Chapter 2
Features of Normal Oral Tissues

Brian G. Murphy1 and Jason W. Soukup2,3

1 Professor, University of California, Davis, California, USA

2 AVDC Founding Fellow - Oral and Maxillofacial Surgery

3 Clinical Professor, University of Wisconsin-Madison, Madison, Wisconsin, USA

Chapter Menu

1. 2.1 Introduction
2. 2.2 Oral Mucosa
3. 2.3 Tonsil
4. 2.4 Diffuse Salivary Gland
5. 2.5 Lip
6. 2.6 Incisive Papilla and Ducts
7. 2.7 Lateral Nasal Gland
8. 2.8 Teeth and Associated Tissues
9. 2.9 Bone

2.1 Introduction

Histopathological evaluation of oral tissues requires a solid understanding and familiarity with normal anatomy. Several tissues, such as teeth, are anatomically unique to the oral cavity. The authors hope that this chapter will assist readers with acquiring this basic understanding and enable them to interpret tissue changes accurately and meaningfully. We recommend Ten Cate's Oral Histology [1] as a highly detailed and authoritative resource for learning more about the unique histology, development, and physiology of oral tissues.

2.2 Oral Mucosa

The entire oral cavity, including the tongue, is lined by a variably pigmented, moist mucous membrane referred to as the oral mucosa, or more simply, mucosa. The oral mucosa is relatively impermeable and serves primarily as a protective barrier, but also has important sensory, secretory, and thermoregulatory roles. In dogs, the oral mucosa plays a critical role in thermal regulation. In contrast to the skin, the oral mucosa has no associated adnexa other than minor salivary glands, and features a variably keratinized, stratified squamous surface epithelium and underlying connective tissue or lamina propria. Dependent upon anatomic location, the mucosa varies in thickness and is divided into three different types: the more delicate lining mucosa (buccal, sublingual, and soft palate), the durable masticatory mucosa (gingiva and hard palate), and specialized mucosa (dorsal surface of the tongue). The masticatory mucosa, a type of mucoperiosteum, is mostly keratinized, immovable, and tightly bound to the periosteum of the underlying bone. The lining mucosa is nonkeratinized and is more flexible and expansile. The mucosa covering the dorsal surface of the tongue includes unique anatomic structures like papillae and neurosensory taste buds, and is therefore referred to as specialized mucosa.

Depending on the anatomic location, the oral mucosa varies from soft (lining mucosa) to firm (mucoperiosteum). The lining mucosa tends to gape when incised, while the masticatory mucosa does not. These differences have important clinical implications for surgical biopsies and local injections. The epithelium of the normal oral mucosa may contain small numbers of melanocytes, Langerhans cells, Merkel cells, and leukocytes; these cells are collectively referred to as nonkeratinocytes and may exhibit a clear halo. Neural crest-derived melanocytes are intraepithelial pigmented cells containing granules of melanin. Melanin-containing cells within the superficial lamina propria are more likely to be melanophages containing phagocytosed cytoplasmic granules of melanin. The precise identity of these intraepithelial nonkeratinocytes is best determined through special stains or immunohistochemical assays.

2.2.1 Buccal, Sublingual, and Soft Palate (Lining Mucosa)

The relatively delicate lining mucosa covering the sublingual/floor of mouth and soft palate is both flexible and distensible. In contrast to the mucoperiosteum of the gingiva, the lining mucosa is not tightly bound to the underlying bone and may exhibit a poorly delineated submucosa. The buccal and alveolar mucosa are thicker than the other lining mucosae; like the tongue, the buccal mucosa has submucosal skeletal musculature. The surface epithelium is nonkeratinized stratified squamous and is comprised of four layers, although each layer may not be visible histologically if the epithelium is attenuated. From deep to superficial, these layers include the basal cell layer, prickle cell layer, intermediate cell layer, and superficial layer. The immediately subjacent connective tissue is the lamina propria, below which lies the submucosa. The lamina propria is divided into two layers - papillary and reticular. The papillary layer interdigitates with epithelial pegs and has capillary loops proximate to the epithelial basement membrane. With small vessels and bundles of collagen, the deeper reticular layer blends into the underlying submucosa (Figure 2.1). In contrast to the submucosa of the gut, which is clearly separated from the lamina propria by the muscularis mucosa, the oral submucosa is less easy to recognize as there is no muscularis mucosa in the tissues of the oral cavity. The submucosa may contain adipose tissue, larger vessels, and nerves. The submucosa varies in density and amount of collagen matrix. Minor salivary glands and their associated ducts are widely distributed throughout the submucosa (Figures 2.2 and 2.3).

