Arch. Histol. Cytol., Vol. 56, No. 4 (1993)p. 417-421

The Existence and Distribution of Melanocytes in the

Periodontal Ligament of the Mongrel Dog*

Yasunori TAKEDAI and Kazuyuki UYENO2

Departments of Oral Pathology1 and Periodontology2, Iwate Medical University School of Dentistry, Morioka, Japan

Received July 12, 1993

Summary. The existence and distribution of melano-cytes in the periodontal ligament of the mongrel dog were investigated. Melanocytes were found in the peri-odontal ligament of the maxillary incisor and premolar segments. Melanocytes in the periodontal ligament varied in number, and generally appeared as dendritic or elongated cells with several, long cytoplasmic proces-ses. These melanocytes were always distributed closer to the tooth root side of the periodontal ligament than the alveolar bone side, but did not contact the cementum. Melanocytes were longitudinally distributed in the apical half of the periodontal ligament. There were no findings suggestive of a relationship between melano-cytes and epithelial rests of Malassez or other cellular elements of the periodontal ligament. Although some parts of the gingival mucosa of all dogs in this study showed pigmentation, no relationship was seen between melanocytes in the periodontal ligament and those in the gingival mucosa.

This is the first report of melanocytes in the peri-odontal ligament under normal conditions.

Melanocytes are derived from the neural crest, from where most of their precursors travel during early embryonal life to the epidermis and hair follicles in mammals, while small numbers reach other locations such as the dermis, sebaceous glands and lactiferous ducts of the nipple, mucous membranes of upper aero-digestive tract, genital organs and anus, orbital cavity, leptomeninges, inner ear, and occasionally some visceral organs (MOOI and KRAUSZ, 1992). In additon, melano-cytes can be embedded within the bone by melano-blasts which have failed to complete their migration from the neural crest to the periphery (ZIMMERMANand BECKER, 1959), such as in the bones of fowls (HIRANO, 1990). However, virtually nothing has been

published in regard to the postnatal existence or distribution of melanocytes in the bones of mammals in normal condition. The occurrence and distribution of melanocytes in intraosseous odontogenic lesions of the jaw have been studied, and the possible histo-

genesis and pathological significance of these melano-cytes discussed (TAKEDA et al., 1985a,b, 1987, 1988, 1989a, b, c, 1990). The present paper reports on the existence and distribution of melanocytes, which have been found during a study on the histogenesis of melanocytes in intraosseous odontogenic lesions of the jaw, in the periodontal ligament of the mongrel dog. This is the first report of melanocytes in normal

periodontal ligaments.

MATERIALS AND METHODS

Mandibles and maxillae obtained from 10 young mongrel dogs, weighing 10-12 kg, with complete dentition of permanent teeth and healthy gingiva, were fixed in 10 % neutral-buffered f ormalin for 3 weeks, then divided into smaller incisor, canine,

premolar, and molar segments. They were demineral-ized in Plank-Rychlo's solution at 4C for 2 weeks,dehydrated and embedded in celloidin. Longitudinal serial sections 15um thick were made along bucco-lingual or mesio-distal direction through the apical area of all teeth. The sections were stained routinly with hematoxylin and eosin. The sections were fur-ther treated with 10% hydrogen peroxide solution when cells with brown to black pigmentation were found in hematoxylin and eosin-stained sections. The brown to black pigmentation in cells with elongated branches disappeared after treatment with 10

*This study was supported in part by a Grant -in-Aid for Developmental Scientific Research (02670811) from the Ministry

of Education, Science and Culture, Japan.

417

418 Y. TAKEDA and K. UYENO:

hydrogen peroxide solution, these pigmented cells being regarded as melanocytes (LILLIE and FULLMER, 1976).

In addition, the upper and lower jaws of two dog fetuses (mongrel dogs), 12 to 13 weeks of gestation, were examined histologically by the methods men-tioned above.

RESULTS

Melanocytes were found in the periodontal ligament of the maxillae of 4 dogs, and not in the mandibles. The segmental locations of the periodontal ligaments containing melanocytes in each dog were as follows: right premolar, left incisor and left premolar regions in Case 1; right incisor region in Case 2; right and left incisor regions in Case 3; and right incisor region in Case 4. Excepting the periodontal ligament, no melano-cytes were found in any other areas of the jaw bone.

Melanocytes in the periodontal ligament varied in number, e.g., a solitary cell, groups of a few cells, or

groups of numerous cells; these cells generally ap-peared as dendritic or elongated cells with several long cytoplasmic processes (Figs. 1, 2). Neither melano-

phages nor pigmentation in cellular elements of the periodontal ligament were seen. Melanocytes were always distributed closer to the tooth side of the

periodontal ligament than the alveolar bone side, but not in contact with the cementum. These melano-

cytes were arranged parallel to the long axis of the tooth root (Fig. 2), and their elongated cytoplasmic

processes also ran parallel to this axis (Fig. 2b). Melanocytes were longitudinally distributed in the apical half of the periodontal ligament, and varied numbers of melanocytes were also scattered in the loose fibrous connective tissue of the periapical area

(Fig. 3). There were no findings suggestive of a morphological intercellular relationship between melanocytes and cells of epithelial rests of Malassez or other cellular elements of the periodontal liga-ment. Although some parts of the gingival mucosa of all the dogs examined showed pigmentation, serial sections failed to reveal any relationships between melanocytes in the periodontal ligament and those in the gingival mucosa.

