Splenic plasticity at the arid limit: dehydration is associated with microanatomical remodelling and B-cell depletion in the dromedary camel (Camelus dromedarius)

The dromedary camel ( Camelus dromedarius ) is the exemplar of mammalian adaptation to hyper-arid environments, possessing specialized physiological mechanisms to tolerate extreme osmotic stress and hemoconcentration. While renal and thermoregulatory adaptations are well-characterized, the costs of these survival strategies on immune microanatomy remain poorly defined. Structural remodeling of lymphoid organs may accompany hydric and metabolic stress in xeric specialists, but whether such changes represent adaptive reallocation or a nonspecific consequence of physiological stress remains uncertain. This study investigates the plasticity of the splenic marginal zone (MZ), a critical reservoir for innate-like B cells, under naturally occurring dehydration gradients. We conducted a cross-sectional morphophysiological study on C. dromedarius ( n = 100) in the hyper-arid El Oued region of the Algerian Sahara. Hydration status was defined via serum osmolality and hematocrit. We utilized quantitative histomorphometry and immunohistochemistry to assess MZ thickness and B-cell density. Multivariable linear regression was employed to control for covariates (age, sex, body condition). We applied statistical mediation analysis to examine whether MZ remodeling statistically accounted for part of the association between dehydration and B-cell density. Dehydrated camels exhibited significant splenic plasticity, characterized by a marked reduction in MZ thickness (157.9 ± 10.8 μm vs. 183.1 ± 9.6 μm in hydrated controls; p < 0.001) and a concomitant 24% decline in B-cell density. MZ atrophy was strongly correlated with systemic hydration markers (serum osmolality: r = -0.72). Mediation modeling indicated that 59% of the dehydration–B-cell density association was statistically accounted for by the MZ-thickness pathway (ACME: -196.2 cells/mm²). Furthermore, animals from the hyper-arid Sahara micro-region displayed greater MZ contraction than those from the transitional Sahel, suggesting environmental modulation of splenic architecture. These findings demonstrate marked dehydration-associated plasticity of the splenic MZ in the dromedary camel. The concurrent reduction in MZ thickness and B-cell density, together with their statistical association, identifies a previously undercharacterized relationship between systemic hydration status and splenic microanatomy. The present data do not establish that lymphoid remodeling is itself beneficial, adaptive, or specific to water deprivation; it may also reflect a broader physiological response to metabolic, endocrine, hemodynamic, nutritional, or other stressors. The findings therefore provide a basis for further investigation of the mechanisms and functional significance of splenic remodeling during dehydration.