Oral mucosae, periodontal tissues and alveolar bone

• The oral mucosa is a membrane that continues anteriorly with the skin near the lips and posteriorly with the pharynx and the anterior pillars of fauces
• Mucosa arises partly from the ectoderm (lips, gingiva, cheeks, palate, etc.) and partly from the endoderm (tongue)
• The oral mucosa is composed of an epithelium (usually a stratified squamous epithelium, may be keratinised or non-keratinised) and a connective tissue component (lamina propria and/or submucosa)
• Depending on its function, oral mucosa can be classified into 3 types: masticatory, lining, and specialised
• Masticatory oral mucosa: mucosa that lines the masticatory surfaces (they are exposed to masticatory forces), including the gingiva and hard palate; has a keratinised epithelium
• Lining oral mucosa: has a basic lining function and occupies the whole oral cavity except for masticatory surfaces and the dorsum of the tongue; it is soft, pliable and has a non-keratinised epithelium
• Over time, non-keratinised epithelium may become keratinised if exposed to masticatory forces
• Specialised oral mucosa: lines the dorsum of the tongue, is specialised to receive taste sensation
• Mucosa mainly serves a barrier function and protects tissues against mechanical forces (e.g. compressive/shearing forces during mastication), microorganisms and toxins
• The mucosa is adapted to receive different sensations such as temperature, touch, pain, and taste to initiate reflexes such as swallowing, gagging, or salivation
• Oral mucosa also contains minor salivary glands so that the surfaces of the oral cavity can be kept moist, and helps in pH buffering as well as defense as the saliva contains antimicrobial components
• Certain lining mucosa may be especially permeable to certain drugs/molecules, e.g. the floor of the mouth
• Mucosa can also act as an immunological barrier for both humoral and cell-mediated functions
• Mucosa can become thinner and regress on the cervical region of the teeth with age, and also may become less moist due to reduced activity of the salivary glands
• Four types of nerve receptors are present in the mucosa (heat, cold, pain, touch) and they are most numerous in the lips and tip of the tongue
• Layers of oral mucosa: oral epithelium (stratified squamous, keratinised/non-keratinised), lamina propria (connective tissue of mainly collagen), submucosa (looser connective tissue with fatty tissue, blood vessels, and salivary glands)
• The main cells present in the oral epithelium are keratinocytes, but around 10% of non-epithelial cells are non-keratinocytes, such as melanocytes (produces pigment), Langerhan cells (antigen-presenting cells), Merkel cells (associated with nerve fibers to help transmit sensation), and inflammatory cells
• Keratinised epithelium consists of four layers: stratum basale (basal cell layer), stratum spinosum (prickle cell layer), stratum granulosum (granular cell layer), stratum corneum (surface keratinised layer)
• Stratum basale: layer closest to the lamina propria, it is made of a single layer of cuboidal/low columnar cells ans is proliferative; cells divide and migrate towards the surface of the epithelium, allowing it to regenerate
• Stratum spinosum: has a spiky appearance due to desmosomes that connect adjacent cells
• Stratum granulosum: cells containing keratohyaline granules in their cytoplasm
• Stratum corneum: keratinised layer composed of very flat cells with no nucleus; the keratin helps the epithelium withstand shearing forces and it is a non-permeable barrier for further protection
• Non-keratinised oral epithelium has no clear granular or keratin layer, instead it has a stratum intermedium made of stacks of plumper cells with more cytoplasm, and the most superficial layer is the stratum superficiale with similarly large cells except the outermost cells are flattened
• Lamina propria is composed of mainly collagen bundles, but may also have nerve fibers, blood vessels, and cells like fibroblasts
• The oral epithelium is avascular, so its metabolic needs are served via vessels of the lamina propria
• Lamina propria can be divided into the papillary layer (superficial with thinner and irregular collagen fibers, prominent in masticatory mucosa) and a reticular layer (thicker and organised collagen fibers, prominent in lining mucosa)
• The submucosa plays a role in nutrition, defense, and attaching lamina propria to the bone/muscle to withstand masticatory forces; it contains fatty tissue, large blood vessels, nerves, lymphatics, and minor salivary glands
• n the cheek, the underlying submucosa contains fat cells and small mixed salivary glands
• Not all areas of the oral cavity have submucosa, e.g. it is not present in attached gingiva and the hard palate
• The oral epithelium is linked to the underlying lamina propria by hemidesmosomes; there are hemidesomosomes along the length of the basal cell membrane and cytoplasmic tonofilaments insert into them while traversing filaments pierce into the cell membrane, immediately adjacent is the basal lamina (composed of lamina lucida and lamina densa) which binds with anchoring fibrils that interdigitates with collagen fibrils in the extracellular matrix
• Epithelium and lamina propria interface

  A) basal cell

  B) anchoring fibril

  C) lamina densa

  D) lamina lucida

  E) tonofilaments

  F) hemidesmosome

  G) collagen fibrils

  
• Gingiva can be free (thin, smooth part lining the cervical region of the tooth, not attached to the enamel surface) or attached (stipling appearance that is attached to the underlying tooth)
• Gingival sulcus: space created between the free gingiva and the tooth
• Interdental papilla: gingiva between two teeth
• Free gingival groove: microscopic feature, occurs because the free gingiva is freely hanging above the attached gingiva and demarcates their junction
• Dental col: region of gingiva located between two teeth, so it is more susceptible to plaque formation and inflammation
• Gingiva

  A) attached gingiva

  B) alveolar mucosa

  C) submucosa

  D) free gingiva

  E) free gingival groove

  F) gingival margin

  G) gingival sulcus

  H) junctional epithelium

  I) crevicular epithelium

  J) alveolar mucosa

  K) free gingival groove

  L) mucogingival junction

  M) attached gingiva

  N) free gingiva

  O) interdental papilla

  P) frenulum

  
• Attachment of the gingival epithelium to the tooth occurs at the junctional epithelium, which extends from the cementum-enamel junction to the bottom of the gingival crevice
• The junctional epithelium has two basal cell layers (one towards the connective tissue interface, one towards the enamel surface), and hemidesmosomes of basal cells attach to the basement membrane and the basement membrane is then attached to the dental cuticle of the enamel surface
• The cells of the junctional epithelium adjacent to the tooth attach by hemidesomosomes and a basal lamina (known as the internal basal lamina), while the basal lamina adjacent to the lamina propria is the external basal lamina