Crystals and crystalloids in oral pathology - An overview

INTRODUCTION Crystals and crystalloids are frequently encountered in neoplastic and non-neoplastic lesions of the salivary glands and can serve as a diagnostic aid.[1] The identification of various crystalloid forms will also help in determining the nature of salivary gland diseases.[2] Crystals are solid and highly structured with birefringence that are created by molecules repeating in three dimensions whereas crystalloid structures are non-birefringent with no recognizable internal structures.[3] Crystals and crystalloids are most often identified in Fine Needle Aspiration Cytology (FNAC) of salivary gland tumours. Crystalloids The most common crystalloids seen in the fine needle aspiration cytology of salivary gland lesions include Amylase, Tyrosine and Collagenous crystalloids. Amylase crystalloids Amylase crystalloids, otherwise known as nontyrosine crystalloids, can have tabular structures with frequent pointed ends, sometimes rhomboid, rectangular, or plate-like or needle-like shapes.[1,4] α-amylase crystalloids range in size from 5 μm to 500 μm.[2] They stain orange with Papanicolaou (PAP) stain, deep blue with Romanowsky stain, deep blue by May–Grünwald–Giemsa stain, and pink by haematoxylin-eosin stains.[2] Sometimes, psammoma body-like structures may be identified in association with amylase crystalloids.[1] Salivary stasis contribute to cystic space formation which leads to the formation of amylase crystalloids. This is due to condensation of α-amylase in supersaturated saliva that undergoes crystallization in the cystic space.[1] Amylase crystalloids are usually found in older individuals and frequently associated with duct obstruction, sialadenitis, cystic lesions such as oncocytic cysts or cystadenomas, developmental cysts, lymphoepithelial cysts, oncocytic metaplasia, and benign neoplasm like Warthin’s tumor.[1] While amylase crystalloids on FNAC are not specific for a particular lesion, they have not been linked to malignancies, which is a reassuring characteristic indicating a benign non-neoplastic or benign neoplastic lesion [Figures 1a and 2a].[1]Figure 1: Hand drawn illustrations of (a) Amylase crystalloids: tabular structures with pointed ends, Needle shaped, rhomboid, and rectangular. (b) Tyrsoine crystalloids: multiple floret or petal shaped appearance. (c) Collagenous crystalloids: radially arranged clusters of needle-shaped fibers surrounded by modified myoepithelial cells. (d) Intraluminal crystalloids: geometrically shaped dense amorphous materialFigure 2: Histopathological images of (a) amylase crystalloids, (b) tyrosine crystalloids, and (c) intraluminal crystalloidsTyrosine crystalloids Tyrosine crystalloids are clusters of eosinophilic crystalloids.[5] The most distinctive morphologic pattern consists of central, translucent, glassy, round core with multiple floret, afibrillar appearance arranged in radially symmetrical pattern, sunburst or petal shaped morphology with blunt ends or rosette-like appearance.[1,5] They stain orange with PAP stain and deep blue with Romanowsky type stains.[1] These structures exhibit strongly positive staining with both the Millon reagent and the protein diazotization coupling reaction.[5] Tyrosine crystalloids are associated with the stromal and myoepithelial cells of neoplastic processes; hence, they are believed to be formed from secretion by these neoplastic components.[1] They are associated with pleomorphic adenoma, Warthin’s tumor, oncocytoma and also with malignancies, including carcinoma ex pleomorphic adenoma, adenoid cystic carcinoma, and polymorphous adenocarcinoma. Tyrosine crystalloids are also associated with non-neoplastic processes like lymphoepithelial cysts and even fibrous tissue following surgery [Figures 1b and 2b].[1] Collagenous crystalloids Collagenous crystalloids appeared as eosinophilic radially arranged needle-shaped fibers of collagen (Type I and Type III) around a clear central area surrounded by modified myoepithelial cells.[1,2,6] Crystalloids are stained blue by the Masson trichrome method as they constitute collagen. Collagenous crystalloids are surrounded by neoplastic myoepithelial cells. Neoplastic myoepithelial cells may be associated with the formation of these structures.[6] They are commonly seen in pleomorphic adenomas and myoepitheliomas [Figure 1c].[1,7] Intraluminal crystalloids Intraluminal crystalloids are large geometrically shaped crystalline structures composed of dense amorphous eosinophilic material. They have been reported in cystic spaces of both neoplastic and non-neoplastic salivary oncocytic lesions. They are considered to be a product of oncocytic cell secretion.[2] They are found in malignant salivary gland tumors.[2,8] These crystalloids were mainly composed of carbon and sulfur, which are chemical components of the proteins [Figures 1d and 2c].[2,8,9] Crystals Crystals are solid highly ordered structures that are created by molecules repeating in three dimensions. They are found to be refractile, polarizable, and birefringence. Calcium oxalate and cholesterol crystals are most commonly seen in intraoral lesions.[3] Calcium oxalate crystals Calcium oxalate crystals are square to rectangular in shape. They are true crystals and hence refractile and polarizable.[1] They have been frequently associated with pleomorphic adenomas and rarely in parotid sarcoidosis.[1,10] Increased 1,25(OH) D which is the biologic active form of vitamin D synthesized by macrophages and other cells associated with granulomatous tissues causes hypercalcemia, which might lead to the formation of calcium oxalate crystals [Figure 3a].[10]Figure 3: Hand drawn illustration of (a) calcium oxalate crystals: dumbbell shape, square, and rectangular. (b) Cholesterol crystals: irregular rectangular shaped with notched corners. (c) H&E image of cholesterol cleftsCholesterol crystals Cholesterol crystals have a typical rhomboid shape. In paraffin sections, the cholesterol crystals are dissolved by the fat solvents in tissue processing and leave needle-like clefts known as “Cholesterin clefts” within the cyst wall and the cyst cavity.[11,12] In cytology, it can be in the form of flat, parallelogram-shaped crystals in a range of sizes, often in aggregates. The background may show hemosiderin-laden macrophages, as observed in hemorrhagic processes.[1] Local tissue damage appears to be a requisite for cholesterol accumulation. The cholesterol crystals are deposited in the cyst cavity by means of a foreign body giant cell reaction within the fibrous cyst wall [Figure 3b and c].[11,12] CONCLUSION Crystals and crystalloids serve as noncellular morphological markers that are useful for the clinicians in differentiating benign and malignant lesions of the salivary glands. It also helps in understanding the nature of lesions and most often observed in minimally invasive FNAC samples of salivary glands. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.