Focal Densities Research Articles

Leiomyoblastoma of the uterus: an immunohistochemical and electron microscopic study of distinctive tumours with immature smooth muscle cell differentiation mimicking fetal uterine myocytes.

We present three distinctive uterine tumours which exhibited immature smooth muscle differentiation mimicking smooth muscle cells of the fetal uterus. The patients were 45, 46 and 49 years old, and all of them had simple hysterectomies. Grossly, all tumours were present in the uterine body, and two of the three tumours were well demarcated 60-mm and 85-mm lesions, and the other tumour was a small 25-mm incidental lesion within multiple conventional leiomyomas. The tumours had varied histological features and were composed of round epithelioid, rhabdoid and large vacuolated cells intermingled with spindle-shaped cells to various degrees. Although their round vesicular nuclei showed mild to moderate variation in size, prominent nuclear atypia was not seen. Necrosis and mitotic figures suggesting biological aggressiveness were not present in any of the tumours. Immunohistochemically, tumour cells were intensely positive for desmin and alpha-smooth muscle actin, whereas positivity for heavy molecular weight caldesmon was restricted. In addition, two cases were positive for non-muscle myosin heavy chain (SMemb). Ultrastructurally, most tumour cells contained various amounts of intermediate filaments which were occasionally abundant and aggregated as in rhabdoid cells. Well-developed myofilaments with focal densities were observed in only a few tumour cells. Intermediate filaments and bundles of thin filaments without dense bodies were often intermingled and they occasionally formed distinctive complexes with many irregular dense body-like structures and crystalloid bodies. Other cytoplasmic organelles including rather rich mitochondria, some rough endoplasmic reticulum and free ribosomes were also common. These findings support their immature smooth muscle cell differentiation which mimics the mesenchymal cells of fetal uterus during 14-26 weeks of gestation. The term 'uterine leiomyoblastoma' is thought to be appropriate for describing these distinctive immature smooth muscle tumours.

Undifferentiated (embryonal) sarcoma of the liver in middle-aged adults: Smooth muscle differentiation determined by immunohistochemistry and electron microscopy

Undifferentiated (embryonal) sarcoma of the liver (UESL) is a rare pediatric liver malignancy that is extremely uncommon in middle-aged individuals. We studied 2 cases of UESL in middle-aged adults (1 case in a 49-year-old woman and the other in a 62-year-old man) by histology, immunohistochemistry, and electron microscopy to clarify the cellular characteristics of this peculiar tumor. One tumor showed a mixture of spindle cells, polygonal cells, and multinucleated giant cells within a myxoid matrix and also revealed focal areas of a storiform pattern in a metastatic lesion. The other tumor was composed mainly of anaplastic large cells admixed with few fibrous or spindle-shaped components and many multinucleated giant cells. In both cases, some tumor cells contained eosinophilic hyaline globules that were diastase resistant and periodic acid-Schiff positive. Immunohistochemically, the tumor cells showed positive staining for smooth muscle markers, such as desmin, alpha-smooth muscle actin, and muscle-specific actin, and also for histiocytic markers, such as alpha-1-antitrypsin, alpha-1-antichymotrypsin, and CD68. Electron microscope examination revealed thin myofilaments with focal densities and intermediate filaments in the cytoplasm of tumor cells. Our studies suggest that UESL exhibits at least a partial smooth muscle phenotype in middle-aged adults, and this specific differentiation may be more common in this age group than in children. Tumor cells of UESL with smooth muscle differentiation in middle-aged adults show phenotypic diversity comparable to those of malignant fibrous histiocytoma with myofibroblastic differentiation. Hum Pathol 34:246-252. Copyright 2003 Elsevier Inc. All rights reserved.

Orientation of mineral crystallites and mineral density during skeletal development in mice deficient in tissue nonspecific alkaline phosphatase.

