Q&A: How does the oral epithelium attach to the connective tissue?
Author: Sanketh DS, MDS
The two main components of the oral mucosa are the stratified squamous epithelium and the underlying connective tissue called lamina propria. The epithelium is of two types – the keratinized and non-keratinized epithelium. While the gingival and palatal mucosae are keratinized, the non-keratinized epithelium lines the rest of the oral mucosa.
Below the epithelium is the lamina propria or the connective tissue. For descriptive purposes, the lamina propria is divided into a papillary layer associated with the epithelial ridges and a reticular layer below it.
So, how does the epithelium sit on the connective tissue without ripping apart? Well, one reason is that the epithelial –connective tissue interface is not flat! The interface is rather undulating or wavy in which the connective tissue papillae interlock with the epithelial rete ridges. Scanning electron micrographs show that connective tissue has numerous conical papillae jutting out and penetrating the epithelium, thus giving it a firm attachment.
Well, that being said, the interlocking alone does not provide for a firm attachment. The basal cells of the epithelium do not sit directly on the connective tissue but is rather attached to it with the help of inter-cellular junctions called hemi-desmosomes. In the light microscope, when the tissue is stained with a special stain called Periodic Acid Schiff, the basement membrane can be visualized as a thin line between the epithelium and the connective tissue. At the ultrastructural level, the basement membrane is called basal lamina and consists of lamina lucida and lamina densa. Basal lamina consist of the proteins making the hemi-desmosomal junction.
Hemi-desmosomes are composed primarily of three proteins: transmembrane proteins, cytoplasmic adapter proteins and cytoskeletal filament. In the hemidesmosomal junction Bullous Pemphigoid antigen 180 (BP 180) and α6β4 integrins form the transmembrane proteins, BP 230 and plectin form the cytoplasmic adapter proteins and tonofilaments/cytokeratins form the cytoskeletal proteins. Together, these proteins along with other constituents of the basal lamina, like laminin and type VII collagen otherwise called the anchoring fibrils, help in firm attachment of the epithelium to the connective tissue.
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