Abstract:
Newborn marsupials do not have a thyroid gland at birth. The gland developswhile the young marsupial is in the mother’s pouch. The young brushtailpossum initiates secretion of thyroid hormones from its own thyroid at aboutDay 65 post partum. However, during the first threeweeks of pouch life thyroxine is passed from the mother to the young via themilk. To determine if this maternal thyroxine can effect organ development inthe young possum before it initiates secretion of thyroxine from its ownthyroid, the ontogeny of thyroid hormone receptors was determined in nuclearextracts of lung, liver and kidney by radioreceptor assay,using<emph type="7">125I-labelled tri-iodothyronine as ligand. Receptordensity was calculated for tissues removed from young possums at Days 25(n = 5), 50 (n = 4),100 (n = 3) and 150 (n= 4) and from adults (n = 5). Receptorswere found in possums of all age groups, including the small 25-day pouchyoung. Significant differences were not found in the receptor density betweendifferent tissues or at various ages. The association constantK<emph type="8">a (4 ·0 ± 2·6 L nmol<emph type="7">-1 for lung) was similar in different tissuesand at the various ages examined. The passage of thyroid hormones from themother to the developing marsupial via the milk may have a role in the slowdevelopment of organ systems early in pouch life by acting on thyroidreceptors in the pouch young. However, the functional maturation of thethyroid gland of the young possum, not an increase in receptors, appears tocoincide with the rapid increase in the rate of growth and development whichoccurs in later pouch life.