Tissue nonspecific alkaline phosphatase (TNALP) is thought to play an important role in mineralization processes, although its exact working mechanism is not known. In the present investigation we have studied mineral crystal characteristics in the developing skeleton of TNALP-deficient mice. Null mutants (n = 7) and their wild-type littermates (n = 7) were bred and killed between 8 and 22 days after birth. Skeletal tissues were processed to assess mineral characteristics (small angle X-ray scattering, quantitative backscattered electron imaging), and to analyze bone by light microscopy and immunolabeling. The results showed a reduced longitudinal growth and a strongly delayed epiphyseal ossification in the null mutants. This was accompanied by disturbances in mineralization pattern, in that crystallites were not orderly aligned with respect to the longitudinal axis of the cortical bone. Among the null mutants, a great variability in the mineralization parameters was noticed. Also, immunolabeling of osteopontin (OPN) revealed an abnormal distribution pattern of the protein within the bone matrix. Whereas in the wild-type animals OPN was predominantly observed in cement and reversal lines, in the null mutants, OPN was also randomly dispersed throughout the nonmineralized matrix, with focal densities. In contrast, the distribution pattern of osteocalcin (OC) was comparable in both types of animals. It is concluded that ablation of TNALP results not only in hypomineralization of the skeleton, but also in a severe disorder of the mineral crystal alignment pattern in the corticalis of growing long bone in association with a disordered matrix architecture, presumably as a result of impaired bone remodeling and maturation.

Intestitial Cells of Cajal in the Human Small Intestine: Immunochemical and Ultrastructural Study

The stem cell kinase CD117 has recently been found to play an important role in the development of interstitial cells of Cajal (ICC), which are currently regarded as pacemaker cells of the gastrointestinal tract. CD117 is expressed in both gastrointestinal stromal tumors (GIST) and ICC, with the latter regarded by many as the progenitor cells of GIST. The authors investigated immunoreactivity of 25 normal surgically removed small intestinal tissues and correlated the findings with electron microscopy (EM) on 12 cases. In all cases CD117-positive cells were frequently seen around the myenteric plexi either singly or in groups. CD117-positive cells on immunostained sections corresponded to the cells appearing as fibroblast-like or undifferentiated primitive mesenchymal cells around the myenteric ganglia and interstitial spaces by EM. In contrast, S-100 stain revealed a fine network of positive staining throughout the muscularis. Branches of nonmyelinated axons and nerve endings were found regularly between myocytes with direct contact with muscle cells by EM. The cells that we could depict as ICC because of their distribution andstaining pattern of CD117 were limited to the nonmuscular mesenchymal cells. No muscle cell-like ICC were found. Instead, the muscle cells in direct contact with nerve endings were often disfigured and the cytoarchitectural contents for muscle cells became less distinct because of lighter staining and loss of definite focal densities among actin filaments. However, these latter cells did maintain most muscle cell features, such as continuous external lamina, caveolae, and some of the peripheral densities. These findings raise a possibility that previous investigators could have included these altered muscle cells into the ICC group. It was also found that intestinal muscularis not only was richly endowed with an elaborate neural network of delicate axonal extensions and dense-core granule containing nerve endings traversing through and between myocytes, but also showed frequent synapse-like direct contact between nerve endings and muscle cells. These findings indicate that enteric nerves may play a major role in the control of intestinal motility, while CD117-positive cells play an accessory role as cells of Cajal as originally speculated. Further studies are necessary to better define and characterize interstitial cells of Cajal, which will be useful in the correlation of the vast number of data concerning the possible role of CD117-positive ICC in the pacemaker function of the intestine and oncogenesis of GIST.

Focal asymmetric densities seen at mammography: US and pathologic correlation.

