STRUCTURAL AND MORPHOLOGICAL CHANGES OF THE THYMUS AND SPLEEN IN AN EXPERIMENTAL MODEL OF ACUTE DESTRUCTIVE PANCREATITIS

SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 213 STRUCTURAL AND MORPHOLOGICAL CHANGES OF THE THYMUS AND SPLEEN IN AN EXPERIMENTAL MODEL OF ACUTE DESTRUCTIVE PANCREATITIS M. A. Islamova Tashkent State Medical University (TSMU) https://doi.org/10.5281/zenodo.17860828 Abstract. In an experimental model of acute destructive pancreatitis (ADP) reproduced by a cryogenic method in rats, a morphological study of the thymus and spleen was performed. Pronounced structural and morphological changes in the lymphoid organs were established, including a reduction of splenic lymphoid follicles and dystrophic alterations in the thymus. A wave-like activation of the T-cell component of immunity was recorded, accompanied by the expansion and hyperplasia of the periarteriolar lymphoid sheaths. By the 14th day, signs of immune response exhaustion and possible secondary immunodeficiency were observed. The obtained data emphasize the importance of early immunomodulatory therapy in acute destructive pancreatitis. Keywords: pancreatitis, thymus, spleen, lymphocytes, immunity, morphology, cryomodel, inflammation. Introduction. The study of the morphofunctional state of lymphoid organs in pancreatic pathologies remains an insufficiently developed and debatable area of modern medicine. The key pathogenetic mechanism in acute pancreatitis is the formation of the systemic inflammatory response syndrome (SIRS), which represents a complex of clinical, biochemical, and morphological changes that affect not only the parenchyma of the pancreas but also adjacent organs and tissues. According to M.R. Sapin and D.B. Nikityuk [3,7], the presence of lymphoid follicles in the organs of the immune system is a reliable morphological indicator of their functional activity. The structural and functional analysis of various components of the spleen and thymus, as well as the assessment of their cellular composition, is carried out in an ontogenetic aspect, taking into account age-related, individual, and species-specific variability [1,2,8]. Nevertheless, data on the morphological changes of lymphoid structures during acute inflammatory processes of the pancreas remain limited. Currently, acute pancreatitis (AP) occupies a leading position among pancreatic diseases. The relevance of this study is determined by the high incidence, the duration of therapeutic intervention, and the significant level of postoperative mortality in destructive forms of pancreatitis, which reflects not only medical but also socio-economic aspects of this pathology. The pathogenesis of acute pancreatitis is characterized by the systemic involvement of organs and tissues, including lymphoid structures of the thymus and spleen, leading to immune system dysfunction [4–6]. The aim of the study was to evaluate the dynamics of morphological changes in the thymus and spleen of rats in an experimental model of acute destructive pancreatitis induced by cryogenic exposure. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 214 Materials and methods. The experimental model of acute destructive pancreatitis (ADP) was reproduced in 40 outbred white rats of both sexes weighing 180–220 g by cryogenic treatment of the splenic segment of the pancreas using a chloroethyl agent, in accordance with the “Regulations on the Use of Experimental Animals” (Group 1). The control group consisted of 5 animals subjected to laparotomy without additional intervention (Group 2). Animals were kept on a standard laboratory diet with unrestricted access to water before and after the intervention. In Group 1, under aseptic conditions and ether anesthesia, an upper midline laparotomy was performed followed by mobilization of the pancreas. The splenic segment of the pancreas and the spleen were exteriorized into the wound and isolated from the abdominal cavity with a polyethylene film. Cryoexposure lasted approximately 1 minute until frost appeared on the surface of the cooled area, after which spontaneous thawing of the tissue occurred within 2–3 minutes. Euthanasia of the animals in Group 1 was carried out after 1 hour, and on the 1st, 3rd, 7th, and 14th days after surgery, followed by morphological examination of the thymus and spleen. The animals of the control group were euthanized after 1 hour and on the 1st day for morphological control. From the autopsy material obtained at various time points, histological sections were prepared and stained with hematoxylin and