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Epididymal Histiocytic Sarcomas Identified in B6C3F1 Mouse Carcinogenicity
Studies Conducted by the National Toxicology Program
John Curtis Seely1, Kyathanahalli S. Janardhan2, Jerry F. Hardisty1, Gabrielle Willson1, and Ron Herbert3
1Experimental Pathology Laboratories, Inc., Research Triangle Park, North Carolina, USA
2Integrated Laboratory Systems, Inc., Research Triangle Park, North Carolina, USA
3National Toxicology Program, NIEHS, Research Triangle Park, USA
Spontaneous primary or metastatic neoplasms of the
mouse epididymis are uncommon in historical control
data (Haseman et al. 1998; Baldrick and Reeve 2007).
Histiocytic sarcomas (HS) of the epididymis have been
reported in B6C3F1 mice (Itagaki et al. 1993; Shiga
1994) and described in the INHAND document on the
male reproductive system (Creasy et al. 2012). The
recognition and incidence of epididymal HS may become
important when evaluating an unusual or potential test
article-related incidence observed in a carcinogenicity
study (Yano et al. 2008). In addition, primary epididymal
HS in the mouse may present as a confounding
histological appearance in a tissue not commonly
associated with the usual tissue distribution of HS.
Therefore, this retrospective study was conducted to
provide additional information regarding the morphology
and incidence of epididymal HS in the B6C3F1 mouse.
A database search was conducted for the presence of
epididymal HS in B6C3F1 mouse studies from the NTP’s
Toxicology Data Management System Enterprise
(TDMSE). The database surveyed cases of epididymal
HS from 1982 onward in long term carcinogenicity
studies. The database included mice from both control
and treated groups. Individual animal study records and
selected H&E stained slides were retrieved from the NTP
Archives. From the study records and for categorization,
epididymal HS were determined to be a primary
neoplasm when HS was only diagnosed in the epididymis
or part of a multi-site neoplastic process when tissues
other than the epididymis contained HS.
In order to further investigate the histogenesis of the
epididymal lesions 21 selected cases diagnosed as
epididymal HS were further studied by F4/80 antibody
Primary or metastatic neoplasms of the mouse
epididymis are uncommonly reported from National
Toxicology Program (NTP) long term carcinogenicity
studies. However, epididymal histiocytic sarcomas (HS)
have been diagnosed as either a primary neoplasm or
part of a multi-site neoplastic disease. HS in mice are
usually recognized as infiltrative tumors largely limited to
organs such as the liver, skin, lymph nodes, or uterus. In
some cases, HS becomes wide spread involving many
sites. This retrospective study was undertaken to
investigate the presence and incidence of epididymal HS
in NTP carcinogenicity studies using the B6C3F1 mouse.
The histologic and immunohisto-chemical (IHC) staining
pattern of epididymal HS from NTP cases was consistent
with previous reports of the appearance and
immunostaining patterns of HS. In general, epididymal
HS in the B6C3F1 mouse appears to have a strong
predilection for the epididymal cauda as observed in
other mouse strains.
• 67 cases of epididymal HS were identified from 40,
4046 male B6C3F1 mice, including controls and
treated animals, from long term NTP studies. Overall,
the incidence was 0.17%.
• The 67 cases were categorized as either a primary
neoplasm or as part of a multi-site neoplastic disease.
53 cases were associated with HS in other tissues and
14 cases were diagnosed as primary neoplasms.
• 14 epididymal HS cases were identified from 10,651
control animals. 4 of these cases were diagnosed as
• Histological features of epididymal HS varied but
resembled typical HS observed in other tissues.
• Immunohistochemistry using F4/80 antibody was
positive in the HS cases examined. IHC is
recommended in problematic cases where cell
identification is challenging. F4/80 is a reliable marker
The TDMSE database search for epididymal HS surveyed
approximately 40, 046 male B6C3F1 mice from long term
carcinogenicity studies. The database included 10,651
controls and 29,395 test article treated animals exposed to a
wide range of chemicals. The routes of administration
represented gavage, respiratory exposure whole body, dosed
water, dosed feed, skin application, in-utero, dermal solution,
and magnetic fields.
67 cases of epididymal HS, primary and multi-site, were
identified. Out of those 67 cases, 53 cases were diagnosed
as part of a multi-site neoplastic disease and 14 cases were
diagnosed as primary epididymal neoplasms. The overall
incidence of epididymal HS was approximately 0.17%.
