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.
Cell proliferation and programmed cell death, or apoptosis, are critical steps in shaping the immune repertoire in higher vertebrate thymi. The majority of proliferative activity was found in thymic cortex, as it is in higher vertebrates. Similarly, using a method to detect DNA strand breaks, apoptosis was observed throughout the thymus, but appeared to be more prevalent in the thymic cortex. TNC may be involved with removal of apoptotic cells from the thymus, as well as with proliferation and maturation of thymocytes. Understanding the function and significance of the thymus and TNC in a phylogenetically primitive vertebrate contributes to understanding the evolution of lymphocytes and lymphocyte maturation processes in higher vertebrate immune systems.