Shift from Pro- to Anti-Inflammatory Phase in Pelvic Floor Muscles at Postpartum Matches Histological Signs of Regeneration in Multiparous Rabbits

Abstract

Background and Objectives: Pelvic floor muscles (PFM) play a core role in defecation and micturition. Weakening of PFM underlies urogynecological disorders such as pelvic organ prolapse and stress urinary incontinence. Vaginal delivery damages PFM. Muscle trauma implies an inflammatory response mediated by myeloid cells, essential for subsequent recovery. Molecular signaling characterizing the pro-inflammatory phase shifts M1 macrophages to M2 macrophages, which modulate muscle repair. The present study aimed to evaluate histological characteristics and the presence of M1 and M2 macrophages in bulbospongiosus (Bsm) and pubococcygeus muscles (Pcm). Materials and Methods: Muscles from young nulliparous (N) and multiparous rabbits on postpartum days three (M3) and twenty (M20) were excised and histologically processed to measure the myofiber cross-sectional area (CSA) and count the centralized myonuclei in hematoxylin-eosinstained sections. Using immunohistochemistry, M1 and M2 macrophages were estimated in muscle sections. Kruskal–Wallis or one-way ANOVA testing, followed by post hoc tests, were conducted to identify significant differences (p < 0.05). Results: The myofiber CSA of both the Bsm and Pcm of the M3 group were more extensive than those of the N and M20 groups. Centralized myonuclei estimated in sections from both muscles of M20 rabbits were higher than those of N rabbits. Such histological outcomes matched significant increases in HLA-DR immunostaining in M3 rabbits with the CD206 immunostaining in muscle sections from M20 rabbits. Conclusions: A shift from the pro- to anti-inflammatory phase in the bulbospongiosus and pubococcygeus muscles of multiparous rabbits matches with centralized myonuclei, suggesting the ongoing regeneration of muscles.