Type-selective muscular degeneration promotes infiltrative growth of intramuscular lipoma

Abstract

Background: Intramuscular lipoma is a relatively common benign neoplasm that is occasionally described as an infiltrating lipoma. Typical benign tumors show a clear margin, however, the infiltrative growth pattern of this lipoma mimics that of a malignant tumor. Although its growth has an effect on muscle bundles and it is known to never metastasize, the mechanism of infiltrative growth is not well understood. Previously, little attention has been paid to pathogenic features of muscle fibers around an intramuscular lipoma.

Methods: In the present study, we focused on pathologic changes of the surrounding skeletal muscles especially to the degenerative features of involving muscular types, and evaluate the role of type-selective muscular degeneration for the infiltrative growth of intramuscular lipomas. Following a review of the medical records in our institute, 17 lesions containing muscle tissues in their specimens (15 infiltrating lipomas, 2 well-circumscribed lipomas) were analyzed immunohistochemically. The tumor from the most recent case was also subjected to ultrastructural analysis. Two cases of the traumatic muscle damage were also evaluated as the control experiments.

Results: These analyses revealed type-selective muscle involution in 11 of 17 intramuscular lipomas and in 10 of 11 of the infiltrative type, with an involving pattern that resembled that of a neurogenic or myogenic disorder. Immunoreactivity to cathepsin-D, a lysosomal catabolic enzyme, was increased in the involved muscle fibers. Subsarcolemmal vacuoles in the muscle fibers of the peripheral areas were also positive for cathepsin-D, while degenerative findings were not visually apparent in these areas. Ultrastructural analysis revealed degenerative changes in those fibers. Neither positive staining for cathepsin-D nor type-selective atrophy was detected in the sections of traumatic muscle damage.

Conclusions: Our findings suggest that type-selective muscular degeneration and endomysial fatty growth as a result of atrophy may modulate the infiltrating growth characteristic of intramuscular lipoma.

Figure 1

Intramuscular lipoma specimen. Various degrees of muscle atrophy, as well as pyknotic nuclei or nuclear swelling, shown in the specimen (hematoxylin-eosin stain, ×200).

Figure 2

Immunohistochemical study for the involving muscle fibers. (A) SERCA1-positive fast muscle tissues are dominantly preserved, with the other type prominently decreased (×125). (B) Slow muscle fiber tissues reacting to SERCA2 are conserved, and SERCA2-negative fibers are smaller and angulated (×125). (C) Fast muscle tissues stained by troponin T are shown dominantly atrophic and replaced with fatty tissues (×125).

Figure 3

Cathepsin-D immunohistochemical study. Muscle regions developed sequentially from small subsarcolemmal vacuoles are positive for cathepsin-D (×250).

Figure 4

Histological analyses for the traumatic muscle damage. (A) No atypical finding was demonstrated in the specimen (hematoxylin-eosin stain, ×100). In the immunohistochemical study, neither type-selective atrophy in SERCA1 (×100) (B) nor positive staining for cathepsin-D (×200) (C) was detected in the specimens.

Figure 5

Ultrastructural study. (A) Mitochondria are shown increased in the sarcoplasm (×4000). (B) Glycogen is abundantly accumulated in degenerated muscle fibers (×4000).