eosin. Morphological analysis of lymphoid organs was performed using light microscopy. Results of the study. Microscopic examination of the pancreas, thymus, and spleen at various time intervals after modeling acute destructive pancreatitis (ADP) revealed the following morphological changes. One hour after cryogenic exposure, a clear distinction between the cortical and medullary layers of the thymus was preserved, with a large number of small thymic windows observed. Hassall’s corpuscles exhibited signs of dystrophic changes, while their quantitative density remained unchanged. Pronounced vascular hyperemia was noted (Fig. 1B). In the spleen, lymphoid follicles of various shapes and sizes were identified, while germinal centers were absent in most of them. The red pulp was characterized by moderate congestion. In the white pulp of the rat spleen, a statistically significant increase in the mean area of periarteriolar lymphoid sheaths (PALS), representing T-cell zones, was recorded — from 0.009 ± 0.006 mm² (control) to 0.014 ± 0.005 mm². In this zone, an increase in the number of lymphocytes was also observed — from 23.6 ± 6.6 to 115.6 ± 29.5 cells. A B A. Spleen of a rat 1 hour after induction of acute destructive pancreatitis. Lymphoid follicle. Hematoxylin and eosin staining, magnification ×400. B. Thymus of a rat 1 hour after induction of acute destructive pancreatitis. Multiple thymic windows. Hematoxylin and eosin staining, magnification ×200. 24 after the induction of acute destructive pancreatitis, focal-diffuse venous-capillary congestion with moderately expressed stromal edema was revealed in the pancreas. The vascular SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 215 walls showed no morphological alterations. The islets of Langerhans were represented by structures of medium size (Fig. 2). The histological pattern of the thymus on the first day was characterized by indistinct differentiation between the cortical and medullary layers. Thymic windows were represented by small elements, Hassall’s corpuscles exhibited signs of dystrophic changes, and the vessels were moderately hyperemic. In the spleen, a reduction in the size of the lymphoid follicles of the white pulp was observed; these follicles were round in shape and contained reactive centers. The red pulp and vascular bed were characterized by moderate congestion (Fig. 3). In the periarteriolar lymphoid sheaths (PALS) during the first 24 hours of pancreatitis pathogenesis, a statistically significant increase in the cellular population was noted — on average 51.2 ± 3.0 cells (Spearman’s rank correlation coefficient, p < 0.05). The mean area of this structural component of the white pulp increased to 0.054 ± 0.01 mm². Fig. 2. Microphotograph of the pancreas of a rat in an experimental model of acute destructive pancreatitis on the 1st day. In the center of the image, the lumen of the duct and a compressed vein are visible; the interlobular connective tissue is characterized by pronounced edema. Hematoxylin and eosin staining, magnification ×400. Fig. 3. Microphotograph of the spleen of a rat on the 1st day of acute destructive pancreatitis. The lymphoid follicles of the white pulp are reduced in size, and moderate venous-capillary congestion is observed. Hematoxylin and eosin staining, magnification ×400 On the third day, the inflammatory process in the pancreas continued to show a progressive course. Stromal edema was of moderate intensity, and the vascular bed was characterized by pronounced congestion. The presence of the sludge phenomenon and a large focus of necrosis was noted. In the center of the specimen, the lumen of the duct and a collapsed vein were visualized, while the lobules were surrounded by edematous connective tissue. The islets of Langerhans were represented by small and medium-sized structures with preserved morphological organization (Fig. 4). In the spleen on the third day, the mean total area of the periarteriolar lymphoid sheaths (PALS) significantly increased compared to intact animals and reached 0.063 ± 0.01 mm². The number of lymphocytes in the PALS was 101.0 ± 24.6, which significantly exceeded the values of the control group (p < 0.05). The lymphoid follicles of the spleen were of medium size, round or irregular in shape, with the germinal centers absent; the red pulp was characterized by marked congestion (Fig. 5). The morphological picture of the thymus on the third day was similar to that observed on the first day. