Ultrastructural cytology of human osteosarcoma cells

Microenvironment matters: insights from the FOSTER consortium on microenvironment-driven approaches to osteosarcoma therapy

Osteosarcoma (OS), a prevalent malignant bone tumor, has seen limited progress in treatment efficacy and patient outcomes over decades. Recent insights into the tumor microenvironment (TME) have revealed its crucial role in tumor progression and therapeutic resistance, particularly in OS. This review offers a comprehensive exploration of the OS microenvironment, meticulously dissecting its crucial components: the mesenchymal stromal TME, the immune microenvironment, hypoxia-induced adaptations, and the impact of the physical microenvironment. By demonstrating how these elements collectively drive tumor proliferation, immune evasion, and invasion, this review explores the intricate molecular and cellular dynamics at play. Furthermore, innovative approaches targeting the OS microenvironment, such as immunotherapies, are presented. This review highlights the importance of the TME in OS progression and its potential as a source of novel therapeutic strategies, offering new hope for improved patient outcomes.

h-Caldesmon as a specific marker for smooth muscle tumors. Comparison with other smooth muscle markers in bone tumors

Caldesmon is a protein widely distributed in smooth and non-smooth muscle cells and is thought to regulate cellular contraction. Its isoform, high-molecular-weight caldesmon (h-CD), was demonstrated to be specific for smooth muscle cells and smooth muscle tumors of the soft tissue and to never be expressed in myofibroblasts. We performed an immunohistochemical study to examine h-CD expression in the following bone tumors: conventional and non-conventional osteosarcoma, 13; malignant fibrous histiocytoma of bone, 5; giant cell tumors of bone, 5; chondroblastoma, 3; metastatic leiomyosarcoma, 2; and rhabdomyosarcoma, 1. Frequent immunoreactivity for muscle actin (alpha-smooth muscle actin or muscle-specific actin) was seen in 11 of 13 osteosarcomas and all other tumors, whereas h-CD was expressed intensely only in 2 leiomyosarcomas. h-CD is considered a specific and useful marker to distinguish smooth muscle tumor from bone tumors with myoid differentiation.

Small cell osteogenic sarcoma of the ribs: cytological, immunohistochemical, and ultrastructural study with literature review

Small cell osteosarcoma (OS) is a rare variant of OS that is composed of small cells resembling those of Ewing's sarcoma (ES) with recognizable osteoid. This type of tumor often creates difficulty in making a diagnosis when tissue samples do not include osteoid. The frequent sites are long bones and until now there have been no reported cases arising in the ribs. A case is reported here of small cell OS occurring in the ribs of a 37-year-old female with its aspiration cytologic and electron microscopic characteristics. In the cytologic smear, the small round neoplastic cells were individually scattered or arranged in small nests. The nuclei were hyperchromatic and oval with no visible nucleoli. Ultrastructurally, the nuclei had a round or oval euchromatic chromatin pattern and occasional nucleoli. The scanty cytoplasm contained a small quantity of organelles including either tubular or dilated cisternae of RER, a few mitochondria, and free or polyribosomes. Other organelles were absent. Although the electron microscope sample of this case did not include bone mineral (hydroxy apatite), the electron microscopic features of the tumor cells were unique and useful for exclusion of other small round cell neoplasms.

Mandibular osteosarcoma with unusual expression of alpha-actin smooth muscle antibody

Mandibular osteosarcoma is a rare neoplasm, appearing in young adults as a mass often accompanied by pain, occasionally with paraesthesias, gingival haemorrhages and mobility of teeth, and rarely with ulceration of the skin or mucosa. Roentgenological lytic and/or sclerotic findings are non-specific. Some patients have a previous history of Paget's disease, antecedent radiation treatment, or fibrous dysplasia. Malignant osteoid production can be minimal, resulting in diagnostic difficulties. We report a case of a 33 year-old woman, who, in the absence of a clinical soil associated with the condition, was presented with advanced disease. Immunohistochemical study using alpha-actin smooth muscle antibody disclosed bizarre tumor cells with "dendritic" appearance, to our knowledge, never described thus far. This report draws attention to this uncommon neoplasm and reviews the pertinent literature related to this entity.

