|Year : 2014 | Volume
| Issue : 1 | Page : 40-43
Odontogenic ghost cells: Realities behind the shadow….
Thippeswamy Shamenahalli Halappa, Jiji George, Abhilasha Shukla
Department of Oral Pathology and Microbiology, Babu Banarasi Das College of Dental Sciences, Lucknow, Uttar Pradesh, India
|Date of Web Publication||5-Sep-2014|
Thippeswamy Shamenahalli Halappa
Department of Oral Pathology and Microbiology, Babu Banarasi Das College of Dental Sciences, Lucknow - 227 105, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Ghost cells are pale anucleate cells with homogeneous pale eosinophilic cytoplasm with very pale to clear central areas in place of a basophilic nucleus; which are seen in small clusters or large masses. Few odontogenic and nonodontogenic tumors exhibit the presence of these transparent or shadow cells as a typical feature. Many attempts have been made in the past by several investigators regarding the true characteristics of ghost cells in these lesions, in spite of which they seem to be mysterious. Here, we have made an attempt to clear the illusions and controversies surrounding ghost cells.
Keywords: Calcifying cystic odontogenic tumor, ghost cell odontogenic tumor, shadow cell
|How to cite this article:|
Halappa TS, George J, Shukla A. Odontogenic ghost cells: Realities behind the shadow…. J Oral Res Rev 2014;6:40-3
| Introduction|| |
In general, Ghost cells are described as pale eosinophilic, balloon shaped, elliptic epithelial cells that have lost their nuclei, leaving only a faint outline, hence the term "ghost." Although the cell outlines are usually well-defined, they may sometimes be blurred giving the group of ghost cells a fused appearance [Figure 1], [Figure 2], [Figure 3].  Ghost cells have also been described in several other odontogenic lesions, which include calcifying ghost cell odontogenic cysts/tumor, odontomas, ameloblastic fibro-odontomas, and solid/multicystic ameloblastomas. Furthermore, ghost cells with similar histomorphologic appearance to those in odontogenic lesions are found in craniopharyngiomas, and in cutaneous epithelioma of Malherbe. 
|Figure 2: Photomicrograph showing fused ghost cells revealing blurred nuclear remnants (×40)|
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|Figure 3: Photomicrograph showing typical microscopic characteristics ghost cells: (a) Pale eosinophilic cytoplasm. (b) Balloon shaped elliptical cell with well-defined cellular outline. (c) Clear central area with faint nuclear outline|
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| Histochemical/Immunohistochemical Characteristics of Ghost Cells|| |
Many investigators have made efforts to clarify the nature of ghost cells by employing methods ranging from conventional histochemistry to immunocytochemistry. However, the nature of ghost cells has been illusional and many hypotheses have been proposed and debated upon. Ghost cells are believed to be transformed odontogenic epithelial cells, the mechanism of which is still unclear. First description on ghost cells was cited by Highman and Ogden  in 1936. They described ghost cells as dyskeratotic cells and criticized the term degeneration or necrosis for these cells as their form and coherence were retained.
Gorlin et al.  in 1964 concluded that ghost cells in calcifying odontogenic cysts (COCs), pilomatrixomas and craniopharyngiomas represent a form of abnormal keratinization; a concept, which was supported by many authors over the years. The changes in ghost cells were interpreted as aberrant, incomplete or true keratinization. ,, However, many immunohistochemical studies on cytokeratins in ghost cells failed to demonstrate positive staining for different kinds of keratin.
Levy  suggested that ghost cells represent squamous metaplasia with subsequent calcification caused by ischemia. Sedano and Pindborg  thought that the ghost cells represented different stages of normal and aberrant keratin formation and were derived from the metaplastic transformation of odontogenic epithelium. Hong et al.  proposed that the characteristics of ghost cells are compatible with the features of coagulative necrosis visualized in odontogenic epithelium.
Takata et al.  in their study using polyclonal antibodies against a wide spectrum of cytokeratins found that ghost cells in COCs showed only faint or no positivity, while adjacent nonghost epithelial cells were obviously positive, which led to the conclusion that aberrant keratinization seems to make only a minor contribution to the formation of ghost cells and biologic properties of ghost cells are different from keratinocytes.
Kim et al.  performed an immunohistochemical study for ghost cells using cytokeratins, involucrin, and apoptosis-related proteins such as Bcl-2, Bcl-X L , and Bax. Their study demonstrated expression of cytokeratins and involucrins in the nucleated cells adjacent to the ghost cells, whereas the ghost cells showed no reaction; but, they exhibited Bax proteins, which made them conclude that ghost cells undergo abnormal terminal differentiation.
Further, Takata et al.  examined the immunoreactivity of ghost cells in COCs and dermal calcifying epitheliomas, with antibodies against amelogenin, enamelin, sheath protein (sheathlin) and enamelysin in the cytoplasm of ghost cells. They found a distinct immunolocalization of the enamel-related proteins within ghost cells of COC, while similar areas in the calcifying epitheliomas of the skin showed a negative reaction. These findings strengthen the belief that ghost cells seen associated with odontogenic lesions are different from those seen in lesions of skin origin.
