The distribution of nerves in human primary teeth.

Human primary teeth in the stages of late root formation, mot completion and root resorption were impregnated with silver to allow study of the distribution of nerves in the pulp. Neural trunks composed of many individual, myelinated nerve fibers enter the apical foramen of primary teeth and pass in a coronal direction, often in close association with blood vessels. Neural trunks in the crowns of the teeth divide into smaller branches, which divide further into myelinated nerve fibers, which become interwoven to form the parietal layer (plexus of Raschkow). An occasional unmyelinated fiber leaves this zone and passes into the odontoblastic layer of cells where it appears to terminate with no specialized ending apparent. No nerves were seen to enter the predentin or dentin. As primary teeth begin to undergo root resorption, degenerative changes such as thickenings, varicosities and fragmentation appear in the nerves. The quantity of neural tissue also decreases. The greater the amount of resorption, the greater are the degenerative changes. In teeth in which the roots are almost completely resorbed, only a small number of nerves remain. The pattern of distribution of nerves in primary teeth is observed to be similar to that of permanent teeth. The density of the innervation of the primary teeth, however, is not found to be as great as that of permanent teeth. The innervation of human primary teeth has received little attention when compared to similar research on permanent teeth. The lack of interest in the distribution of pulpal nerves in the primary dentition may result from the belief that the pulps are similar to those of permanent teeth or that primary teeth are not a lasting component of the oral cavity. Clinicians have long been aware of a lower sensitivity to pain in primary teeth than in permanent teeth which may be due to differences in number and/or distribution of their neural components. Another area in which information is lacking involves changes in the neural tissue of the primary teeth during the period of root resorption (and exfoliation). The present investigation is concerned, therefore, with the distribution of nerves in primary teeth during stages of root formation, root completion, and root resorption. Bradlaw ('36) studied neural degenerative changes in both human and animal primary teeth using a variety of silver impregnation techniques. He observed these changes to occur prior to exfoliation in some teeth but not in others. Mohuiddin ANAT. REC.. 159: 89-104. ('50) studied the primary and permanent teeth of the cat by means of both the Romanes' and Bielschowsky silver methods. The permanent and nonresorbing primary teeth exhibited patterns of innervation similar to those observed for human permanent teeth by Rapp, Avery and Rector ('51). All of these investigators observed neural fibers within the pulp to form arborizations or a parietal layer (plexus of Raschkow) adjacent to the dentin and then terminate between or among the odontoblasts. A few nerves were found in the predentin and dentin, but none were observed coursing within the dentinal tubules. Mohuiddin was interested in whether neural changes preceded or followed the onset of pulpal disorganization during exfoliation. He found degenerative neural changes appear prior to evidence of root resorption. He noted also that degenerative changes were limited to nerves 1 !-I or 1This study was supported in part by U.S.P.H.S. reasearch grant DE 01604 from the National Institute of Dental Research, National Institutes of Health, Bethesda Maryland and ant 695 of the H. Rackham Fund of ;he University oFMichigan. 2 Present address: Department of Pedodontics, School of Dentist?, University of Pittsburgh, Yittsburgh, Pennsylvania 15213.