Bull. Org. mond. San )11974, 50, 101-110Bull. Wid Hith Org.j

VIII. Tumours of the soft (mesenchymal) tissuesE. WEISS 1

This is a classification oftumours offibrous tissue, fat, muscle, blood and lymph vessels,and mast cells, irrespective of the region of the body in which they arise. Tumours offibroustissue are divided into fibroma, fibrosarcoma (including " canine haemangiopericytoma "),other sarcomas, equine sarcoid, and various tumour-like lesions. The histological appearanceof the tamours is described and illustrated with photographs.

For the purpose of this classification " soft tis-sues" are defined as including all nonepithelialextraskeletal tissues of the body with the exception ofthe haematopoietic and lymphoid tissues, the glia,the neuroectodermal tissues of the peripheral and

autonomic nervous system, the paraganglionic struc-tures, and the mesothelial and synovial tissues.

This classification was developed together withthat of the skin (Part VII, page 79), and in describingsome of the tumours reference is made to the skin.

HISTOLOGICAL CLASSIFICATION AND NOMENCLATURE OF TUMOURSOF THE SOFT (MESENCHYMAL) TISSUES

I. TUMOURS OF FIBROUS TISSUE

A. FIBROMA1. Fibroma durum2. Fibroma molle3. Myxoma (myxofibroma)

B. FIBROSARCOMA1. Fibrosarcoma2. " Canine haemangiopericytoma"

C. OTHER SARCOMASD. EQUINE SARCOID

E. TUMOUR-LIKE LESIONS

1. Cutaneous fibrous polyp2. Keloid and hyperplastic scar3. Calcinosis circumscripta

II. TUMOURS OF FAT TISSUE

A. LIPOMA

B. LIPOSARCOMA

III. TUMOURS OF MUSCLE TISSUE

A. LEIOMYOMA

B. LEIOMYOSARCOMA

C. RHABDOMYOMAD. RHABDOMYOSARCOMA

IV. TUMOURS OF BLOOD ANDLYMPH VESSELS

A. CAVERNOUS HAEMANGIOMAB. MALIGNANT HAEMANGIOENDOTHELIOMA (ANGIO-

SARCOMA)C. GLOMUS TUMOUR

D. LYMPHANGIOMAE. LYMPHANGIOSARCOMA (MALIGNANT LYMPH-

ANGIOMA)F. TUMOUR-LIKE LESIONS

V. MESENCHYMAL TUMOURS OFPERIPHERAL NERVES

VI. MAST CELL TUMOUR(MASTOCYTOMA)

VII. CANINE HISTIOCYTOMAVIII. RETICULOSARCOMA(CUTANEOUS RETICULOSIS)

IX. MISCELLANEOUS TUMOURSX. UNCLASSIFIED TUMOURS

1 Veterinar-Pathologisches Institut der Justus Liebig-Universitat, Frankfurterstrasse 94, 6300 Giessen, FederalRepublic of Germany.

a Enzinger, F. M. et al. Histological typing of soft tissuetumours, Geneva, WHO, 1963.

However, as the classification is applicable to softtissue tumours in all parts of the body, it is publishedseparately, as in the case of the WHO classificationof the human tumours.a It is based on the study ofsome 2 500 tumours, mostly from dogs.

3170 101 -8

E. WEISS

DESCRIPTION OF TUMOURS

I. TUMOURS OF FIBROUS TISSUE

A. Fibroma1. Fibroma durum (Fig. 1). This benign tumour

consists of a dense growth of mature fibrocytes andcollagenous fibrous connective tissue. The spindle-shaped or ovoid nuclei of the tumour cells are

relatively poor in chromatin. The collagenous fibresare usually arranged in fascicles, more rarely inwhorls.

2. Fibroma molle (Fig. 2). This benign tumour issoft and has less dense connective tissue. The tumourcells are often stellate. Fat tissue may be present.

