Signalment:  

Adult male Long-Evans rats (Rattus norvegicus)As part of a cerebral aneurysm study, rats were made hypertensive by ligation of left renal artery, fed a high salt diet, and implanted with pellets of deoxycorticosterone acetate (DOCA). To further cerebral aneurysm formation, the right carotid artery was surgically occluded, and rats were fed beta-aminopropionitrile (BAPN).(3) Animals had been on study for at least two months at the time of death.

Gross Description:  

On radiographs and gross examination bilateral irregular firm white nodular masses, approximately 4 x 5 x 7 mm, were noted on the medial proximal femoral diaphyses and at the site of insertion of the pectineus and adductor longus muscles. The masses were intimately associated with these muscles. Both rats died of exsanguination due to ruptured abdominal arterial aneurysms and hemoabdomen.

Histopathologic Description:

Submitted is a longitudinal section of the proximal femur including femoral head and adjacent soft tissues. Arising from the proximal medial cortex of the diaphysis and separating, surrounding and dissecting myocytes of adjacent skeletal muscle is a nodular, unencapsulated, moderately cellular mass consisting of short streaming bundles of plump spindloid cells in a coarse fibrous matrix. There is a well-differentiated proliferation of immature and mature bone adjacent to the cortex, which is multifocally absent (remodeling). These proliferations often incorporate hematopoietic elements within marrow spaces. In some sections, there are foci of hyaline cartilage, some of which transition to bone. At the soft tissue edges of the mass, there are numerous haphazardly arranged, variably sized, but generally small, spicules of osteoid. Spindle cells are plump with a proliferative appearance, but are cytologically relatively bland. There is minimal anisocytosis or anisokaryosis, and mitotic figures are rare (one per ten 400x HPF). There is secondary atrophy of entrapped myocytes.

Morphologic Diagnosis:  

Femurs, proximal, bilateral, periosteal fibroplasia, florid, atypical, with cartilaginous and osseous differentiation and marrow formation (exostoses), chronic, moderate to severe, consistent with osteolathyrism.

Condition:  

Lathyrism

Contributor Comment:  

Beta-aminopropionitrile (BAPN) is a principle toxic agent of the sweet pea, Lathyrus odoratus, and is an irreversible inhibitor of lysyl oxidase.(5) Lysyl oxidase is required for the oxidative deamination of hydroxylysine residues in collagens and lysine residues in collagens and elastin. These deaminated aldehyde derivatives form spontaneous intermolecular covalent bonds, cross-linking molecules and providing strength. Copper is a necessary cofactor for lysyl oxidase, and thus copper deficient diets result in functional inhibition of lysyl oxidase. BAPN intoxication and copper deficiency are phenotypically similar, although BAPN does not produce anemia.

Yeager and Hamre provide excellent descriptions of the evolution of the lesions of osteolathyrism.(7) Briefly, the exostoses develop at the site of the insertion of the tendons of the pectineus and adductor longus muscles on the periosteum of the femur through two distinct phases. In the preliminary proliferative stage, the inner layer of the periosteum becomes a highly cellular and less organized mass of tissue resembling a fibroma or fibrosarcoma. The second osteogenic stage occurs as soon as 7 to 8 days after initiation of a sweet pea diet, and consists of intra-membranous ossification at the periphery of the mass. No distant metastases were noted in any osteolathyrism studies, despite the infiltrative growth pattern, high cell density, and immature cytologic characteristics of the fibroblasts. Lesions may also develop at the deltoid ridge of the humerus, the gluteal trochanter of the femur, and the lamboidal ridge of the skull.

Although Yeager and Hamre observed no gross or histologic evidence of trauma at the site of the exostoses (7), additional studies noted that muscle tension was required for lesion development, as transected or denervated muscles failed to produce lesions, while lesions did develop at the sites of insertion of muscles that provided functional compensation for transected muscles.(1) Ponseti suggests that lifting of the periosteum is required for exostosis formation.(4)

JPC Diagnosis:  

Long bone, femur, proximal epiphysis, metaphysis and diaphysis: Atypical periosteal fibroplasia, focally extensive, moderate to marked, with reactive new bone and osseous metaplasia.

Conference Comment:  

The contributor provides an excellent review of the effects of BAPN in adult rats. Although this is a rare condition, the case provided an excellent exercise in descriptive pathology, and conference attendees found this to be a very challenging slide. For conference attendees, the predominant differential diagnosis was osteosarcoma based on the proliferation of spindled cells and presence of osteoid. Although a reasonable consideration, especially without knowledge of the case history, the bland nature of the spindle cells, paucity of mitotic figures, and the presence of well-differentiated bone containing normal bone marrow in the nodular masses are not consistent with osteosarcoma. The bilaterally symmetric distribution, of which conference attendees were not aware prior to the conference, also is more consistent with a non-neoplastic process. 

