Comparative analysis of the morphological heterogeneity of smooth muscle cells in two avian species: the muscovy Duck (Cairina moschata) and the Japanese quail (Coturnix japonica): ultrastructural and histochemical study.

Smooth muscle tissues exhibit significant functional diversity across various organ systems, but their cellular heterogeneity remains poorly understood. Recent ultrastructural investigations have identified two distinct populations of smooth muscle cells (SMCs), dark and light cells suggesting potential specialization in their roles. This study aims to comprehensively characterize these SMC subpopulations using a detailed morphological approach and histochemical techniques. This study characterizes two morphologically and functionally distinct smooth muscle cell (SMC) populations-light and dark cells-in avian intestinal and pulmonary tissues through comprehensive histochemical (H&E, Giemsa, Mallory/Crossmon's trichrome, silver stain, alcian blue, toluidine/methylene blue, PAS, Orange G) and ultrastructural (TEM) analyses. Light cells, identified by electron-lucent cytoplasm and secretory vesicles, and dark cells, marked by electron-dense cytoplasm and lysosomes, were consistently segregated within the intestinal muscular tunic and bronchovascular walls. Both subtypes contained dense bodies, confirming contractile capacity while suggesting specialized roles-light cells in secretory functions (e.g., extracellular matrix modulation) and dark cells in lysosome-mediated tissue remodeling. In pulmonary tissues, these cells populated the bronchial walls and the arterial tunica media, implicating subtype-specific contributions to airway resistance and vascular tone. The conserved presence of these populations across organs highlights their fundamental role in motility regulation, with clinical relevance to SMC pathologies: light cell dysfunction may underlie secretory disorders (e.g., mucus hypersecretion in asthma), while dark cell abnormalities could drive hypercontractile states (e.g., hypertension, achalasia). These findings establish avian models as powerful tools for investigating SMC heterogeneity, offering insights into phenotype-specific mechanisms in motility diseases and paving the way for targeted therapies that selectively modulate secretory or contractile SMC subpopulations to restore tissue homeostasis.