Unique insights into morphological characterization and functional adaptation of the scaly shank skin in aquatic and terrestrial birds

The avian skin is a vital barrier against external effects and undergoes modification to adapt to the different ecosystems. The current study focused on the comprehensive study of the scaly shank skin of aquatic birds, Egyptian Balady Duck (EBD) as well as terrestrial birds, Broad Breasted White Turkey (BBWT) via gross anatomy, histology, and scanning electron microscopy with ED-XRF analysis. The shank skin color was yellow to black in EBD and creamy-white in BBWT. Gross anatomy exhibited two types of scales around the shank: scute and scutella scales in EBD and four types: scute, scutella, reticula, and cancella (interstitial) scales in BBWT. Most scales were non-overlapped and separated by sulci in both birds except those on the dorsum of the shank of BBWT. SEM of the descaled skin revealed an irregular surface due to keratinocytes defining a polygonal texture in EBD and Langerhans cells (a large oval-shaped cell body with abundant long projections) attached to neighboring keratinocytes in BBWT. Histologically, the epidermal and dermal layers varied among the different skin aspects of the shanks of both birds. Langerhans cells were seen within the basal cell layer of the stratum germinativum and collagen fibers of the dermal stratum superficiale. Melanocytes were observed in the stratum basale in EBD. In both birds, abundant telocytes and fibroblasts were distributed within the dermal layers, with excessive adipose tissue in the dermis of the shank skin of EBD. According to the results of the ED-XRF analysis of the scaly shank skin, elements and oxides were present in both species. In conclusion, the findings of the present study reflect the correlations between the functional morphology of shank skin and the bird habitats.