The American College of Radiology (ACR) Breast Imaging Reporting and Data System (BI-RADS) defines four different types of asymmetric breast findings: asymmetric breast tissue, densities seen in one projection, architectural distortion, and focal asymmetric densities. These lesions are frequently encountered at screening and diagnostic mammography and are significant because they may indicate a neoplasm, especially if an associated palpable mass is present. Once these lesions are detected at standard mammography, supplementary breast imaging with additional mammographic views and ultrasonography (US) can be a key aspect of work-up. The role of US in this setting has not been clearly defined. However, a positive US finding such as a solid mass or an area of focal shadowing increases the level of suspicion for malignancy. A thorough knowledge of the patient's clinical history, along with a fundamental understanding of the ACR BI-RADS lexicon and the role and limitations of supplementary breast imaging, will allow more accurate interpretation of these potentially perplexing soft-tissue findings.

Fibronexus in "Malignant Fibrous Histiocytoma" of the Bone: A Case Report of Pleomorphic Myofibrosarcoma

The fibronexus (fibronexus junction) has been thought to be a characteristic ultrastructural feature of myofibroblasts, but it is controversial whether the fibronexus is also a characteristic of various myofibroblastic tumors. We report here a case of pleomorphic bone sarcoma (pleomorphic/storiform malignant fibrous histiocytoma) with fibronexus junctions arising in the head of the left humerus of a 70-year-old woman. By light microscopy the tumor was composed of large spindle or polygonal cells occasionally arranged in fascicles. Foamy giant cells with bizarre nuclei were not uncommon. Immunohistochemically, the tumor cells were positive diffusely for vimentin and focally for muscle actin ( HHF35 ) and alpha-smooth muscle actin but were negative for desmin, high-molecular weight caldesmon, and S-100 protein. Ultrastructurally, the tumor cells had indented nuclei and spindle or polygonal cytoplasm, with little rough endoplasmic reticulum and small vesicles, and rather numerous mitochondria. The tumor cells had myofilaments with focal densities in the periphery, fibronectin fibrils adjacent to intracellular myofilaments, and by definition, therefore, fibronexus junctions. These findings suggest myofibroblastic differentiation, and a diagnosis of pleomorphic myofibrosarcoma is thought to be appropriate. We believe that the fibronexus is a characteristic and useful ultrastructural feature for differentiating myofibroblastic tumors from other pleomorphic myogenic sarcomas.

Leiomyosarcoma Showing Partial Synthetic-Phase Differentiation

An unusual leiomyosarcoma in the thigh of a 53-year-old woman is described. Tumor cells were spindled and positive for muscle immuno-markers. Some cells exhibited typical smooth-muscle ultrastructure - abundant myofilaments, focal densities, attachment plaques, plasmalemmal caveolae, and lamina. Others had fewer myofilaments and prominent rough endoplasmic reticulum. These features indicated a variable phenotype that included synthetic-phase (matrigenic) differentiation. Synthetic-phase cells were distinguished from myofibroblasts by typical smooth-muscle surface features in the absence of fibronexus junctions. In a subset of cells myofilaments were absent, but structures resembling solitary focal densities were identified. These are discussed as possible indicators of a primitive level of smooth-muscle differentiation.

Contribution of Electron Microscopy to Understanding Cellular Differentiation in Mesenchymal Tumors of the Gastrointestinal Tract: A Study of 82 Tumors