3. Myxoma (myxofibroma). This benign tumouris similar to fibroma molle, but shows mucinousmaterial in varying amounts.

B. Fibrosarcoma (Fig. 3)1. Fibrosarcoma. This is a circumscribed or

infiltrating tumour composed of collagenous andreticulin fibres and tumour cells. The tumour cells,spindle-shaped or stellate, are irregularly arranged.They show anisomorphic nuclei and varying num-

bers of mitotic figures. So-called spindle cell sarco-

mas are included under this heading, but the termhas been abandoned in favour of " fibrosarcoma ",since the former always-at least in some areas-

shows collagenous fibres. Usually the diagnosis offibrosarcoma is made only on the basis of thehistological criteria mentioned above, but it shouldbe kept in mind that most of these tumours, althoughthey look malignant, are actually benign. Unfortu-nately the number of canine fibrosarcomas followedup over a long period was too small to allow conclu-sions as to the real degree of malignancy of thesetumours.

2. " Canine haemangiopericytoma" (fibrosarcomawith perithelioma-like structures, perithelioma, haem-angiosarcoma) (Fig. 4-6). Histologically, this tumouris' characterized by spindle-shaped or fibrocyte-like cells arranged in curls, whorls, or finger-print-like patterns. The nuclei are ovoid and hypo-chromatic, and they have a distinct and finely-granu-lated nuclear membrane. The typical feature of thistumour is the onion-skin arrangement of tumourcells around small vessels. The vessels have an openor narrow lumen owing to hyalinization of the vesselwall. However, these structures are present only in

relatively few areas of the tumour. The main tumourmass is similar to a fibrosarcoma. Therefore thetumour should not be called " haemangiosarcoma ".Furthermore the designations " haemangiopericy-toma " and " malignant pericytoma " are not cor-rect, because the onion-skin structures alone furnishno real proof of the origin of the tumour cells frompericytes. A variant of the tumour shows radialarrangement (cuffing) of spindle-shaped hyperchro-matic cells around small blood-vessels (not capil-laries) within areas of mucinous degeneration ornecrosis. The " cuffed " blood-vessels always showopen lumina and frequently hyalinized walls. Theperipheral cells of these " tumour-islands " invadethe surrounding necrotic area like fibroblasts intissue culture. The perithelioma-like structures can beexplained as growth of tumour cells along bloodvessels within necrotic areas. The so-called " caninehaemangiopericytoma" is most often seen in olddogs. The tumour may be up to 25 cm in diameter,greyish-white, firm, and lobulated. It is commonwithin the soft tissue of the skin and muscles,predominantly of the thighs and trunk. Recurrencesafter excision are common (25-50%), whereas meta-stases are frequent only in the necrotic type.

C. Other sarcomasHistologically, these sarcomas are characterized by

the pleomorphism of the tumour cells. Cells similarto undifferentiated reticulum cells, epitheloid cells,and giant cells with one or more nuclei are present.Mitotic figures are common. Collagenous and reti-culin fibres are encountered to varying degrees indifferent parts of the tumour. Larger tumours oftenshow areas of central necrosis. Infiltrative growth oftumour cells into lymphatic clefts and lymph andblood vessels takes place, and metastases occur. Itseems possible that even the ill-defined pleomorphicsarcomas without any fibres are of fibroblasticorigin.

D. Equine sarcoid (Fig. 7, 8)

Histologically, this tumour looks like a fibrosar-coma. In contrast to papilloma, the mesodermalproliferation is not only an adaptation to epidermalgrowth but the predominant part of the tumour. Theoverlying epidermis is commonly ulcerated and sendsdeep pegs into the fibrous mass. Most frequently thetumour is found in the skin about the base of

102

X - :qF N~Fig. 1. Fibroma durum (dog).

Fig. 4. " Canine haemangiopericytoma ".

Fig. 2. Fibroma molle (dog).

Fig. 3. Fibrosarcoma (dog).

Fig. 5. " Canine haemangiopericytoma ". Fig. 6. " Canine haemangiopericytoma ", perivasculargrowth in necrotic area; Gomori's silver stain.

