[Various magnocellular nuclei in the Merino sheep hypothalamus]

Light microscopic studies in the Nissl-picture have been carried out on the supraoptical nucleus, the paraventricular nucleus and the caudal part of the tuberomamillar nucleus. These studies have been conducted in neonates, 2 to 6 month-old lambs and 9 month-old Merino sheep. The above structures were found to undergo age-related changes which become evident in the following way: By increasing structure size and the increasing amount of the Nissl-substance. These changes were most marked at the caudal part of the tuberomamillar nucleus in which the neurons, mostly smooth up to the age of 9 months, acquire an "indented" shape. Simultaneously with these changes, vascularization of the supraoptical nucleus and the paraventricular nucleus increases. The relation of the nuclei under study to other structures of the hypothalamus, in particular to the commissura rostralis has evidently changed.