Postnatal ossification centers of the atlas and axis in Miniature Schnauzers

Alastair G. Watson From the Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078 (Watson), and the College of Veterinary Medicine, University of Florida, Gainesville, FL 32610 (Watson, Stewart).

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 BVSc, MAgrSc, PhD
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James S. Stewart From the Department of Physiological Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK 74078 (Watson), and the College of Veterinary Medicine, University of Florida, Gainesville, FL 32610 (Watson, Stewart).

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 MS, DVM

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SUMMARY

Postnatal ossification of the atlas and axis was studied in Miniature Schnauzers by examining alizarin-stained bone clearings, cleaned dry bones, radiographs, histologic sections, and arterially perfused bone clearings. Sixty-two pups (1 day to 16 weeks old) and 4 adults were examined.

In 1-day-old pups, the atlas consisted of 3 separate ossification centers: a left and right neural arch center and midventrally, the intercentrum 1, which formed the body of the atlas. The axis contained 4 separate ossification centers: a left and right neural arch center; centrum 2 in the main part of the axis body; and centrum 1, which formed the caudal part of the dens and the cranial part of the axis body. By 6 weeks of age, the epiphysis on the caudal end of the axis body had begun to ossify. At this time, the intercentrum 2, which developed as a cuboidal ossification center intercalated between centrum 1 and centrum 2 in the middle of the cranial half of the axis body, also began to ossify. The centrum of the proatlas, which formed the apex of the dens, was first seen ossified in a 9-week-old pup. These 10 ossification centers were seen as constant and separate elements.

In all dogs, the dens developed from 2 separate ossification centers: the centrum of the proatlas formed the cranial one-quarter, and centrum 1 formed the caudal three-quarters. Dens dysplasia is unlikely to be a result of failure of development of one of the ossification centers for the dens. Seemingly, vascular-related ischemia may lead to postnatal resorption of at least the middle part of the dens and result in dens dysplasia, with subsequent atlantoaxial subluxation. This condition in dogs may be a suitable animal model for the study of the pathogenesis of os odontoideum and related conditions in man.

SUMMARY

Postnatal ossification of the atlas and axis was studied in Miniature Schnauzers by examining alizarin-stained bone clearings, cleaned dry bones, radiographs, histologic sections, and arterially perfused bone clearings. Sixty-two pups (1 day to 16 weeks old) and 4 adults were examined.

In 1-day-old pups, the atlas consisted of 3 separate ossification centers: a left and right neural arch center and midventrally, the intercentrum 1, which formed the body of the atlas. The axis contained 4 separate ossification centers: a left and right neural arch center; centrum 2 in the main part of the axis body; and centrum 1, which formed the caudal part of the dens and the cranial part of the axis body. By 6 weeks of age, the epiphysis on the caudal end of the axis body had begun to ossify. At this time, the intercentrum 2, which developed as a cuboidal ossification center intercalated between centrum 1 and centrum 2 in the middle of the cranial half of the axis body, also began to ossify. The centrum of the proatlas, which formed the apex of the dens, was first seen ossified in a 9-week-old pup. These 10 ossification centers were seen as constant and separate elements.

In all dogs, the dens developed from 2 separate ossification centers: the centrum of the proatlas formed the cranial one-quarter, and centrum 1 formed the caudal three-quarters. Dens dysplasia is unlikely to be a result of failure of development of one of the ossification centers for the dens. Seemingly, vascular-related ischemia may lead to postnatal resorption of at least the middle part of the dens and result in dens dysplasia, with subsequent atlantoaxial subluxation. This condition in dogs may be a suitable animal model for the study of the pathogenesis of os odontoideum and related conditions in man.

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