Comparative Morphology of Trigeminal Ganglion in Some Species of Vertebrates

Introduction:  The mammalian digital end organ has developed in three basic functional forms, i.e., claw, hoof and nail. Whereas detailed information on the ontogeny and phylogeny of the equine and bovine hooves, and the human nail, respectively, are on hand, such data are still not available for the canine claw. Because phylogenetically the carnivore claw is considered as a primary form of digital end organ, data on the organogenesis of the canine claw will complement current knowledge on onto- and phylogeny of the carnivore claw, and digital end organs in general.
Materials and Methods:  Light and electron microscopy of 18 canine fetuses of 40–150 mm crown-rump-length and five puppies aged 1 day–4 weeks post-partum.
Results:  The development of the canine claw can be divided into four periods. The first stage is initiated by the differentiation of the connective tissue layers of the claw anlage. The second stage is characterized by formation of the specific claw shape, and the third stage by further regional function-related modifications resulting in the development of the five different segments of the digital end organ. The fourth stage proceeds post-natally when the influence of increasing body weight and weight bearing leads to the final functional adaptation of the papillary body.
Conclusion:  The organogenesis of the canine claw resembles that of the feline claw in many aspects, but also shows some specific deviations. In particular, the development of the dorsal ridge – characteristic for the carnivore claw – differs between both species. The results of our study therefore raise the question whether the current hypothesis on allocation of the different segments of the canine claw has to be reconsidered.