The structure and protein histochemistry of the hinge epidermis of the chequered water snake, Natrix piscator, throughout the sloughing cycle has been described.
Hinge epidermis, like the scale epidermis, undergoes cyclic changes in its organization and six consecutive stages are itemized during one complete cycle.
In contrast to the scale epidermis, the beta layer, alpha layer, presumptive beta layer and presumptive alpha layer could not be distinguished in hinge epidermis throughout the sloughing cycle. The stratum germinativum, 1–2 layers of undifferentiated epidermal cells, mesos layer and Oberhautchen constitute the epidermis in stage 1. In stage 2, the undifferentiated epidermal cell layers increase in number and a few eosinophilic granular cells have appeared. In stage 3, the lacunar tissue, consisting of large swollen cells arranged in several layers, and the clear layer are differentiated. In stage 4, the presumptive Oberhautchen and presumptive mesos layer are delineated and the lacunar tissue richly infiltrated by eosinophilic granular cells. The presumptive Oberhautchen matures and the lacunar tissue shows further degeneration in stage 5, and in stage 6 the presumptive mesos layer is mature, the lacunar tissue degenerates and the outer epidermal generation invariably is detached.
Histochemically, the Oberhautchen is weakly eosinophilic, stains orange in MTS (Mallory's triple stain), strongly for cystine-bound -SS groups, moderately for cysteine-bound -SH groups and weakly for basic protein, protein-bound-NH2 groups, tyrosine, tryptophan and bound calcium. The mesos layer is strongly eosinophilic, stains red in MTS and displays strong reactions for the various protein end groups. Both the Oberhautchen and mesos layer show very strong DNase and R Nase resistant fluorescence with congo red—titan yellow/thioflavin T, rhodamine B, thioflavin T and titan yellow. In contrast, the lacunar tissue, clear layer, undifferentiated epidermal cells and stratum germinativum cells are weakly eosinophilic and show weak reactions for -NH2 groups and bound calcium and do not show D Nase and R Nase resistant fluorescence with various fluorochromes.
High amounts of disulphide groups confer mechanical strength and durability to the Oberhautchen and the mesos layer, and the outer layers of the epidermis. The functional value of tyrosine and tryptophan-rich proteins in the keratinized layers of the epidermis is not immediately apparent.
In the epidermis of the hinge, unlike that of the scale, the keratinized layers are thin and folded, and vertical striations are absent in the Oberhautchen and presumptive Oberhautchen, for the skin in this region is well protected between overlapping scales. In addition, thin and folded keratinized layers, together with large swollen lacunar tissue cells confer elasticity and stretching to the hinge epidermis, so as to permit free movements by the snake.
The eosinophilic granular cells probably act as scavengers to clear the debris from the autolysed lacunar tissue cells, and they play an important role in the local defence mechanism to protect snakes from invasion by micro-organisms.