Fig. 7. Equine sarcoid. Fig. 8. Equine sarcoid.

~~~~~~~~~~~~~~~~~~~~~~~~O

Fig. 1 0. Calcinosis circumscripta (dog).

Fig. 12. Liposarcoma (dog).

7, -I .

Fig. 9. Cutaneous fibrous polyp (dog).

Fig. 1 1. Lipoma (dog).

Fig. 13. Leiomyoma (dog). Fig. 14. Rhabdomyosarcoma (dog).

Fig. 15. Cavernous haemangioma (dog).

Fig.16 .Malignant haemangioendothe.Iom (dg)

Fig . 1 6. Malignant haemangioendothelioma (dog) .

Fig. 17. Mast cell tumour; toluidine blue (dog).

1 v hiticta F. 2

Fig. 19. Canine histiocytoma.

Mast cell tumour; toluidine blue (cat).

.-Am

Fig. 20. Canine histiocytoma.

SOFr (MESENCHYMAL) TISSUES

the ears, but it can occur anywhere on the skin. Theinitial lesion is warty (not horny), often multiple, andinfiltrates later in dermis and subcutis. Recurrenceafter excision is common, but metastases do notoccur. The tumour is autotransplantable to scarifiedskin and is presumably virus-induced. In many regionsabout half the skin tumours in horses are sarcoids.

E. Tumour-like lesions1. Cutaneous fibrous polyp (Fig. 9). The cutaneous

fibrous polyp is hairless and stalked or fungoid inshape. Its base consists of solid collagenous fibreswith few cells. The covering epidermis shows super-ficial ulceration and long rete pegs. This lesion is veryrare.

2. Keloid and hyperplastic scar. Both lesions arecharacterized by dense collagenous tissue with ex-tremely few nuclei and pronounced hyalinization offibre fascicles that becomes particularly evident inslides stained according to van Gieson's method. Thecovering epidermis is hyperkeratotic. Clear differen-tiation from true neoplasms such as fibromas ispossible if elastic fibres can be shown by properstaining methods; true neoplasms of connective tis-sue do not show elastic fibres.

3. Calcinosis circumscripta (calcium gout, apocrinecystic calcinosis (Fig. 10). Central deposits of coarsebasophilic granules are surrounded mainly by ma-crophages and giant cells. This is a granulomatouslesion common in young dogs of large breeds and itis most frequent in the skin of the limbs, especiallythe subcutis of the footpads, over bony prominences,but also occasionally in the oral cavity, e.g., thetongue.

II. TUMOURS OF FAT TISSUE

A. Lipoma (Fig. 11)This benign tumour is composed of mature fat

cells. Sometimes there are broad bands of denseconnective tissue between the cells. In large lipomas,areas of necrosis and dystrophic calcification arequite common. Myxoid changes may occur. Lipomashave a predilection for the thoracic region and thighsand are more common in bitches.

B. Liposarcoma (Fig. 12)This malignant tumour is rich in hypochromatic,

ovoid or round nuclei and mitotic figures. Well-differentiated and pleomorphic types occur. Thelatter are more malignant and metastasize morefrequently.

III. TUMOURS OF MUSCLE TISSUE

A. Leiomyoma (Fig. 13)This benign tumour is composed of mature, non-

striated muscle fibres with long match-like nuclei.Collagen formation is present in varying degrees.Special stains (van Gieson's, Mallory's, azan) arehelpful in differentiation from fibromas. The tumouris rare in the skin but common in the female genitaltract and in the alimentary tract.

B. LeiomyosarcomaThis malignant tumour is less well differentiated

and more cellular than leiomyoma. The main histo-logical features are: long hypochromatic and spindle-shaped nuclei, multinuclear giant cells, mitotic figures,and areas of necrosis. Special stains are helpful inmaking a proper diagnosis. The tumour is rare inanimals.

C. RhabdomyomaThis benign tumour is composed of long cells with

vesicular hypochromatic nuclei in syncytial forma-tion. Cross-striations are not always easy to demon-strate. With azan, the cytoplasm is stained yellow;with Mallory's stain, bright red. Glycogen-contain-ing cells are frequent and show a vacuolated pattern.The tumour is rare in animals.

D. Rhabdomyosarcoma (Fig. 14)Well-differentiated and pleomorphic types have

been described. Often cross-striations are not visible.This malignant tumour is very rare in animals.

IV. TUMOURS OF BLOOD AND LYMPH VESSELS

A. Cavernous haemangioma (Fig. 15)Typical histological features of this benign tumour

