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Anatomy & physiology

Age-dependent changes in gross and histological morphology of the thyroid gland in South Australian koalas (Phascolarctos cinereus)

Graham, C, Woolford, L, Johnson, L & Speight, KN 2014, Australian Journal of Zoology, vol. 62, pp. 360-365.

This study investigated the morphological changes of the koala thyroid gland, which were determined to be linked to the age of the koala, rather than health status or sex. Juvenile koalas were mostly observed to exhibit typical thyroid gland morphology, while most adult koalas presented with colloidal goitre. The authors suggest an association of the findings with the low metabolism of koalas or dietary goitrogen exposure.

  A cohort of 36 euthanised koalas from Mount Lofty, consisting of eight juveniles and 28 adults, was examined for morphology of the thyroid gland. Colloidal goitre was observed in 69% of koalas, indicated by the presence of large colloidal macrofollicles on the thyroid glands, which were lined with flattened or low cuboidal epithelial linings. Twenty-five per cent of koalas had thyroid glands that had a mixture of typical follicles and large colloidal macrofollicles, indicative of a transitional stage from typical morphology to colloidal goitre. The development of colloidal goitre was determined to be associated with the age of koalas, rather than health status or sex, evident from a significant proportion of thyroid glands from adult koalas that were morphologically classified as colloidal goitre.

  The findings of this study provide a first-hand observation that development of colloidal goitre is largely age-dependent. The requirement of thyroid hormones may be reduced as koalas age, leading to a decrease in thyroid activity, which could be associated with colloidal goitre. Additionally, as metabolism is regulated by the thyroid gland, the low metabolism of koalas could be linked to low thyroid activity. The authors also suggest that the koalas could have been exposed to a goitre-inducing agent (goitrogen) in their diet. These may be present in the form of cyanogenic glycosides in the preferred eucalypt leaf species of Mount Lofty koalas. Ingestion of these goitrogens may have gradually caused colloidal goitre in the koalas in this study.

  Prior to this study, the koala thyroid gland had been relatively undescribed.  This study presents the first step in understanding the link between morphology and function of the koala thyroid gland.

 

Summarised by Daniel Chew

 

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