Immunohistochemical detection of bone morphogenetic proteins in bone and soft-tissue sarcomas

BACKGROUND

Bone morphogenetic proteins (BMPs) are potent inducers of bone formation. Functional and immunohistochemical studies have identified BMPs in a subset of osteosarcomas. In the present study, the authors extend the analysis of BMP expression to other bone and soft tissue sarcomas.

METHODS

Monoclonal antibody AbH3b2/17 against human BMP-2 and BMP-4 was used in avidin-biotin-immunoperoxidase assays with frozen sections of bone tumors (71 specimens), soft tissue sarcomas (69 specimens), and normal tissues.

RESULTS

Among bone tumors, BMP was detected in osteosarcomas (17 of 29 samples), malignant fibrous histiocytomas (MFHs) (6 of 6), and the spindle cell sarcomatous components of spindle cell (dedifferentiated) chondrosarcomas (4 of 4), but not in conventional chondrosarcomas (0 of 10) or Ewing's sarcomas (0 of 14). Histologic subtypes of osteosarcoma differed for BMP expression, with 8 of 9 fibrohistiocytic, 9 of 13 osteoblastic, and 0 of 5 chondroblastic lesions showing immunostaining. In all BMP-positive bone tumors, immunostaining was localized in the cytoplasm of primitive mesenchymal cells, with little or no staining in tumor matrix and more mature osteoblastic/chondrocytic cells. Among soft tissue sarcomas, MFHs (11 of 12), liposarcomas (5 of 11), leiomyosarcomas (3 of 9), and malignant schwannomas (3 of 8) showed cytoplasmic BMP immunostaining. Synovial sarcomas (0 of 9), rhabdomyosarcomas (0 of 8), and fibrosarcomas (0 of 7) were BMP-negative. All normal human tissues tested, including the tissues of a 16-week-old fetus, lacked BMP immunoreactivity.

CONCLUSIONS

Bone morphogenetic protein is expressed in a subset of osteosarcomas, a high proportion of MFHs of bone and soft tissue, and in spindle cell chondrosarcomas. In these tumors, BMP is localized predominantly to the cytoplasm of malignant cells with primitive mesenchymal features; no or little BMP is detected in the more differentiated elements of bone and soft tissue sarcomas. Different histologic types of bone and soft tissue sarcomas may mimic discrete stages of mesenchymal differentiation as defined by BMP expression and histologic criteria.

Expression of bone morphogenetic proteins in human osteosarcoma. Immunohistochemical detection with monoclonal antibody

BACKGROUND

Bone morphogenetic proteins (BMP) induce ectopic bone formation in vivo and may play a role in normal bone development. In addition, bone morphogenetic activity, as measured in a bone-forming assay in immunodeficient, athymic nu/nu mice, is present in a proportion of osteosarcomas; this activity, which may be mediated by BMP, is correlated with a poor prognosis.

METHODS

The development of a monoclonal antibody against recombinant human BMP-2, AbH3b2/17, has allowed immunohistochemical localization of BMP in tumor tissues. Cryostat sections of osteosarcomas (21 tumor samples), chondrosarcomas (5 samples), and Ewing's sarcomas of bone (5 samples) were examined with AbH3b2/17 using the avidin-biotin-immunoperoxidase method.

RESULTS

The authors found AbH3b2/17 immunoreactivity in 12 of the 21 osteosarcoma samples (57% sensitivity) obtained from 20 patients. For one patient, samples of the primary lesion and a subsequent metastasis were tested, and only the latter showed AbH3b2/17 immunoreactivity. The chondrosarcomas and Ewing's sarcomas examined showed no immunoreactivity. In antigen-positive osteosarcomas, AbH3b2/17 immunostaining was localized predominantly in the cytoplasm of tumor cells. Moreover, the proportion of AbH3b2/17-reactive cells varied among osteosarcomas with disparate histologic features.

CONCLUSIONS

The authors identified a rapid and widely applicable method for detecting BMP expression in intact tissues, which may complement and enhance the bone-forming assay in nu/nu mice as a prognostic procedure in osteosarcomas.

