Regional histomorphometric characterization of rhesus monkey (Macaca mulatta) skin: anatomical variability and dermatological implications

Abstract

The use of non-human primates (NHPs) in research is an invaluable tool for advancing knowledge of numerous human diseases. It is imperative, however, that their use as biomodels strictly complies with current regulations. In this context, morphological studies of species used in Laboratory Animal Science (LAS) are highly relevant to scientific progress, with the rhesus monkey (Macaca mulatta) among the most widely used models. Despite its importance, the literature on integumentary histology in NHPs remains scarce, with most information dating back to the 1970s. The present study aimed to describe the skin histology of Macaca mulatta and determine the average epidermal thickness. Detailed histological and morphometric analyses were performed on skin samples collected from different anatomical sites of six ex situ adult rhesus monkeys (both males and females). The regions examined included: Regio parietalis (RPA), Regio colli dorsalis (RCD), Regio presternali (RPE), Regio vertebralis thoracis (RVT), Regio abdominis caudalis (RAC), Regio sacralis (RSC), Regio palmaris manus sinistra (RPM), Regio plantaris pedis sinistra (RPP), and Regio ischiadica (RIS). Samples were embedded in paraffin, stained with hematoxylin and eosin (H&E), and analyzed using ImageJ software. Marked regional differences were observed in the architecture of the epidermis and dermis, with thicker epidermis and stratum corneum areas subjected to greater friction. Epidermal thickness ranged from 11.25 µm in the ventral region to 347.32 µm in the ischiatic calluses, while the stratum corneum varied from 2.18 µm to 256.15 µm. Despite these morphological disparities, statistical analysis revealed no significant differences in epidermal thickness among regions. Thin skin areas exhibited three to four cell layers in the epidermis and a balanced distribution of skin appendages, with a higher density of sweat glands in the upper parietal region. In contrast, thick skin regions presented 8 to 20 cell layers in the spinous layer. Palmar and plantar regions displayed well-defined epidermal ridges, whereas in the ischiatic calluses, the epidermis formed parallel layers over the dermis. Additionally, subclinical dermatitis was identified in the palmar, plantar, and ischiatic regions. These findings highlight the structural specialization of primate skin and reinforce the importance of carefully selecting sampling sites in dermatological studies involving non-human primates.