A fine structural study of divalent cation-mediated epithelial union with connective tissue in human oral mucosa.

Following incubation in an isotonic saline solution containing 20 mM EDTA, human oral mucosa may be separated into its epithelial and connective tissue components. Ultrastructural study of the separated tissues reveals that the plane of separation is through the lamina lucida. Hemidesmosomal structure is altered by the separation process: the peripheral density is absent but a fine, generally filamentous material remains associated with the outer membrane leaflet of the hemidesmosome. Desmosome structure is not altered. An intact lamina densa remains attached to the connective tissue fragment. Oral mucosa incubated in EDTA-saline containing calcium, or its return to a divalent cationsupplemented medium after treatment with EDTA, prevents separation. By maintaining the structural integrity of the hemidesmosome, divalent cations appear to play a principal role in uniting oral mucosal epithelium to the lamina propria. Contact between stratified squamous epithelium and connective tissue occurs at the basal lamina. Adherence of the epithelial sheet to the connective tissue is belived to be mediated via specialized attachments (i.e., hemidesmosomes) located on the inferior membrane surface of the basal cell (Porter, '54; Weiss and Ferris, '54; Selby, '56; Odland, '58; Stern, '65; Kelly, '66). Fine hemidesmosomal filaments traverse an electron-lucent zone (lamina lucida) and terminate in an incompletely understood fashion in the lamina densa.' These structures are not the equivalent of the light microscopic basement membrane visualized following application of the periodic acid-Schiff procedure. The basement membrane of light microscopy is thought to be due to periodate-engendered aldehydes associated with unit collagen fibrils and protein-polysaccharides immediately beneath the lamina densa (Swift and Saxon, '67 ) . The region of contact between stratified squamous epithelium and connective tissue has been the subject of considerable attention since i t is intimately involved in the process of wound healing (Krawczyk, '71) and in disease processes such as AM. J. ANAT., 133: 431-454. carcinomatous invasion (Frithiof, '69) and severe blistering diseases (Pearson, '67; Wilgram, '67). Additionally, the contact region is of fundamental importance in biological phenomena such as transport of metabolites to the epithelium (Slavkin et al., '71), biogenesis of the basal lamina (Pierce and Nakane, '69; Nadol and Gibbins, '70), and the influence of connective tissue on epithelial behavior (Wessells, '64; Dodson, '67a,b). Mammalian basal cell connections (via hemidesmosomes) to the lamina densa appear tenacious, since separation does not occur with moderate pulling (Charles and 1 In this paper the term lamina lucida applies to the generally electron-lucent zone between the plasma membrane of the basal cell and the electron-dense zone commonly called the basal lamina which separates the epithelium from the connective tissue. The basal lamina will be referred to as the lamina densa. We are aware of at least six names applied to the basal lamina during the evolution of electron microscopic cytology. The first joint use of the terms lamina lucida and lamina densa was, as far as we can determine, by Low ('61) who, in the same paper, advocated the term "boundary membrane" be applied to the lamina densa. Stern ( ' 6 5 ) reintroduced the terms lamina lucida and lamina densa in a paper concerned with oral mucosal basal cells and the basal lamina. Subsequently, this terminology appears to be used almost exclusively among those studying the ultrastructure of oral mucosa (lamina lucida as described: lamina densa and basal lamina are used interchangeably). The terms lamina lucida and lamina densa are used here for convenience and without prejudice as to the exact interrelationship between the two zones.