THE ANATOMICAL RECORD 236:586-590 (1993) Incidence and Histology of Human Accessory Parotid Glands HIDETOSHI TOH, JUN KODAMA, JINICHI FUKUDA, BARRY RITTMAN, AND IAN MACKENZIE Second Department of Oral Anatomy, Fukuoka Dental College (H.T., J.K.), Department of Dentistry and Oral Surgery, School of Medicine (J.F.), Fukuoka University, Fukuoka, Japan; University of Texas Health Science Center, Dental Branch, Houston, Texas (B.R., I.M.) ABSTRACT The parotid glands of 228 Japanese human cadavers were examined to determine the incidence and histological features of accessory parotid glands. The incidence was found to be 56% with no differences between right and left sides or between sexes. Thirty parotid glands and their associated accessory glands were examined histologically: eight of these accessory glands were found to be mixed secretory glands (i.e., containing both serous and mucous acini).Thus, the pattern of differentiation of a significant fraction of accessory glands differs from that of the main parotid gland: it appears that mixed acini, present in the early stages of development, persist into later life, and their presence may be related to tumors developing at these sites. o 1993 Wiley-Liss, Inc. Key words: Accessory parotid glands, Japanese, Adult human Since the end of the last century, many reports have described the anatomical and histological features of the normal human parotid gland (Jonnesco and Juvara, 1896; Testut and Jacobs, 1905); and the presence of accessory parotid glands, which are collections of salivary tissue separate from the main gland and located on the anterior portion of the masseter muscle, is well established (Ogawa, 1918; Batsakis, 1988; Ellis et al., 1991). Studies of accessory parotid glands have reported their incidence t o range from 21% (Frommer, 1977)to 69% (Toh et al., 1984)and have described their histology (Frommer, 1977; Toh et al., 1984; Takeda et al., 1985). Human accessory parotid glands are generally regarded as serous in nature, but Zimmermann (18981, Akiyoshi (1929), and Takeishi (1966) have indicated the presence of both mucous and serous acini in the newborn parotid, and Toh et al. (1984) and Takeda et al. (1985) have reported that accessory parotid glands in the adult may also contain both mucous and serous acini. The presently described work investigated a series of 228 pairs of Japanese human parotid glands to provide better data concerning the incidence and histologic features of accessory parotid glands. MATERIALS AND METHODS The accessory parotid glands studied were derived from dissection of 107 male and 121 female Japanese cadavers (Table 1). From the 128 cadavers showing the presence of accessory glands, 30 main parotid glands, together with the accessory parotid glands, were removed and the glandular capsule carefully stripped free, keeping the facial nerve and blood vessels intact. Specimens were refixed in 10% neutral formalin, processed to paraffin wax for sectioning at 6 pm, and stained for examination by light microscopy with hematoxylin-eosin, with Azan trichrome (Cason, 19501, and with mucicarmine (Lillie, 1965). 0 1993 WILEY-LISS, INC. RESULTS Morphology Figure l a illustrates the common appearance of a human accessory parotid gland as a small gland separated from the main parotid gland and lying above the parotid duct. The accessory glands are usually round or ellipsoidal in shape (Fig. lb,c) but occasionally may be triangular (Fig. Id) or of indeterminate shape (Fig. le). Seven of the glands examined showed very small “ricelike” accessory glands lying on or above the duct (Fig. If) and at the buccinator side of the anterior border of the masseter muscle without being covered by the buccal fat pad (Fig. 2). Generally, accessory parotid glands lie on the masseter muscle, with the zygomatic and buccal branches of the parotid plexus of the facial nerve being found both within the lobes and overlying them as a net-like covering (Fig. 3). Branches of the transverse facial artery are usually buried in the lobes of the accessory glands and there is a clear boundary between the main parotid gland and the accessory glands. The “rice-like” glands showed no consistent relationship to either the parotid plexus of the facial nerve or the transverse facial artery. incidence Table 2 shows the incidence of accessory parotid glands. Of the 228 cadavers examined, 32 cadavers (14%) showed accessory parotid glands on both sides, 52 (23%)on the left side only, and 44 (19%)on the right side only. The incidence of accessory parotid glands in males was 55% and glands were present on both sides in 13%, on the left side only in 21%, and an the right Received August 27, 1992; accepted December 22, 1992. Address reprint requests to Dr. Ian C. Mackenzie, University of Texas Health Science Center, Dental Branch, P.O. Box 20068, Houston, TX 77225. ACCESSORY HUMAN PAROTID GLANDS TABLE 1. Age and sex distribution Age 30-39 40-49 50-59 60-69 70-79 80-89 90-99 2100 Total Male Female 3 16 45 37 5 107 Total 1 0 1 3 6 22 34 63 17 1 79 100 121 228 22 1 Fig. 1. Illustrations of the variable morphology of accessory parotid glands (arrows). a: Typical accessory parotid gland, male aged 84 years. b: Round shape, female aged 77 years. c: Ellipsoidal shape, female aged 83 years. d: Triangular shape, female aged 83 years. e: 587 side only in 21% of the cadavers. The incidence