Granular cell tumors are reported in a wide range of species and in several anatomic locations. Regardless of species and anatomic location, the majority of tumors have a homogeneous morphologic appearance and benign clinical course.35,38,54
In our study, the immunohistochemical and ultrastructural findings support a heterogeneous population despite their homogeneous light microscopic appearance. As granular cell tumors grow, they may differentiate and take on characteristics that vary between neoplasms. Granular cell tumors were once thought to be myoblastomas because of their relationship to muscle. To date, there is no consensus as to their pathogenesis, and conflicting reports regarding immunostaining of granular cell tumors from any site are common.
In the present study, 3 tumors reacted positively to muscle markers and none of the 4 reacted to the neural or epithelial markers. This suggests a possible myoblastic origin of these cervical granular cell tumors in mice. To address the issue of skeletal muscle origin, PTAH was used for detection of cross striations in the tumor cells compared with a skeletal muscle–positive control. All 4 granular cell tumors were negative when stained with PTAH and did not show cross striations. These data show that the tumors are not striated and suggest a smooth muscle origin.
Our results were similar to previous studies. For example, in 2 avian studies and a canine study there was a positive reaction for the muscle marker desmin.39,40
In a report on cervical granular cells in mice, the Schwann-cell origin was questioned because of the absence of S-100 staining and lack of basal lamina. Another study demonstrated no reactivity to S-100 in the tumor cells of the meninges in Wistar rats.27
In contrast to our findings, uterine cervical and vaginal granular cell tumors in 2 female aged (110-week-old) Wistar rats stained positively for S-100 and negatively for desmin.5
Our results were not completely consistent with the description that Cooper and Valentine reserved for the diagnosis of granular cell tumors in multiple species. They described neoplasms composed of cells containing abundant eosinophilic, PAS-positive and diastase-resistant granules. These neoplasms were immunohistochemically negative for muscle markers and positive for NSE or S-100.4
We verified the negative S-100 and NSE staining of the granular cell tumors in our study using positive control brain sections from each animal. This demonstrated the integrity of these stains despite prolonged formalin fixation of the tissues. Moreover, small amounts of nerve tissue peripheral to 2 of the tumors stained positive for S-100 (mice Nos. 1 and 4) and NSE (mice No. 4), acting as a positive internal control.
In our study, multinucleated giant cells were observed in 2 granular cell tumors and were numerous in 1 of the tumors. The mouse macrophage stain F4/80 was negative in all 4 tumors. Positive internal control macrophages were found in mice Nos. 1 and 4, whereas in mice Nos. 2 and 3 the entire section, consisting mainly of tumor tissue, was negative. Therefore, the multinucleated cells found rarely in mice No. 2 and frequently in mice No. 3 were negative either because they were not macrophage-derived, or because they failed to stain because of long-term exposure to formalin during archival storage.
Two of the tumors in our study were found in untreated control animals from 2 NTP chronic inhalation studies.32,33
Prior to these studies, granular cell tumors in mice were most often observed in the uterine cervix of animals treated with estrogen.17
The data indicate that uterine granular cell tumors are rare in untreated mice and can be produced by treatment with estrogens. The NTP chronic inhalation studies were the first reported studies in which spontaneously occurring granular cell tumors were reported. Subsequently, another study described 9 cases of granular cell tumors in treated B6C3F1 mice from 16 carcinogenicity studies, and the authors considered the tumors to have occurred spontaneously since there was no estrogenic effect.27
However, it was noted these mice were not control animals. Spontaneous uterine granular cell tumors have been described in 1 female Fischer 344 rat and 2 female Wistar rats.5,34
It has been reported that up to 23% of control female rats in carcinogenicity studies have granular cell lesions in the distal reproductive tract.23
In a previous study, the ultrastructure of the granules were described to have amorphous dense material, microvesicles, or lamellae, all representing disorganized material suggestive of digestion as observed in secondary lysosomes.6
Secondary lysosomes are sites of current or past digestion. In mouse No. 2 the cytoplasm was dense with variably sized cytoplasmic granules, each containing heterogeneous electron-dense material. These secondary lysosomes were a distinguishing characteristic of the granular cell tumors evaluated.
The diagnostic criteria of a schwannoma rely on the ultrastructural demonstration of an external basal lamina around neoplastic Schwann cells and their cell processes, as well as desmosomes.6
In schwannoma cells, the basal lamina is reported to be thickened and often folded into long redundant loops. The lack of a distinct basal lamina and absence of desmosomes, in addition to immuno-histochemical findings, supports the contention that these mouse granular cell tumors are not of Schwann cell origin. We did not observe angulate bodies in our study, which have been described in human granular cell tumors.3
Another study found no basement membranes around the tumor cells of meningeal granular cell tumors of rats. The authors concluded that a Schwann cell origin was unlikely, and their findings were more consistent with a meningeal arachnoid cell origin.26
Immunohistochemical and ultrastructural characterizations of a series of granular cell lesions from the distal reproductive tract of female rats did not result in new insights as to the pathogenesis of the lesion. However, additional data were provided in support of these lesions being similar to those of humans, and most likely of neural origin.23
In contrast, the granular cell tumors from the reproductive tracts of mice in our study showed no immunohistochemical (reaction to S-100 or neuron-specific enolase) or ultrastructural characteristics to support a neural origin.
Ultrastructural findings for mice Nos. 2 and 3 also support a nonhistiocytic lineage for these tumors, despite multinucleated giant cells observed in mouse No. 3. This rules out histiocytic sarcoma as an alternative diagnosis for these neoplasms. Histiocytes, especially activated macrophages, might be expected to have numerous secondary lysosomes, but unlike the neoplastic cells described here, the cytoplasm would likely also contain increased numbers of mitochondria, pinocytotic vesicles, enlarged Golgi, and have an undulating membrane. None of these characteristics was seen in these cells.
No ultrastructural characteristics of smooth muscle cells such as spindle shape, bundles of cytoplasmic filaments, or membrane-associated dense bodies were seen. This challenges our suggestion of smooth muscle histogenesis from a strictly ultrastructural view. However, the histochemical and immunohistochemical findings were strongly supportive of smooth muscle origin, and therefore immunohistochemistry may be a better tool for exploring the histogenesis of these tumors.
In conclusion, our study is in agreement with previous findings confirming that granular cell tumors have a large degree of variation despite their homogeneous morphologic appearance and provides additional data to support a possible myogenic origin for these tumors.