Structure of the thyroid gland, serum thyroid hormones, and the reproductive cycle of the Atlantic stingray, Dasyatis sabina.

This study examines the role of the thyroid gland in the control of reproduction in the viviparous Atlantic stingray, Dasyatis sabina. Thyroid activity in individuals in different reproductive stages was assessed both by microscopic examination of the gland, and by analysis of circulating levels of thyroid hormones from the same individuals. The thyroid gland is a cylindric organ, embedded in a connective tissue capsule, and composed of follicles, i.e., monolayer spheres of thyroid epithelial cells. Stingray follicular cells possess several characteristic features, namely apical cilia and a well-developed endoplasmic reticulum. Cells vary in size and shape, according to the activity of the gland. No structural differences were observed between the thyroid glands of the two sexes. Both thyroid hormones, triiodothyronine, [T(3)], and thyroxine, [T(4)], were detected in the serum of all animals examined. Levels ranged from 1.3-2.6 microg/100 ml for total T(4), and from 1.2-2.6 ng/ml for total T(3). The T(4) levels did not vary significantly in any group. Immature individuals and females undergoing oogenesis had the lowest levels of circulating T(3) and mature females from ovulation throughout gestation had high thyroid gland activity and high levels of circulating T(3). J. Exp. Zool. 284:505-516, 1999.