Previous studies have found that during aging a large number of biological structures such as tissues and organs loses its complexity and that such loss leads to reduced ability to adapt to physiological stress. However, so far there is no information on whether similar structural changes occur in individual cells and their genetic material. Structural complexity can be quantified in several ways. Recent studies have determined that the chromatin, as well as many other biological structures exhibit fractal characteristics in their morphology. The concept of fractals is based on the principle of self-similarity, or the fact that the lower parts of a physical or biological system resemble the system as a whole. The complexity of fractal structures can be measured by determining the fractal dimension and lacunarity as the two most important parameters of fractal analysis. In addition to the fractal analysis, textural analysis as a method is also frequently used. Textural analysis can determine the parameters of textural heterogeneity and disorganization (i.e. entropy) of biological structures. In our study, on the mouse experimental model, we studied age-related changes in chromatin structural complexity in the total of 10 cell populations in the thymus, spleen, kidney and liver. Also, we described changes in the complexity of the nuclear structure in a cell culture after the induction of DNA damage by UV radiation. The study was conducted on 80 male mice, originating from the experimental animal vivarium of the Institute of Medical Physiology, Faculty of Medicine, Belgrade. The pilot study was conducted on a group of 16 mice in which the first animal was a newborn, and each of the following animals was 30 days older than the previous one (the oldest animals were 450 days old). The main part of the study in laboratory animals, was conducted on a total of 64 animals which were divided into 8 age groups (n = 8): 0, 10, 20, 30, 120, 210, 300 and 390 days old. The experimental protocol was designed according to the following procedure: measurement of body mass, sacrifice by cervical dislocation, extirpation of the thymus, spleen, kidney and liver, fixation in Carnoy fixative, embedding in Paraplast, making of slices of thickness 5 microns. Slices of all four organs were then stained by hematoxylin / eosin and toluidine blue technique. Spleen and thymus was further stained by Gimza technique and kidney tissue with Azo carmine aniline blue (AZAN) method. After staining, we made digital micrographs of the tissues using the Olympus C-5060 Wide Zoom camera mounted on Olympus BX41 light microscope (1000x magnification, 100x immersion lens). Digital micrographs of tissue were made and processed similarly to our previous study (Pantic & Pantic, 2012). For all examined tissues, with the help of special software ImageJ of the National Institutes of Health (USA) and integrated plugins, we determined the mean value of fractal dimension, lacunarity, entropy, variance, angular second moment, textural contrast, textural correlation and inverse difference moment. We measured the structural complexity of chromatin in the following cell populations stained with hematoxylin / eosin technique: in thymus cortical and medullar lymphocytes and stromal cells; spleen follicular lymphocytes, and spleen erythroid precursor cells of the subcapsulary haematopoietic tissue; hepatocytes in the liver; kidney cells of proximal and distal tubules , collecting duct cells as well as macula densa cells. Analysis of individual nuclei was performed on the model of a similar protocol described in our previous studies (Pantic et al. 2012a, Pantic et al. 2012b). In the in vitro part of the study, cell line U251 was treated with UVB rays of wavelength 312 nm with a total duration of 15 minutes. A sample of 32 individual cells (paired sample) was monitored in 12 time points: before UV treatment, immediately after UV treatment, and 30min, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h and 5h after the treatment. The first time point (immediately before UV treatment) was the control. With the help of ImageJ software for each cell and its nucleus at each time point the following parameters were determined: fractal dimension, lakunarnosti, entropy, angular second moment, textural contrasts, textural correlation, variance and inverse difference moment, in accordance with the above text. The results of our study indicate that the fractal complexity of nuclear chromatin decreases in postnatal development and aging in the cell populations of cortical and medullar thymic lymphocytes, epithelial cells of the proximal renal tubules, macula densa cells and in hepatocytes in the liver. Disorder of nuclear chromatin in spleen erythroid precursor cells, measured by textural entropy increased with age. In postnatal development and aging significant reduction of tissue architecture complexity (measured by fractal dimension) was detected in the cortex and medulla of thymus, the cortex