Figure 2.1 Histologic images of the hard palate of a 5-month-old male boxer dog. (a) There is a compact fibrous submucosa (bracket) between the palatal mucosa and the periosteum (asterisks) of the palatine bone. Neurovascular bundles (arrows) course within this layer. (b) The mucosal epithelium of the hard palate is thick relative to many other areas of the oral cavity, and the surface layers may be parakeratinized. The papillary (P) layer of the lamina propria interdigitates with the epithelium, and the reticular (R) layer contains more prominent vasculature.

Figure 2.2 Histologic image of the soft palate mucosa from a 7-year-old, spayed female domestic short-haired (DSH) cat. Minor salivary glands are abundant throughout the oral mucosa. These glands open directly onto the mucosal surface via small ducts.

Figure 2.3 Histologic image of the tongue and soft palate of a 7-year-old, spayed female DSH cat. At the level of the soft palate, the caudal oral soft tissues in the cat are very rich in salivary tissue. Mucous salivary glands are abundant dorsal to the palatal mucosa and within the submucosa and superficial skeletal muscles of the tongue. The tongue has both pale-staining mucous salivary glands and more basophilic serous salivary glands (arrows). In the center of the image, the lingual mucosa forms a fungiform papilla. The lingual mucosa is otherwise undulant, which correlates with mucosal ridges that can be seen grossly on the caudal dorsal surface of cat tongues.

2.2.2 Gingiva and Hard Palate (Masticatory Mucosa)

The relatively durable mucosa of the gingiva is subject to the potentially traumatic forces of compression, shear, and abrasion resulting from the mastication of food. As such, the gingiva has relatively thick, keratinized stratified squamous epithelium, consisting of the basal cell layer, prickle cell layer, granular cell layer, and a keratinized orthokeratotic to parakeratotic layer. The granular cell layer is generally indistinct because, unlike in the epidermis, parakeratinization is the normal pattern of squamous cell maturation and differentiation in the gingiva. In many veterinary species, the mucosa lining the hard palate features a series of grossly identifiable transverse ridges, or rugae. The epithelium of the hard palate is thicker than that of the gingiva and demonstrates occasional keratinization.

The junction between the mucosal epithelium and lamina propria is highly convoluted, with alternating epithelial rete pegs and connective tissue papillae. The fibrous stroma of the lamina propria is poorly cellular and contains densely organized, intersecting robust bundles of fibrillar collagen. The masticatory mucosa (mucoperiosteum) is directly attached to the periosteum of the jawbone without an intervening submucosa. The mucosa of the hard palate may have collections of minor salivary glands and adipose tissue in regions where it overlies neurovascular bundles.

A unique feature of the gingiva is that teeth penetrate through it. Because of its direct association with the tooth crown, the gingiva has several unique anatomic features. The attached gingiva is an essentially flat layer that directly covers the alveolar bone, specifically the bone of the alveolar margin. Mucoperiosteum of the attached gingiva is organized as described above. The free gingiva represents the unattached margin of this mucoperiosteum encircling the tooth crown, meeting and transitioning to tissues that line the sulcus, and attaching to the tooth near the cementoenamel junction (CEJ) (Figure 2.4). The mucogingival junction is the point where the attached gingiva meets the looser alveolar mucosa (Figure 2.5).

Figure 2.4 Histologic image of a normal premolar tooth from a 7-year-old dog. The free gingiva (FG) surrounds the base of the tooth crown, forming a sulcus (S). The depth of the sulcus (bracket) is from the gingival margin (asterisk) to the point where the sulcular epithelium attaches to the tooth (arrow). The attached gingiva (AG) covers the alveolar bone. Tissues within the pulp canal (PC) are distorted due to artifacts.

Figure 2.5 Gross image of canine free and attached gingiva.

Source: B. Murphy, University of California, Davis.

The free gingiva is comprised of...