are cavities lined by a monolayer of endothelium andseparated by varying amounts of fibrous stroma.Very frequently, especially after traumatic insults,thrombi in various stages of organization are en-countered.

B. Malignanthaemangioendothelioma (angiosarcoma)(Fig. 16)These highly malignant tumours have a tendency

towards solid growth of atypical, polymorphic endo-thelial cells and therefore they show a certain simi-larity to fibrosarcomas. The presence of erythrocyteswithin spaces formed by malignant endothelial cells,which always exist in at least some areas of thetumour, establishes the diagnosis. Mitotic figures are

9

109

110 E. WEISS

numerous. Metastases in various organs are com-mon. Malignant haemangioendothelioma is not anundifferentiated anaplastic form of cavernous haem-angioma, but a neoplasm sui generis.

C. Glomus tumourEpithelioid, angioma-like, and neuroma-like types

can be distinguished in man. The cells show a pinkcytoplasm and large oval hypochromatic nuclei, andhence are similar to epithelioid cells. In angioma-liketypes, the tumour cells are arranged around bloodvessels consisting only of endothelium, and, at theearliest stages, resemble normal glomus structures.These benign tumours are very rare in animals.Neuroma-like types have not yet been recognized indogs and there is some doubt whether the otherforms of glomus tumour occur in this species.

D. LymphangiomaThis benign and rare tumour is composed of lymph

vessels forming capillary, cavernous, or cystictumours. Congenital lymphangiomas (malforma-tions) are known in animals as well as in man.

E. Lymphangiosarcoma (malignant lymphangioendo-thelioma)This malignant tumour is extremely rare in ani-

mals.

F. Tumour-like lesionsVaricosis and ectasia of scrotal veins can be con-

sidered as tumour-like lesions of blood-vessels. Theyshow some similarities with angiokeratoma (Mibelli)of man. Especially ectasia of veins can be confusedwith cavernous haemangioma. The typical localiza-tion of the lesions in the scrotum is an importantcriterion for differential diagnosis.

V. MESENCHYMAL TUMOURS OF PERIPHERAL NERVES

(See Part V. Tumours of the nervous system,page 53.)

VI. MAST CELL TUMOUR (MASTOCYTOMA)This malignant tumour (Fig. 17, 18) is composed

of mast cells showing a wide range of granulationboth among and within cases. The cells are round,

oval, polygonal, or elongated and have oval nuclei.The cytoplasm appears pale red and slightly granularin haemotoxylin and eosin sections. Some tumoursshow very poor granule formation, and diagnosis ishelped by staining with toluidine blue. In less welldifferentiated mast cell tumours, bizarre cells-whichmay be multinucleated-may occur. Metachromasiaof granules is often slight.Mast cell tumours are the most common mesen-

chymal cutaneous neoplasms of dogs and occur veryfrequently in the boxer breed. They occur also in catsand less frequently in other species. Mast cell tumoursmay be multifocal, and recurrences and metastasescommonly lead to systemic involvement (malignantmastocytosis).a

VII. CANINE HISTIOCYTOMA

The cells of this benign tumour (Fig. 19, 20) arerelatively small and seem to form a syncytium. Thenuclei are mostly round and hypochromatic. Mitoticfigures and regressive changes are common. Thetumour is frequent in young dogs, especially on thehead and extremities. Ordinarily, spontaneous re-gression occurs.

VIII. RETICULOSARCOMA (CUTANEOUS RETICULOSIS)

These malignant tumours, which may be single ormultiple, are szen in old dogs. They resemble histiocy-tomas (VII), but the nuclei of the tumour cells aremore polymorphic and hyperchromatic. Probablythese tumours are special forms of lymphosarcoma.a

IX. MISCELLANEOUS TUMOURS

Certain tumours classified as mainly affectingother body sites or systems sometimes involve themesenchymal tissues of the skin. Lymphosarcoma,especially in cattle, is sometimes localized in theskin.a Canine transmissible venereal tumour, al-though most commonly located on the genitalia, insome cases occurs in the skin (this tumour will beincluded in the classification of the male genitalsystem, to be published later).

a See Part II. Neoplastic diseases of the haematopoieticand lymphoid tissues, page 21.