Abstract
Background:
Although the epididymal duct (ED) supports sperm maturation and storage, its ultrastructural and immunohistochemical adaptations in seasonal breeders, such as the Meriz buck, remain unclear.

Aim:
This study aimed to investigate seasonal adaptations in the ultrastructure and immunohistochemical profile of the ED in Meriz buck during mating and non-mating seasons.

Methods:
Twenty-four Meriz bucks from Duhok Province, Iraq, at the College of Veterinary Medicine were restudied. Epididymal tissues were examined using Transmission Electron Microscopy and immunohistochemistry α-smooth muscle actin (α-SMA), S-100, and von Willebrand factor (VWF) to characterize cellular localization during the mating and non-mating seasons.

Results:
The epithelium of the ED comprised principal cells (PCs), basal cells, apical cells, narrow cells (NCs), and clear cells. Ultrastructurally, PCs displayed Abundant microvilli, apical blebs, and well-developed endocytotic apparatuses (vesicles and multivesicular bodies), which were more prominent in autumn (October) than in other seasons. Adjacent PCs were joined by occluding junctions, forming the Blood-Epididymal Barrier. During autumn, the supranuclear region of PCs exhibited a larger Golgi apparatus, extensive rough endoplasmic reticulum, and numerous mitochondria, whereas other epithelial cells exhibited notable seasonal variations. Immunohistochemical, α-SMA expression intensified in the peritubular and vascular smooth muscle layers during autumn (9.3 ± 0.01, p < 0.01), suggesting enhanced contractile activity or smooth muscle remodeling associated with reproductive seasonality. S-100 immunoreactivity was strongest in the PCs and NCs (7.5 ± 0.002, p < 0.01), whereas VWF expression in vascular endothelial cells increased significantly in autumn compared to other seasons (0.99 ± 0.01, p < 0.01).

Conclusion:
The study concludes that in October, increased ultrastructural activity and elevated expression of α- SMA, S-100, and VWF in the ED indicate increased reproductive function. Suggesting that this period corresponds to the Meriz bucks' mating season.

Key words: Epididymal duct; Ultrastructure; Immunohistochemistry; Seasonal variation; Meriz Buck.