The Clavicle of Newborn Dogs

Cern y H., S. C i z i na u sk a s: The Clavicle of Newborn Dogs. Acta vet. Brno 1995,64: 139-145. The morphology, topography and structure of the clavicle and its adnexa, forming together the clavicular complex, were investigated in 21 newborn dogs of both sexes and various breeds by dissection, radiography and histological techniques. The reduced clavicle, I to 2 mm in size, was oval in most cases. Horseshoe-shaped and annular clavicles were less frequent. The clavicle was located outside the clavicular intersection being linked with it by a fascicle of collagenous fibres. A similar fascicle linked the clavicle with the caudal margin of the scapula. No links of the clavicle with the sternum or the perimysium of the pectoral muscles were observed in the early postnatal period. The histological pattern was typical of desmogenic ossification. Osteoid and osseous trabeculae formed an incoherent cortical surface lamella and the formation of a cavity was evident in the centre of the clavicle. The osseous trabeculae aimed radially forming an anastomosing network. The surface of the clavicle was covered by a coherent layer of connective tissue with arranged fascicles of collagenous fibres which, on both ends of the clavicle, passed continuously into fibrous fascicles connecting the clavicle with the surrounding structures and forming together a functional unit. An oval basis of an embryonic cartilage, which we considered to be the preclavium, was observed at the sternal end of the clavicle in more than one half of the dogs. The function of the markedly reduced canine clavicle in the early postnatal period, in which the formation of the associated structures is not yet influenced by locomotion, is discussed. We assume that some of the links joining the clavicle with the adjacent structures develop only after birth in dependence on the locomotion. Newborn dog, early postnatal ontogenesis, clavicle, junction, morphology, ossification The clavicle is one of the components of the bone skeleton of the pectoral girdle connecting the scapula with the sternum. Its primary function is to reinforce the moving extremity. In most mammalian species, the clavicle develops as an important element of the skeleton. On the other hand, it is subject to regression in other species and its function has become limited with the advancing adaptation. The phylogenetic transformation of the skeleton of the pectoral girdle is causally linked with the development of locomotion. The accelerating locomotion and the increased motility of mammals were associated with the turning of both limbs towards the trunk. The limbs aim ventrally and the trunk is hung on them. The change of the overall arrangement of the mammalian skeleton was accompanied by a reduction of the ventral part of the pectoral girdle skeleton consisting of the coracoid and the clavicle, while the dorsal part, i.e. the scapula, has developed fully. The supportive function of the ventral part of the mammalian pectoral girdle skeleton has been taken on by a single bone the clavicle. The coracoid has disappeared as a separate bone and persists as a vestigial prominence (processus coracoideus) of the scapula. The advancing regressive development has affected the clavicle which is found as a markedly reduced and very small bone in some mammalian species and has become extinct in others. Various degrees of the clavicular regression can be observed in Ungulata, Carnivora, Cetacea, and Sirenia (S tar c k 1979).