Immunohistochemical study of autophagy associated molecules and cell adhesion molecules in canine intracranial granular cell tumors

Granular cell tumors (GCTs) are characterized by abundant eosinophilic cytoplasmic granules. Based on the hypothesis that canine intracranial GCT is a subtype of meningioma and its cytoplasmic granules are formed through autophagy processes, histopathological and immunohistochemical examination were performed on biopsy samples from 7 cases of canine intracranial GCTs and 15 cases of conventional meningiomas. Histopathologically, 7/7 cases of GCTs involved the meninges; foci of meningothelial-like cells were observed in 3/7 cases; brain invasion was observed in 2/7 cases. Immunohistochemically, neoplastic cells of GCTs were positive for E-cadherin and negative for S100, cytokeratin, CD204, and β-catenin in 7/7 cases. Neoplastic cells of 15/15 cases of meningiomas were positive for E-cadherin, and negative for S100 and CD204. Immunoreactivity of meningiomas for cytokeratin and β-catenin was observed in 6/15 cases and 8/15 cases, respectively. Cytoplasmic granules of GCTs were positive for ubiquitin (5/7), p62 (5/7), and LC3 (7/7). Compared to GCTs, the ratios of ubiquitin (6/15) and p62 (3/15) positive cases were lower in meningiomas, and 15/15 cases were negative for LC3. These findings indicate that the biological natures of GCTs including anatomical location, histopathological features and immunoreactivity for E-cadherin are almost in conformity with those of meningiomas. The immunoreactivity for autophagy associated molecules may suggest the possible involvement of autophagy in cytoplasmic granule formation of canine intracranial GCTs.     

Immunohistochemical and ultrastructural investigation of granular cell tumours in dog, cat, and horse.

Six canine, one feline and one equine granular cell tumours (GCTs) were investigated electron microscopically and immunohistochemically. The tumours were tested for reactivity with monoclonal antibodies against vimentin and desmin and with polyclonal antibodies against cytokeratin, S-100 protein, glial fibrillary acidic protein (GFAP) and neuron specific enolase (NSE). All GCTs were characterized by their PAS positive cytoplasmic granules in light microscopy, which in electron microscopy appeared as lysosome-like granules. In each case two canine GCTs were stained by the antibody against cytokeratin, vimentin and S-100 protein. Cells of the equine GCT showed reactivity with the antiserum against S-100 protein. In the feline GCT no reactivity with any of the antibodies tested was observed. These differences of the immunohistochemical reactions of GCTs suggest a nonuniform histogenesis of GCTs in domestic animals. The reactivity of the tumour cells with the antiserum against NSE is discussed.     

Ultrastructural Features of the Bovine Cecal Mucosa

The cecal surface epithelium of cattle was lined entirely by columnar cells except near the openings of the glands where a few partially depleted goblet cells were encountered. Surface columnar cells with pale cytoplasm, indented (sometimes dome-shaped) apical border and irregular microvilli, were also found near the gland orifices. Surface columnar cells -near the openings of the glands contained large cytoplasmic vacuoles and lysosomal-like bodies. Near the extrusion zone, however, the columnar cells contained highly indented and apically displaced nuclei. The glands were usually long and straight and lined by an epithelium concisting mainly of undifferentiated and goblet cells. There were few entero-endocrine and non-epithelial cells (intra-epithelial lymphocytes and globule leukocytes). Paneth cells were absent. The undifferentiated cells contained many free ribosomes and apical secretory granules. Some of these cells were undergoing mitotic division. Goblet cells exhibited a typical brandy-glass appearance and showed dark and light mucigenous granules. Endocrine cells with polymorphic secretory granules and cells with more uniform secretory granules were identified in the basal part of the epithelium. Globule leukocytes and intra-epithelial lymphocytes were usually encountered near the basal part of the epithelium. The lamina propria was highly cellular. It contained many plasma cells, mast cells and small lymphocytes, and few eosinophils, neutrophils and globule leukocytes. Some of the plasma cells contained large, dense Russell bodies within the cisternae of the rough-surfaced endoplasmic reticulum. The neutrophils displayed distinct populations of small and large cytoplasmic granules. Some of the eosinophilic granules showed narrow crystalline cores. Large lymphocytes were found in the lamina propria, but occurred less frequently than small lymphocytes.     

