Figure 1.

The physical, chemical, cellular, and microbiological innate immune barriers of frog skin. Frog skin, which is mucosal in nature, contains physical, chemical, cellular, and microbiological barriers that work together in defence against pathogen assault. Frog skin is composed of an epidermal and dermal layer, containing resident immune cells throughout the layers. The epidermis is comprised of stratified squamous epithelial cells in three distinct layers: the stratum corneum (1), stratum spinosum (2), and stratum germinativum. The dermis is largely comprised of connective tissue formed by collagenous fibres (black lines) in two layers, the spongious dermis (4) and the compact dermis (6), connected by collagenous columns (white star). In mainly terrestrial frogs, the Eberth-Kastschenko layer (5, thick blue line) separates the spongious dermal layer and compact dermal layer. Glands within the dermal layer include granular glands (a), mucosal glands (b), and small mixed glands (c) that secrete a slew of compounds, including mucus and antimicrobial peptides. Commensal bacteria overlay the frog skin layers, forming the microbiological barrier. The image was, in part, created with the aid of BioRender.