Eighty-two mesenchymal tumors of the gastrointestinal tract were examined by electron microscopy for the purposes of subtyping for diagnostic precision and of understanding cellular differentiation. Tumors were subclassified into leiomyoma/leiomyosarcoma, tumors of the interstitial cell of Cajal (equivalent to traditionally defined GISTs [Miettinen et al. Hum Pathol. 1999; 30:1213-1220; Mod Pathol. 2000; 13:1134-1142]), gastrointestinal autonomic nerve tumors (GANTs), and fibroblastic and myofibroblastic tumors, using criteria from the literature. Leiomyoma/leiomyosarcoma were diagnosed by myofilaments, attachment plaques, plasmalemmal caveolae, and lamina; GIST by processes or cell bodies full of intermediate filaments, solitary focal densities amid intermediate filaments, attachment plaques with incomplete lamina, scarce myofilaments, and smooth endoplasmic reticulum; GANTs by neuroendocrine granules, cell bodies/processes full of intermediate filaments (more rarely microtubules), and smooth endoplasmic reticulum; fibroblastic/myofibroblastic tumors by abundant rough endoplasmic reticulum, myofilaments, and fibronexuses. Seventy-three tumors (89%) were successfully subclassified, as 5 leiomyoma/leiomyosarcoma (6%), 36 GISTs (44%), 22 GANTs (27%), 10 fibroblastic and myofibroblastic tumors (12%). Results indicated overlap between poorly differentiated leiomyosarcoma and GIST, and between GIST and GANT. GANT is emphasized as a neuronal tumor identifiable by electron microscopy, and thereby distinguishable from GIST.

Marching at the front and dragging behind: differential alphaVbeta3-integrin turnover regulates focal adhesion behavior.

Integrins are cell–substrate adhesion molecules that provide the essential link between the actin cytoskeleton and the extracellular matrix during cell migration. We have analyzed αVβ3-integrin dynamics in migrating cells using a green fluorescent protein–tagged β3-integrin chain. At the cell front, adhesion sites containing αVβ3-integrin remain stationary, whereas at the rear of the cell they slide inward. The integrin fluorescence intensity within these different focal adhesions, and hence the relative integrin density, is directly related to their mobility. Integrin density is as much as threefold higher in sliding compared with stationary focal adhesions. High intracellular tension under the control of RhoA induced the formation of high-density contacts. Low-density adhesion sites were induced by Rac1 and low intracellular tension. Photobleaching experiments demonstrated a slow turnover of β3-integrins in low-density contacts, which may account for their stationary nature. In contrast, the fast β3-integrin turnover observed in high-density contacts suggests that their apparent sliding may be caused by a polarized renewal of focal contacts. Therefore, differential acto-myosin–dependent integrin turnover and focal adhesion densities may explain the mechanical and behavioral differences between cell adhesion sites formed at the front, and those that move in the retracting rear of migrating cells.

Filopodial initiation and a novel filament-organizing center, the focal ring.

This study examines filopodial initiation and implicates a putative actin filament organizer, the focal ring. Filopodia were optically recorded as they emerged from veils, the active lamellar extensions of growth cones. Motile histories revealed three events that consistently preceded filopodial emergence: an influx of cytoplasm into adjacent filopodia, a focal increase in phase density at veil margins, and protrusion of nubs that transform into filopodia. The cytoplasmic influx probably supplies materials needed for initiation. In correlated time lapse-immunocytochemistry, these focal phase densities corresponded to adhesions. These adhesions persisted at filopodial bases, regardless of subsequent movements. In correlated time lapse-electron microscopy, these adhesion sites contained a focal ring (an oblate, donut-shaped structure approximately 120 nm in diameter) with radiating actin filaments. Filament geometry may explain filopodial emergence at 30 degree angles relative to adjacent filopodia. A model is proposed in which focal rings play a vital role in initiating and stabilizing filopodia: 1) they anchor actin filaments at adhesions, thereby facilitating tension development and filopodial emergence; 2) "axial" filaments connect focal rings to nub tips, thereby organizing filament bundling and ensuring the bundle intersects an adhesion; and 3) "lateral" filaments interconnect focal rings and filament bundles, thereby helping stabilize lamellar margins and filopodia.

Short-term follow-up results in 795 nonpalpable probably benign lesions detected at screening mammography.

