Estrous cycle-dependent changes in platelet indices in healthy mares

Platelets (PLTs) are key regulators of hemostasis, inflammation, and tissue remodeling, and their phenotype is known to fluctuate with sex steroid concentrations in several species. However, the influence of the estrous cycle on PLT characteristics in mares has not been investigated. We hypothesized that PLT indices vary between reproductive phases in association with changes in estradiol-17β (E2) and progesterone (P4). The aim of this study was to characterize PLT indices in estrus and diestrus and to determine how these hematological parameters relate to circulating E2 and P4 concentrations. PLT indices were evaluated in 25 healthy cyclic mares sampled during estrus (Day 0) and diestrus (Day + 12), and their associations with circulating ovarian steroids were assessed. Quantitative parameters (PLT count, plateletcrit [PCT], mean platelet volume [MPV], large platelets [PLCR], and platelet clumps [AGREG]) and qualitative indices of PLT size and heterogeneity-related indices (platelet distribution width [PDW], mean platelet mass [MPM], platelet component distribution width [PCDW], and platelet mass distribution width [PMDW]) were measured using an ADVIA 2120i analyzer. Plasma E2 and P4 were quantified by ELISA and RIA. PLT count, PCT, MPM and MPC did not differ between phases. MPV, PLCR, AGREG, and heterogeneity-related indices (PDW, PCDW, PMDW) were significantly highest during diestrus (P < 0.05), indicating a larger and more variable PLT population. Both hormones showed positive correlations with indices of PLT size and heterogeneity, particularly P4 (P < 0.05). In conclusion, PLT indices in mares vary across the estrous cycle in parallel with ovarian steroid fluctuations.