Viperous fangs: Development and evolution of the venom canal

Abstract

Fangs are specialised long teeth that contain either a superficial groove (Gila monster, Beaded lizard, some colubrid snakes), along which the venom runs, or an enclosed canal (viperid, elapid and atractaspid), down which the venom flows inside the tooth. The fangs of viperid snakes are the most effective venom-delivery structures among vertebrates and have been the focus of scientific interests for more than 200 years. Despite this interest the questions of how the canal at the centre of the fang forms remains unresolved. Two different hypotheses have been suggested. The mainstream hypothesis claims that the venom-conducting canal develops by the invagination of the epithelial wall of the developing tooth germ. The sides of this invagination make contact and finally fuse to form the enclosed canal. The second hypothesis, known as the "brick chimney", claims the venom-conducting canal develops directly by successive dentine deposition as the tooth develops. The fang is thus built up from the tip to the base, without any folding of the tooth surface. In an attempt to cast further light on this subject the early development of the fangs was followed in a pit viper, Trimeresurus albolabris, using the expression of Sonic hedgehog (Shh). We demonstrate that the canal is indeed formed by an early folding event, resulting from an invagination of epithelial cells into the dental mesenchyme. The epithelial cells proliferate to enlarge the canal and then the cells die by apoptosis, forming an empty tube through which the poison runs. The entrance and discharge orifices at either end of the canal develop by a similar invagination but the initial width of the invagination is very different from that in the middle of the tooth, and is associated with higher proliferation. The two sides of the invaginating epithelium never come into contact, leaving the orifice open. The mechanism by which the orifices form can be likened to that observed in reptiles with an open groove along their fangs, such as the boomslang. It is thus tempting to speculate that the process of orifice formation in viperids represents the ancestral pleisomorphic state, and that enclosed canals developed by a change in the shape and size of the initial invagination.

Evolution and developmental diversity of tooth regeneration

“…At birth 3-4 developing generations of teeth are found linked to the dental lamina, making a developmental series ready to replace the functional tooth. In fanged snakes this series of replacement teeth is even longer, with approximately eight teeth ready to place the fangs [49], similar to the number of replacement teeth observed in the chondrichthyans. In the crocodilia, only one developing replacement tooth is observed behind the functional tooth, and together with the dental lamina they form a family unit [46].…”

Tooth shape formation and tooth renewal: evolving with the same signals

“…Most elaborated fangs have an enclosed canal, and Zahradnicek et al (Zahradnicek et al, 2008) have shown that this canal forms during development by proliferation-induced invagination of the epithelium, resulting in a groove that eventually closes off from the surface. The molecular regulation of this epithelial folding seen in fangs, however, remains unknown.…”

The viper fangs: clinical anatomy, principles of physical examination and therapy (a review)

“…This canal communicates with the outside through two orifices, proximal oval foramen and distal oval foramen (Zahradnicek et al 2008). Venom passes from the venom gland into the fangs entering the proximal oval foramen and is delivered through the distal foramen into the prey (Jackson 2002;Zahradnicek et al 2008). The triangular maxilla contacts dorsally the prefrontal bone, caudally the ectopterygoid bone and cranially the septomaxilla.…”

“…Fangs in vipers (Viperinae) and pit vipers (Crotalinae) are highly modified fangs (solenoglyphous), have a smooth surface and host the venom canal (VC). This canal communicates with the outside through two orifices, proximal oval foramen and distal oval foramen (Zahradnicek et al 2008). Venom passes from the venom gland into the fangs entering the proximal oval foramen and is delivered through the distal foramen into the prey (Jackson 2002;Zahradnicek et al 2008).…”

“…Functional fangs are periodically shed and replaced by a successive generation (Deufel and Cundall 2006). In Trimeresurus albolabris eight generations of venom fangs are present at the same time on the maxilla; seven calcified fangs serve as the replacement for the functional one (Zahradnicek et al 2008). Before the functional tooth is shed, its neighbor becomes ankylosed and connected with the venom duct.…”