Lucchese et al.  analyzed the ghost cells using confocal laser scanning microscope. Based on the different fluorescent effects, they divided ghost cells into three types:
- Scarcely detectable,
- Well-resolved and
- Cells with excellent resolution.
They represent three different stages of ghost cell maturation, which indicates accumulation of hard keratin in the cell during pathological transformation, which suggests that ghost cells might represent differentiation into hair.
The recent WHO classification of odontogenic tumors considered ghost cells as transitory squamous cells at various stages of differentiation.  However, until date, nature and process of formation of ghost cells remains elusive.
| Ultra-structural Findings|| |
Fejerskov and Krogh  explained the ultra-structure of ghost cells in COCs; where they found coarse, thick tonofilament bundles in their cytoplasm, thus distinguishing it from the keratin pattern of the epidermis or oral epithelium. Endoplasmic reticulum, mitochondria, Golgi apparatus and ribosomes could not be identified. In general, oral epithelium exhibits evenly distributed, fine tonofilaments in the cytoplasm. These findings were supported by Regezi et al.,  who suggested that ghost cells represent an unusual or aberrant form of keratin and not the true keratin.
| Staining Characteristics|| |
Various stains, e.g., Goldner stain, Van Gieson [Figure 4], Masson's trichrome, Mallory and Rhodamine B for fluorescence microscopy, may be useful in distinguishing ghost cells and other acidophilic masses  [Table 1]. Phloxin-Tartrazine stain can be used to differentiate ghost cells from similar looking dentinoid areas.  The staining reaction of the ghost cells suggests that they are keratinizing, and they are entirely Thioflavine T-negative. 
|Figure 4: Photomicrograph of ghost cells stained yellow with Van Geison (×10)|
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| Origin, Patterns and Significance|| |
These cells are always epithelial in origin and many investigators believed that these can originate from any layer of the epithelium, such as basal, spinous, or superficial. In calcifying cystic odontogenic tumor (CCOT) based on differentiation of epithelial cells, it can arise from either squamoid or stellate reticulum like cells. Ghost cells differ from normal keratotic squames as they are larger, often vacuolated with prominent nuclear membrane. This may be due to intracellular edema and dilated degenerated membranous organelles present in them; moreover, a ghost cells do not show intercellular bridges. , Sometimes these cells appear to be fused involving larger areas with blurred nuclear remnants; such areas are sometimes referred to as ghost cell keratinization.
Abrams and Howell  identified two ways of the degeneration process leading to ghost cell formation. (i) Transformation of a large mural squames into eosinophilic cells retaining only the outline of the original nucleus. (ii) Individual or small groups of stellate and basal cells enlarge displacing their nuclei to periphery and disappearance of nucleus thereafter. Such cells apparently account for the actual breach of epithelial membrane. Ghost cells when in contact with the connective tissue, evokes a foreign body reaction with the formation of multinucleated giant cells. However, this phenomenon has very less role in the prognosis of the lesion. 
Dystrophic calcification may occur in some of the ghost cells, which are initially seen as fine basophilic granules and later as small spherical bodies. Satomura et al.  studied the initial calcification in ghost cells and found that a variety of vesicles were scattered among the tonofilament bundles. Some vesicles contained needle like crystals, which were considered as initial calcification sites.
Very few studies have been performed to know the significance of ghost cells in odontogenic lesions. The presence of ghost cells within the proliferative odontogenic epithelium is the essential characteristic for the diagnosis of calcifying cystic odontogenic cysts/tumors. However, the ghost cells alone are not sufficient for the diagnosis of these lesions nor is it pathognomonic.
| Role of Wingless/Int-1 (Wnt) and Notch Signaling Pathways in Ghost Cells|| |
Wnt pathway has well-established role in hair formation as well as in odontogenesis, including enamel protein formation. Several studies suggested the possible role of Wnt and Notch signaling pathways in the pathogenesis of ghost cell associated lesions. These two pathways synergistically play a part in maintaining tissue homeostasis, controlling cell fate, patterning and morphogenesis during embryogenesis. Thus, many authors believe that aberrant Wnt pathways are held responsible for similar enamel proteins in ghost cells of craniopharyngioma and CCOT. ,,
Siar et al.  investigated the role of Notch signaling in the fate specification of ghost cells in CCOT, and they demonstrated overexpression of Notch 1 and Jagged 1 in ghost cells, which suggests Notch 1 and Jagged 1 signaling might serve as the main transduction mechanism in cell fate decision for ghost cells in CCOT. Their study also demonstrated positivity of mineralized ghost cells for Notch 1 and jagged 1, which implicates that calcification process might be associated with upregulation of these molecules.
| Conclusion|| |
Ghost cells are typical characteristic of many odontogenic and nonodontogenic lesions, but if their presence has any pathognomonic role in the pathogenesis of these lesions is still debatable. True nature of ghost cells remains elusive and controversial, though their most accepted nature is aberrant keratinization. Recent studies are directed toward the role of Wnt and Notch signaling pathways in tumorigenesis and assigning cells to particular fate. However, further extensive studies on these aspects will give us the possible clue in determining the role of these 'shadow cells' in many odontogenic as well as nonodontogenic lesions.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]