Thyroid development in the marsupial bandicoot, Isoodon macrourus

The postnatal structural development of the thyroid gland of the Australian native bandicoot, Isoodon macrourus, was monitored and the onset of thyroid function (i.e., the secretion of thyroid hormones) was determined. Thyroid glands were obtained from bandicoots at days 1, 3, 11, 12, 13, 21, 25, 30, 34, 35, 39, 46, 48, 50, 59, 61, 75, 79, 83, and 163 of age and from adult animals, and the tissues were examined with the transmission electron microscope. The thyroid gland of the newborn bandicoot consisted of undifferentiated cells with no lumen. Follicles were first observed at day 12 postpartum, after which time rapid follicular growth occurred. The first signs of thyroid hormone secretion were seen at day 30 postpartum. A peak in thyroid activity seemed to occur around day 50 postpartum, and it correlated with the rapid rate of development of thermoregulatory capacity and hair development which occurred over the last 2 weeks of pouch life.