MORPHOLOGY OF THE THYROID IN COASTAL AND NONCOASTAL POPULATIONS OF THE KOALA (PHASCOLARCTOS-CINEREUS) IN QUEENSLAND

The gross morphology, histology, and ultrastructure of the thyroid gla nd of the koala, Phascolarctos cinereus, is described. Generally, the glands were found to contain large-diameter follicles in association w ith an epithelium of low height. Morphometric analysis demonstrated a high relative thyroid weight (0.3 +/- 0.2 g/kg) for koalas compared wi th the 0.07-0.24 g/kg typical of eutherian mammals and 0.03-0.1 g/kg f ound in other marsupials. The relative thyroid weight of glands (0.33 +/- 0.21 g/kg) from the coastal population (less than 28 km from the c oastline) was found to be significantly higher (ANOVA: P = 0.007, sign ificant at the 1% level) than that for glands (0.21 +/- 0.11 g/kg) of noncoastal koalas (greater than 28 km from the coastline). Follicle si ze was positively correlated (at the 0.1% level) with relative thyroid weight in the overall koala sample. The presence of C cells, occurrin g singly in the epithelial layer, was demonstrated in electron microgr aphs. Structural features such as low epithelial height, large follicl e length and width, and large intercellular spaces in association with low concentrations of free TS (3.3 +/- 2.1 pM) and free T-3 (1.4 +/- 0.9 pM) as reported previously (Lawson et al., 1996) are consistent wi th an unusually low level of glandular activity in the koala thyroid e ven though iodine concentrations in the thyroid gland [4.7 +/- 1.6 mg/ g (dry weight)] as well as leaf [0.8 +/- 0.3 mu g (dry weight)] and so il samples [3.8 mu g/g (dry weight)] from the koalas' habitat appear u nremarkable. (C) 1998 Academic Press.