Morphological characterization of a newly established human osteosarcoma cell line, HS-Os-1, revealing its distinct osteoblastic nature

A newly established human osteosarcoma cell line, HS-Os-1, from an osteoblastic tumor arising in the left humerus of an 11-year-old girl was morphologically characterized in vitro and in vivo. HS-Os-1 cells in a monolayer have been maintained for more than 2 years since the initial cultivation, and were round or polygonal in shape with marked pleomorphism. Their cytoplasm was strongly positive for specific markers of osteoblasts, such as alkaline phosphatase and osteocalcin. Tumors induced in nude mice by HS-Os-1 cell inoculation at passage 12 or 23 revealed typical histological features of osteoblastic osteosarcoma, similar to those observed in the original tumor, producing prominent osteoid matrix with calcification. Ultrastructurally, HS-Os-1 cells in vitro and tumor cells in vivo showed similar well-developed, markedly dilated rough endoplasmic reticulum, polysomes and microfilaments in their cytoplasm. Additionally, many collagen fibers associated with deposition of electron-dense material were detected in the stroma featuring osteoid matrix. Thus, the HS-Os-1 cell line was shown to exhibit its osteoblastic nature in vitro and in vivo, and therefore might become an extremely useful tool for various pathomorphological investigations on human osteosarcomas.

Immunophenotypic heterogeneity in osteosarcomas

Eighteen osteosarcomas were studied immunohistochemically. The tumors were classified into the following six histologic subtypes: five osteoblastic, four chondroblastic, four malignant fibrous histiocytoma-like, two telangiectatic, two low-grade central, and one giant cell-rich. Variable amounts of osteocalcin immunoreactivity were found in all tumors. Factor XIIIa-positive cells, which may be of fibrohistiocytic lineage, were present in three tumors of the malignant fibrous histiocytoma-like type, one of the telangiectatic type, one of the low-grade central type, and the tumor of the giant cell-rich type. One tumor of the osteoblastic type showed cytokeratin and epithelial membrane antigen immunoreactivities. The positive reactions for desmin in four tumors, for alpha-smooth muscle actin in 11 tumors, and for type IV collagen in one tumor seemed to indicate myofibroblastic differentiation of some tumor cells. S-100 protein-positive tumor cells were detected not only in all four tumors of the chondroblastic type, but also in three of the osteoblastic type, one of the low-grade central type, and in the tumor of the giant cell-rich type. These immunohistochemical results suggest that osteosarcomas are composed of heterogeneous cell populations, such as those of the osteoblastic, chondroblastic, myofibroblastic, and fibrohistiocytic types, and occasionally also of cells with epithelial features.

Immunohistochemical and biochemical studies on the collagenous proteins of human osteosarcomas

The distribution of type I, II, III, IV, V and VI collagens in 20 cases of osteosarcoma was demonstrated immunohistochemically using monospecific antibodies to different collagen types. In addition, biochemical analysis was made on collagenous proteins synthesized by tumor cells in short-term cultures obtained from seven representative cases and compared with dermal fibroblasts. In osteoblastic areas, most of the tumor osteoid consisted exclusively of type I collagen. Type V collagen was associated in some of them. Type III and type VI collagens were mainly localized in the perivascular fibrous stroma. Cultured tumor cells from osteoblastic osteosarcomas produced type I collagen exclusively and small amount of type V collagen constantly, while the synthetic activity of type III collagen was extremely low. In contrast, fibroblastic areas were characterized by the codistribution of type I, III, VI collagens and chondroblastic areas by type I, V, VI collagens as well as type II. Furthermore, type IV collagen was demonstrated in the stroma, other than the basement membrane region of blood vessels, in fibroblastic, intramedullary well-differentiated and telangiectatic osteosarcomas. In vitro, the production of variable amounts of type IV collagen, which was not detected in cultured dermal fibroblasts, was also recognized in the osteoblastic, fibroblastic, undifferentiated and intramedullary well-differentiated osteosarcomas examined. These findings suggest that the immunohistochemical approach using monospecific antibodies to different collagen types is useful not only in identifying some specific organoid components, such as tumor osteoid, but also in disclosing the biological properties of osteosarcoma cells with diverse differentiation.

