Human cellular mixed stratified cementum: a tissue with alternating layers of acellular extrinsic- and cellular intrinsic fiber cementum.

Following the first description of the formal genesis of acellular extrinsic- and cellular intrinsic fiber cementum on growing human teeth, the present paper introduces cellular mixed stratified cementum as a particular tissue. Based on a large and diverse material (human premolars and molars) that was examined by various methods, such as microradiography, light microscopy using transmitted and polarized light, and scanning- and transmission-electron microscopy, the composition and structure of this type of cementum is described. Cellular mixed stratified cementum covers the apex, the apical third of the roots or more, and the furcational aspects of human teeth. With all methods employed, it was shown that this tissue consists in principle of acellular extrinsic- and cellular intrinsic fiber cementum, i.e. two major types of cementum that alternate or are in irregular sequences deposited upon one another. The dynamics of this layering and the fact, that, temporarily, root surfaces may remain unsupported by Sharpey's fibers as a consequence of this layering, are discussed.