From the 67 cases identified 14 cases of epididymal HS were
present in control males. Out of the 14 control cases, 4 cases
of HS represented primary epididymal HS.
During the review of the study records, epididymal HS were
often grossly apparent as a traceable gross lesion. In the
majority of cases, the epididymis was described as enlarged
and, in general, appeared as unilateral lesions.
Microscopically, epididymal HS appeared as irregularly sized
densely cellular masses that were nearly always located within
or near to the epididymal cauda. Large neoplasms often
contained hemorrhage and necrosis (Fig. 1). Generally,
smaller tumors were well contained within the epididymis and
lacked significant hemorrhage and necrosis (Figs. 2 and 3).
Tumor cells infiltrated interstitial tissue gradually displacing
tubules as the neoplasm expanded in size (Fig. 4). HS cells
appeared as densely packed round to oval to spindle-shaped
mononuclear cells with variable amounts of indistinct pale
eosinophilic cytoplasm (Fig. 5). Nuclei were generally oval to
pleomorphic with one or more nucleoli. Multinucleated tumor
cells were present but their numbers were inconsistent
throughout sections. The presence of tumor cell mitoses was
variable. In some cases, neoplastic cells were seen to have
invaded the capsule spreading to capsular fat (Fig. 6).
Other microscopic findings associated with epididymal HS
included mononuclear cell infiltrates and infrequent cases of
sperm granulomas (Fig. 7).
All epididymal neoplasms with a diagnosis of HS stained
positively with the F4/80 antibody. Staining was cytoplasmic
and membranous. In most instances the staining was
diffusely present in the majority of the neoplastic cell
population (Fig. 8).
The authors would like to thank the following people who helped with
various phases of this study: Kylie Brockenfelt and Johnnie Carden
(EPL) for retrieval of the NTP study material, Emily Singletary (EPL)
for photography support, Lois Wyrick of Image Associates for the
graphic design of this poster, and the histology and immunohisto-
chemistry core laboratories, Cellular and Molecular Pathology
Branch for their help.
Histiocytic sarcoma in the mouse comprises a
pathological entity that is generally characterized as a
primary to multi-site neoplasm which can either involve
one or more specific organs such as the liver, skin, lymph
nodes, and uterus or can be widely disseminated (Frith
1990). HS has been recognized as a neoplasm of the
mononuclear phagocytic system by the immunohisto-
chemical expression of specific antigens (Ward and
Sheldon, 1993; Hao, et al. 2010).
Epididymal HS have only been observed in mice and are
considered to be equivalent to the mouse uterine HS
(Creasy et al. 2012).
The histological features of epididymal HS appear similar
to HS occurring in other organs or tissues. The strong
predilection of the tumor cells for the epididymal cauda
Interstitial cell (Leydig) tumors of the mouse epididymis
which described histological features closely resembling
HS tumors were reported in animals which lacked
primary testicular tumors (Mitsumori et al. 1989).
However, the male reproductive INHAND document
suggests that the reported Leydig cell tumors may be
epididymal HS and IHC markers are recommended to
provide a definitive diagnosis (Creasy et al. 2012).
Antibodies to F4/80 antigen have been shown to be a
reliable, sensitive and specific marker to identify
macrophages in mice and to help with identification of HS
(Austyn and Gordon 1981; Hao, et al. 2010).
Figure 1. Epididymal HS (control, multi-site
neoplasm). Large mass involving the
epididymal cauda. H&E, original
Figure 2. Epididymal HS (control, multi-site
neoplasm). Neoplastic cells infil-trating and
separating tubules. H&E, original
Figure 3. Epididymal HS (control, primary
neoplasm). Similar infiltrative pattern as
Figure 2. H&E, original magnification 4X
Figure 4. Epididymal HS (control, primary
neoplasm). Dense population of neoplastic
cells with focal mononuclear cell infiltrates.
H&E, original magnification 10X
Figure 5. Epididymal HS (control, multi-
site neoplasm). Monomorphic cell
population with indistinct eosinophilic
cytoplasm and slight nuclear
pleomorphism. H&E, original magnification
Figure 6. Epididymal HS (control, multi-
site neoplasm). Capsular invasion and
infiltration of epididymal fat by HS cells.
H&E, original magnification 20X
Figure 7. Epididymal HS (low dose, multi-
site neoplasm). Sperm granuloma (upper
left of image). H&E, original magnification
Figure 8. Epididymal HS (control, multi-site
neoplasm). Positive cytoplasmic and membranous
staining. F4-80 IHC, original magnification 40X