Significance of the localization of large granules in the thyrogonadotrophs of the musk shrew (Suncus murinus).

In order to examine the significance of small round and large granules in the thyrogonadotrophs of the musk shrew, we investigated the differences of numbers, distribution patterns and/or immunoreactivities of the large granules in the thyrogonadotrophs among the newborn, infant and adult musk shrews. As a result, we observed that the thyrogonadotrophs exist in newborn musk shrew, but the large granules were few in number and immunoreactivity on the granules was weak. The ratio of the number of large granule-containing cells to all LH beta positive cells was very low (6.2%) in newborn musk shrew, but high (98.2%) in adult male animals. We assume that thyrogonadotrophs of newborn shrews may not yet synthesize active beta-subunits, that the synthesis of beta-subunits is gradually increased with aging, and excessive products of beta-subunits may be stored in the large granules. It is known that female musk shrews are induced ovulators; therefore, we investigated morphological changes in the large and small granules in the thyrogonadotrophs at the LH surge which occurs after mating. We observed that the small granules were markedly decreased in number, or had almost vanished, 2 h after mating, but that the large granules still remained. Consequently, we assume that the large granules may be storage sites of excessive products of beta-subunits in older animals. On the other hand, small granules (release type granules) may be exhausted by release of a large quantity of gonadotrophin 2 h after mating, and this acute exhaustion of small granules may induce a sharp depression of the LH surge.