To evaluate short-term follow-up of nonpalpable probably benign lesions in a 2-year mammographic screening. Of 13,790 women aged 45-65 years who underwent first-round screening, 795 (5.8%) underwent short-term mammographic follow-up (every 6 months for 2 years) of nonpalpable probably benign lesions (eg, masses, focal asymmetric densities, and calcifications) previously assessed at an additional imaging evaluation, including ultrasonography. When no changes were found at short-term mammographic follow-up, women were assigned to the 2-year screening interval. Needle localization and surgical biopsy were performed when the lesion progressed (was enlarged or had an increased number or size of calcifications or modification of their initial characteristics). The effectiveness of this approach was evaluated with statistical analysis. Of 795 lesions, 788 (99%) remained stable, and seven (1%) had changes prompting surgical biopsy. Two cancers (0.3%), one microinvasive intraductal carcinoma and one 7-mm invasive ductal carcinoma without positive nodes, were found. Four of the five benign histologic results were probably benign calcifications with progression at short-term follow-up. The sensitivity, specificity, accuracy, and positive and negative predictive values were 100%, 99%, 99%, 29%, and 100%, respectively. The benign nature of most nonpalpable probably benign lesions can be typified with short-term mammographic follow-up. This approach permitted identification of a few low-stage carcinomas, but progression in the probably benign calcifications was usually unrelated to malignancy.

The Myofibroblast: An Assessment of Controversial Issues and a Definition Useful in Diagnosis and Research

Some interpretational problems associated with the myofibroblast, which affect how this cell is identified, are discussed. Questions addressed include distinguishing between "external" lamina ("basement membrane") and the fibronectin fibril of the fibronexus; the nature of stress fibers (bundles of smooth-muscle myofilaments with focal densities); the utility of some of these features to distinguish between myofibroblastic and smooth-muscle cell surfaces; and cytoskeletal immunophenotype. The following points are emphasized. Myofibroblasts have a surface characterized by prominent fibronectin fibrils and fibronexus junctions, which are distinct from lamina ("basement membrane"). This can permit a distinction to be made between smooth-muscle and myofibroblastic lesions and tumors. Myofibroblasts are typically positive for vimentin and alpha-smooth-muscle actin, but desmin is not a useful discriminant between smooth-muscle and myofibroblastic lesions. The main features for defining the myofibroblast are abundant rough endoplasmic reticulum; modestly developed peripheral myofilaments with focal densities (stress fibers); fibronexus junctions; vimentin and smooth-muscle actin immunostaining. Other features include a Golgi apparatus and collagen secretion granules, gap junctions, and actin-associated nondesmosomal junctions. Illustrations of the usefulness of these criteria in the diagnosis of soft-tissue lesions (myofibrosarcoma, so-called myofibroblastoma) are given.

The fibronexus in reactive and tumoral myofibroblasts: further characterisation by electron microscopy.

Forty two surgical specimens containing myofibroblasts were studied to clarify the criteria for identifying the fibronexus, an ultrastructural feature regarded as a marker for myofibroblastic differentiation. Granulation tissue, tumour stroma, fibro-proliferative lesions (nodular fasciitis, myofibromatosis, inflammatory myofibroblastic tumour) and malignancies (myofibrosarcoma and fibrosarcoma) were studied. Comparable results were found throughout these specimens, although fibronexus junctions were better developed in reactive compared with tumoral myofibroblasts. By electron microscopy, myofibroblasts were identified by abundant rough endoplasmic reticulum, peripheral smooth-muscle myofilaments with focal densities, and fibronexus junctions. The latter were recognised as the points of convergence on the myofibroblast surfaces of intracellular myofilaments and extracellular fibronectin fibrils. The fibronectin fibrils were often co-linear with myofilaments. Also, fibronectin fibrils were dark-staining, straight and rigid-looking, and had a longitudinal filamentous substructure. A striking feature was the tendency of fibronectin fibrils to project into the surrounding extracellular space, away from the myofibroblast surface: in these respects, they differed significantly from lamina ("basement membrane"). The presence of fibronectin fibrils correlated positively with fibronectin immunostaining by light and electron microscopy. Laminin and collagen IV showed variable and weak staining in the intercellular spaces in a minority of cases and never strongly stained myofibroblast surfaces. The data emphasise that the fibronexus has a number of distinctive features permitting identification, and constitute a reference-point for pathologists wishing to use electron microscopy to refine light microscopy diagnoses of putative myofibroblastic lesions. The role of the fibronexus in the definition of the myofibroblast is discussed.

