[Rhythmicity of structural components of the thyroid gland at different stages of postnatal ontogenesis (morphometric...

Age evolution of biological rhythms in the activity of the thyroid gland has been studied at organic, tissue and cellular levels. The thyroid glands have been studied in 144 male mice, A/He strain, of three age groups: 14--17-day-old (young premature), 55--60-day-old (mature) and 160--170-day-old (ageing animals). The mean height of follicular cells, stereological index of resorption, relative volumes of follicular and interfollicular epithelium, colloid and stroma have been considered, as well as relations of volumetric fractions in certain tissue components. The data obtained have been processed with a computer. Multicomponent character of the rhythms for all indices studied has been revealed, circadian, ultradiurnal and infradiurnal components including. Spectral analysis of the rhythms has revealed steady tendency towards decreasing relative power of ultradiurnal fluctuations with age while circadian rhythmics become more pronounced. Amplitude of the circadian component of all the rhythms studied increases with growth and development of the animal and decreases for a number of indices in ageing mice. Maximal activity of the organ, as stated the data of morphometric analysis, in all three groups corresponds to the night time.