Ultrastructure of the Integument of the Domestic Pig (Sus scroh) from One through Fourteen Weeks of Age

The biopsies of normal lumbar and dorsal thoracic pig epidermis and dermis are described ultrastructurally at one, three, five, seven, nine and fourteen weeks and compared to the human integument. In the epidermis, basal cell projections extended into the dermis and increased slightly with age. Intraepidermal axons were seen. The basal lamina thickened opposite the hemidesmosomes of the epidermal-dermal junction. Small anchoring fibrils extended from the basal lamina into the dermis. Mitochondria were concentrated beneath the cellular nuclei of the stratum basale. Upper stratum spinosum and stratum granulosum cells contained membrane coating granules. The stratum corneum contained cells of varying density. The only intraepidermal nonkeratinocytes observed were melanocytes and Langerhans-like cells. Both were similar to those of humans. In the dermis, the interfollicular muscle resembled the arrector pili and smooth muscle found in other body areas. Nerve fibers were similar to those described in humans except subepidermal nerve endings contained mitochondria. One unexpected finding was the presence of mast cells which were enveloped by a zone of loose floccular material which stained positive with PAS from seven through fourteen weeks of age. Mast cell population density peaked at seven weeks and was greater in the papillary dermis than in the reticular dermis. Generally, the pig integument is morphologically similar to that of man and would serve as a useful animal model when age changes are accounted for.     

Ultrastruct of the juxtaglomerular apparatus in the dog in a sodium-balanced state.

The ultrastructure of juxtaglomerular apparatuses (JGA) from four dogs in a state of sodium balance were examined. Plasma renin activity and urinary sodium excretion in the dogs were within normal values. At the vascular pole of the glomerulus, circumferentially oriented smooth muscle cells in the wall of the glomerular arteriole were abruptly replaced by modified smooth muscle cells with complicated interwoven cell processes. These modified cells were indistinguishable from the juxtaglomerular cells of the lacis region (extraglomerular mesangium). The basal lamina surrounding these modified smooth muscle cells in the wall of the glomerular arteriole was continuous with the basal lamina of the juxtaglomerular cells of the lacis (extraglomerular mesangium) and the macula densa. The most striking differences in the JGA of the dog, when compared with the JGA of other laboratory animals, are related to the observation that cytoplasmic secretory granules are less abundant in the dog, are generally smaller than the practical resolving capability of the light microscope, and are restricted to cells of the lacis area in the JGA.     

Histology, immunohistochemistry and ultrastructure of the equine palatine tonsil

The palatine tonsils of five young horses formed 10-12 cm elongated follicular structures extending from the root of the tongue on either side to the base of the epiglottis and lateral to the glossoepiglottic fold. The stratified squamous non-keratinized epithelium of the outer surface was modified into crypts as reticular epithelium by heavy infiltration of lymphoid cells from underlying lymphoid follicles. In places, lymphoid tissue reaching almost to the surface and with only one to two cell layers intact was identified as the lymphoepithelium. Langerhans cells with Birbeck granules were interspersed between epithelial cells. Lymphoid tissue organized in lymphoid follicles constituted the parenchyma of the palatine tonsil. CD4-positive cells were more numerous and CD8-positive lymphocytes less numerous compared with their distribution in the lingual tonsil. B cells and macrophages were also more numerous than in the lingual tonsil and lectins showed a different pattern of attachment. M cells were not observed. High endothelial venules with well-developed vesiculo-vacuolar organelle had structural evidence of transendothelial and interendothelial migration of lymphocytes. Striated muscles as seen in the deeper lamina propria mucosae of the lingual tonsil were absent. The immunohistological and ultrastructural characteristics of the equine palatine tonsil are similar to those of humans but differ from those of the lingual tonsil and are consistent with a role as an effector and inductor immunological organ.     

Epithelium lining the rostral portion of the porcine nasal mucosa

Epithelial tissues from the rostral half of the pig nasal cavity were prepared for light and electron microscopy. The predominant epithelial cell type at the luminal surface was cuboidal with surface microplicae or microvilli and a multilobate nucleus. Pinocytotic vesicles were a common feature of the adluminal cytoplasmic zone of these superficial cuboidal cells. Other cell types included basal cells, intermediate cells and, occasionally, goblet and ciliated cells. Basal cells contained vesicles located adjacent to the basal lamina. This transitional mucosa may be specialised for sampling substances from the luminal surface preliminary to releasing cytokines and, or, presenting immunogens to intraepithelial lymphocytes.     