Epithelioid Leiomyosarcoma in a Non-Immunocompromised Infant: Additional Differential Diagnosis of Pediatric “Round Cell Tumors”

We report an 18-month-old Japanese girl with purely epithelioid leiomyosarcoma presenting as a huge intraabdominal mass. The patient had been well from birth and had shown no signs of immunodeficiency. She was negative for human immunodeficiency virus. Blood examination revealed elevated serum neuron specific enolase (NSE). Histologically, the tumor was comprised of solid growths of round or polygonal cells with vesicular nuclei and often vacuolated cytoplasm rich in glycogen. The tumor cells were positive for vimentin, NSE, and MIC2, and were negative for desmin and neurofilament. The age, clinical presentation, and histologic findings mostly favored Ewing's sarcoma/primitive neuroectodermal tumor. Silver stain, however, demonstrated well-developed reticulin fibers often outlining individual tumor cells. An expanded panel of immunostains showed that the tumor cells were intensely positive for smooth muscle actin, and ultrastructural study revealed abundant fine cytoplasmic filaments with focal subsarcolemmal densities, various amounts of glycogen, and irregularly arranged, thick basal lamina. The diagnosis of epithelioid leiomyosarcoma was made. Following reduction in tumor size by chemotherapy, the serum NSE level was normalized. From the surgical finding, the primary site was presumed to be the urachus or the urinary bladder dome. Although extremely rare, epithelioid leiomyosarcoma should be added in the list of differential diagnoses of pediatric “round cell tumors.”

Electronic Properties and Interlayer Coupling in Magnetic Semiconductor Heterostructures

Heterostructures composed of magnetic and nonmagnetic semiconductor layers are investigated within the framework of the tight-binding model. The band structure and focal densities of states (DOS) are calculated. Spin-dependent modifications are found in DOS of nonmagnetic layers for thin spacers. A coupling between magnetic moments across a nonmagnetic spacer is discussed.

Breast tumors with myofibroblastic differentiation: clinico-pathological observations in myofibroblastoma and myofibrosarcoma

This report describes the clinico-pathological features of myofibroblastic tumors of the breast in six patients. Four women and one man presented with a benign myofibroblastoma. The sixth patient was a woman with myofibrosarcoma. All myofibroblastomas were composed of a fascicular arrangement of spindle cells embedded in dense bundles of collagen. Tumors differed with respect to their proportion of neoplastic cells and collagenous stroma as well as cellular pleomorphism. Based on this variation, the tumors could be subclassified as classic, collagenized, epithelioid and cellular myofibroblastoma. Immunohistological staining confirmed myofibroblastic differentiation by strong expression of either desmin or smooth muscle actin with coexpression of vimentin. In addition, numerous cells reacted with antibodies to CD68. Proliferative activity was rather low in the myofibroblastoma with an average of 0-2 mitotic figures per 10 HPF. DNA cytometric analysis was performed in two cases and showed diploid stem lines with minor S-phase fractions (1% and 3%). In the myofibrosarcoma, cells contained pleomorphic nuclei with some giant cells and numerous mitotic figures (6-7/10 HPF) and had infiltrating margins that were apparent even grossly. Immunohistochemically, tumor cells strongly expressed vimentin, smooth muscle actin and fibronectin. Ultrastructurally, neoplastic cells met the criteria of myofibroblasts, i.e. contained abundant intermediate filaments and myofilament bundles with focal densities as well as fibronexus junctions. DNA cytometric analysis exhibited again a diploid stemline but marked proliferative activity was present as indicated by an S-phase fraction of 20%. In conclusion, in benign myofibroblastoma there may be some cellular pleomorphism but mitotic activity is always low. The malignant counterpart, myofibrosarcoma, is characterized by marked cellular pleomorphism, infiltrating margins and high mitotic rate.

