributes to immune dysfunction in the elderly. 10,29 Evidence of early and continual decreases in vaccine responses with age is broadly consistent with the process of thymic involution, which begins in early adulthood. By 50 y of age around 80% of functional thymus tissue has been lost.…”

Development of immune organs and functioning in humans and test animals: Implications for immune intervention studies

“…Postnatal. At birth the thymus is fully formed morphologically, its size relative to body size is at its peak and it has maximal thymocyte output at early postnatal life [41,42]. This may be followed by a temporary decrease until late infancy, with continuous growth thereafter until involution (including replacement of thymus parenchyma by adipose tissue) in adolescence (reviewed by [42]).…”

Role of Dendritic Cells in Autoimmunity in Aged Humans

“…Aged humans develop a variety of autoantibodies, and display poor response to vaccines and increased susceptibility to both viral and bacterial infections. The progressive impairment in immune functions include progressive T cell deficiency, which is contributed by thymic involution [1], increased apoptosis of T cells and T cell subsets [2][3][4], and impaired priming of T cells by DCs [5][6][7]; B cell dysfunctions are revealed by impaired specific antibody response to vaccines, and development of autoimmunity [8][9][10][11][12].…”