Evolution of the thymus size in response to physiological and random events throughout life.
During embryogenesis and in the early stages of life, the thymus is a crucial organ for the generation of the T cell repertoire. T cells are generated from hematopoietic stem cells already differentiated to precursor T cells in the bone marrow. These cells enter the thymus guided by chemotactic factors secreted by this organ. The complex maturation process takes place that ensures self-tolerance and homeostasis. Thymocytes that show autoreactivity do not leave the thymus, but rather die by apoptosis. The final percentage of mature T cells that survive to migrate from the thymus to the periphery is very low: at most 5%, under optimal conditions. The highest migration occurs in childhood and adulthood, at least in mice and humans; however, it declines throughout life and is minimal in the elderly. Under normal circumstances, the thymus commences involution soon after birth, and this involution correlates with the capacity to export mature T cells to the periphery. Hormones, cytokines, and neurotransmitters all play a role in this age-associated process, but the reasons for and mechanisms of this involution remain unknown. Apart from physiological conditions that change throughout life and govern age-related thymus evolution, random states and events provoked by intrinsic or extrinsic factors can induce either thymus involution, as in reversible transient thymic hypoplasias, or thymic hyperplasias. The age-associated involution, unlike transient involutions, follows a regular pattern for all individuals, though there are clear differences between the sexes. Nevertheless, even the age-associated involution seems to be reversible, raising the possibility of therapeutic strategies aimed at enhancing thymus function in the elderly.
Domínguez-Gerpe
Lourdes
L
Departamento de Bioquímica y Biología Molecular, Facultad de Biología, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, La Coruña, Spain. bnldomin@usc.es
Rey-Méndez
Manuel
M
D000222:Adaptation, Physiological; D000375:Aging / Q000502:physiology*; D000818:Animals; D005075:Biological Evolution; D002454:Cell Differentiation; D005260:Female; D018869:Heat-Shock Response; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009929:Organ Size; D013950:Thymus Gland / Q000166:cytology / Q000276:immunology / Q000502:physiology*