This study carried out to examine the role of the thyroid gland in the control of reproduction in the viviparous Atlantic stingray, Dasyatis sabina. Thyroid activity of individuals in different reproductive states was assessed both by histological and ultrastructural examination of the gland, and by analysis of circulating levels of thyroid hormones. Stingray follicular cells possess characteristic features such as a single apical cilium, and a well-developed endoplasmic reticulum. They vary in size and shape, according to the activity of the gland. Immature individuals have resting glands and low levels of circulating thyroid hormones.
Females undergoing oogenesis have low serum hormone levels and moderately active glands.
High thyroid gland activity, and high levels of circulating thyroid hormones is seen in females, from ovulation until mid-gestation. Hormonal levels and gland activity decline during the second half of gestation and in post-partum females. The decline in activity of the maternal gland may correspond to the beginning of autonomous function of the fetal thyroid gland, i.e. the time at which the maternal supply of thyroid hormones is no longer needed by the embryo. Walsh, Cathy J., Luer, Carl A. Preliminary investigation of cell proliferation and programmed cell death in elasmobranch thymus. Elasmobranchs represent the earliest phylogenetic appearance of a clearly defined thymus.