In addition to the detailed description of the lesion provided by the contributor, the moderator and attendees noted foci of new bone formation in the area of the trochanteric fossa and the lateral aspect of the proximal femur. In agreement with the contributors description, the Armed Forces Institute of Pathologys Department of Musculoskeletal and Soft Tissue Pathology summarized the lesion as a broad-based periosteal proliferation of new bone with a poorly-formed fibrocartilagenous cap and an overlying area composed of a large cellular proliferation of fibroblastic and myofibroblastic type cells that in one area blend with tendon and muscle cells; the possibility of an exuberant myofibroblastic reaction to an avulsive injury at a tendon insertion site was considered. 

Ectopic ossification is the neoformation of non-neoplastic trabecular bone at extraosseous sites, and is distinct from ectopic or heterotopic mineralization, which lacks bone formation. Most occurrences of ectopic ossification are common, clinically insignificant findings, e.g. dural ossification (ossifying pachymeningitis) in aged dogs and ectopic bone formation in the lungs of dogs and cattle; or occur in the supporting stroma of certain neoplasms, such as mammary carcinoma in dogs. Two specific diseases associated with ossification of soft tissues are fibrodysplasia ossificans progressiva and myositis ossificans; the former is characterized by progressive, symmetrical ossification of the subcuticular and epimysial connective tissue of the neck, dorsum, and limbs. In contrast, myositis ossificans is characterized by localized and asymmetric lesions containing a peripheral zone of orderly maturation from fibrous tissue to mineralized osteoid, which is replaced by lamellar bone.(6)

The contributor provides a useful description of the effects of BAPN on lysyl oxidase and its similarity to the pathologic effects of copper deficiency. During the conference, the effects of BAPN and similar toxins produced by members of the genus Lathyrus on domestic animals and humans were discussed. The Lathyrus genus is composed of variety of species (e.g. Lathyrus sylvestris, L. sativus); toxins produced by this genus include BAPN (L. odoratus), diamino-butyric acid (L. sylvestris), and beta-oxalyl-diamino-propionic acid (L. sativus). In limited amounts, Lathyrus species of legumes are a nutritious source of protein for domestic animals and humans; however, consumption of large quantities of these legumes over prolonged periods (weeks to months) results in the disease condition referred to as lathyrism. Because of the plants ability to survive in poor soils, flood and drought, outbreaks of lathyrism often occur in impoverished areas of the world during prolonged drought conditions.(3)

In contrast to rats, which develop skeletal deformities referred to as osteolathyrism, ingestion of Lathyrus spp. in humans and most domestic animals results in neurologic disorders termed neurolathyrism. Neurolathyrism is characterized by degeneration and loss of neurons in the spinal cord, resulting in gradual paralysis of the posterior limbs in cattle, horses, pigs, humans, and other species. Peripheral nerves, such as the vagus and recurrent laryngeal, are also affected. In cattle, toxicosis also leads to blindness, torticollis, and skin anesthesia. In horses, paralysis of the recurrent laryngeal nerve results in roaring. Horses and pigs appear to be more susceptible than cattle. Death usually results from respiratory paralysis. Teratogenic effects in sheep and other species and aortic aneurysm in rats and turkeys are other pathologic effects of Lathyrus intoxication.(3)

References:

1. Hamre CJ, Yeager VL: Influence of denervated muscles on exostoses of rats fed a sweet-pea diet. AMA Arch Pathol 65:215-227, 1958
2. Hashimoto N, Handa H, Nagata I, Hazama F: Saccular cerebral aneurysms in rats. Am J Pathol 110:397-399, 1983
3. Jones TC, Hunt RD, King NW: Veterinary Pathology, 6th ed., p. 757. Williams and Wilkins, Baltimore, MD 1997
4. Ponseti IV: Lesions of the skeleton and of other mesodermal tissues in rats fed sweet-pea (Lathyrus odoratus) seeds. J Bone Joint Surg Am 36-A:1031-1058, 1954
5. Tinker D, Rucker RB: Role of selected nutrients in synthesis, accumulation, and chemical modification of connective tissue proteins. Physiol Rev 65:607-657, 1985
6. Thompson K: Bones and joints. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 1, pp. 107-108, 127-129. Elsevier Saunders, Philadelphia, PA, 2007
7. Yeager VL, Hamre CJ: Histology of lathyrus-induced exostoses of rats; the initial changes at tendon-bone junction. AMA Arch Pathol 64:171-185

Click the slide to view.

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2-1. Pelvic radiographs

2-2. Pelvic radiograph

2-3. Bone, femur

2-4. Bone, femur

2-5. Bone, femur

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