Multicentric benign peripheral nerve sheath tumors in two related bearded dragons, Pogona vitticeps

Multiple subcutaneous masses from two sibling bearded dragons were removed. Nodules were well demarcated, restricted to the subcutis, and soft, white to yellow, resembling adipose tissue. Histologically, the masses were composed of short interlacing streams and bundles of spindle cells, with regions of vague nuclear palisading. Two of the tumors contained a subpopulation of polygonal cells with abundant periodic acid-Schiff (PAS)-positive cytoplasmic granules. Neoplastic cells were immunohistochemically positive for S100 and neuron-specific enolase (NSE) but negative for desmin and smooth muscle actin. Electron microscopy and reticulin stains demonstrated a continuous basal lamina separating intertwining cells. Histologic, ultrastructural, and immunohistochemical features were consistent with a peripheral nerve sheath origin. At 1 year postexcision, local reoccurrence of a single incompletely excised mass from the left shoulder was noted.     

Distribution and ultrastructure of mast cells in the equine respiratory tract

Mast cells in the equine respiratory mucosa were studied at both light--and transmission electron--microscope levels. Mast cells were identified at all levels of the tract, with the greatest cell density in the nasopharynx. The majority (57 to 94 per cent) of this cell population were located within the connective tissue of the lamina propria. Up to 20 per cent of these cells were associated with the mucosal glandular tissue, whilst small numbers were present within the surface epithelium and in association with nodular lymphoid tissue. In the peripheral lung tissue 20 per cent of the mast cell population was associated with the small airways, 25 per cent with the pleura, and 32 per cent with blood vessels. The fine and ultrastructural features of these mast cells resemble those described in other species, and their granules consist of two types which resemble human mast cells.     

Ultrastructure of feline mammary hypertrophy

The ultrastructure of feline mammary hypertrophy was studied in a five-month-old female which had aborted recently, a ten-year-old female which was one month postestrus, and a four-year-old progestin-treated neutered male. Morphologic comparisons were made to normal mammary tissue from a one-year-old female cat. Hypertrophied mammary tissue had the same cell types and spatial relationships as did the normal gland. Major differences included a more highly developed duct system composed of metabolically active cells which often were arranged in multiple cell layers, and periductular stroma with increased fibroblasts and vascularization. Hypertrophied epithelial cells were characterized generally by smooth-contoured nuclear membranes, more evenly dispersed heterochromatin, prominent nucleoli, increased polyribosomes, and elongated mitochondria. Secretory activity was developed significantly only in the cat that had aborted recently. Modifications in myoepithelial cells included: more evenly dispersed nuclear heterochromatin, thicker bundles of cytoplasmic filaments, more straight plasma membranes along the basal lamina, and elongated hemidesmosomes. Multilayering of the basal lamina was accentuated. Stromal fibroblasts had nuclear heterochromatin distributed similarly to that of epithelial and myoepithelial cells, and increased rough endoplasmic reticulum. Myoepithelial cells did not contribute to the increased stromal cellularity. No significant ultrastructural differences were noted between mammary hypertrophy in young, old, and progestin-treated cats.     

Ultrastructural and ultracytochemical cyclic changes in the bovine uterine tube (oviduct) epithelium.

Ultrastructural and ultracytochemical features of the uterine tube (oviduct) infundibulum were studied in 8 Hereford cows, which were slaughtered in pairs on days 1 (estrus), 3, 9 or 10, and 18 of the estrous cycle. Fibrous granules (60 to 80 nm), which are supposedly related to basal body replication, were observed in the apical cytoplasm of ciliated cells. Close association between basal bodies and fibrous granules was apparent, especially during the follicular phase. Cilia were observed throughout of estrous cycle, although degeneration of cilia was not observed at any phase of the cycle. Prominent, striated rootlets were observed during both the follicular and luteal phases of the cycle. Maximum secretory cell differentiation was apparent during the follicular phase, at which time these cells were characterized by having a well-developed, rough endoplasmic reticulum with dilated cisternae, numerous ribosomes, and secretory granules of varied size and density. A prominent feature of the secretory granules was their membranous structure, consisting of concentric lamellae of equal dimensions. During the luteal phase, cytoplasmic protrusions were prominent, and extruded nuclei along with other cytoplasmic organelles were present in the tubal lumen. The presence of a well-developed, rough endoplasmic reticulum and numerous secretory granules during the follicular phase indicates that secretory activity of the uterine tube infundibulum may be stimulated by estrogen. During estrus, the cytoplasm of the stromal cells displayed abundant, rough endoplasmic reticulum with dilated cisternae. The increased and extensively dilated rough endoplasmic reticulum at the time of estrus probably indicates increased protein synthesis by the stromal cells. The presence of adenosine triphosphatase activity on the membrane of cilia suggests that this enzyme is involved in energy-forming reactions related to the vigorous action of cilia. The presence of acid phosphatase activity on the cell membrane of the epithelium, microvilli, and secretory granules may indicate involvement in the secretory mechanism of the cell.     

