Nutritional Secondary Hyperparathyroidism in the Horse

68 Expcrimcntal Nutritional Secondary I-Iypcrparathyroidism in the tIorse . . .
The resorption from VOLKMANN’S canals was liliewise very inconspicuous
in the beginning. In more advanced stages the widening
of the canal could be impressive (Figs. 33,34 and 37). As the resorption
from VOLKMANN’S canals proceeded, the disruption of lamellae of
HAVERSIAN systems became very prominent (Figs. 34 and 36). Osteoclastic
resorption of interstitial lamellae occurred, it appeared, only as a
result of lateral expansion of resorptive processes originating in
HAVERSIAN and VOLKMANN’S canals or subperiosteallg (Figs. 34, 36,
38, and 40).
Longitudinal sections of compact bone showed that the resorption
was very irregular. The width of the HAVERSIAN canals varied within a
wide range, and localized encroachment upon adjacent osteons and
interstitial lamellae was frequently encountered.
The outer circumferential lamellae of compact bone were also
sites of resorption. The normal structure (Fig. 39) was never present in
NSH horses, and in most sections there were hardly any remnants of
such lamellae (Fig. 40). Resorption usually expanded into adjacent
osteons and interstitial lamellae (Fig. 40). In other instances a haphazard
resorption was going on, and numerous lacunae filled with osteoclasts
were then observed. The resorption was often found to be very much
accentuated locally with large excavations into the compact bone and
loss of the osseus support of the periosteum (Fig. 38).
The morphology of the osteoclasts varied. Actively eroding
osteoclasts were usually present in HOWSHIP’S lacunae (Figs. 29, 33,
35, 38) or flattened along trabecular (Fig. 29). Their shape varied
accordingly ; in the former case they were irregularly rounded, in the
latter elongated. The nuclei were relatively large and diffusely scattered
throughout the cells. In areas where mineralized bone was no longer
available for resorption, the nuclei of the osteoclasts were pyltnotic and
retracted centrally or toward one end of the cell (Fig. 30). Inactive
osteoclasts trapped in fibrous tissue sometimes showed vacuolization
of the cytoplasm. In rare instances a phagocytized erythrocyte was
seen in osteoclasts in hemorrhage areas in loose connective tissue.
Fibrosis. The fibrous tissue replacing the resorbed bone varied
considerably in tinctorial properties in different areas. In the jawbones
it was rather well collagenized with a deep-red staining reaction with
van Gieson. Differences in collagenization of the fibrous tissue did
occur, however, in areas of long-standing resorption. The fibrous
tissue replacing the resorbed outer circumferential lamellae was usually
well collagenized (Fig. 37), but the birefringent material (Fig. 40)
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