Keratinizing Potential of Human Crevicular Epithelium

T H E MORPHOLOGY of the crevicular epithelium in man differs from that of gingival epithelium. Gingival epi­ thelium is composed of stratified squamous epithelium normally exhibiting a keratinized or a parakeratinized surface, 1 , 2 while crevicular epithelium consists of a thin layer (5-15 cells) of nonkeratinized squamous epithe­ lium. 2 Human crevicular epithelium does not possess either a stratum granulosum or a stratum corneum, although these layers have been observed in the cre­ vicular epithelium of other species including the rat and hamster. 3 , 4 Since morphologic appearance is a major means of characterizing the state of keratinization of the epithelium, it is not surprising that most theories of keratin formation are based on the sequence of events seen in histologic and electron microscopic prepara­ tions. The nonkeratinizing nature of human crevicular epithelium has been reported by both light and electron microscopic investigations. 3 , 7 McHugh , 3 using polariz­ ing microscopy as well as histochemical analyses for the distribution of sulphydryl and disulphides at the surface of epithelium, concluded that crevicular epithelium in man and monkey is not keratinized. In a recent ultrastructural study of human gingiva, Takarada, et a l . , 8 did not observe any discernible keratin pattern in the crevicular epithelium and the coronal aspect of pocket epithelium. However, some of their specimens showed epithelia with one or more of the features associated with the process of keratinization, i.e., keratohyaline granules, birefringence of the spinous cells, membrane coating granules, and thickening of the plasma mem­ brane. Although the exact role of keratohyaline gran­ ules has not been well established, recent autoradi­ ographic and biochemical data strongly suggest that they are important precursors in the formation of the kerat in . 9 , 1 0 More recently, Listgarten provided further evidence that the human crevicular epithelium has a tendency towards keratinization. His observations were also supported by the appearance of keratohyaline and membrane coating granules. Absence of keratinization has been related to the hypothesis that nonfunctioning lining epithelium does not keratinize in the absence of functional stimuli. 1 1 However, conversion of a nonkeratinized epit' elium to a keratinized epithelium may not be related to direct physical stimulation of the epithelium, 1 2 but rather de­ pend on the environment in which it exists. Some recent studies notably those of Innes, 1 3 B a n o c z y 1 4 and Caffesse and Karring 1 5 have indicated that environ­ mental changes may indeed alter the keratinization potential of crevicular epithelium. Since the nonkera­ tinized nature of the crevicular epithelium has been considered as the "weak link" in the gingival unit, it may serve as the site of the primary gingival les ion. 1 6 2 4 Clinicians, therefore, have sought therapeutic meas­ ures (such as intrasulcular brushing) 2 5 to enhance the keratinization of the crevicular epithelium, thereby in­ creasing its effectiveness as a protective barrier. O n the other hand, recent embryologic studies on epithelialmesenchymal interactions 2 6 3 0 and grafting 3 1 3 3 experi­ ments have suggested that keratinization of this tissue may be limited by the dermal specificity of the underly­ ing connective tissue. 4 Thus, therapeutic measures de­ signed to enhance keratinization would prove to be unsuccessful. The popularity of the concept of "connective tissue as determinant" may be partly due to the difficulty of designing and assessing experiments to demonstrate the opposite. In any event, proving that connective tissue determines epithelial morphology does not altogether discount the possibility that epithelium also determines connective tissue morphology, or at least, its own mor­ phology. It is this possibility that has led some workers to postulate that mutual determination or an interac­ tion between both connective tissue and epithelium may control dermal specificity. 3 5 3 7 For example, A l vares and Myer 3 8 showed that there were differences between different types of orthokeratinized epithelia, such as that from rat cheek and rat palate, almost as great as those between keratinized and nonkeratinized epithelia. This reinforces Weinmann's 3 9 suggestion that the process of keratinization might be considered as a spectrum, varying in small steps between the extremes of orthokeratinization and nonkeratinization, rather than an all-or-none process. In order to further elucidate the keratinization po­ tential of human crevicular epithelium, we undertook a histologic study of this epithelium when its local envi­ ronment was drastically altered.