AFIP Wednesday Slide Conference - No. 16
January 12, 2000

Conference Moderator:
Dr. Steven E. Weisbrode, Diplomate, ACVP
The Ohio State University
Department of Veterinary Biosciences
Columbus, OH 43210
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Case I - 99-1848 (AFIP 2679493)
Signalment: One-year-old, male thoroughbred horse.
History: Ataxia of one-month duration. Static narrowing of cervical spinal canal at C3-4 seen radiographically.
Case 16-1. Proximal radius. Reddish-brown areas bordered by pale gray cartilage are the synovial fossae. Indistinct vertical streaks in the cartilage are small grooves (wear lines) which appear histologically as areas of cartilage compression and chondrocyte necrosis.
Gross Pathology: Mid sagittal diameter of spinal canal at C3-4 was 13 mm. Radial-ulnar joint had developing synovial fossae and mild degenerative joint disease (linear "scoring").
Contributor's Diagnoses and Comments: Specimen is proximal radius. There is a focal depression in articular cartilage and corresponding subchondral bone. Cartilage in this region has a thickened superficial fibrous layer. In some areas, chondrocytes in the tangential layer have a stellate or myxomatous appearance. Multifocally there is hypocellularity or coagulation necrosis in the radial layers. A tidemark is more prominent in cartilage in the depression compared with adjacent cartilage, and subchondral bone beneath the depressed cartilage is more porous. In areas corresponding to scoring or wear lines in the non-depressed cartilage, there is mild degenerative joint disease characterized by cartilage hypocellularity, reactive chondrocyte clusters and variable fibrillation of the matrix.
1. Normal synovial fossa
2. Mild degenerative joint disease ("wear lines")
The gross and microscopic appearances of the irregular depressed regions are characteristic of early stages of synovial fossae. These are grooves of uncertain function found on articular surfaces of ruminants, horses, pigs and dogs. It has been suggested that they might act as reservoirs for synovial fluid or might reflect disuse atrophy in non-weight bearing regions of the joint. Although in some locations in cattle these fossae begin to develop prenatally, in most species and locations, they are not apparent at birth but progressively develop in the first several months of life. With complete development, the articular cartilage of the fossa is replaced by fibrous tissue which directly communicates with the marrow through the relatively porous subchondral bone. The scoring or wear lines in the gross photo are common in the elbow joints of horses and are often subclinical. They have been described as being associated with cases of cervical vertebral stenotic myelopathy (as in the current case). It is speculated that ataxia might cause increased turbulence of synovial fluid resulting in these wear lines.
AFIP Diagnoses:
1. Bone, articular cartilage, superficial and tangential: Chondrocyte necrosis, multifocal, thoroughbred, equine.
2. Bone, articular cartilage: Synovial fossa (normal)
Conference Notes: Histologically the score lines that are visible adjacent to the fossa are characterized by focal areas of chondrocyte necrosis. Although grossly there are visible and generally palpable depressions or scores, histologically the cartilage is not scored and is only intermittently depressed over these areas. Physiologically these depressions are caused by necrosis of chondrocytes resulting in decreased glycosaminoglycans that in turn results in decreased water binding and subsequent cell shrinkage. Processing frequently obscures this subtle lesion.
Contributor: Department of Veterinary Biosciences, The Ohio State University, 1925 Coffey Road, Columbus, OH 43210
1. Palmer N: Bones and Joints. In: Pathology of Domestic Animals, eds. Jubb KVR, Kennedy PC, Palmer N, 4th ed., vol. 1, p. 144. Academic Press, San Diego, 1993
2. Rooney JR, Robertson JL: Foreleg. In: Equine Pathology, p. 155. Iowa State University Press, Ames, IA, 1996
3. Wagner KM, Heje N-I, Aarestrup FM, Ravn BT, and Osterby J: The morphology of synovial grooves (Fossae synoviales) in joints of cattle of different age groups. J Vet Med Assoc 40:359, 1993
Case II - 99-1413 (AFIP 2693021)
Signalment: 4-year-old Scottish terrier, male, castrated dog
History: Presented with a 2cm long oral mass of the left rostral mandible surrounding and caudal to the canine tooth. The surgeon's report indicated the mass "shelled out". There has been no recurrence 1.5 mos. after surgery.
