Functional Histology of Salivary Gland Pleomorphic Adenoma: An Appraisal

Abstract

The complex microstructure of salivary gland pleomorphic adenoma is examined in relation to function. Events related to secretion of macromolecules and absorption, responses to the altered microenvironment and controversies concerning epithelial-mesenchymal transition versus modified myoepithelial differentiation are explored. Their effects on tumor cell phenotypes and arrangements are emphasized. Heterotopic differentiation and attempts at organogenesis are also considered. The approach allows interpreting microstructure independently of histogenetic perceptions, envisaging the tumor cells as a continuum, endorsing luminal structures as the principal components, and defining pleomorphic adenoma as a benign epithelial tumour characterized by variable epithelial-mesenchymal transition, secretion/differentiation and metaplasia.

Fig. 1

Scanned histological sections of parotid PAs. a Bosselated, circumscribed, solid tumor with an equal epithelial (E): stromal (S) ratio, which is incompletely surrounded by capsule (C). b Largely cystic tumor divided in two; the arrow indicates a typical area. c Multinodular, recurrent tumor; while most of the nodules are stroma-rich and myxoid (asterisk), other are cell-rich and of a monomorphic appearance (arrow) (Unless otherwise specified, the pictures are from sections stained with hematoxylin and eosin. Zooming on the electronic format of the picture would allow appreciation of detail difficult to be seen in print)

Fig. 2

Typical histology of PA. Luminal structures are present at the upper part of the pictures. a Myxoid stroma (asterisk). b Chondroid stroma (asterisk)

Fig. 3

a Bilayered, luminal structures of PA (arrowheads) showing an outer layer of clear cells. L Lumen. b Ductule and related cluster of acini from a case of simple fibrocystic disease of the breasts. While clear, rounded myoepithelial cells are on the outer aspect of acini (arrow), elongated myoepithelial cells with eosinophilic cytoplasm/differentiated myofibrils surround the ductule (arrowhead). Comparison of the figures explains why clear cells in PA had been interpreted as myoepithelial

Fig. 4

PA stained by Alcian Blue at pH 2.5 followed by periodic acid-Schiff (AB pH 2.5-PAS). Luminal structures containing a mixture of neutral and acid glycoproteins staining royal blue are seen at the left part of the picture. The asterisk indicates stromal glycosaminoglycan staining sky blue

Fig. 5

PA stained by elastic—van Gieson. Copious, newly formed elastin staining black is seen. Part of the collagenous capsule that stains red is at the upper left of the picture

Fig. 6

a Collagenous, stellate fibrillar structures. b Multiple tyrosine crystalloids of floral appearance (arrowheads) in myxoid stroma. An aggregate of dyscohesive, non-luminal, tumor cells is at the upper left part of the picture

Fig. 7

Subcapsular venous structures (V). C capsule

Fig. 8

A small subcapsular nerve fascicle (N). C capsule

Fig. 9

PA stained by high-iron diamine followed by AB at pH 2.5 (HID-AB). Carboxylated, non-sulfated glycoproteins (sky blue) are seen in the apical cytoplasm of luminal cells around a lumen (L). Sulfated glycosaminoglycan (brown) is present in the stroma (S)

Fig. 10

Cryostat section of PA stained with glyoxal bis(2-hydroxyanil) for ionized and ionizable calcium. Weak stain (arrow) partly outlines a luminal structure (L)

Fig. 11

Cohesive luminal structures lined by short columnar tumor cells that contain adluminal small amphophilic secretory granules (arrows). The increased height and cytoplasmic granulation distinguish those cells from normal intercalated ductal cells

Fig. 12

Adluminal mucoid material (arrow). The arrowhead indicates basal cell arrangements rimmed by hyalinised material; they resemble those of salivary basal cell adenomas and cutaneous cylindroma and illustrate similarities between salivary and dermal adnexal tumors

Fig. 13

a Tumor mucous cells in acinar arrangements (arrows). b Staining by HID-AB shows carboxylated, non-sulfated glycoproteins (sky blue) in the mucous cells, which contrast with the stromal, sulfated glycosaminoglycan (brown)

