Some Differences in Thymus Morphology in Immature Insectivorous Mammals: Sorex araneus, Sorex caecutiens, Neomys fodiens, and Erinaceus roumanicus.

The thymus structure of four species of immature insectivorous mammals belonging to the families Soricidae and Erinaceidae (Eulipotyphla) has been studied. Representatives of the studied families are characterized by contrary surviving strategies, significantly differing in the intensity of metabolism and the activity of the animal. As these differences are presumably reflected in the morphological parameters of the thymus, this work aimed at a comparative study of these parameters of the thymus in representatives of the above two families. A light microscopy technique has been applied. Sections of thymus lobes 5 µm thick have been stained with hematoxylin and eosin, as well as picrofuchsin, according to van Gieson's method, and with azure-eosin, according to Romanovskii–Giemsa. During the processing of the material, the mass index and the cortical-medullar index of the thymus have been determined. The area of the connective and lymphoid tissues on the thymus section has been identified. The number of thymocytes, as well as the number and area of the vessels of the microvasculature of both the thymus cortex and the medulla have been counted within the conventional unit area. The percentage of thymocytes in mitosis has been calculated. The results of the study show that the shrews have a higher thymus mass index than the northern white-breasted hedgehog. This leads to significant changes in the syntopy of the thymic lobules. In comparison to the northern white-breasted hedgehog, the thymus of shrews is characterized by an increased cortical-medullar index, a higher density of the arrangement of thymocytes per unit area, and a higher number and relative area of the vessels of the microvasculature. At the same time, all studied immature representatives of insectivorous mammalian species have equally high relative areas of lymphoid tissue. This indicates an active functional state of the thymus at this life stage in all Eulipotyphla representatives. The relative area of the thymus connective tissue is directly related to the absolute dimensions of the organ, which is necessary for the implementation of the frame function. The values of the mitotic index of the thymus medulla in Eulipotyphla representatives are higher than expected and may indicate the need for increasing the pool of thymocytes at a very late stage of differentiation. The patterns revealed indicate that the morphology of the thymus depends on the biological characteristics of representatives of different families of Eulipotyphla, has a certain adaptive value, and deserves further study. [ABSTRACT FROM AUTHOR]