Case 16-2. Radiograph of mandible. A nodular mass of low radiodensity extends from the lateral aspect of the mandible (lower right corner), but does not deform it.
Gross Pathology: Three, white, firm to hard, smooth multilobulated fragments of the gingival mass measure up to 3 X 1.7 cm.
Laboratory Results: Radiograph of left mandibular ramus (see 2x2) illustrates a soft tissue mass with subtle mottling; there is a smooth border along the mandible-tumor interface. Interpretation by Drs. Gregory Daniel and Kari Anderson: "Left rostral mandibular soft tissue neoplasia with no evidence of bony destruction. Primary consideration is given to ossifying epulis".
Contributor's Diagnosis and Comments: Aggressive osteoblastoma, left mandible, canine.
The non-decalcified, rounded mass has 3 remarkable elements consisting of more superficial osteoblasts merging with more and more mineralized and non-mineralized matrix deeper in the tumor: The outer portion is more cellular as it is dominated by epithelioid osteoblasts; these cells surround mineralized bony fragments which gradually become branching trabeculae with limited lattice formation deep in the mass; fragments and trabeculae often have wide osteoid seams. A hyaline matrix (osteoid), sometimes stippled with mineralization is adjacent to foci of confluent mineralization. Very occasional deep trabeculae contain chondrocytes (not in all sections).
The epithelioid component has cells with a variable amount of lightly eosinophilic to basophilic cytoplasm with discrete margins in some regions. Nuclei of vary in size, and are oval to irregular with fine chromatin and indistinct nucleoli. Very occasional normal mitotic figures are present. The nuclei of these cells become more elongate and their cytoplasm less conspicuous as they are surrounded by more and more hyaline matrix (osteoid). The most discrete epithelioid cells are surrounded by a deeply basophilic material (mucin). Occasional multinucleated giant cells (with oval nuclei resembling those of the epithelioid population but smaller) are present within the trabecular region usually unassociated with the surfaces of the trabeculae.
The tumor has undergone coagulation necrosis in a region at the periphery of the basal-most portion. Tumor extends to the surgical margin. The original diagnosis was parosteal osteogenic sarcoma reflecting Roy Pool's description (not the Atlas of Tumor Pathology's description) in order to emphasize an expected less aggressive behavior. The AFIP fascicle describes osteoma, osteoid osteoma and osteoblastoma. This tumor somewhat resembles a variant of osteoblastoma, the aggressive osteoblastoma. This tumor is not an osteoma which usually has well-formed trabeculae surrounded by pavemented osteoblasts; trabecular spaces in an osteoma typically contain fibrovascular tissue, not the epithelioid cells seen here. Cellular atypia and bony invasion were not present to suggest the osteoblastic variant of osteogenic sarcoma.
Note: 8 months following the original surgery the mass recurred and a hemimandibulectomy was performed. A smooth firm sessile mass (1.5 x 1.2 cm) was present caudal and medial to the canine and surrounding the first premolar. A similar mass 1.5 x .6 cm mass was located lateral to the canine tooth. Histologically the mass is very similar to the original biopsy.
AFIP Diagnosis: Gingiva: Benign fibro-osseous neoplasm, Scottish terrier, canine.
Conference Notes: This unusual lesion was the subject of a lengthy discussion. While this lesion is moderately cellular, has multifocal mild nuclear atypia and occasional mitotic figures, the small size, absence of bone destruction and absence of greater atypia are more consistent with a benign process. The Departments of Orthopedic Pathology and Oral Pathology were consulted and both favor a benign process. The Department of Orthopedic Pathology noted that "the absence of vascularity, osteoclastic and blastic activity does not fit an osteoblastoma". Conference participants and the 2 consulted departments could not definitively determine whether the lesion was purely peripheral or intraosseous with perforation. The Department of Oral Pathology favors a variant of ossifying fibroma if the lesion is central and peripheral ossifying fibroma if the lesion is peripheral.