Osteosarcoma

Osteosarcoma is the most common bone tumor of children and adolescents. The peak incidence of the disease is in the 15 to 19 year age group. The disease is more commonly seen in males than females. While several factors, including exposure to radiation, genetic disorders such as retinoblastoma, and high rate of bone growth, have been associated with osteosarcoma, in most cases no definite etiology can be established. Osteosarcoma usually originates in the metaphyseal region of long bones and extends through the cortex, causing varying degrees of bone destruction and expansion of periosteum. The radiographic appearance caused by this process is often referred to as "sun burst" sign. Positive diagnosis of osteosarcoma is made by histopathology. The histopathological classification of osteosarcoma can also predict the degree of aggressive behavior of this tumor and thus has prognostic significance. Surgery, including amputation or limb-salvage procedure, is the mainstay of treatment of osteosarcoma. It is now unequivocally established that adjuvant chemotherapy will prolong the survival of patients with this disease. Chemotherapy agents often used include platinum derivates, methotrexate, vincristine, cyclophosphamide, adriamycin, actinomycin D, bleomycin and DTIC. Depending on surgical decision, these agents can be used prior to or after the operation. Immediate fitting with prosthesis and provision of appropriate medical and psychological support in the care of these patients is essential.

Osteosarcoma. Ultrastructural and immunohistochemical studies on alkaline phosphatase-positive tumor cells constituting a variety of histologic types

The osteosarcomas were subclassified into osteoblastic, fibroblastic, chondroblastic and telangiectatic types and examined by electron microscopy. Their immunohistochemical reactions were also studied. In an overall survey of the above types, fibroblast-like cells revealed poorly developed cytoplasmic organelles with rather short, branching rough endoplasmic reticulum, mixed with osteoblast-like cells that were hardly distinguishable from the former. They appeared to be an early stage of an osteoblastic cell lineage from the distribution and development of their cell organelles and highly positive vimentin activity. The tumor cells in malignant cartilage varied in appearance from chondroblast-like to osteoblast-like cells. All types of tumor cells expressed alkaline phosphatase activity to a significant degree. Immunohistochemical staining showed a mixture of procollagen type I-positive cells among the cells positive for both procollagen type II and S-100 protein in the malignant cartilage. Irrespective of any ultrastructural differences between these various tumor cell types, they all revealed a significant degree of ALPase activity unlike other types of bone tumors, suggesting that the tumor cells which constitute the various types of osteosarcoma are derived from a common precursor cell.

Cellular junctions in a spectrum of human malignant tumors

Cellular junctions in tumors are often considered a hallmark of epithelial differentiation. However, junctions are also seen in tumors having a different differentiation. This observation prompted us to study cellular junctions in malignant nonepithelial tumors. We found a variety of cellular junctions in such tumors, although the majority were poorly formed. This observation is of importance for diagnostic purposes. We have also tried to clarify the nomenclature of cellular junctions as applied in tumor diagnosis by proposing a systematic categorization of terms in everyday use by pathologists and by referring more extensively to the term paired subplasmalemmal densities (PSD) for non-well-formed junctions.

Differential histologic diagnosis of osteoid. A study on human osteosarcoma collagen by the histochemical picrosirius-polarization method

Using only one histologic preparation and under the light microscope, the simple method described here permits a distinction between osteoid and other tissue components that can be mistaken for osteoid. Osteoid from both normal osteogenesis and osteosarcomas is disclosed by the picrosirius-polarization method as a three-dimensional network of randomly arranged, thin, short, weakly birefringent collagenous structures which shine against a dark background. These morphologic features can be used as a precise diagnostic criterion for the differential diagnosis between osteoid and other materials which resemble osteoid by other staining techniques. This precise characterization of osteoid is of great importance since the presence of osteoid is used as a criterion for the differential diagnosis of osteosarcomas. Not only does this method permit the precise characterization of osteoid but it is also useful for studying collagen distribution in osteosarcomas. In this regard the foregoing results agree with the immunofluorescence observations recorded in the literature.