in females was 57%,with the presence on both sides at 15%, the left side only 25% and the right side only 17%. Histologic Features The 30 parotid and accessory parotid glands that were stained with hematoxylin-eosin and with Trichrome stain showed the presence of serous cells and fat cells. However, right accessory parotid glands, unlike the main parotid glands, showed characteristic mucous cells with light cytoplasm and nuclei com- Indeterminate gland, female aged 66 years. f: Rice-like shape, male aged 71 years. D: duct; F: facial artery; N: facial nerve; 0: obicularis oculi; P: parotid gland; Z zygomaticus minor and major. H. TOH ET AL. 588 Fig. 2. A deeply lying accessory parotid gland (arrow) a t the buccinator side of the anterior border of the masseter muscle. Female aged 89 years. pressed toward the base (Fig. 4a). The cytoplasm of these cells showed strong staining with mucicarmine, further evidence of their mucous nature. The mixed nature of the accessory parotid glands was also indicated by the presence of serous demilunes and goblet cells in the striated duct epithelium (Fig. 4b). DISCUSSION The reported incidence of accessory parotid glands ranges from 21% (Frommer, 1977) to 69% (Toh et al., 1984) and is generally regarded to be below 50%. The high percentage found in the present and previous studies by this group is believed to be the result of detailed examination of a greater number of samples. In particular, the existence of “rice-like” accessory glands at the front edge of the masseter muscle close to the fat pad must be taken into account when studies of the incidence of accessory parotid glands are undertaken. The anterior processes of parotid glands are described in older textbooks (Testut and Jacobs, 1905; Jonnesco and Juvara, 1896,) as lobes protruding to the front edge of the masseter muscle. Ao and Okada (1965) and Toh et al. (1984) noted that more than half of the accessory parotid glands were either round or triangular in shape and that all showed anastomosis of the zygomatic and buccal branches of the facial nerve buried within them. Most accessory parotid glands showed a net-like covering by this nerve. The rapid embryonic development of the anterior processes of the parotid gland may influence the size and distribution of the developing facial nerve. During development the parotid plexus of the facial nerve branches extensively and the parotid duct lies under the anastomosis of its buccal branch. According to McCormack et al. (1945),there is usually anastomosis between the zygomatic and buccal branches of the facial nerve on the fat pad a t the anterior edge of the masseter muscle with only 13% of facial nerves lacking anastomosis of the zygomatic and buccal branches. Gasser (1970) reported the presence of accessory parotid glands in the 8-week human fetus (27 mm CR length): these were considered to be extensions of the main gland and might therefore be expected to have a similar cellular composition, i.e., to be serous in na- Fig. 3. Accessory parotid glands (arrows) surrounded by a rich network of the parotid plexus of the facial nerve. a: Male aged 78 years. b Male aged 91 years, TABLE 2. Frequency of the accessory parotid glands Subjects M F Totals N R (%I 107 121 228 23 21 (21) (17) (19) 44 L 22 30 52 (%I Both (%) (21) (25) (23) 14 18 (13) (15) (14) 32 Total 70 55 57 128 ACCESSORY HUMAN PAROTID GLANDS 589 Fig. 4. Photomicrographs of accessory parotid glands showing mucous (MA) and serous (SA) acini, several intralobular striated ducts (D), fat cells (F), and serous demilunes (arrows). a Female aged 85 years. H & E stain. b: Male aged 77 years. Mucicarmine stain. ture. However, the present study confirms previous reports of the presence of mixed acini both in the adult (Zimmermann, 1927; Du Plessis, 1957; Evans and Cruikshank, 1970; Iwata et al., 1977; Komori et al., 1979; Toh et al., 1984; Takeda et al., 1985) and in the terminal portion of newborn human accessory parotid glands (Zimmermann, 1898). According to Taga and Sesso (1979), the rat parotid gland also has mixed acini during its early development. The surgical anatomy of accessory parotid glands is of importance in several clinical situations. For example, failure of removal of involved accessory glands may be associated with recurrence of neoplastic lesions (Ellis et al., 1991) or sialadenitis (Ferguson and McDonald, 1978). Further, the cellular composition of the accessory glands may differ from that of the main gland and this too could be of clinical significance; more work is needed to clarify the extent to which mixed acini persist from the fetal stage of development and about the cells of origin of salivary tumors (Batsakis et al., 1989). However, the most common epithelial tumors arising in the salivary glands of children are benign pleomorphic adenomas and mucoepidermoid carcinomas (Kamori, 1977; Greer et al., 1983; Ellis et al., 1991), and the mixed acini persisting from the early stages of development could influence the patterns of development of such tumors. LITERATURE CITED Akiyoshi, Y. 1929 Histologische studien der Glandula parotis des menschlichen Embryos und Sauglings. Nagasaki Igakkai Zassi, 7:106-115. (in Japanese) Ao, H. and N. Okada 1956 Anatomical study of accessory parotid glands. Tokyo shika daigaku kaibougaku gyousekisyu, 1 :1-5. 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