ORAL EPITHELIUM – PART II: LAYERS OF KERATINIZED AND NON-KERATINIZED EPITHELIUM
Author: Sanketh DS, MDS
The term oral mucosa or oral mucous membrane is used to describe the moist lining of the oral cavity. Oral mucosa is essentially made of an epithelium and an underlying connective tissue and is classified into 3 types based on its function as 1) masticatory mucosa, 2) lining mucosa and 3) specialized mucosa. The gingiva and hard palate constitute the masticatory mucosa, and bears the forces of mastication. The dorsum of the tongue forms the specialized mucosa because of the presence of taste buds which have a sensory function. The rest of the oral cavity amounting to at least 60% is lined by the lining mucosa.
The process of maturation may form 2 different types of stratified epithelium; keratinized and non-keratinized epithelium.
Keratinized epithelium lines the masticatory mucosa comprising of the gingiva and hard palate and also some parts of the specialised mucosa of the tongue. It consists of 4 layers or strata namely 1) the basal layer or stratum basale, 2) the prickle cell layer or stratum spinosum, 3) the granular layer or stratum granulosum and 4) the corneal/keratinized layer or stratum corneum.
Stratum basale/Basal layer
The basal layer is the lower most layer consisting of cuboidal or columnar cells just above the basal lamina. It consists of the progenitor and the maturing population responsible for continuous renewal of cells in the epithelium. It is attached to the connective tissue below with the help of hemi-desmosomes. Ultra-structurally, it consists of tonofilaments packed or aggregated into bundles called tonofibrils.
Stratum spinosum/Spinous/Prickle cell layer
The prickle cell layer or the stratum spinosum is so called because of the prickle like or spiny appearance this layer has, as a result of the cells shrinking during histologic preparations. As the cells shrink, it can be seen that the cells are joined by intercellular bridges which are ultra-structurally known to be desmosomes.
The spinous layers form the bulk of the epithelium and are large ovoid to polygonal cells. There is a significant increase in size of the cells from the basal layers also accompanied by more protein synthesis in the form of conspicuous tonofibrils.
An organelle called membrane coating granule/ lamellate granule or Odland body appears in the uppermost spinous layers. This is an elongated organelle with parallel lamellae and contains glycolipid.
Stratum granulosum/Granular layer
Cells in the granular layer are flat but have greater volume than cells in the spinous layer. Cells in this layer also possess the lamellate granule which fuse with the cell membrane and release their lipid content into the intercellular space between the granular and the corneal layer, forming a permeability barrier.
The granular layer is so called due to the presence of basophilic kerato-hyaline ganules. These granules are produced by ribosomes and contain proteins filagrin and loricrin.
These proteins along with involucrin form strong cross-links with tonofilaments. This aggregation of proteins form an envelope called “cornified cell envelope” inside the cells just below the plasma membrane and is an effective barrier to chemical solvents.
The intercellular lipid content produced by the lamellar granules along with the cornified cell envelope form an effective water barrier.
Stratum corneum/Corneal layer
The corneal layer is eosinophilic microscopically and consists of flat hexagonal cells called squames that are dehydrated and have lost all their organelles and nuclei. This layer essentially consists of tightly packed tonofilaments cross-linked with di-sulphide bonds aggregated in a matrix of filagrin and would hence possess a dense eosinophilic appearance under the microscope. The corneal cells possess modified desmosomes called corneo-desmosomes and thicker plasma membranes than cells in other layers. Corneal cells or squames are lost or shed actively and this process is called desquamation. Once shed, they are replaced by cells from the underlying layers. The corneal layer acts as a barrier to mechanical and chemical damage.
When the cells in the corneal layers have lost their organelles and nuclei, the stratified epithelium is referred to as an orthokeratinized epithelium. However, there are parts of the gingiva or palate, where the corneal layers have not lost all organelles and possess pyknotic nuclei. Such a pattern of maturation is called parakeratinization.
The non-keratinized epithelium lines about 60% of the oral mucosa that includes the buccal mucosa, labial mucosa, soft palate, alveolar mucosa, ventral surface of tongue and floor of mouth. It consists of a basal and a prickle cell layer and the outer 2 layers are called intermediate layer or stratum intermedium and superficial layer or stratum superficiale.
Basal and Spinous layer (Also referred to as Stratum Basale)
The basal and spinous layers are similar to the keratinized epithelium although the inter-cellular bridges in the spinous layer are not as visible as compared to the keratinized epithelium. Hence some prefer to avoid the use of the term prickle cell or spinous layer and consider only 3 layers in the non-keratinzed epithelium.
Stratum intermedium/Intermediate layer
The intermediate cells are bigger than the granular cells but have dispersed tonofilaments that are not aggregated into tonofibrils.
Lamellate granules or membrane coating granules are present in the intermediate cells and are circular in contrast to the elongated granule in the keratinized epithelium. These granules also secrete lipid material into the intercellular space between the intermediate and superficial layer, but do not form as effective a barrier as the one in the keratinized epithelium. The intermediate cells may sometimes harbour keratohyaline granules.
Stratum superficiale/Superficial layer
The superficial layer consist of flat cells that are not dehydrated, have lesser organelles and possess nuclei and dispersed tonofilaments. This layer thus is flexible and can withstand compression and other forces applied.
Under the microscope, the non-keratinized epithelium would possess a thin superficial layer of flat cells with nuclei in contrast to the dense eosinophilic appearance a keratinized epithelium would have.
Nanci A. Tencate’s Oral Histology. Development, Structure and Function. 8th ed. Elsevier;2013.
Kumar GS. Orban’s Oral Histology and Embryology.13th ed. Elsevier;2011.
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