Human osteogenic sarcoma: fine structure of the fibroblastic type

The fine structure of representative regions of nine fibroblastic osteogenic sarcomas was studied. As judged by light microscopic criteria, the tumors represented both highly malignant (grades 3-4) and less malignant (grade 2) varieties. By electron microscopy, six basic cell types were found in the selected regions (fibroblastlike, histiocytelike, and myofibroblastlike cells, along with xanthoma cells, multinucleated giant cells, and undifferentiated cells). In addition, occasional osteoblastlike cells were encountered. Fibroblastlike cells in general, and especially in grade 2 tumors, showed a fine structure, enabling differentiation from osteoblastlike cells. Multinucleated giant cells were of two morphologic types, and the fine structure appeared to be related to the malignancy potential and differentiation of the tumors. Many multinucleated giant cells in grade 2 tumors had a ruffled border and appeared to be highly active in digestive events (especially phagocytosis of whole cells and portions of cells). Accumulation of variable amounts of lipid in droplet form was common in the various types of cells present in the tissues. The observations were discussed with particular emphasis on the interrelationships and functional roles of the cells.

Human osteogenic sarcoma: fine structure of the chondroblastic type

The fine structure of representative regions of four chondroblastic osteogenic sarcomas was studied. These regions contained four morphologically distinguishable subtypes of chondroblastlike cells. In addition, multinucleated giant cells, fibroblastlike cells, and macrophagelike cells were present, along with small populations of unclassifiable cells forming at least two subgroups of cells likely to be of a neoplastic nature. With only one exception, all types of chondroblastlike cells were separated by wide zones of extracellular matrix. The large multinucleated cells showed a fine structure that differed from that seen in multinucleated giant cells of other tissues. The evidence suggested that the multinucleated cells in the chondroblastic osteogenic sarcomas were active in phagocytic functions. It is not clear whether or not they are neoplastic in nature. Osteoblastlike cells were not encountered in the chondroid areas of the osteogenic sarcomas studied. On the basis of the findings it is concluded that the observed fine structural polymorphism of the chondroblastlike cells may reflect differences in maturation and differentiation among these cells. The most well-differentiated cells (type 1) appear to be able to exert secretory functions.

Biologic characterization of human bone tumors. II. Distribution of different collagen types in osteosarcoma--a combined histologic, immunofluorescence and electron microscopic study

Sixteen cases of typical highly malignant osteosarcoma were investigated by light, electron, and immunofluorescence microscopy to demonstrate the presence of collagen types I-III. It was shown that, in light-microscopically anaplastic areas of the tumor, collagen type III predominates, while only very few membranes of collagen type I are observed. Ultrastructurally, the cells are characterized by numerous free ribosomes in their cytoplasm and only a few membranes of granular endoplasmic reticulum (ER). In osteoblastic areas, collagen type I is increased, while type-III collagen is decreased. The cytoplasm of cells contains markedly more granular ER. An increasing mineralization of matrix is observed. In fibroblastic areas of the tumors, collagen types I and III are codistributed. Tumor cells have a fibroblast appearance with elongated nuclei and well developed granular ER. The chondroblastic areas, characterized by immature neoplastic cartilage, contain varying amounts of collagen type II. Chondroblast-like tumor cells have typical ring-shaped membranes of granular ER in their cytoplasm. The evidence of different collagen types in osteosarcomas lends additional support to the concept that a pluripotent mesenchymal cell is the stem cell of osteosarcomas.

Human osteogenic sarcoma: fine structure of the osteoblastic type

The fine structure of representative regions of 13 osteoblastic osteogenic sarcomas was studied. These regions contained four morphologically distinguishable subtypes of osteoblastlike cells. In addition, fibroblastlike and chondroblastlike cells were present, along with multinucleated giant cells, leukocytes, macrophagelike cells, and small populations of histogenetically unclassifiable (but probably neoplastic) cells. The morphologic evidence was compatible with the view that the variations in appearance among the subgroups of osteoblastlike cells reflected differences in maturation and differentiation of these cells. In at least one subgroup, the morphologic findings suggested that the cells were capable of manufacturing a secretory product. The multinucleated giant cells occurring in genuine tumor areas appeared to be closely related to neoplastic osteoblasts. The presence of chondroblastlike cells in the tissues illustrates that cells with a diverging differentiation can occur in an osteoblast-dominated cell population. This agrees with the view that the neoplastic cells originate from a mesenchymal stem cell with potential for multifaceted differentiation.