Myofibroblastom a of Breast: Evidence Favoring Smooth-Muscle Rather Than Myofibroblastic Differentiation

A histopathological study of two cases of the tumor known in the literature as myofibroblastoma of the breast is presented. The tumors occurred in Caucasian males aged 57 and 62 years. Histologically, these were moderately cellular, lobulated spindle-cell lesions, each with a reasonably well-delineated edge with surrounding fatty connective tissue. No breast ducts or lobules were present. Tumor cell nuclei were bland, with small nucleoli and some nuclear grooving. Nuclear atypia and mitoses were absent. Immunostaining revealed positivity for alpha -smoothmuscle actin, desmin, and CD34. Tumor cells contained rough endoplasmic reticulum, bundles of myofilaments with focal densities, intermediate filaments, attachment plaques alternating with plasmalemmal caveolae, and focal lamina. Ultrastructural findings pointed to true smooth-muscle differentiation, and the cell-surface in particular lacked surface features of myofibroblasts (fibronectin fibrils [microtendons] and fibronexus junctions). These and published data suggest that at least some of the lesions referred to in the literature as myofibroblastoma may not be myofibroblastic and may be better designated as myogenic stromal tumors or as variants of leiomyoma.

Haemangiopericytoma in a calf

After castration, a Japanese Black male calf developed subcutaneous tumours near the scrotum. They were surgically excised, and no recurrence was detected two years after surgery. The tumours were characterised by neoplastic cells concentrically arranged around blood vessels of various sizes. The neoplastic cells contained many cytofilaments, and the majority of the cells expressed smooth muscle actin, but focal densities were not found. On the basis of the immunohistochemical and ultrastructural features. the tumours were considered to be of pericyte origin.

Ultrastructure of Leiomyosarcoma Invasive Phenotype

Abstract Malignant tumors are known to have an heterogeneous cell population. Metastases are supposed to be formed by subpopulations with high metastatic capability. These metastatic cells constitute the so called invasive phenotype. In that order of ideas, metastases could be pathologically different from their parent tumors if the invasive phenotype had distinct morphological features. The similarities or differences between primary tumors and their metastases have not been adequately studied at the ultrastructural level. In this work we report an electron microscopic study of liver leiomyosarcoma metastases which shows alterations not described in primary tumors of that kind. Biopsies of liver metastases from a colon leiomyosarcoma were surgically obtained. Samples were processed with routine techniques for transmission electron microscopy and observed in a Hitachi H-500 electron microscope. Some alterations usually observed in primary leiomyosarcomas were seen. They were presence of myofilaments with focal densities, nuclear changes, swollen mitochondria, and abundance of rough endoplasmic reticulum (Fig. 1).

Muscle-specific actin expression in chondroblastomas

Chondroblastomas are unusual cartilage benign lesions of bone that have well-characterized histological features. We reviewed and immunohistochemically examined the paraffin block material of 20 cases, and in seven tumors of this collection we found distinct cytoplasmic muscle-specific actin positivity of some tumor chondroblasts and chondrocytes. Muscle-specific actin-positive cells had the histological and ultrastructural features typical of chondroblasts. Moreover, in their cytoplasm they contained bundles of microfilaments with focal densities, as is typical of myofilaments. We did not observe any basal lamina around these cells, which were surrounded by intercellular matrix of the cartilage tissue type. Therefore, we suppose that muscle-specific actin-positive cells occurring in some chondroblastomas do not represent an admixture of myofibroblasts but chondroblasts with actin expression. The unusual immunophenotype of some chondroblasts might be the result of aberrant actin expression or of the plasticity of their phenotype modulated by microenvironmental stimuli. It is a question of whether, analogously to the terminology of myofibroblasts, such cells could be designated as myochondroblasts.