The mechanoreceptors in hatchling and adult Elasmobranch skin

Abstract

The skin is the most extensive organ in vertebrates, composed of two layers: the epidermis and the dermis. Sensory axons originating from the dorsal root ganglia innervate the skin mechanoreceptors in the dermis. Elasmobranchs, which appeared 380 million years ago, are characterized by rough skin composed of dermal denticles. While we know about the epidermis and dermis of elasmobranchs, we do not know much about the presence or abundance of mechanoreceptors in their skin. Using the classic histological hematoxylin and eosin method, we examined the skins of hatchling embryos and adults Batoidea (skates and rays) and Selachimorpha (modern sharks). Our histology findings provide substantial evidence to identify structures with similar morphology to traditional mammalian and reptilian mechanoreceptors like Pacinian and Meissner corpuscles. An interesting observation was the presence of Pacinian in the skin of Batoidea but not in the skin of a Selachimorpha Squalus shark.

Figure 1. A Hematoxylin & Eosin of elasmobranchs skin.

A Hematoxylin & Eosin of elasmobranchs skin. A-D: skin from embryo/hatchling from Leucoraja (Batoidea) and Squalus (Selachimorpha). A and C show the developing Ampullae de Lorenzini (white arrows). In C1, we observed a developing mechanoreceptor (black arrow). The trunk of both elasmobranchs B1 and D1 (red arrow) shows what might be developing structures. E-H: skin from adult Urobatis (Batoidea) and Squalus (Selachimorpha). The epidermis and dermis (reticulate layers) are fully developed at this stage, and mechanoreceptors can be distinguished. In Urobatis head (E, E1), we observed unknown structures (red arrow). In the Squalus head (G, G1), we found Ampullae de Lorenzini ( red arrow) and an encapsulated Ruffini mechanoreceptor (G1). In the trunk of Urobatis (F, F1), we identified several Pacinian mechanoreceptors (F1). In the trunk of Squalus , we observed several Ruffini mechanoreceptors (black arrow in H) and a Meissner mechanoreceptor (H1). The cartoons summarize the structures we identified (K, L) and the location within the skins (I, J). Red circles correspond to head tissues, and blue to the trunk. K and L cartoons summarize how the elasmobranch skins organize (in the epidermis and reticular layers). We also made cartoons of the mechanoreceptors we observed in their skins. The scalebar dimension for all images is 50 μm.