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 216 Fig. 4. Pancreas of a rat with acute destructive pancreatitis on the 3rd day. Hematoxylin and eosin staining, magnification ×100. Fig. 5. Spleen of a rat on the 3rd day of acute destructive pancreatitis. Medium-sized lymphoid follicles with the formation of a secondary lymphoid nodule; the red pulp shows marked congestion. Hematoxylin and eosin staining, magnification ×400. On the 7th day, the pancreas showed pronounced diffuse capillary congestion accompanied by the formation of granulation tissue. In the stroma, disorganized elastic fibers were identified, reflecting marked destructive-regenerative changes. Moderately expressed focal lipomatosis was diagnosed. The islets of Langerhans were predominantly of medium size, exhibiting signs of hyperplasia and interstitial edema. In the interlobular connective tissue, active processes of neoangiogenesis and fibroblastic proliferation were visualized, indicating the development of granulation tissue (Figs. 6A, 6B). On the 7th day, the thymus showed a blurred boundary between the cortical and medullary substances. Thymic corpuscles were increased in number and characterized by pronounced dystrophic changes. Thymic windows had irregular shapes and varied from small to medium size. The vascular bed of the organ retained a moderate degree of congestion. Fig. 6. Histological structure of the pancreas of a rat on the 7th day after modeling acute destructive pancreatitis. Pronounced capillary congestion, the presence of granulation tissue, and areas of lipomatosis are observed. Hematoxylin and eosin staining, ×400. In the spleen, the lymphoid follicles of the white pulp were markedly reduced in area, had lost their distinct shape, and lacked light (germinal) centers. The red pulp was characterized by hypoperfusion, and the arterioles showed thickening and edema of the vascular wall (Fig. 7). At this stage of the inflammatory process in the pancreas, the immune response from the spleen was reflected in a significant increase in the quantitative composition of lymphocytes within the periarteriolar lymphoid sheaths (PALS), with a mean value of 109.8 ± 17.4 cells. The area of the T-dependent zone of the white pulp also increased, reaching 0.066 ± 0.009 mm², which was statistically significantly higher than in intact animals. On the 14th day, in the interlobular connective tissue of the pancreas, well-formed areas of granulation tissue and localized zones of lipomatosis were visualized. The vessel walls were thickened, and the lumina of the ducts were narrowed as a result of hyalinosis. The islets of SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 217 Langerhans were few in number, with their structural organization partially preserved. Pronounced diffuse stromal edema was observed. In the stroma, moderate reticular (mesh-like) sclerosis accompanied by mild to moderate focal lymphoid-histiocytic infiltration was recorded. In some microscopic fields, medium-sized areas of coagulative necrosis were identified (Fig. 8). Fig. 7. Spleen of a rat, 7th day of acute destructive pancreatitis. Reduction of lymphoid follicles, anemia (hypoperfusion) of the red pulp, and thickening of arteriolar walls. Hematoxylin and eosin staining, ×400. Fig. 8. Pancreas of a rat, 14th day of acute destructive pancreatitis. Necrosis, lipomatosis, stromal edema, and formation of granulation tissue. Hematoxylin and eosin staining, ×200. On the 14th day, the histological structure of the thymus did not display a distinct boundary between the cortical and medullary layers. The number of Hassall’s corpuscles remained at the previous level; however, pronounced dystrophic changes in epithelial cells were observed. The thymic lacunae were represented by small and medium-sized formations (Fig. 9B). Fig. 9. A. Spleen of a rat, 14th day after modeling acute destructive pancreatitis. Enlarged lymphoid follicle with the absence of a germinal center. Moderate vascular congestion in the red pulp. Hematoxylin and eosin staining, ×400. B. Thymus of a rat, 14th day after modeling acute destructive pancreatitis. Pronounced thymic lacunae of irregular shape; striated hemorrhages are noted in the interstitial stroma. Hematoxylin and eosin staining, ×200 In the spleen, the lymphoid follicles were enlarged; however, germinal (light) centers were absent. The red pulp was characterized by moderate vascular congestion, and phenomena of stasis, sludge, as well as single polymorphonuclear leukocytes, were detected in the vascular lumina (Fig. 9A). By the end of the 14th day of the experiment, the mean area of the periarteriolar lymphoid sheaths (PALS) in the white pulp of the spleen reached 0.055 ± 0.006 mm², which statistically insignificantly exceeded the values of the control group. The number of immunocompetent cells in these zones was 88.6 ± 10.1 (p < 0.05), indicating persistent activation of the T-cell component of the immune response. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 218 Discussion. Histological examination of the thymus and spleen in the control (laparotomy) group revealed only minor changes, whereas in the model of acute destructive pancreatitis (ADP), pronounced dynamic transformations were observed. In the thymus, as early as 24 hours after intervention, a decrease in the number of lymphocytes in the cortical zone was noted, with a progressive decline by day 14. In the medullary layer, signs of reticuloendothelial remodeling were observed, including an increased number of Hassall’s corpuscles, which may indicate the onset of accidental involution of the thymus. In the spleen, a reduction in the size of lymphoid follicles was observed, with the most pronounced effect on the 7th day. By the 3rd day, the disappearance of light centers indicated insufficient proliferation of B-lymphocytes and plasma cells. However, by the 14th day, the follicles began to enlarge again due to the predominance of the dark zone of the germinal center, which may reflect the activation of immunotropic mechanisms. In the red pulp, microcirculatory disturbances were recorded, manifested by distorted vascular walls. During ADP, a predominant activation of the T-cell component of immunity was observed, manifested by the expansion of the periarteriolar lymphoid sheaths (PALS) and an increase in their cellular composition. The nature of the dynamics—wave-like, with peaks at the 1st hour and the 7th day—reflects the phasic nature of the immune response under genotoxic stress. The observed discrepancy between the activation of the Tand B-cell zones may indicate an incomplete immune response under conditions of severe intoxication. Cryo-induced pancreatic injury provokes a pronounced quantitative restructuring of the T-cell component of the immune system in the thymus and spleen. The tendency toward reparative recovery is manifested by an increase in the total pool of T-lymphocytes—morphologically expressed as increased density of the cortical zone of the thymus and suppression of Hassall’s corpuscle formation, and in the spleen—as a gradual enrichment of follicles with immunocompetent cells. However, generalized T-cell hyperplasia under prolonged exposure may be accompanied by secondary immunodeficiency due to an imbalance between immunoregulatory lymphocyte subpopulations. In particular, suppression of the Th1 compartment and activation of Th2 subpopulations are possible, leading to decreased production of proinflammatory cytokines and a shift of the inflammatory response into a “sluggish” state. The data obtained provide a deeper understanding of the morphological manifestations of lymphoid organ damage under systemic inflammation and may serve as a rationale for the timely introduction of immunomodulatory and anti-inflammatory therapeutic interventions in acute destructive pancreatitis. Conclusion. Experimental acute destructive pancreatitis is accompanied by pronounced morphofunctional changes in the lymphoid organs, particularly in the thymus and spleen. A decrease in the number of lymphocytes in the thymic cortex and a reduction in the size of splenic lymphoid follicles were observed, indicating a disturbance of immune homeostasis. Activation of the T-cell component of the immune system shows a wave-like pattern and is accompanied by signs of T-cell hyperplasia; however, subsequent dysfunction of immunoregulatory lymphocyte subpopulations indicates the development of secondary immunodeficiency. These morphological alterations emphasize the necessity for early diagnostic and therapeutic measures aimed at modulating the immune response to prevent the progression of inflammation and complications in acute pancreatitis. REFERENCES SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 11 NOVEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 219 1. Akzhigitov G. N. Acute Pancreatitis. Moscow: Meditsina; 1974. 168 p. 2. Boger M. M. Pancreatitis (Physiological and Pathophysiological Aspects). Novosibirsk: Nauka; 1984. 216 p. 3. Voloshin N. A., Yakhnitsa A. G. The state of the thymus gland in rats after antenatal antigenic stimulation. Archives of Anatomy, Histology, and Embryology. 1998; 82(5): 83–89. 4. 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