In the fetuses, melanocytes were found in mesen-chymal tissue around the dental anlage of the upper and lower jaws (Fig. 4), and melanocytes appeared neither in the oral squamous epithelium nor in the epithelial element of the dental anlage at this fetal stage.

DISCUSSION

The periodontal ligament is a fibrous connective tissue attaching the tooth to the alveolar bone. Its

primary function is to support the tooth in the alveolus and to maintain the physiological relation

Fig. 1 a. Melanocyte (arrow) with dendritic appearance in the periodontal ligament. CC cellular cementum. x 40. b. High magnification of melanocytes (arrow) shown in a. x 400

Melanocytes in Periodontal Ligament 419

between the cementum and the bone. It also has

nutritive, sensory and homeostatic properties. The

periodontal ligamnent is composed of cells and an extracellular component of fibers and ground sub-stances. The cells of the healthy, functioning peri-

odontal ligament are as follows: synthetic cells such

as fibroblasts, osteoblasts, cementoblasts; resorptive

cells such as osteoclasts, cementoclasts, fibroclasts; undifferentiated mesenchymal cells; epithelial rests

of Malassez; other types of connective tissue cells

such as mast cells and macrophages; cells of blood

vessels, lymphatics and nerves. However, there have been no descriptions of the existence of melanocytes

in the periodontal ligament of mammals.

Melanocytes are widely distributed in the skin, the nervous system, certain types of mucosa and other

sites, but are not normally present within the bones of mammals. Pathologically, there are very few descrip-tions of intraosseous melanocyte-containing lesions other than metastases of malignant melanoma, and all reported examples have occurred within the jaw bone (TAKEDA et al., 1985a, b, 1987, 1988, 1989, 1990; TAKEDA, 1989; TAKEDA and YAMAMOTO, 1989). Since the presence of melanocytes in the oral mucosa is not uncommon in mammals, and since the dental anlage originates from the primitive oral lining, the occa-sional presence of melanocytes in intraosseous lesions of the jaw must be expected. LAWSON et al.

(1976) have studied the distribution of melanocytes in the dental primordium of human fetuses, and found melanocytes within both the dental lamina and tooth bud, and in the squamous epithelium that lines the

Fig. 2 a and b. Melanocytes (arrow heads), arranged parallel to the long axis of the tooth root in the periodontal ligament. Their elongated cytoplasmic processes are also run parallel to the long axis of the tooth root. Malanocytes are distributed closer to the tooth side of the periodontal ligament than the alveolar bone side, but do not contact the cementum. B alveolar bone, AC acellular cementum, D dentin, CC cellular cementum. X 100

420 Y. TAKEDA and K. UYENO:

oral cavity of fetuses, which had melanocytes in the dental anlage. Such evidence of melanocytes in dental anlage may also explain the histogenesis of melano-cytes in intraosseous lesions of the jaw. However, the fate of melanocytes presented in the dental anlage of fetus has not been studied.

Another possible origin of intraosseous melanocytes of the jaw is that melanocytes migrate through the mesenchyme, not within the ectoderm. It has been indicated that the cells composing the sheath of the

peripheral nerve bundles are derived from the neural crest (SHANTHA and BOURNE, 1968), and that the peri-

pheral nerve ending is often of neural crest origin and very close to melanocytic differentiation (ACKERMAN, 1978; CRAMER, 1984). In addition, it has been establish-ed that not only melanocytes but also maxillofacial skeletal cells are derived from the neural crest in mammals (LEDOUARIN, 1982). From such evidence, the postulation is made that the melanocytes migrate through the mesenchyme and reach the intraosseous

region of the jaw. In fact, the present study showed that melanocytes exist in mesenchymal tissue around the dental anlage of the upper and lower jaws in dog fetuses, and that melanocytes appear neither in the oral squamous epithelium nor in the epithelial ele-ment of the dental anlage at this fetal stage. Such melanocytes found in the mesenchyme around the dental anlage during the fetal stage may be related to the melanocytes in the periodontal ligament of the

jaw, since the periodontal ligament originates from mesenchyme around the dental anlage.

The present study employed light microscopy with conventional staining procedures, and no conclusion can be drawn as to the physiological and pathological significances of melanocytes in the periodontal liga-ment of the dog. However, further histochemical studies to detect melanocytes in the periodontal liga-ment will clarify such points as their detailed distri-bution, frequency of occurrence, or behavior under both normal and pathological conditions.

Fig. 3. Scattered melanocytes and their cytoplasmic processes (arrows) in loose fibrous connective tissue of the

periapical region. CC cellular cementum of tooth root-apex. X 100

Fig. 4. Melanocyte (arrow) found in the mesenchymal tissue around the epithelial element of the dental anlage

(EP) of the lower jaw at an early fetal stage. x200

Melanocytes in Periodontal Ligament 421

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Dr. Yasunori TAKEDA Department of Oral Pathology Iwate Medical University School of Dentistry 19-1 Uchimaru, Morioka 020 Japan

武 田 泰 典020盛 岡市内丸19-1

岩手医科大学歯学部口腔病理学講座