The ultrastructure of conventional, parosteal, and periosteal osteosarcomas

Twenty-seven osteosarcomas of bone were examined by electron microscopy. Eighteen cases were conventional osteosarcomas, one case was an osteosarcoma that developed in Paget's disease of bone, two cases were metastasis to the lungs of conventional osteosarcomas, five were parosteal osteosarcomas (and the recurrence of one of them), and one was periosteal osteosarcoma. In addition to the basic neoplastic cell, the osteoblast-like tumor cell, seven tumor cell types could be differentiated in varying numbers in the 27 cases examined: (1) chondroblast-like; (2) fibroblast-like; (3) histiocyte-like; (4) myofibroblast; (5) osteoclast-like; (6) malignant multinucleated osteoblast-like; and (7) atypical primitive mesenchymal cells. Moreover, endothelial cells and pericytes were also present. The tumor cells showed common neoplastic features in relation with general cell configuration and fine structures. No cytoplasmic organelle was considered to be cell-specific, and only the amount and distribution of the organelles allowed us to differentiate the above mentioned cell types. The presence of malignant appearing tumor cells with abundant and dilated cisternae of RER, in an intercellular matrix containing collagen fibers with areas of focal collections or puffs of hydroxyapatite crystals, are the most characteristic ultrastructural features of osteosarcoma. However, those features are not pathognomonic and do not allow in all cases the differential diagnosis with osteoid-osteoma and benign osteoblastoma. Fields of chondroblastic, fibroblastic, or fibrohitiocytic character are indistinguishable of chondrosarcomas, fibrosarcomas, or malignant fibrous histiocytoma of bone, respectively. The comparative study of parosteal and conventional osteosarcomas revealed a marked difference in the cell population: fibroblast-like cells and myofibroblasts were the main cell types in parosteal osteosarcoma; on the contrary, myofibroblasts were very scarce or absent in conventional osteosarcomas. This difference may correlate with the better outcome of parosteal osteosarcoma. The ultrastructural features of a periosteal osteosarcoma are reported for the first time. The tumor was comprised mostly of chondroblast-like tumor cells. However, in the more cellular peripheric areas of the neoplasm, the cells were predominantly of the fibroblast-like type and showed anaplastic features.

Ultrastructure of human osteosarcoma. Malignant transformation of a multipotential connective tissue cell

Ten cases of osteosarcoma were studied by electron microscopy. The tumors consisted of six cell types: fibroblastic, myofibroblastic, chondroblastic, osteoblastic, unclassified and histiocytic cells. Disturbed structure of dilated endoplasmic reticulum was a common feature. The neoplastic character of myofibroblastic and histiocytic cells is controversial. Myofibroblastic differentiation was most abundant in parosteal osteosarcoma and in fibrosarcomatous intraosseal osteosarcoma. The malignant cells sometimes formed giant cells and many aggregates of these cells were seen. Osteoclasts and other reactive cells were encountered and this may indicate host reaction against the tumor cells. Formation of collagenous and cartilaginous ground substance was poor, and the capacity of collagen to mineralize was decreased. It is concluded that osteosarcoma is the malignancy of a multipotential connective tissue cell which forms callus in normal osteogenesis.

Human osteogenic sarcoma. Study of the ultrastructure, with special notes on the localization of alkaline and acid phosphatase

The morphology of 26 cases of osteogenic sarcoma was studied using electron microscopic techniques, and the localization of acid and alkaline phosphatase activity at the ultrastructural level elucidated. Four different cells were present in the tumours: osteoblast-like, fibroblast-like, chondroblast-like, and multinucleated giant cells. The osteoblast-like cell was present in most of the tumours studied. Acid phosphatase activity was present in lysosome-like structures of almost all the cell-types studied. Alkaline phosphatase activity was noted in or on the plasma membranes and associated vesicles of osteoblast-like, fibroblast-like, and multinucleated giant cells. The abundant reaction product deposition of alkaline phosphatase as compared with the lower acid phosphatase activity is in agreement with the nature of this bone-forming tumour. The results of the histochemical studies have added to the understanding of the pathobiology of the different cells composing osteogenic sarcomas.

