2
Histological Features of Normal Oral Tissues

Histopathological evaluation of oral tissues requires a solid understanding and familiarity with the normal microscopic anatomy. Several tissues, such as teeth, are anatomically unique to the oral cavity. The authors hope that this brief chapter will assist readers with acquiring this basic understanding and enable them to interpret tissue changes accurately and meaningfully. We highly 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.

2.1 Oral Mucosa

Oral mucosa covers the entire surface of the oral cavity with the exception of the gingiva, which is a specialized form of mucoperiosteum (discussed below). The mucosa consists of surface epithelium and lamina propria. In most areas, there is also a distinct submucosa of fibrous connective tissues that is looser (usually) and supports other structures such as glands, muscle, and larger vessels and nerves. Depending on the location within the oral cavity, the mucosa varies in thickness and extent of keratinization. For example, particularly delicate buccal mucosa (also called vestibular mucosa) and sublingual mucosa line the inner surface of the cheeks and the floor of the mouth, respectively. In contrast, the palatal mucosa lining the hard palate is more durable, with thicker epithelium, occasional keratinization, and compact collagen within the lamina propria (Figure 2.1).

Figure 2.1 Histologic images of the hard palate of a five-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 area 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.

The non-keratinized epithelium (e.g. buccal and sublingual mucosa) has four layers that are distinguished by morphological differences - from deep to superficial: basal cell layer, prickle cell layer, intermediate cell layer, and superficial layer. Identification of layers is slightly different with keratinized epithelium (e.g. hard palate and dorsal tongue): basal cell layer, prickle cell layer, granular cell layer, and keratinized (usually parakeratinized) layer. In the oral cavity, unlike the epidermis, parakeratinization can be a normal pattern of squamous cell maturation/ differentiation. The lamina propria (also called mucosa propria) is divided into two layers - papillary and reticular. The papillary layer interdigitates with epithelial pegs and has capillary loops in close proximity to the epithelial basement membrane. With small vessels and bundles of collagen, the deeper reticular layer blends into the underlying submucosa (Figure 2.1).

The submucosa also varies in density and amount of collagen matrix. Minor salivary glands and their associated ducts are widely distributed throughout the oral submucosa (Figure 2.2). Occasionally, organized lymphoid tissue is present within the submucosa, particularly in the soft palate of young animals. As in the dermis, inflammation is often superficial, and cellular infiltrates are densely distributed within the lamina propria (see Figure 2.3).

Figure 2.2 Histologic image of the soft palate mucosa from a seven 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 seven-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 muscle 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.

Specialized mucosa of the tongue includes various types of papillae (filiform, fungiform, vellate). Some species, especially cats, also have filiform keratin projections on the hard palate mucosa. The rostral hard palate has an incisive papilla composed of cartilage that should not be mistaken for pathology.

2.2 Gingiva

The gingiva is a unique and distinct tissue that is characterized by surface epithelium and its underlying fibrous stroma which, together, comprise a mucoperiosteum. In the gingiva, there is no distinct lamina propria or submucosa, since the fibrous stroma extends from the epithelial basement membrane to the periosteal surface of the underlying alveolar bone of the jaw. The fibrous stroma is poorly cellular and overwhelmingly composed of densely organized, intersecting robust bundles of fibrillar collagen. Gingival stroma has an indistinct transition to periosteum - these two tissues are firmly connected by abundant extracellular fibers.

Like the oral mucosa, the gingiva has variation among specific anatomical sites. This variation is not as much a function of location along the dental arcades as it is location with respect to individual teeth. The attached gingiva is a more or less flat layer that covers alveolar bone; it is mucoperiosteum and organized as described above. The free gingiva represents the unattached margin of this mucoperiosteum as it meets and transitions to tissues that line the sulcus, then attaches near the cementoenamel junction of the tooth (Figure 2.4).

Figure 2.4 Histologic image of a normal premolar tooth from a seven 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 alveolar bone. Tissues within the pulp canal (PC) are distorted due to artifact.

The free gingiva is comprised of tissue from two different embryological sources, and this difference is readily appreciated in the mature animal. The outer surface is derived from gingiva, whereas the lining of the sulcus is derived from odontogenic origin, specifically the reduced enamel epithelium and dental follicle that once surrounded the crown of the tooth prior to eruption. As such, the free gingiva does not exist prior to tooth eruption. The sulcular epithelium has features of odontogenic epithelium, and the fibrovascular connective tissue under the sulcular epithelium is more similar to periodontal ligament than it is to fibrous gingival stroma. At the base of the sulcus, the site of attachment is particularly rich in fibers, similar to the attachment interface between gingival stroma and periosteum. The epithelium of the sulcus also attaches to the tooth at the base of the crown - this attachment is less obvious histologically, but some sections will show a thin epithelium reflecting onto the tooth surface (Figure 2.5). Loss of this epithelial attachment is an important step in the development of periodontal disease. To clarify, the attached gingiva is not the same as the attached epithelium. Attached gingiva covers alveolar bone and contrasts with the free gingiva. Attached epithelium is toward the base of the gingival sulcus where the gingiva anchors at the base of a tooth crown.

Figure 2.5 Histologic images of a normal tooth from a five-month-old, male Boxer dog. (a) A narrow cleft of clear space separates the free gingiva (FG) from the tooth crown (C), which is an artifact of processing. (b) Higher-magnification (area within the box of a) shows continuity between the sulcular epithelium and epithelium that attaches broadly and directly to the tooth surface. The broad zone of attachment is exaggerated in this case because the tooth is not fully erupted, but histological evidence of attachment is rarely seen in decalcified sections because, in the natural state, the epithelium adheres to enamel.

Gingival fiber bundles are abundant and unequally distributed within the tissues, tending to be most abundant where a ligamentous attachment is necessary between gingiva and bone, as well as between gingiva and the cervical portion of the tooth. Collagen fibers are by far most abundant (Figure 2.6). In addition to collagen fibers, the gingiva has fibers with elastic properties, including both elastin fibers and oxytalan fibers. These fibers are abundant within the cervical portion of the attached gingiva and are often organized parallel to collagen fiber bundles (Figures 2.7 and 2.8).

Figure 2.6 Histologic images of a normal maxillary premolar tooth from a five-month-old, male Boxer dog....