Contributor: Department of Pathology, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37901-1071
1. Fechner RE, Mills SE: Atlas of Tumor Pathology, Third Series, Fascicle 8, Tumors of the Bones and Joints, pp. 26-38. Armed Forces Institute of Pathology, Washington, DC, 1993
2. Hoffman S, Jacoway JR, Krolls SO: Atlas of Tumor Pathology, Second Series, Fascicle 24, Intraossoeous and Parosteal Tumors of the Jaw, eds Hartmann WH, Sobin LH, pp. 203-216. Armed Forces Institute of Pathology, Washington, DC, 1985
3. Pool RR: Tumors of Bone and Cartilage In: Tumors in Domestic Animals, ed. Moulton JE, 3rd ed., pp. 159-163;193-194;222-225. University of California Press, Berkeley, CA, 1990
Case III - MP9B (AFIP 2679485)
Signalment: Adult, male, C57BL6 mouse (Mus musculus)
History: Eleven days following experimental, intravenous inoculation with Mycoplasma pulmonis, the mice exhibited lameness with reddening and swelling around multiple synovial joints of the limbs, and edema of the hind feet.
Gross Pathology: Purulent fluid was in multiple joint spaces. The periarticular tissue of affected joints was swollen and edematous.
Contributor's Diagnosis and Comments: Tarsus: marked, suppurative polyarthritis and chronic active periarthritis with synovial proliferation, osteolysis, and subperiosteal bone proliferation.
Etiology: Mycoplasma pulmonis
The section consists of a decalcified sagittal section through the tarsus. Affected joints have distended synovial spaces containing proteinaceous material and degenerate neutrophils. Proliferative synovial cells lined the thickened synovial membrane, often interrupted by areas of erosion. The periarticular soft tissue contained organized layers of histiocytic and neutrophilic inflammation with scattered focal accumulation of degenerate neutrophils into microabscesses. Early fibroplasia of the synovia and fascia was suggestive of burgeoning ankylosis. Articular inflammation frequently undermined articular cartilage, destroyed subchondral bone, and entered the metaphyseal medullary cavity. Lytic areas in metaphyses were filled with histiocytic inflammation and proliferative fibrocartilage, and also lined by new cancellous bone. Along the periosteal surface of cortical bone adjacent to the joints, there was prominent reactive bone proliferation. Chronic, mixed inflammation of tendons, entheses, and tendon sheaths was also present.
Mycoplasmas are responsible for multiple, naturally occurring inflammatory diseases including pneumonia, polyserositis, and arthritis in many species, including rodents. Mycoplasma arthritidis is the agent typically responsible for mycoplasmal polyarthritis in the rat and mouse. Experimental polyarthritis similar to that produced by M. arthritidis can be produced by Mycoplasma pulmonis in the mouse by intravenous inoculation, although M. pulmonis is usually responsible for lymphocytic pneumonia in the rodent. By intravenous exposure, the pathogenesis and outcome of the arthritic lesion of M. pulmonis is similar to that of M. arthritidis (Lindsay, 1978). In general, infected mice fail to clear the mycoplasma organism and develop chronic proliferative arthritis characterized by periods of remission and exacerbation (Kono, 1980).
Histologically, an acute phase (1 week post inoculation) consists of suppurative inflammation and edema of the articular and periarticular tissues with sporadic tendonitis. Two to three weeks post inoculation, affected joints display acute and chronic features of the inflammatory process, and by 4 weeks and onward, the chronic phase consists of hyperplasia of the synovial membrane, mononuclear cell infiltration, granuloma formation, pannus, destruction and proliferation of subchondral cortical bone, and destruction of articular cartilage (Harwick, 1976). This pattern resembles the lesions of rheumatoid arthritis in humans, for which this experimental disease is a suitable model.
AFIP Diagnosis: Tarsus: Synovitis and tenosynovitis, chronic, suppurative, multifocal, moderate, with pannus and osteophytes, C57BL6 mouse (Mus musculus), rodent.
Conference Notes: Conference participants favored Mycoplasma sp. as the most likely etiology of this lesion. Participants discussed the need to exclude other possible causes including bacterial and viral infections and autoimmune diseases.
