Morphology of the Thyroid in Coastal and Noncoastal Populations of the Koala (Phascolarctos cinereus) in Queensland
Veronica J. Lawson and Frank N. Carrick
Koala Study Program, Department of Zoology, University of Queensland, St Lucia, Queensland, Australia, 4072
The gross morphology, histology, and ultrastructure of the thyroid gland of the koala, Phascolarctos cinereus, is described. Generally, the glands were found to contain large-diameter follicles in association with an epithelium of low height. Morphometric analysis demonstrated a high relative thyroid weight (0.3 6 0.2 g/kg) for koalas compared with the 0.07–0.24 g/kg typical of eutherian mammals and 0.03–0.1 g/kg found in other marsupials. The relative thyroid weight of glands (0.33 6 0.21 g/kg) from the coastal population (less than 28 km from the coastline) was found to be significantly higher (ANOVA: P 5 0.007, significant at the 1% level) than that for glands (0.21 6 0.11 g/kg) of noncoastal koalas (greater than 28 km from the coastline). Follicle size was positively correlated (at the 0.1% level) with relative thyroid weight in the overall koala sample. The presence of C cells, occurring singly in the epithelial layer, was demonstrated in electron micrographs. Structural features such as low epithelial height, large follicle length and width, and large intercellular spaces in association with low concentrations of free T4 (3.3 6 2.1 pM) and free T3 (1.4 6 0.9 pM) as reported previously (Lawson et al., 1996) are consistent with an unusually low level of glandular activity in the koala thyroid even though iodine concentrations in the thyroid gland [4.7 6 1.6 mg/g (dry weight)] as well as leaf [0.8 6 0.3 mg/g (dry weight)] and soil samples [3.8 mg/g (dry weight)] from the koalas’ habitat appear unremarkable.