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An electron microscopic survey was carried out on the human periodontal ligament (PDL), including a part of the gingival connective tissue attached to extracted tooth roots (11 functioning premolars and 6 nonfunctioning third molars) in order to examine the characteristics of microfilaments (6 nm) in cementoblasts and PDL fibroblasts. Microfilaments which were grouped in bundles with semiperiodic dense nodes or in meshworks just beneath the cell membrane were seen predominantly in the cells characterized by their ultrastructurally immature appearance. These microfilaments were more commonly observed in third molar PDL than in premolar PDL and, in general, more conspicuous in cementoblasts than in fibroblasts. The significance of microfilaments in human PDL is discussed, particularly in relation to cell differentiation and morphogenesis.

In order to elucidate the cytological characteristics of human cementoblasts which distinguish them from periodontal fibroblasts, the periodontal ligament and gingival connective tissue attached to 37 extracted teeth from 27 patients (ages 10-67) were analyzed by electron microscopy coupled with a morphometric procedure. The cementoblasts largely consisted of either immature or resting types of collagen-producing cells (CPC), both of which were poor in the rough endoplasmic reticulum and Golgi complex; these organelles were well-developed in the relatively less common active cementoblasts. The cementoblasts consistently revealed a higher mean-volume-density per tissue unit than the fibroblasts and sometimes were grouped into clusters with the formation of junctional apparatuses. The relative volume of glycogen particles per cytoplasm was significantly higher in the cementoblasts, whereas the rough endoplasmic reticulum was higher in the fibroblasts. From the present study, it is suggested that the cementoblasts are functionally less active CPC than the periodontal fibroblasts.