抄録
The age of six mature and one juvenile Stejneger's beaked whales Mesoplodon stejnegeri were estimated by examining the growth layers appearing in ground thin sections of tooth cementum under the light microscope. In order to determine a reliable observation method for counting the growth layers of tooth cementum, serial thin slices of root cementum were cut out from one well-grown tooth and examined using various histological methods. Observation of ground thin sections under dark field illumination was shown to give the highest contrast of growth layers of various dimensions, and hence chosen as the method to examine whole ground sections of the tooth samples. Using this method, growth layer groups (GLGs), or growth layers of the first order, most probably representing yearly deposition of cementum, were clearly identified and shown to decrease in width toward the root surface. The number of GLGs thus counted in the tooth cementum of each whale ranged from 15 to 35.5 for the adults, and two for the juvenile. Furthermore, analysis of root elongation rate and its relation to GLG counts of the individual teeth indicated a wide variety of growth patterns in tooth development, and that the extent of characteristic wear on the mesial edge of the tooth represents the period after eruption, and may not reflect the actual age of the whales.