Contributor: Lilly Research Laboratories, PO Box 708,Greenfield, IN 46140
1. Harwick HJ, Mahoney AD, Kalmanson GM, Guze LB: Arthritis in mice due to infection with Mycoplasma pulmonis. II. Serological and histological features. J Infect Dis 133(2):103-112, 1976
2. Kono M, Tanaka H, Yayoshi M, Araake M, Yochioka M, Imai M: Mycoplasma pulmonis arthritis in congenitally athymic (nude) mice. Histologic features. Microbiol Immunol 24(5):381-391, 1980
3. Lindsay JR, Cassell GH, Baker HJ: Diseases due to Mycoplasmas and Rickettsias. In: Pathology of Laboratory Animals, eds. Benirschke K, Garner FM, Jones TC, vol II, pp. 1507-1513. Springer-Verlag, New York, 1978
4. Palmer N: Bones and Joints. In: Pathology of Domestic Animals eds. Jubb KVR, Kennedy PC, Palmer N, 4th ed., vol. 1, p. 144. Academic Press, San Diego, 1993
Case IV - 8903/99 (AFIP 2698157)
Signalment: A 42-day-old male Ross broiler chicken.
History: This bird was culled due to lameness and submitted with other lame broilers to the laboratory for necropsy.
Gross Pathology: On examination of a mid-line frontal section of the proximal tibiotarsus, a small zone of pale/yellow tissue was observed in the region of the growth plate.
Laboratory Results: A profuse growth of Staphylococcus aureus was recovered following bacterial culture of one half of the affected proximal tibiotarsus.
Contributor's Diagnosis and Comments: Bacterial chondronecrosis of the physeal cartilage with osteomyelitis due to infection with S. aureus.
The pale stained physeal chondrocytes and matrix may indicate the edge of a chondronecrotic lesion that contrasts with the dark blue, normally stained zone. Note also clumps of bacteria closely associated with necrotic cartilage and accumulating in the metaphyseal blood vessels. This condition is a common cause of lameness in fast-growing broiler chickens. It is most frequently detected in the proximal femur and proximal tibiotarsus.
AFIP Diagnosis: Tibiotarsus: Osteomyelitis, necrotizing, heterophilic, diffuse, moderate, with focally extensive physitis and epiphysitis, necrotizing vasculitis, congestion, hemorrhage, and numerous colonies of cocci, Ross broiler chicken, avian.
Conference Notes: Bacterial osteomyelitis affecting the proximal tibiotarsus and the distal femur is an important cause of lameness in commercial poultry. Staphylococcus aureus is the most common cause of chondronecrosis and osteomyelitis in chickens, although Mycoplasma sp, Escherichia coli, Staphylococcus xylosus, and other bacteria have been isolated from affected birds. McNamee et al reported an increased frequency of lesions in association with high nutritional levels and dual infection by chicken anemia virus and infectious bursal disease virus, thus providing a model for study of the natural disease.

A possible pathogenesis for localization of infection in the tibia and femur is deposition of bacteria in the growing ends of metaphyseal vessels following development of a continuous bacteremia after a primary infection of the respiratory tract. Predisposing factors discussed in conference include lack of anastomoses in metaphyseal vessels, an incomplete endothelial lining,and locally decreased phagocytic activity.
Contributor: Veterinary Sciences Division, 43 Beltany Road, Omagh, N. Ireland BT78 5NF
1. McNamee PT, McCullagh JJ, Thorp BH, Ball HJ, Graham DA, McConaghy D, Smyth JA: A longitudinal investigation of leg weakness in two commercial broiler flocks. Vet Rec 143:131-135, 1988
2. McNamee PT, McCullagh JJ, Thorp BH, Ball HJ, Connor T, McConaghy D, Smyth JA: Development of an experimental model of bacterial chondronecrosis in broilers following exposure to Staphylococcus aureus by aerosol, and inoculation with chicken anaemia and infectious bursal disease viruses. Avi Pathol 28:26-35, 1998
J Scot Estep, DVM
Captain, United States Army
Registry of Veterinary Pathology*
Department of Veterinary Pathology
Armed Forces Institute of Pathology
(202)782-2615; DSN: 662-2615
* The American Veterinary Medical Association and the American College of Veterinary Pathologists are co-sponsors of the Registry of Veterinary Pathology. The C.L. Davis Foundation also provides substantial support for the Registry.
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