The mechanism of secretion in the thyroid gland

The thyroid gland presents a peculiar problem in its mechanism of secretion. In previous publications we have shown that during rest the colloid is secreted into the centre of an alveolus-an enclosed space in which it accumulates. When the gland is stimulated to activity by exposure to cold, or in sympathetic fever, there is a disappearance of the colloid from this enclosed space, together with an opening up of the inter-alveolar and intra-alveolar capillaires. What happens now when the gland continues its activity after most of the colloid has disappeared from the alveolar lumen? Do the thyroid cells continue to pass their specific hormone into the alveolar lumen, from which it is then poured into the circulation ? Or, do they reverse the direction and pass the specific hormone direclly into the cirulation ? In the second part of this paper we shall show that such a reversal occurs under pathological conditions, so that it is a matter of importance to determine whether it takes place also under physiological conditions. Information on this point can be obtained by a study of the Golgi apparatus in the thyroid cells. Recent cytological work has shown that the Golgi apparatus is intimately related to certain phases of the process of secretion. In gland cells the specific products of secretion accumulate in relationship with the Golgi apparatus, which in these cells is always situated at the side of the nucleus directed towards the secreting surface of the cell. At the onset of secretory activity the Golgi apparatus enlarges, spreading out away from the nucleus towards the lumen of the alveolus. Droplets of secretion make their appearance in contact with the surface of the Golgi apparatus, and, breaking away from it, pass out into the cytoplasm towards the lumen. The position of the Golgi apparatus its polarity gives, therefore, a clue as to the direction of the process of secretion. In a previous paper we have shown that the Golgi apparatus of the thyroid cells is always situated between the nucleus and the alveolar lumen, that it undergoes the changes just described when the gland is stimulated to activity, and that no reversal of polarity can be obtained under the conditions which we studied.