ORAL EPITHELIUM – PART I: INTRODUCTION, ULTRASTRUCTURE AND PROLIFERATION
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 two main components of the oral mucosa are the stratified squamous epithelium or the oral epithelium and the underlying connective tissue called lamina propria. The epithelium and connective tissue interface is wavy and undulating with the connective tissue papillae interdigitating with the epithelial ridges or pegs. The junction between the epithelium and connective tissue is called the basement membrane and is referred to as the basal lamina ultra- structurally.
Below the lamina propria is another layer of connective tissue called the submucosa consisting of minor salivary glands, fatty tissue, major blood vessels and nerves which attaches the overlying tissues to the underlying bone or muscle. However, it is absent in the gingiva or parts of the hard palate where the oral mucosa is directly attached to the underlying bone giving it a firm consistency.
The epithelial cells are called keratinocytes due to the presence of cytokeratins which are bundled into tonofilaments. Cytokeratins along with microtubules and microfilaments form the cytoskeleton of these cells.
Keratinocytes are firmly attached to each other with the help of intercellular-bridges called desmosomes which are a group of cellular junctions that are composed of transmembrane and cytoplasmic adapter proteins attaching with cytoskeletal filaments or tonofilaments.
Similarly, another group of inter-cellular junctions called hemi-desmosomes provide adhesion between keratinocytes and connective tissue. Hemi-desmosomes also have a set of transmembrane and cytoplasmic adapter proteins attaching to tonofilaments. Forces being applied on the oral epithelium are uniformly distributed over a wide area with the help of desmosomes and hemi-desmosomes.
The oral epithelium is a stratified squamous epithelium; meaning, the epithelial cells are arranged in different layers or strata.
Oral epithelium consists of cells that are continuously shed and renewed; shed from the surface or upper most layers and renewed by the lower layers. The cells in the lower layers divide and produce new cells, which in turn undergo maturation as they keep moving up layers and replace cells that are shed.
It is believed that, there are essentially 2 types of cells in the lower basal layers – the progenitor cells and maturing cells. The progenitor cells produce new cells and the maturing cells(the new cells produced) undergo maturation and differentiation. The progenitor population further is believed to consist of a stem cell group and an amplifying group. The stem cells function to produce basal cells and retain their proliferative potential and the amplifying group function to increase the number of cells which undergo further maturation.
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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