Electron microscopic studies of estrogen-induced ciliogenesis and secretion in uterine tube of the gilt.

The time required for occurrence of estrogen-induced uterine tubal (oviductal) ciliogenesis and for differentiation of secretory cells was studied, utilizing electron microscopy procedures. Sixteen cycling gilts were ovariectomized; 3 to 4 months later, 12 principal gilts were each given subcutaneous injections of 17 beta-estradiol in 0.5 ml of corn oil at the rate of 200 mug/day, and 4 control gilts were given injections of corn oil only at the rate of 0.5 ml/day. Two principals each were killed on days 1, 2, 3, 4, 5, and 7 after start of treatment. The epithelial heights were low and completely atrophied 3 to 4 months after ovariectomy. Uterine tubal cilia were absent in all the control gilts. Cytologic changes were not seen in the atrophied epithelium of ovariectomized gilts 1 day after estradiol treatment, but definite proliferative elements consisting of an extensive fibrillar meshwork encrusted with granules (60 to 80 nm) were observed in close association with the nuclear envelope and in the apical cytoplasm after 2 days of estradiol treatment. By day 3, enlarged electron-opaque granules referred to as condensation forms, undergoing various stages of depletion, were closely associated with radially arranged procentrioles. These associations have been referred to as generative complexes. The presence of many generative complexes indicates that maximal production of basal bodies can be expected after 3 days of treatment with estradiol. The depletion of the condensation forms produced hollow spheres with thin walls as the procentrioles grew in length and assembled their microtubules. Enlarged mature-appearing basal bodies were abundant in the cytoplasm after 3 days of estradiol treatment. These bodies aligned themselves linearly along the luminal surface of the cell. Small ciliary buds were then formed above the cell surface, and ciliary filamentogenesis occurred in the bud. Motile cilia were observed on day 3, but cilia numbers increased markedly between day 4 and days 5 and 7. Procentrioles were generated from the diplosomal centriole after 2 days of estradiol treatment. These observations have provided evidence for both ancentriolar and centriolar basal body replication in the ciliated cells of uterine tube of the gilt. Maximal secretory cell differentiation occurred after 3 days of estradiol treatment. Hypertrophy of cytoplasmic organelles was evident on day 3, but the number of secretory granules and amount of rough endoplasmic reticulum increased markedly on days 5 and 7. Close association of secretory granules, Golgi apparatus, and endoplasmic reticulum was evident after estadiol treatment. These data indicate that both ciliated and secretory cells are sensitive to estrogen.     

A malar granular cell tumor in a djungarian hamster

'Granular cell' tumor observed in the malar subcutis of a Djungarian hamster was examined to determine its cellular origin. Histologically, the tumor consisted of a solid growth of oval or spindle-shaped large cells with abundant cytoplasm filled with eosinophilic granules that were periodic acid-Schiff-positive and diastase-resistant. Immunohistochemically, the tumor cells were positive to anti-vimentin and anti-desmin antibodies and a few cells showed positivity to anti-actin antibody as well. They did not react to myoglobin, S-100 protein, and glial fibrillary acidic protein (GFAP). Electron microscopic studies revealed that the tumor cells had pinocytotic vesicles, dense plaque and microfilaments. The first granular cell variant of myogenic tumor reported here in Djungarian hamsters was differentiated from granular cell tumor of Schwann cell origin.     

