A Comparative Neuro-Histological Assessment of Gluteal Skin Thickness and Cutaneous Nociceptor Distribution in Horses and Humans

Abstract

The current project aims to build on knowledge of the nociceptive capability of equine skin to detect superficial acute pain, particularly in comparison to human skin. Post-mortem samples of gluteal skin were taken from men (n = 5) and women (n = 5), thoroughbreds and thoroughbred types (mares, n = 11; geldings, n = 9). Only sections that contained epidermis and dermis through to the hypodermis were analysed. Epidermal depth, dermal depth and epidermal nerve counts were conducted by a veterinary pathologist. The results revealed no significant difference between the epidermal nerve counts of humans and horses (t = 0.051, p = 0.960). There were no significant differences between epidermal thickness of humans (26.8 µm) and horses (31.6 µm) for reference (left side) samples (t = 0.117, p = 0.908). The human dermis was significantly thinner than the horse dermis (t = −2.946, p = 0.007). Epidermal samples were thicker on the right than on the left, but only significantly so for horses (t = 2.291, p = 0.023), not for humans (t = 0.694, p = 0.489). The thicker collagenous dermis of horse skin may afford some resilience versus external mechanical trauma, though as this is below the pain-detecting nerve endings, it is not considered protective from external cutaneous pain. The superficial pain-sensitive epidermal layer of horse skin is as richly innervated and is of equivalent thickness as human skin, demonstrating that humans and horses have the equivalent basic anatomic structures to detect cutaneous pain. This finding challenges assumptions about the physical capacity of horses to feel pain particularly in comparison to humans, and presents physical evidence to inform the discussion and debate regarding the ethics of whipping horses.

The cannabinoid receptors system in horses: Tissue distribution and cellular identification in skin

“…In turn, single nerve terminals were detected almost across the entire skin. In normal skin, epidermal and dermal PGP 9.5 nerve terminals are rare, and their low quantities detected in our studies were consistent with studies on human and equine skin as well as horse intestine 29,53,59 . In contrast to tissue distribution in equine cells and tissues, antibody specificity using WB needs to be included, especially where analyzed target proteins belong to the large superfamily of GCPRs 60 .…”

“…For that reason, PGP 9.5 serves as an excellent connecting marker between neuronal and non‐neuronal cells. 29 , 57 , 58 , 59 Our studies observed heterogeneous PGP 9.5 immunoreactivity in different skin areas, but 2 main staining patterns correspond to cytoplasmic and nerve fiber‐like structures. Cytoplasmic expression detects neuro‐immune‐endocrine cells of the epidermis and single cells within all dermal compartments, mainly of the inner root sheath of hair follicle cells, sebaceous glands, and sweat glands.…”

“…In postnatal skin, PGP 9.5 is still expressed and led to identification of an epithelial neuro‐immuno‐endocrine cell situated in the basal layers of epidermis. For that reason, PGP 9.5 serves as an excellent connecting marker between neuronal and non‐neuronal cells 29,57‐59 . Our studies observed heterogeneous PGP 9.5 immunoreactivity in different skin areas, but 2 main staining patterns correspond to cytoplasmic and nerve fiber‐like structures.…”

“…In normal skin, epidermal and dermal PGP 9.5 nerve terminals are rare, and their low quantities detected in our studies were consistent with studies on human and equine skin as well as horse intestine. 29 , 53 , 59 In contrast to tissue distribution in equine cells and tissues, antibody specificity using WB needs to be included, especially where analyzed target proteins belong to the large superfamily of GCPRs. 60 The anti‐CBR1 antibody provided a single 50 kDa band in the whole equine skin and brain lysates, and those results correspond with those obtained in human endothelial cells, immune cells in mice, and macaque brain tissue.…”

The race that segments a nation: Findings from a convenience poll of attitudes toward the Melbourne Cup Thoroughbred horse race, gambling and animal cruelty

“…But despite its economic and social benefits, Thoroughbred racing in general [ 6 ], and the Melbourne Cup day in particular, potentially carry significant welfare costs to both horses and people. In addition to high profile deaths, such as the euthanasia of racehorse Cliffsofmoher following an injury early in the 2018 Melbourne Cup and the sudden death of Admire Rakti shortly after racing in 2014, In addition to the high profile deaths of Melbourne Cup runners on track or shortly after (7 horses since 2013), Thoroughbred racing is associated with widespread wastage [ 5 ] and acute and chronic pain from musculoskeletal injuries [ 7 ], pulmonary haemorrhages [ 8 ], gastric ulcers [ 9 ] and increasing public distaste for the use and consequence of equipment such aswhips [ 10 – 12 ] and tongue-ties [ 13 ]. Additionally, problem gambling is a widespread financial and mental health issue among Australians [ 14 ].…”