and medulla of the kidney, spleen hematopoietic tissue and liver lobuluses. The fractal dimension of tissue architecture was significantly correlated with the fractal dimension of chromatin in most examined tissues, and cell populations. After the induction of DNA damage in vitro, in cell culture, there was a significant decrease in cellular fractal dimension and chromatin lacunarity. These findings indicate that the intrinsic factors associated with the aging of cells, such as the accumulation of DNA damage and changes in the epigenetic regulation of chromatin play an important role in the aging of the investigated cell populations and tissues. Prethodne studije su utvrdile da tokom starenja veliki broj bioloških struktura kao što su tkiva i organi gubi svoju kompleksnost i da takav gubitak vodi smanjenoj sposobnosti adaptacije na fiziološki stres. Međutim, za sada nema podataka da li se slične strukturne promene dešavaju na individualnim ćelijama i njihovom genetskom materijalu. Strukturna kompleksnost se može kvantifikovati na nekoliko načina. Skorašnje studije su utvrdile da hromatin, kao i mnoge druge biološke strukture u svojoj morfologiji ispoljavaju karakteristike fraktala. Koncept fraktala se u osnovi bazira na principu samosličnosti, odnosno na činjenici da manji delovi nekog fizičkog ili biološkog sistema nalikuju sistemu kao celini. Kompleksnost fraktalnih struktura se može meriti određivanjem fraktalne dimenzije i lakunarnosti kao dva najznačajnija parametra fraktalne analize. Kao dodatak fraktalnoj analizi, danas se često koristi i teksturalna analiza uz pomoć koje se mogu odrediti parametri teksturalne heterogenosti i neuređenosti biološke strukture kao što je entropija. U našoj studiji, na mišijem eksperimentalnom modelu, ispitivane su starosne promene u strukturnoj kompleksnosti nukleusnog hromatina na ukupno 10 ćelijskih populacija u timusu, slezini, bubregu i jetri. Takođe su opisane promene u kompleksnosti nukleusne strukture na kulturi ćelija nakon indukcije DNK oštećenja UV zračenjem. Istraživanje je obavljeno na 80 miševa muškog pola, poreklom iz Vivarijuma za eksperimentalne životinje Instituta za Medicinsku fiziologiju Medicinskog fakulteta u Beogradu. Pilot istraživanje je obavljeno na grupi od 16 miševa u kojoj je prva životinja bila novorođena, a svaka sledeća životinja je bila 30 dana starija od prethodne (najstarija životinja je bila stara 450 dana). Glavni deo istraživanja na eksperimentalnim životinjama je sproveden na ukupno 64 životinje podeljene u 8 starosnih grupa (n=8): 0, 10, 20, 30, 120, 210, 300 i 390 dana. Eksperimentalni protokol je dizajniran po sledećoj proceduri: merenje telesne mase, žrtvovanje životinja cervikalnom dislokacijom, ekstirpacija timusa, slezine, bubrega i jetre, fiksiranje u Carnoy fiksativu, kalupljenje u Paraplastu, pravljenje isečaka debljine 5 μm. Isečci tkiva sva četiri organa su potom obojeni tehnikama Hematoksilin/Eozin i Toluidin plavo. Tkivo slezine i timusa je dodatno obojeno Gimza metodom, a tkivo bubrega Azo carmine aniline blue (AZAN) metodom. Nakon bojenja, pristupilo se pravljenju digitalnih mikrografa tkiva uz pomoć Olympus C-5060 Wide Zoom kamere postavljene na Olympus BX41 svetlosni mikroskop (uveličanje 1000x, imerzioni objektiv 100x). Digitalni mikrografi tkiva su napravljeni i obrađeni po uzoru na našu prethodnu studiju (Pantic & Pantic, 2012). Za svako ispitivano tkivo, uz pomoć specijalnog softvera ImageJ Nacionalnog instituta za zdravlje (SAD) i specijalnih integrisanih podprograma određena je srednja vrednost fraktalne dimenzije, lakunarnosti, entropije, varijanse, angularnog drugog momenta, teksturalnog kontrasta, teksturalne korelacije i inverznog momenta razlike. Za merenje strukturne kompleksnosti hromatina korišćene su sledeće ćelijske populacije obojene hematoksilin/eozin tehnikom: u timusu kortikalni i medularni timociti i stromalne ćelije; u slezini limfociti folikula, i eritroidne prekurzorske ćelije subkapsularnog hematopoeznog tkiva; u jetri hepatociti; u bubregu ćelije proksimalnih i distalnih tubula, ćelije sabirnih kanalića kao i ćelije macule dense. Analiza pojedinačnih jedara je obavljena po uzoru na sličan protokol opisan u našim prethodnim studijama (Pantic et al. 2012a, Pantic et al. 2012b). U in vitro delu studije ćelije linije U251 su tretirane UVB zracima talasne dužine 312 nm u ukupnom trajanju od 15 minuta. Uzorak od 32 individualne ćelije (vezani uzorak) je praćen kroz 12 vremenskih tačaka: pre UV tretmana, odmah nakon UV tretmana, kao i 30min, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h i 5h nakon tretmana. Prva vremenska tačka (neposredno pre UV tretmana) predstavljala je kontrolu. Uz pomoć ImageJ softvera za svaku ćeliju i njeno jedro u svakoj vremenskoj tački određena je vrednost fraktalne dimenzije, lakunarnosti, entropije, angularnog drugog momenta, teksturalnog kontrasta, teksturalne korelacije, varijanse i inverznog momenta razlike, u skladu sa gore navedenim tekstom. Rezultati naše studije su