Malignant fibrous histiocytoma of bone. A clinico-pathological and electronmicroscopical study

The clinicopathological and ultrastructural features of seven Malignant Fibrous Histiocytomas reported to the Spanish Registry of Bone Tumors were studied. Two patients were females (20 and 60 years old) and the other five males (15, 37, 67, 38 and 17 years old). The tumors were located in the lower femur (3 cases), upper tibia, upper humerus, rib and iliac bone. The past medical history revealed previous fracture in one patient and previous radiation of the affected bone in another; in a third patient the tumor associated to a bone infarct. Microscopically all tumors presented a storiform pattern composed of neoplastic fibroblasts, histiocytes, and malignant multinucleated giant cells. There was no evidence of osteoid or bone tissue formation by the tumor cells. Five cases were studied with electronmicroscopy; the tumor cells had features of fibroblasts, myofibroblasts, histiocytes and multinucleated histiocytes. Primary and secondary lysosomes, along with lipid vacuoles were common findings in the tumor cells. The value of the electron-microscopy in the differential diagnosis of this tumor is emphasized.

Tumor cell types in osteosarcoma as revealed by electron microscopy. Implications for histogenesis and subclassification

Ultrastructural studies in 26 osteosarcomas of high grade malignancy which were in diverse locations and of varied histological types revealed seven different tumor cell types. They were characterized by their features as follows: 1) anaplastic cells of malignant blast structure: 2) osteoblastic cells -- some of them with dot-like intranuclear bodies; 3) osteocyte-like cells surrounded by mineralized matrix; 4) fibroblast-like cells; 5) cells of myofibroblastic differentiation; 6) chondroblast-like cells in chondroblastic areas, and even 7) angioblastic cells that may be differentiated from the angioblastic and endothelial structures of capillaries. Histogenetically, osteosarcoma may be derived from stromal mesenchymal cells with a potential for differentiation into these seven tumor cell types, any tumor including, however, the osteoblastic and the osteocyte-like cells with tumor osteoid. This matrix serves as the specific criterion for identifying a tumor as "osteosarcoma", but almost every osteosarcoma of high grade malignancy will show these seven tumor cell types. The predominance of one or the other cell in the population may provide the basic information for achieving a cytologic subclassification of osteosarcoma in order to obtain relevant morphologic criteria in terms of prognosis.

Malignant fibrous histiocytoma of bone. Light microscopic and electron microscopic examination of four cases

Malignant fibrous histiocytoma (MFH) of bone is a well-defined tumor by light microscopy but no agreement has been achieved concerning its histogenesis. We present the light and electron microscopic findings of four cases of MFH of bone. In case 1 multiple bone tumors were observed and in case 4 the tumor developed after irradiation. It was our aim to document the cytological variability and to arrange the findings in a histogenetic concept of primary intraosseous MFH. We observed some undifferentiated cells but mainly histiocyte- and fibroblast-like cells including intermediate forms, and several types of giant cells. We should emphasize the fact that there were also some large cells with a light microscopic resemblance to rhabdomyoblasts and with electron microscopic characteristics of myoblastic differentiation. From the ultrastructural point of view, therefore, MFHs seem to derive from a primitive mesenchymal stem cell rather than from the ordinary histiocyte. It is suggested that osteosarcoma and MFH of bone may have a common progenitor cell but it is important to make a clear clinico-pathological distinction between the tumors because of differing biological behavior.

Ultrastructure of telangiectatic osteosarcoma

Recent investigations have shown that telangiectatic osteosarcoma has a poorer prognosis than other osteosarcomas. To elucidate the histogenesis of TOS two cases were investigated on the electron microscopic level. The results show that besides anaplastic, osteoblast-like, and fibroblast-like tumor cells angiosarcomatous components can be observed in this malignant bone tumor, which are characterized by endothelial cells with pinocytotic vesicles, tight intercellular junctions, fine fibrils, and so-called Weibel-Palade bodies in their cytoplasm. From these results, it is concluded that telangiectatic osteosarcoma is derived from multipotent mesenchymal cells with potential differentiation into various directions, such as osteoblast-like cells, and fibroblast-like cells.