Comparative features of thymus morphology in human and vertebrate animals (Chordata, Vertebrata)

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Abstract

BACKGROUND: It is well known that the thymus structure in the vertebrate phylogeny is characterized by a combination of conservative and highly plastic features. However, the question remains about the causes and patterns of evolutionary similarities and differences in human and animal thymus structure depending on level of organization, habitat, and adaptability.

AIM: The aim of the study was to identify the main patterns of change in the thymus microscopic structure in phylogeny by comparing the thymus structure in humans and various Chordata species.

MATERIALS AND METHODS: Light microscopy was used to determine the cortical/medullary and mitotic indices as well as the area of fibrous tissue, lymphoid tissue, and adipose tissue in thymus sections from 19 vertebrate species and humans. Thymocytes, thymic corpuscles, and the number and area of microcirculatory vessels were counted per conventional area unit. The study was conducted on immature animals of each species as well as on animals that had reached the second stage of maturity.

RESULTS: Comparative analysis shows that immature animals have predominantly similar thymus structures. Significant differences were observed in the parameters of age-related involution, which is characterized by significant magnitude and total fat degeneration in humans compared to animals. The morphological features of the thymus associated with thymocyte migration and maturation have the highest conservatism and include cortical/medullary and mitotic indices, the numerical density of thymocytes in the cortex, the total area of microcirculatory vessels, the relative area of lymphoid tissue. Human thymus, regardless of age, has a higher relative amount of fibrous tissue than vertebrates. In addition, some specific morphological features of the thymus corpuscles also vary.

CONCLUSIONS: The structural features of the human thymus were determined that changed in adaptation to specific conditions of the anthropogenic environment. The revealed morphological differences in human thymus are consistent with immunological hypothesis explaining the causes of age-related thymic involution. They correspond to the main points of Academician A.A. Zavarzin’s theory of parallel development of homologous tissues in vertebrate phylogeny.

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About the authors

Vladislav Ya. Yurchinsky

Smolensk State Medical University

Author for correspondence.
Email: zool72@mail.ru
ORCID iD: 0000-0003-3019-3053
SPIN-code: 8067-8250

Cand. Sci. (Biology), Associate Professor

Russian Federation, Smolensk

Lyudmila M. Erofeeva

Petrovsky National Research Centre of Surgery

Email: gystology@mail.ru
ORCID iD: 0000-0003-2949-1432
SPIN-code: 7217-5030

Dr. Sci. (Biology), Professor

Russian Federation, Moscow

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Supplementary files

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2. Fig. 1. Age-related changes in the amount of fibrous connective tissue in vertebrate thymus: Y-axis — amount of connective tissue in %; RD — right thymic lobe, LD — left thymic lobe

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3. Fig. 2. Thymus of immature representatives of human and vertebrates. Digital images show differences in the thickness of connective tissue septa: a — human; b — grass frog; c — agile lizard; d — gray flycatcher; e — bank vole; f — common shrew. Hematoxylin and eosin staining; scale bar — 100 µm

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4. Fig. 3. The number of microcirculatory vessels of the microvasculature cortical and medulla parts of the vertebrate thymus: Y-axis — number of vessels per unit area of 0,5 mm2: RD — right thymic lobe, LD — left thymic lobe

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5. Fig. 4. Amount of lymphoid tissue in human and vertebrate thymus: Y-axis — relative amount of lymphoid tissue on the thymus section in %

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6. Fig. 5. Adipose tissue content in the thymus of a sexually mature human: a — hematoxylin and eosin staining; b — staining with aldehyde-fuchsin and Halmi mixture according to Gabu–Dyban. Scale bar — 100 µm

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7. Fig. 6. Thymus of sexually mature vertebrates living in the natural environment: a — pond frog; b — ordinary one; c — gray dove; d — house mouse. Hematoxylin and eosin staining; scale bar — 100 µm

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8. Fig. 7. Morphology of human and vertebrate thymus corpuscles: TT — thymus corpuscles; TT I — young thymus corpuscles, phase I; TT II — mature thymus corpuscles, phase II; TT III — aging thymus corpuscles, phase III. All sections were stained with hematoxylin and eosin; human TT III sections were additionally stained with aldehyde-fuchsin according to Gabu–Dyban; scale bar — 20 μm

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9. Fig. 8. Absolute sizes of human and vertebrate thymus corpuscles: a — house mouse; b — agile lizard; c — grass frog; d — blue pigeon; e — human. All sections were stained with hematoxylin and eosin; human sections were additionally stained with aldehyde-fuchsin according to Gabu-Dyban; scale bar — 100 µm

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