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.

Keywords: dermis; epidermis; innervation; nerve cell counts; pain; whipping.

Figure 1

Representative histologic images of horse (A) and human (B) epidermis and superficial dermis of gluteal skin illustrating comparable epidermal thickness (Haematoxylin and Eosin, 400×, 16 μm sections. Bar = 20 μm).

Figure 2

The box-and-whisker plots for the epidermal depth (A) and natural logarithm of epidermal depth (B) of skin from men (n = 5), women (n = 5), mares (n = 11) and geldings (n = 9).

Figure 3

The box-and-whisker plots for the corrected epidermal depth of skin from men (n = 5), women (n = 5), mares (n = 11) and geldings (n = 9). Coefficients and error bars are shown on the original measurement scale for ease of interpretation. This figure illustrates the significant interaction effect found between the side from which the sample was taken and the species from which the sample was taken.

Figure 4

The box-and-whisker plots for the dermal depth of skin from men (n = 5), women (n = 5), mares (n = 11) and geldings (n = 9).

Figure 5

The box-and-whisker plots for the dermal depth of skin from men (n = 5), women (n = 5), mares (n = 11) and geldings (n = 9). Coefficients and error bars are shown on the original measurement scale for ease of interpretation. This figure illustrates the significant interaction effect found between the side from which the sample was taken and the species from which the sample was taken.

Figure 6

Representative skin sections stained with immunohistochemistry using anti-PGP9.5 antibodies from Horse (A) and Human (B) skin. Images include epidermis and superficial dermis. Nerve endings are selectively stained with a red dye (*). Sections illustrate the finding that epidermal nerve endings are found in equivalent density across horse and human skin (anti-PGP9.5 immunohistochemistry, 400× magnification. Bar = 20 μm).

Figure 7

The box-and-whisker plots for the epidermal nerve count (A) and natural logarithm of epidermal depth (B) of skin from men (n = 5), women (n = 5), mares (n = 11) and geldings (n = 9).

Figure 8

The box-and-whisker plots for the epidermal nerve count of men (n = 5), women (n = 5), mares (n = 11) and geldings (n = 9). Coefficients and error bars are shown on the original measurement scale for ease of interpretation. This figure illustrates the significant interaction effect found between the side from which the sample was taken and the species from which the sample was taken.