Mitotic activity in the subodontoblastic cell-rich layer of adult rat molar pulps

Abstract

THE CHARACTERISTIC distribution and mode of development of a subodontoblastic cell-rich layer in the coronal pulp of rat and monkey molar pulps has recently been described (GOTJAMANOS, 1969a, b). This cell layer is analogous to that found in corresponding regions of human pulps and may therefore perform a similar function. STEWART (1965) claimed that human odontoblasts are replaced throughout life by cells from the cell-rich layer. Indeed, isolated cells are often found in the subodontoblastic cell-free zone of human and monkey pulps and these cells may be in the process of replacing degenerated odontoblasts as suggested by Stewart. If such a replacement mechanism exists, then it is possible that maintenance of the cell-rich layer occurs through mitotic activity of its constituent cells. One of the aims of the present investigation was to determine the degree of mitotic activity within the cell-rich layer of rat molar pulps. For this study, a total of 44 male hooded-Wistar rats was used and both the colchicine and tritiated-thymidine-radioautography methods employed for estimating mitotic activity (BERTALANFFY, 1964). In the first series of experiments, twelve 2-month old rats weighing 100-110 g were randomly assigned to one of 4 groups, 3 animals per group. Each animal received 2 intra-peritoneal doses, 1 PC per g body weight, of 3H-thymidine [TRA-61, Thymidine-6-T (n), Radiochemical Centre, Amersham, England]. The first dose was given 6 hr prior and the second 3 hr prior to sacrifice. To detect any diurnal variation in mitotic activity, each group was treated during one of the following periods: noon-6 p.m. ; 6 p.m.-midnight; midnight-6 a.m. ; 6 a.m.-noon. Following sacrifice, each animal’s upper and lower molar quadrants were processed histologically in the manner described previously (GOTJAMANOS, 1969a). Sections 4~ in thickness were coated with Ilford K-2 nuclear emulsion. The emulsion was prepared according to the method of CARO and VAN TUBERGEN (1962) and the sections coated by a modification of the dipping method of MESSIER and LEBLOND (1957). Following a 4 to 8 week exposure period, the radioautographs were developed in Kodak D-19 b developer, fixed, washed, stained with haematoxylin and eosin, mounted and examined under oil immersion fields for the presence of labelled cells. Pieces of tongue epithelium