Fig. 14

Cohesive luminal structures interspersed with serous-like tumor cells (arrows)

Fig. 15

PA with luminal structures resembling striated ducts. The inset shows normal striated ducts with adluminally localized prostate specific antigen

Fig. 16

Nuclear and cytoplasmic S-100 protein immunoreactivity of largely cohesive, non-luminal tumor cells. L lumina. The inset shows minor salivary gland where expression of the protein is confined to demilunes and ductal cells (arrowhead); the demilunar staining accounts for the perception of S-100 protein as a myoepithelial marker

Fig. 17

Nuclear p63 immunoreactivity of largely cohesive, non-luminal tumor cells. L Lumen. The inset shows normal salivary acini with stained nuclei at their periphery, but it is difficult to ascribe them to myoepithelial cells

Fig. 18

Expression of CK14 in luminal tumor cells. L Lumen. The inset shows normal salivary acini embraced by strongly stained myoepithelial cells; this accounts for the popular perception of CK14 as a myoepithelial marker

Fig. 19

Plasmacytoid cells (a). They are variously stained with a milling dye (tannic acid—phosphomolybdic acid—amidoblack; b arrows), and are α-SMA (−) (c, stained vessels are arrowed), S-100 protein (+) (d), CK/5/6 (−/+) (e), CK14 (−) (f), vimentin (+) (g) and WT1 (+) (h)

Fig. 20

a The lining of a luminal structure (L) shows variably widened intercellular spaces (arrows), which indicates decreased cell cohesion. b Adjacent section stained by AB pH 2.5-PAS shows Alcianophilic, sky blue glycosaminoglycan in the intercellular spaces. Glycoproteins staining royal blue are present in the lumen (L)

Fig. 21

Spindled, angular non-luminal cells staining for α-SMA (a) and CK7 (b)

Fig. 22

Lipometaplasia or adipose EMT in PA? Collections of adipocytes (F), luminal structures (arrows), pearly keratinisation (arrowhead) and chondroid stroma (upper left part of the picture) are seen

Fig. 23

Sheets of variably dyscohesive, non-luminal cells strongly stained for the WT1 antigen, stream from luminal structures (L) with unstained, luminal cells. The unstained nuclei of the non-luminal cells can be seen

Fig. 24

Hair follicle in PA? “Normal” associations of “hair shaft” (arrow) and “outer root sheath” (arrowhead)

Fig. 25

Apocrine-like cells showing intraluminally bulging apices (arrowhead). L Lumen

Fig. 26

Luminal cells of PA showing moderate to strong, adluminal CD63 immunoreactivity. The extent of staining varies from cytoplasmic, adluminal rims (arrow) to apical (arrowhead). Mucous cells (M) show weak, cytoplasmic, diffuse immunoreactivity. Non-luminal cells are unstained

Fig. 27

Cellular aggregates interspersed with microliths (arrows)

Fig. 28

Squamous, non-luminal (asterisk) and luminal (arrow) cells in PA

Fig. 29

Oncocytic cells in PA, one of which shows “atypical” nucleus (arrow)

Fig. 30

Osseous metaplasia or EMT in PA? Typical tumor is seen at the upper and right parts of the picture. The arrow indicates osteoclasts

Fig. 31

Selective, strong, cytoplasmic, diffuse HSP27 immunoreactivity of cohesive non-luminal basal cells. Note the unstained luminal cells and nuclei of basal cells. L lumen

Fig. 32

Serous-like cell in a demilunar arrangement (arrow)

Fig. 33

Organogenesis or atrophic salivary parenchyma trapped within a PA?

Fig. 34

MALT-like arrangements in PA? The “homing” lymphocytes are affected by retraction artefact, which results in a “clear” appearance (a–d). They are interspersed between luminal CAM5.2 (+) (b) and non-luminal α-SMA (+) cells (c); and are CD79A (−) (d), CD3 (+) (e) and HLA-DR (+) (f). L lumen

Fig. 35

Biphasic FANCD2 immunoreactivity in PA. Luminal cells show moderate to strong cytoplasmic staining. Non-luminal cells and stroma (asterisk) are unstained. L lumen