Meat Science and Muscle Biology Symposium: stem cell niche and postnatal muscle growth

Stem cell niche plays a critical role in regulating the behavior and function of adult stem cells that underlie tissue growth, maintenance, and regeneration. In the skeletal muscle, stem cells, called satellite cells, contribute to postnatal muscle growth and hypertrophy, and thus, meat production in agricultural animals. Satellite cells are located adjacent to mature muscle fibers underneath a sheath of basal lamina. Microenvironmental signals from extracellular matrix mediated by the basal lamina and from the host myofiber both impinge on satellite cells to regulate their activity. Furthermore, several types of muscle interstitial cells, including intramuscular preadipocytes and connective tissue fibroblasts, have recently been shown to interact with satellite cells and actively regulate the growth and regeneration of postnatal skeletal muscles. From this regard, interstitial adipogenic cells are not only important for marbling and meat quality, but also represent an additional cellular component of the satellite cell niche. At the molecular level, these interstitial cells may interact with satellite cells through cell surface ligands, such as delta-like 1 homolog (Dlk1) protein whose overexpression is thought to be responsible for muscle hypertrophy in callipyge sheep. In fact, extracellular Dlk1 protein has been shown to promote the myogenic differentiation of satellite cells. Understanding the cellular and molecular mechanisms within the stem cell niche that regulate satellite cell differentiation and maintain muscle homeostasis may lead to promising approaches to optimizing muscle growth and composition, thus improving meat production and quality.     

Cytokeratin‐positive folliculo‐stellate cells in chicken adenohypophysis

Folliculo‐stellate (FS) cells are non‐endocrine cells found in the adenohypophysis and are identified in many animals by the S100 protein marker. Although keratin is another FS marker in several animals, there is no information on localization of keratin in the avian adenohypophysis. In this study, localization of cytokeratin in chicken adenohypophyseal cells was investigated immunohistochemically. Basic cytokeratin (bCK)‐positive cells were arranged radially in the cell cords with their cytoplasmic processes reaching the basal lamina. The cell bodies encircled a follicle in the center of the cell cord. Furthermore, the bCK‐positive cells were also S100B‐positive. Growth hormone, prolactin, adrenocorticotrophic hormone, and luteinizing hormone β‐subunit did not co‐localize with the bCK‐positive cells. In addition, the bCK‐positive cells had a laminin‐positive area in their cytoplasm. Transmission electron microscopy observed agranular cells equipped with several microvilli that encircled a follicle. These results indicate that bCK‐positive cells in the chicken adenohypophysis may be a predominant FS cell population and produce laminin. It is suggested that they function as sustentacular cells to sustain the adjacent endocrine cells and the structure of the cell cords in the chicken adenohypophysis.     

Morphogenesis of the Epithelium and the Lamina Propria of the Rumen in Bovine Fetuses and Neonates

Developmental changes in the mucous membrane of the rumen in bovine fetuses were observed by scanning electron microscopy (SEM). The ruminal epitheliums were obtained from 20 bovine fetuses and 4 bovine neonates. The samples were divided into 3 groups. Group 1 was examined in the epithelial surface by SEM. Group 2 was examined in the surface of the lamina propria and the reverse face of the epithelium by SEM, after having separate the epithelium from the lamina propria by Scallata's PBS-EDTA method. Group 3 was examined in histological aspect.
The ruminal papillae appeared first in early in the 5th month of gestation. On the other hand, the papillae of the lamina propria appeared first in early 4th month or late 4th month of gestation. It seemed that the formation of papillae in the lamina propria always preceded that of ruminal papillae. Meanwhile, some epithelial cells were exfoliated from the ruminal surface cells during late 4th month to early 5th month of gestation. Ruminal surface cells came to have keratohyaline granules from the 5th month of gestation onward.     

Ultrastructure of cultured canine oral keratinocytes

Keratinocytes from explants of the oral mucosa of dogs were grown in culture for five passages. The ultrastructure of primary cultures and fully developed subcultures passaged 1, 3, and 5 times was examined. At every stage, the cells had the morphologic characteristics of epithelial cells and formed a multilayered squamous epithelium. The basal cells had the characteristics of metabolically active cells, whereas the suprabasal cells and the cells at the media interface expressed many, but not all, of the organelles and cell surface characteristics associated with keratinocyte differentiation. Keratohyalin granules were located in the suprabasal and superficial cells. Cell size and shape and the relationship between cells in the layers also reflected the morphologic characteristics of the parent tissue. Cells maintained this typical structure through all passages and the cultures changed minimally for up to a week after development.     

