THE ANNUAL THYMUS INVOLUTION IN HIBERNATING ANIMALS: FUTURE TRENDS IN STUDIES OF GERONTOLOGY
AND STEM CELL PROLIFERATION
S. G. Kolaeva,1 * E. G. Novoselova,1 Z. G. Amerkhanov,1
A. V. Kulikov,2 V. G. Ivkov 3
1 Institute of Cell Biophysics RAS, 2 Institute of Theoretical and
Experimental Biophysics RAS, Pushchino, and 3 Pushchino Research Center RAS;
* e-mail: kolaeva@icm.psn.ru
Data on a unique phenomenon of annual involution and neogenesis of thymus gland in hibernating animals are
reviewed. In accordance with morphological findings, the annual thymus involution in hibernating animals is close to the
age-dependent thymus involution occurring in all mammals once in a lifetime. In opposite, thymus involution in hibernating
animals is totally different from the accidental involution. During hibernation, the thymus tissue is substituted by the brown
fat tissue. In the spring, thymus gland neogenesis stats with intensive growth of epithelial tissue followed by lymphocyte
infiltration and exhaustion of brown tissue. Morphological changes in the thymus gland within the annual cycle were compared
with seasonal dynamics of structural and functional changes in peripheral lymphoid organs (spleen, lymphoglandula, peritoneal
fluid). A general regularity was observed involving a decreased functional activity of immune cells in autumn, its sharp
depression during winter hibernation, and obvious increase in summer with the onset of a season of animal activity. It is
supposed that a sharp increase in the tumor necrosis factor (TNF) production observed during short-term awakenings in winter
may serve an important link in this unique immune adaptation mechanism. The season changes in cellular TNF secretion suggest a
mobilization of protective resources in hibernating animals in autumn and winter, i. e. in seasons when the thymus gland activity
is depressed. The annual involution of thymus gland cannot be related to dropings in the environmental or body temperatures,
as it comes long before their fall. Additionally, it is not related to ageing, as it occurs already in young hibernating animals.
The role of hormones, including melatonine and corticosteroids, in mechanisms regulating thymus gland involution in hibernating
animals is disscused.
Key words: thymus, involution, hibernation
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