Submicroscopic changes in the secretory cells of the epithelium of the bovine oviduct during the estrous cycle

The secretory cells of the ampullar part of bovine oviduct are characterized by the secretory granules in cytoplasm. Their apical surface is covered with a small amount of micro-villi. Among the organellae, the Golgi apparatus is well-marked: during the sexual cycle it produces several centres. Its volume and surface reach their maxima before ovulation. The mitochondria are large and have a smaller amount of cristae. Their number culminates in pro-oestrus and their volume and surface area in metoestrus. The tubuli of granulated endoplasmic reticulum dilate during pro-oestrus and oestrus and some of them later disintegrate. They reach the largest volume and surface in pro-oestrus when the highest number of polyribosomes also occurs. Secretory granules are produced throughout the cycle as low-density spherical corpuscles covered with a simple membrane: in the course of maturation they change into high-density corpuscles and some of them break down. Their number and volume culminate in metoestrus when they are secreted on a mass scale. The population of secretory cells is heterogeneous. Besides dense and light cells, thin granules. Some cells lose contact with the basal membrane and their extrusion occurs mainly during metoestrus.     

Histology and ultrastructure of the equine lingual tonsil. I. Crypt epithelium and associated structures

The microstructural and ultrastructural features of the equine lingual tonsil were studied in five young horses. Located at the root of the tongue it presented an irregular surface with rounded elevations, numerous folds and crypts. Stratified squamous non-keratinized epithelium lining its outer surface was modified by heavy infiltration of lymphoid cells to form reticular epithelium within the crypt. The latter implies a role in initiating and maintaining immune responses to incoming infectious agents and antigens. Lamellated structures resembling Hassall's corpuscle were observed towards the outer surface epithelium. Microplicae were visible by scanning electron microscopy on the surface of both the outer and reticular epithelia. No microvillus cells resembling M cells were observed. The stratum superficiale of the reticular epithelium showed strong affinity for Soybean (SBA), Phosphocarpus tetragonolobus 1 (WBA 1), Ulex europaeus (UEA) and Griffonia simplicifolia 1 isolectin-B4 (GS1-B4). The characteristic lectin binding patterns may be useful for embryological and microbiological investigations. Vimentin filaments were not detected consistent with absence of M cells. Mucus glandular acini in the deeper lamina propria mucosae contained glycogen, acidic, neutral and weakly sulphated mucopolysaccharides. Transmission electron microscopy revealed that the layers of the outer surface and reticular epithelia shared characteristic features except the stratum superficiale, which had nuclei of varying shapes and an abundance of cell organelles. A few mast cells with electron lucent granules and myelinated nerve fibres were localized in the deeper portion.     

The expression of p63 and cytokeratin 5 in mixed tumors of the canine mammary gland provides new insights into the histogenesis of these neoplasms

Cytokeratin 5 and p63 have been described as basal and myoepithelial cell markers in human breast. Mixed tumors of the canine mammary gland have been associated with a myoepithelial origin. Cytokeratin 5 expression has not been evaluated in these tumors. We investigated the relation between cytokeratin 5 and p63 double-immunohistochemical expression in 23 mixed tumors of the canine mammary gland (10 benign mixed tumors and 13 carcinomas arising from benign mixed tumors) and their origin. Cytokeratin 5 and p63 co-expression was observed in myoepithelial cells of benign mixed tumors, as well as in squamous differentiation of carcinoma arising from benign mixed tumors. Though a few interstitial spindle cells of the mesenchymal components expressed both p63 and cytokeratin 5, the basal epithelial cells were labeled only by cytokeratin 5. The co-expression of p63 and cytokeratin 5 in myoepithelial cells and squamous differentiation suggest that, like in human breast, cytokeratin 5 can also be considered a myoepithelial- and squamous-cell differentiating marker in canine tumors. The presence of some interstitial spindle cells stained for p63 and cytokeratin 5 might be associated with a myoepithelial origin of the mesenchymal component of mixed tumors of the canine mammary gland. Moreover, contrary to p63, basal epithelial cells were labeled by cytokeratin 5, indicating that cytokeratin 5 may not represent an exclusive myoepithelial cell marker but also a basal epithelial cell marker in canine mixed tumors. According to these data, basal epithelial cells may be related to the origin of the epithelial component of mixed tumors of the canine mammary gland.